Author Archives: Molly

Energy Efficiency and Building with Wood: Six Building Lifecycle Steps

Buildings have an impact on people and the environment throughout their entire lifecycle, starting with extracting resources from the earth to putting them back in the earth, or burning them, at the end of their lives. To evaluate the effect of buildings in this regard, everything from the energy they consume, the waste they generate, and the carbon dioxide (CO₂) they emit must be considered throughout the six major cycles below.

The combination of wood and the Passive House standard is a common-sense approach that can have a very positive lifecycle impact on the environment. In fact, according to a report from the U.S. Forest Service, wood in building products yields fewer greenhouse gases (GHG) than other common materials.*

1. Resource extraction
Everything in buildings comes from natural resources, some of which grow relatively quickly above the ground (e.g. wood), while others take millions of years to form below the ground (e.g. materials derived from fossil fuels). Taking a look at wood, the amount of heat, water, and pollution generated compared to extracting iron to produce steel, or extracting limestone to produce cement is significantly lower.**

The lifecycle of wood has a smaller impact. For example, the sun hits the tree, and the tree grows. It can be cut down with light machinery and a new tree is planted. It absorbs carbon, provides oxygen, and can be used in the future. In this context, it means a more sustainable production, compared to making concrete or steel, where digging for oil, coal, or natural gas and then burning it is a prerequisite to extracting the raw materials from the earth.

2. Manufacturing
The real ‘weight’ of a material—including resources, water, and energy used at the entry point of a manufacturing facility—compared to the material that comes out at the other end is referred to as the ‘ecological backpack.’ This measures the environmental impact of manufacturing products. Common sense suggests it requires less resources and energy to manufacture wood products compared to concrete and steel. Heavy timber and mass timber products can meet the same structural and fire requirements that also govern concrete and steel.

3. Off-site and onsite production
In many cases, the process of constructing buildings is antiquated, relying on manual and labor-intensive onsite processes. Other fields, such as manufacturing automobiles, have advanced considerably using automation and an industrialized system approach to designing and building, where the energy efficiency, in miles per gallon, can be guaranteed and the assembly occurs in a modern factory. Modern wood prefabrication processes can offer new opportunities and better working conditions. In this respect, building with wood can offer fast and efficient options for construction.

4. Operation
The natural resources needed to produce and deliver the energy consumed to heat and cool buildings for lighting, appliances, and water is the highest of all six lifecycle steps. While more efficient lighting and appliances can be specified, the only way to reduce long-term heating and cooling loads is to improve the building envelope. Airtightness is the most important element that has made the Passive House standard succeed. It can easily be achieved using modern wood carpentry, as discussed in this article.

5. Demolition
At the end of a building’s lifecycle, products are usually disposed of in landfills. Using a system approach to construction, buildings can be designed so they can be disassembled and separated for recycling. Design optimization, use of recovered wood, and specifying jobsite waste to be separated and taken to a local recovery center are all ways to reduce, reuse, and recycle.

6. Recycling
Wood from buildings can be recovered for use in other buildings or be employed to create furniture or other products. Even at the end of their second or third ‘life,’ wood products can be burned to generate energy or decompose naturally in the earth.

*See USDA Forest Service’s “Science Supporting the Economic and Environmental Benefits of Using Wood and Wood products in Green Building Construction.”
** For more, see the International Journal of Life Cycle Assessment article, “Wooden Building Products in Comparative LCA: A Literature Review,” by Frank Werner and Klaus Richter. Visit www.vhn.org/pdf/LCA-Wood-algemeen.pdf.

To read the full article, click here.

Energy Efficiency and Building with Wood

Photo © Norman A. Müller

Photo © Norman A. Müller

by Nabih Tahan, AIA

In creating energy-efficient buildings, one of the most important goals is to accurately predict during the design stage how a structure will perform when occupied—not only the natural resources used to produce it, but also the ongoing energy consumed for its regular operation. New opportunities combining modern carpentry techniques and the Passive House standard help achieve these goals.

There are three main ways to make a building energy-efficient—using less energy, generating more energy with renewable resources, or taking a combined approach of the two. In this author’s opinion, doing both is the best option. However, it is first important to eliminate heat losses due to design strategies and construction techniques.

In winter, heat in buildings is often needlessly lost due to conditioned air escaping through cracks in the envelope. To replace this heat, heaters are used. Eliminating air leakage and heat loss in buildings by making them airtight is the most important factor for making buildings energy-efficient.

New wood building systems have been developed to offer greater airtightness to minimize energy consumption. The building industry should focus on combining the two aspects of using renewable building materials and energy efficiency to achieve comfortable buildings, while optimizing indoor air quality (IAQ) and reversing the negative effect of climate change. (See “The Six Lifecycle Steps” to understand the advantages of combining wood and energy efficiency when specifying a building system.)

New building materials created through advanced versions of engineered wood are changing non-residential construction. With the right techniques, they can bring about improved energy effi ciency. Photo courtesy Nabih Tahan

New building materials created through advanced versions of engineered wood are changing  non-residential construction. With the right techniques, they can bring about improved energy efficiency. Photo courtesy Nabih Tahan

Europe is far ahead of North America when it comes to monitoring and reporting energy consumption of buildings and homes. This image comes from Ireland’s Building Energy Rating (BER) program for residences. Image courtesy Sustainable Energy Authority of Ireland

Europe is far ahead of North  America when it comes to  monitoring and reporting energy consumption of buildings and homes. This image comes from Ireland’s Building Energy Rating (BER) program for residences. Image courtesy Sustainable Energy Authority of Ireland

Using and measuring energy
It is not enough to design buildings using energy efficiency strategies—they must also be constructed accordingly and then meet the energy consumption goals of the design during operation. This is similar to a car manufacturer designing and producing a vehicle with a specific fuel economy, and then having the car actually meet that target. Everyone is familiar with comparing cars in terms of ‘miles per gallon,’ and now a similar unit of measurement for buildings is needed. At some point, this metric for energy consumption of a building or apartment might even be included in the Multiple Listing Service (MLS).

The European Union (EU) requires every building have an Energy Performance Certificate listing the energy consumption of space heating and cooling, water heating, lighting, and appliances. The certificate must be available to buyers and tenants when a building is constructed, sold, or leased. In Europe, the unit of measurement used for the certificate is kWh/m2/year.

In the United States, energy consumption in buildings is compared to the local energy code requirements in relative numbers as opposed to a consumption rate. Buildings are described as being a certain percentage better than the prevailing code, rather than having their actual consumption cited. As new energy code updates take effect, a similar unit of measurement comparable to the Energy Performance Certificate will be established. In the meantime, the Passive House standard is a good tool to measure and compare how much energy buildings are consuming.

For new-generation wood projects, walls are simply stood up and windows, siding, and trim are installed. Photo courtesy Nabih Tahan

For new-generation wood projects, walls are simply stood up and windows, siding, and trim are installed. Photo courtesy Nabih Tahan

Preparation for blower door test for LCT ONE—a Passive House-certifi ed, eight-story wood offi ce building in Austria. Photo courtesy Cree GmbH

Preparation for blower door test for LCT ONE—a
Passive House-certified,eight-story wood office
building in Austria. Photo courtesy Cree GmbH

 

 

 

 

 

 

 

 

 

 

Passive House and net-zero energy design
Passive House is a European-developed standard that has recently found its way to North America. The original German name ‘PassivHaus’ refers to both commercial and residential buildings. Rapidly gaining popularity in North America, the standard demands high-performing building envelope assemblies and airtightness.1

Passive House calculates energy consumption (in kWh/sf/year), and includes energy-use from space heating, cooling, and ventilation systems, along with water heating, lighting, and appliances. Under the standard, the maximum energy allowed for heating and cooling is 1.4 kWh/sf/year. The standard has become successful because it has proven it can accurately predict, during the design stages, the building’s eventual energy consumption. (This is comparable to a car company claiming a car will get 30 mpg and proving to be correct.) Predicting the energy consumption in the design stages is done with the Passive House Planning Package—an energy modeling tool.

The Passive House standard is a whole building strategy that harmonizes all aspects of a structure beginning with data on local weather and solar orientation and continuing with the design, layout, foundation, framing, and insulation systems to reduce, or even eliminate, thermal bridging. It also optimizes specification of the openings (i.e. doors, windows, and curtain walls), heating, cooling, and ventilation systems, along with lighting and appliances.

Specific to thermal properties, it is important to incorporate building materials that have low thermal conductivities, and design details that minimize thermal bridging. By nature, wood is ideal for this, made up of thousands of open cells that make it difficult to conduct heat. In fact, the thermal properties of wood products are 400 times better than steel and 10 times better than concrete.2

Most importantly, Passive House has a specific requirement for airtightness, which is where the biggest connection to modern wood carpentry is made. Airtightness is measured with a blower door test.3 Air is either pumped into or sucked out of a building to see how much air is leaked, in both pressurized and depressurized states. This is similar to fixing a leak in a bicycle inner tube. Air is pumped into the tube and placed in water and the leaks are found by following the bubbles. For buildings, smoke or other instruments are used to find leaks during a blower door test. If the test is performed before the building envelope is covered up, the leaks can be sealed to make the building airtight.

Lighting and all appliances (including ovens, cooktops, refrigerators, toasters, and computers), generate some heat. Instead of allowing this heat to escape through a leaky building envelope, it is trapped inside a tight building envelope. A mechanical ventilator, with a heat recovery component, brings in fresh air into the living spaces and removes the same amount of stale air from kitchens and bathrooms.

The heat in the outgoing stale air is transferred to the incoming fresh air inside the ventilator. This recycling (or ‘U-turn’) of ‘free’ heat’ that comes out of everyday appliances and lighting can dramatically reduce a building’s energy consumption. Future energy codes in North America will also be targeting a net-zero energy standard. A net-zero energy building is hooked up to the grid and draws electricity and natural gas from the grid. The building also has a source for generating renewable energy such as solar or wind energy. During a one-year period, the amount of energy a building draws from the grid has to equal the energy it generates from renewable resources. The easiest way to meet the net-zero energy standard is to consume less energy, which is where the strategy of Passive House, in combination with modern carpentry, becomes valuable.

To combine this strategy with the choice of materials and construction methods, use of modern engineered wood products—stable, cut accurately with computerized equipment, and assembled under a controlled environment—results in airtight buildings that are automatically energy-efficient.

Prefabrication and modular wood construction is helping building designers achieve this while increasing the speed of construction and reducing project cost. Energy-efficient design is also becoming more important in North America, as the U.S. Department of Environment (DOE) has a goal of all new commercial buildings being ‘net zero’ by 2025.

This photo shows framing lumber cut with computer numerical control (CNC) machinery. Photo courtesy Nabih Tahan

This photo shows framing lumber cut with computer numerical control (CNC) machinery. Photo courtesy Nabih Tahan

Wall elements are produced on tables. Photos courtesy Cree GmbH

Wall elements are produced on tables. Photos courtesy Cree GmbH

 

 

 

 

 

 

 

 

Wood technology
When talking about wood construction, the reference points are traditionally stick-frame, or light-frame residential construction. Modern wood construction falls under the category of heavy timber, using large-dimensioned posts and beams. A new category of mass timber includes cross-laminated timber (CLT)—sometimes referred to as ‘plywood on steroids.’4

The post-and-beam method of wood construction was prevalent in many cities at the beginning of the last century, before the industrial revolution introduced concrete and steel. Now, wood products are beginning to increase again in popularity due to awareness over some of the negative environmental effect of products extracted and manufactured with intensive use of fossil fuels. Of course, this is not to say one product type is always better than another—each material has special properties and they should be combined to make hybrid buildings.

Modern timber products for structural framing are referred to as engineered lumber. These members use smaller pieces of wood, eliminating the need to harvest large trees. The ends of these smaller pieces are finger-jointed and glued to make longer pieces. Several long pieces are laminated together to make large glued-laminated (glulam) post and beams. This lumber is stable and will not shrink or twist because it is dry, which is a great advantage for airtightness, performing much better than traditional stick frame wood with higher moisture content.

For fire safety, heavy timber is allowed under the 2012 International Building Code (IBC). Wood burns approximately 38 mm (1.5 in.) per hour. Therefore, the fire regulations allow the size of structural members to be increased by 38 mm per hour for each exposed member. If a member requires two-hour fire protection, 76 mm (3 in.) are added to the size required structurally. Light-frame construction is similar to kindling for a fire. Heavy timber construction cannot be ignited without kindling; like throwing a large log into a fire, heavy timber members will char, protecting their structural integrity and strength.5

The modern process of carpentry is based on digital, computerized information. The carpentry company receives the computer-aided design (CAD) drawings from the architect. They transfer the drawings to 3D-CAD/computer-aided manufacturing (CAM) program where the wood frame can be looked at in 3D and the structure can be optimized. This is called optimal value-engineered, or ‘smart,’ framing.

Every piece of wood is placed in an exact location and has a purpose. Unnecessary framing members are eliminated and replaced with insulation to optimize energy performance.

The material is then fed to a wood-cutting machine that uses the computer numerical control (CNC) data to precisely cut all the elements. The engineered lumber is stable and is cut precisely and eliminates waste since the members can be 12.1 to 18.2 m (40 to 60 ft) long. The individual pieces are assembled together in a facility, working on tables and prefabricated into wall, floor, and roof elements. These components can be quickly erected onsite in an airtight manner, ensuring energy-efficient construction.

Instead of asking carpenters to measure, cut, and assemble walls and floors onsite, employing labor-intensive processes, the carpenters are moved in a controlled environment, where they are given the drawings and pieces for each component to be assembled, making use of overhead cranes and forklifts to protect their bodies.

The advantage of this process is it optimizes the construction, guarantees stable material, and accurately cut pieces and assembled components that fit together tightly. Specifically designed tapes and gaskets are used at the intersection of panels to prevent air leakage. As proof of performance, the building can be tested for airtightness by a third party by administering a blower door test to meet the ≤ 0.6 air changes per hour @ 50 Pascal pressure—one of the main requirements of the Passive House standard.6

This wall element was craned in place. These types of components can be completed onsite in an airtight manner.

This wall element was craned in place. These types of components can be completed onsite in an airtight manner.

This is an example of precision cutting using CNC machinery. Photo courtesy Nabih Tahan

This is an example of precision cutting using CNC machinery. Photo courtesy Nabih Tahan

 

 

 

 

 

 

 

 

Conclusion
Combining modern wood technology and Passive House strategies helps save resources and achieves energy efficiency in buildings. The fundamentals of modern carpentry are based on:

  • optimizing the timber structure;
  • uses stable engineered lumber;
  • cutting the material accurately using industrial machinery;
  • prefabricating components under a controlled environment; and
  • assembling them quickly onsite to be cost-competitive, while automatically meeting airtightness requirements.

Using this construction process and modern carpentry skills, the building envelope’s thermal performance and airtightness can be predicted during the design stages. To prove the performance, the envelope is tested for airtightness with a blower door test after assembly. Similarly, the energy performance of a building can be predicted using the Passive House standard during the design stages—through the thousands of certified buildings in Europe, the actual energy consumption during occupancy has been shown to match the predicted values.

Overall, the use of new modern wood technologies can have a positive effect on construction industry from job creation to reduced environmental impact.

The design team, including architects and engineers, can collaborate to ensure wood-based building envelopes can meet these high-performance standards. In collaboration with the Passive House consultant, the design team can determine which layer in the wall/floor/roof assemblies prevent air leakage and include details on how to seal intersections and penetrations.7 They can model heat transfer effect in building components to eliminate or reduce thermal bridging and specify available products for taping and sealing joints proven to be durable and long-lasting.8

Notes
1 There have been dozens of certified projects in North America, and the numbers are growing fast. There are two competing U.S. organizations, and both of them have databases and listings for these projects. The North American Passive House Network (NAPHN)—www.naphnetwork.org—is directly affiliated with the Passive House Institute (PHI) in Germany, while the Passive House Institute US (PHIUS)—phius.org—was founded by someone who used to work with PHI, before branching off.
2 For more information, see Naturally Wood’s “Green Building with Wood–Module 3” at www.naturallywood.com/sites/default/files/Module-3-Energy-Conservation.pdf.
3 The Passive House Institute requires the test to meet the European Standard EN 13829, Thermal performance of buildings: Determination of air permeability of buildings, fan pressurization method. In the United States, ASTM E1927, Standard Guide for Conducting Subjective Pavement Ride Quality Ratings, and ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, would be used with Resnet Protocol Chapter 8, “Enclosure and Air Distribution Leakage Testing.”
4 The limitations of these types of wood assemblies are the current building codes or special approval by a jurisdiction for alternative means of design and construction. Obviously, it is not a one-for-one switch between wood and concrete—the architect and engineers have to run calculations for both materials to ensure the design meets the structural, seismic, acoustic, and thermal requirements. At the moment, the building codes limit wood buildings to 75 ft (i.e. 23 m). The wood industry is in the process of testing new wood products such as CLT for fire and seismic performance. This author’s company is in the process of negotiating a partnership/joint venture agreement to participate in an Expression of Interest for a 16 to 18-story student residence building utilizing advanced wood-based building system, physically demonstrating the applicability of wood in the tall building market.
5 For more on wood and fire resistance, see “Design of Fire-resistive Exposed Wood Members,” by Bradford Douglas, PE, and Jason Smart, PE, which appeared in the July 2014 issue of The Construction Specifier.
6 Visit www.passiv.de/en/02_informations/02_passive-house-requirements/02_passive-house-requirements.htm for more information.
7 This is a consultant that collaborates with the architect, and structural, mechanical, and electrical engineers. During the design stages, the passive house consultant begins the energy modeling in the Passive House Planning Package (PHPP) tool to determine the heating, cooling, and electrical loads. The PH consultant then submits the calculation to either PHI or PHIUS for pre-certification. The MEP engineers use these calculations to design the system, and the architect and structural engineers design the details to eliminate or reduce thermal bridging.
8 For more, see this author’s previous article, “LCT ONE–A Case Study of an Eight-story Wood Office Building,” in the March 2014 issue of The Construction Specifier.

Nabih Tahan, AIA, is an international architect, Passive House consultant, and CEO of Cree Buildings Inc. For more than 30 years, he has honed his knowledge in architecture, energy efficiency, and sustainable timber-based construction methods through work in Austria, Ireland, and the United States. Currently, Tahan is guiding Cree Buildings to establish a systems approach to design and construction, combining wood and energy efficiency strategies to build single-family and multi-family residential projects, along with office buildings. He can be contacted at nabih.tahan@creebuildings.com.

10 Key Questions about Exterior Shading

Photo © Richard Wilson. Photo courtesy Draper Inc.

Photo © Richard Wilson. Photo courtesy Draper Inc.

by Richard Wilson, B.Sc.

Over the last decade, exterior shading has become more popular in the U.S. construction market. However, many architects and building owners still have limited knowledge about these systems and why they should be considered part of the building design.

This article explores 10 frequently asked questions about exterior shading, while providing insight into available systems and how they can be an important part of the building’s environmental control.

1. What exterior shading systems are available?
A wide range of exterior shading systems are available, but they can be broken down into three broad categories of systems:

  • fixed louver;
  • adjustable louver; and
  • retractable.

Fixed louver systems include projecting sunshades generally installed at the head of the glazing (i.e. brise-soleil systems), as well as fixed vertical or horizontal louvers installed in front of the glazing. These systems are designed to remain in place at all times and need to be able to withstand all weather, including wind, ice, and snow. The shading performance varies depending on the system’s projection and the louver profile selected, as well as the angle of the louvers and the spacing between them. These items need to be evaluated during the design process to ensure the system provides sufficient shading during periods when solar gain is an issue.

Brise-soleil systems only address high sun angles and, as a result, they generally will only be effective on south or near south-facing elevations. They also only provide shading during the summer. During the winter months, the low sun angles mean these systems provide little or no shading.

The effectiveness of fixed horizontal or vertical louvers depends on louver size, angle, and spacing. These systems normally only shade higher sun angles in order to allow views to the exterior, and are most effective on south-facing elevations. They can be installed on east and west elevations, but will normally not protect occupants from the low sun in the early morning or late afternoon.

Vertical and horizontal adjustable louver systems can be motorized, allowing louver angles to be adjusted to give more responsive shading, particularly if they are connected to an automated control system. The systems do not retract—they will always remain in front of the glazing—but can be moved between the fully open and closed positions.

The method of control can range from switch operation, where occupants operate the system according to their needs, to a fully automated system that responds to the sun conditions and adjusts the louver angle to prevent any direct sun penetration. The systems are generally controlled independently of the interior lighting systems; ideally, levels are automatically adjusted to supplement natural daylight where required. Since the systems only operate from time-to-time, and only for a few seconds to adjust the louver angle, energy usage is not significant, particularly compared with the savings that can be achieved through a reduction in HVAC requirements.

This project features an exterior venetian blind assembly. Photo courtesy Draper Inc.

This project features an exterior venetian blind assembly. Photo courtesy Draper Inc.

This brise-soleil system was installed at the Southern Alberta Institute of Technology (SAIT) in Calgary, Alberta. Photo © Ralph Wilson. Photo courtesy Draper Inc.

This brise-soleil system was installed at the Southern Alberta Institute of Technology (SAIT)
in Calgary, Alberta. Photo © Ralph Wilson. Photo courtesy Draper Inc.

 

 

 

 

 

 

 

 

 

 

 

 

2. Why is an exterior system more effective than an interior one?
In broad terms, an exterior system is better than an interior one because it prevents a large part of the sun’s energy from reaching the glazing and entering the building. If the solar energy does not get into the building, it does not have to be dealt with.

Energy from the sun is short-wave and carries little heat. Heat is only produced when the solar energy is absorbed by a surface (e.g. carpeting, furniture, clothing, or skin) and is then radiated as long-wave infrared (IR) energy.

An interior shading system can:

  • allow solar energy to pass through;
  • absorb solar energy; and
  • reflect solar energy back through the glazing.

The reflected solar energy is not an issue—it remains short-wave and does not cause any heat gain. The transmitted energy is absorbed by surfaces in the building and is radiated as heat. The energy absorbed by the shading system is then radiated as heat and most of this heat is then trapped inside the building, particularly if low-emissivity (low-e) glazing is used.

An exterior system is similar to an interior one with regards to the transmittance, absorption, and reflection of solar energy. Anything absorbed by the shading system, however, is radiated as heat on the building’s exterior. Since glass is not transparent to long-wave energy, little of this radiated heat gets inside the building. Accordingly, an exterior system eliminates one of the two sources of heat gain, resulting in much greater reduction in solar gain inside the building.

Performance data is readily available for shading fabric. Consider a popular fabric in a grey-white color and a particular type of glazing (e.g. low-e, argon filled, double-glazed unit), the ‘g’ value is 0.13 when the fabric is installed on the exterior, but increases to 0.43 when installed on the interior. The ‘g’ value is the sum of the direct and secondary solar transmittance into the building. The secondary transmittance comprises the amount of solar radiation absorbed by the combination of glazing and shading system which is then convected or radiated into the building. In North America, the ‘g’ value is also known as the solar heat gain coefficient (SHGC).

Even with a white fabric, which has the highest level of reflectance, the comparison is 0.16 for an exterior installation compared with 0.36 for an interior one.

The message is therefore straightforward—for the most effective solar control, the shading system should, wherever possible, be installed on the exterior. There will be some situations where this is not practical—for example, high-rise buildings with 25 floors or more. In these cases, the use of a shading system inside a ventilated double façade is a potential approach, although shading is just one of many influencing factors when pursuing this type of façade construction.

3. What are the main benefits of an exterior shading system?
The primary benefit of an exterior shading system is a reduction in HVAC requirements. As discussed earlier in this article, exterior shading blocks a large part of the solar gain before it comes through the glazing and into the building. If there is less solar gain, then the size of the HVAC system can be reduced. This results in a saving in the initial capital cost—which can wholly or partly offset the shading system’s cost—as well as the ongoing running costs. The most effective shading systems, such as exterior venetian blinds, can block more than 90 percent of solar gain, having notable impact on reducing the HVAC requirements.

Some buildings, however, need to be cooled in the summer, while also have heating requirements in the winter. If a retractable exterior shading system is used, it can be turned off in the winter months, allowing the solar gain into the building and providing an element of free heating. During those months, glare and light control issues would be addressed with an interior shading system such as a roller shade.

Another benefit is natural daylighting. Exterior shade systems can help optimize the use of diffuse daylight to illuminate interiors, reducing the need for artificial lighting. More than 30 percent of the energy costs of an office building relate to artificial lighting, so if lighting needs can be reduced, significant savings can result.

A well-designed shading system also contributes to comfortable working conditions which can lead to increased productivity. A good shading system manages both heat and glare while providing access to outside views. Finally, using exterior shading systems can significantly contribute to a building’s appearance; it can become a design feature as well as one bolstering efficient building performance.

CS_October_2014_HR-44

The orientation of the glazing has a signifi cant impact on what solution works best. Shown above is the impact of orientation on incident solar radiation for a building in Indianapolis, Indiana.

The orientation of the glazing has a significant impact on what solution works best. Shown  above is the impact of orientation on incident solar radiation for a building in Indianapolis, Indiana.

 

 

 

 

 

4. Can exterior shading systems be used on both new and existing buildings?
It is always easier to apply exterior shading systems to a new building, as integration issues can be reviewed and connection details developed during the design phase. Fixed exterior louver systems can exert significant loads on the façade, and if they are being attached to the curtain wall, mullions might need to be reinforced to support them. Even with lighter, retractable systems, such as venetian blinds, it is helpful to be able to discuss attachments with the curtain wall contractor during the design phase so brackets can be specified to avoid problems such as cold bridging.

However, it is possible to apply external shading to existing buildings. While the original building design would not have anticipated exterior shading, structural elements can be incorporated as required, to allow installation onto the existing façade. The structure, rather than the façade, would then accommodate applied loads (i.e. wind, ice, snow) as well as the weight of the system itself.

If an operable system is going to be used with an existing building, it will be necessary to look at the electrical requirements and determine how conduit and electrical cabling can penetrate through the façade to allow connections to be made to the blinds or shades.

5. What are the common methods of attachment to the building façade (and what issues need to be considered)?
With both new and existing buildings, installation of an exterior shading system might involve attaching directly to the curtain wall mullions, to brick or concrete masonry units (CMUs), or through cladding to steel structure. It is probable different brackets will be required for each situation, and these will often be developed to meet the specific project requirements.

These louvers were designed for custom window shapes. Images courtesy Draper Inc.

These louvers were designed for custom window shapes. Images courtesy Draper Inc.

Exterior roller shades and venetian blinds are generally installed just above or at the top of the glazing. They are relatively lightweight and, because they are retracted when the wind speed exceeds a defined level, they do not apply significant loads to the façade. This means lighter aluminum brackets can normally be used to connect the head box to the façade. Pre-tensioned side guide wires are also generally used to prevent movement of the shading system under wind load (the other option is extruded side guides) and each of these will be tensioned to approximately 22.7 kgf (50 lbf).

Since exterior louver and brise-soleil systems remain fixed in place in all weather conditions, they apply more significant loads to the façade. The brackets for the system will therefore be designed in accordance with the loads defined in local building codes, and bolts or other fasteners will also be selected based on the maximum loads. If the systems are being connected to the curtain wall, it is possible the mullions will need to be reinforced with steel. This is particularly the case with brise-soleil systems, which project some distance from the façade and, as a result, generate significant turning moments and shear forces at the connection points. With these types of systems, structural calculations will always be undertaken to determine the applied loads and the impact on the façade design and building connections.

Other issues that need to be considered include separation of dissimilar metals, cold bridging, and water penetration, as well as relative expansion and contraction between the shading system and the façade. Given these issues, it is strongly recommended the shading requirements are reviewed and discussed during the early stages of the design process.

6. Will the building’s location and the glazing’s orientation influence the choice of exterior shading system?
There are many factors influencing the choice of an exterior shading system. Two significant ones are building location and glazing orientation.

As seen in Figure 1, the movement of the sun during the year (shown by the blue lines) is significantly different between two extremes in the United States—Miami, Florida, and Anchorage, Alaska.

In Miami, the sun angle is approximately 86 degrees, and almost vertical in the sky, at 12:00 p.m. on June 21. In Anchorage, the sun has a peak altitude angle of approximately 51 degrees, which is not much greater than the highest winter sun angle in Miami of 41 degrees. The sun also sets much further to the south in Anchorage during the winter compared to Miami.

Given the differences in sun movement, the optimal shading strategies will be different. In Miami, fixed projections will be effective; while in Anchorage, retractable and adjustable systems offer much more flexibility in controlling solar gain.

The glazing’s orientation will also have a significant impact on system choice. The graph in Figure 2 shows the incidental solar radiation on different orientations of glazing for a building in Indianapolis, Indiana. As expected, the solar radiation on the north elevation is the lowest as there is no direct sun. However, the background radiation is still reasonably significant, particularly in the summer.

The solar radiation on the east and west elevations is similar, with the maximum values occurring in the summer. Interestingly, the maximum solar radiation on the south elevation occurs during the colder months. In the middle of the summer, the high sun angles mean the incident radiation falls. The maximum exposure to solar radiation, however, occurs at the roof. Therefore, any skylights will potentially cause significant solar issues.

Given the variations by façade, fixed systems might work on the south elevation, but operable ones will be better east and west. Although vertical louvers might work on the east and west elevations, horizontal ones are generally better for controlling the solar gain and allowing views to the exterior.

This installation in Holland features solid-screen, ‘zip’ system installation. Photo © AVZ. Photo courtesy Draper Inc.

This installation in Holland features solid-screen, ‘zip’ system installation. Photo © AVZ. Photo courtesy Draper Inc.

7. How do exterior shading systems cope with adverse weather conditions?
As previously highlighted, fixed louver systems are designed to take account of the maximum applied loads. With brise-soleil systems, the loads at the attachment points might be significant, particularly if projections are substantial. If this is the case, diagonal brace rods might be incorporated into the design to allow the load to be shared between two attachment points. With fixed systems, ice buildup and the risk of falling ice must also be considered. Therefore, brise-soleil systems might be inappropriate for tall buildings in urban areas.

Retractable systems such as exterior roller shades and venetian blinds are more lightweight than fixed systems and are designed to retract when the wind speeds are high. Standard roller shades need to be retracted at relatively low wind speeds (up to a maximum of about 32 km/h [20 mph]) and will not be appropriate for windy locations or on tall buildings. There is, however, a generic version known as a ‘zip system,’ which allows the fabric to be locked into side tracks. This type can operate in wind speeds of up to 144 km/h (90 mph) and is suitable for tall buildings.

Ice is also a potential issue, but should not be a problem if the systems are protected in the raised position. Automated controls will ensure the systems are only deployed when there is sun. Temperature and humidity sensors can also be used to stop the blinds or shades from being operated when there is a risk of icing. In locations with a cold winter climate, buildings generally require heating in the winter months. It may be appropriate to leave the exterior shading systems in the retracted position during these periods and allow the solar gain into the building as a free source of heating.

8. What maintenance is required?
Most exterior shading systems require little or no maintenance. Fixed louver systems need to be cleaned periodically to maintain the warranty on the paint finish, but no other maintenance work is required.

Adjustable and retractable systems also require little or no maintenance. Nevertheless, it is recommended they be inspected on a periodic basis to check the systems are correctly operating, guide cables (where used) are adequately tensioned, and there is no evidence of damage or general wear and tear to components.

9. How can exterior shading contribute toward achieving LEED certification?
There are numerous areas where the use of exterior shading system can help achieve credits for Leadership in Energy and Environmental Design (LEED) certification. These include:

  • minimum energy performance: use of exterior shading systems can assist in achieving a five percent reduction in building performance compared with the baseline building (in many cases, the reduction achieved is substantially more);
  • optimize building performance: using exterior shading systems can help in achieving reductions beyond the minimum requirement;
  • thermal comfort: exterior shading systems can potentially assist in achieving the requirements of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 55-2010, Thermal Comfort for Human Occupancy; and
  • daylight: to achieve this credit it is necessary to provide manual or automatic (with manual override) glare-control devices for all regularly occupied spaces (exterior shading systems—possibly in combination with interior ones—allow this to be achieved).

10. Do exterior shading systems make sense in terms of costs and benefits?
To justify using exterior shading systems, it needs to be demonstrated it makes economic sense to do so. Determining the cost of an exterior shading system is a straightforward exercise, but measuring the benefits can be more difficult. It is therefore important the shading system be considered in the context of the building as a whole, rather than as an isolated system, as it can impact several areas of building performance—notably lighting and the HVAC system.

This building in Erbendorf, Germany features an exterior venetian blind system. Photo © Faltenbacher. Photo courtesy Draper Inc.

This building in Erbendorf, Germany features an exterior venetian blind system. Photo © Faltenbacher. Photo courtesy Draper Inc.

In the past, it has often been the case the shading system’s performance was not taken into account when sizing the HVAC system. In this case, it is difficult to justify the use of exterior shading since the potential cost savings from reducing the size of the HVAC system will not be achieved. However, the mechanical consultants who deal with the heating and ventilation systems are now much more aware of the impact of effective shading, and are generally able to take this into account in their calculations.

Conclusion
The traditional approach to windows has been to use interior shading systems to control light and glare, and to address solar heat gain through the HVAC system. Increasing energy costs, requirements for improved façade performance, and greater environmental awareness are leading architects to look for alternative solutions.

Exterior shading systems will not be appropriate for all buildings; where they are used, however, they can make a significant contribution to the building’s performance as well as the building aesthetic. There is no question more architects are considering exterior shading, and, as understanding grows, exterior shading systems will become an important element in the design of high-performance buildings.

Richard Wilson, B.Sc., is a consultant to Draper Inc., and has been working with the company to introduce a range of exterior and specialty shading systems. He has been involved in the solar shading industry for more than 20 years. Wilson can be contacted by e-mail at rwilson@draperinc.com.

Don’t Seal Your Fate: Considerations for parking garage surface treatments

All photos © Hoffmann Architects Inc.

All photos © Hoffmann Architects Inc.

by Lawrence E. Keenan, PE, AIA and Robert A. Marsoli Jr., EIT

Elastomeric traffic-bearing membranes have soared in popularity over the past decade. But, what should designers know before specifying one at a parking facility?

It is true parking decks must be protected from the harmful effects of moisture and chlorides, but there is a growing misconception installing a traffic-bearing membrane is a one-way ticket to the garage equivalent of immortality. While a traffic-bearing membrane may be the best option for many situations, it is a big-ticket item, and thorough consideration is necessary to determine whether this costly investment is suited to the garage’s needs.

In order to withstand the punishing abrasion which a parking deck must endure, the traffic-bearing membrane must be hard and durable. At the same time, the membrane must be soft and flexible to bridge over moving cracks and joints without failure. However, traffic-bearing membranes are not perfect. Since hard membranes are generally inflexible, and more pliable membranes do not hold up well to abrasion, choosing the right membrane is a balancing act. Additionally, there are locations where no membrane performs well, such as those areas requiring a flexible membrane, yet are subject to snow plows.

Identifying product properties and applying appropriate selection criteria can guide the specifier in developing a customized system that will provide immediate protection, while also considering future treatment options.

For this parking lot, cracks are routed and sealed as part of a concrete repair project.

For this parking lot, cracks are routed and sealed as part of a concrete repair project.

This test core shows epoxy penetration to the bottom of a crack, as indicated by the arrows on the concrete.

This test core shows epoxy penetration to the
bottom of a crack, as indicated by the arrows
on the concrete.

 

 

 

 

 

 

 

 

Sources of deterioration
Since the interior and exterior of parking structures are exposed to the elements, they are more susceptible than other types of buildings to deterioration due to moisture, temperature cycles, and contaminants. Even the best designed and constructed garages need help to survive this onslaught of corrosive forces.

Water is at the heart of most parking deck deterioration. Moisture can facilitate reactions between certain aggregates and alkali hydroxides in the concrete, creating a cycle of expansion, cracking, and further moisture intrusion. Alkali-silica reaction (ASR) is difficult to stop once it has developed. Other minerals, notably sulfates, migrate via penetrating moisture and can lead to formation of gypsum, which can lead to softening and loss of concrete strength, and ettringite, a crystalline mineral the formation of which can result in an increase in solid volume, creating expansive forces that cause cracking and a loss of cohesion and strength in the concrete.

In northern climates, parking decks are subjected to extreme corrosive and deteriorating environments. Moisture, laden with chlorides from de-icing chemicals, tracks into garages and ultimately soaks into the concrete surface. The dissolved chlorides then migrate to embedded steel reinforcement through the pores in the concrete or penetrate through cracks. Once they reach the steel, the salts cause expansive corrosion, ultimately resulting in unsightly, destructive, and costly deterioration.

Moisture’s ability to transport corrosive chlorides is not its only damaging property. Coupled with cold weather, water can damage concrete decks as it expands and contracts during freeze-thaw cycles. Air entrainment, the deliberate incorporation of microscopic air voids in concrete, releases the internal pressure created by freezing water by permitting moisture to flow from void to void. Although this solution to freeze-thaw degradation has been known for years, garages may inadvertently be constructed with insufficient air entrainment, leading to premature concrete breakdown as freezing water destroys it from the inside out.

Applying an epoxy healer/sealer to a concrete deck can be a quick, effective, lowmaintenance option.

Applying an epoxy healer/sealer to a concrete deck can be a quick, effective, low-maintenance option.

Gravity-feeding an epoxy healer/sealer can repair cracks on a concrete deck.

Gravity-feeding an epoxy healer/sealer can repair cracks on a concrete deck.

 

 

 

 

 

 

 

 

 

 

Deck protection: product types
Technological advances in the chemical industry over the past 30 years have brought concrete sealers a long way from the boiled linseed oil previously used. Today, an industry dedicated solely to concrete protection offers a dizzying array of products to treat concrete before, during, and after production.

Ultimately, the goal of parking garage protection is to stop water from getting into the deck. This may be an over-simplification, but no water means significantly reduced deterioration. The tricky part is water comes in multiple forms. Liquid water is an obvious ‘villain,’ as is the expansive force of ice and snow, but water vapor can be just as damaging.

For example, a chloride-laden deck can actually draw moisture from the air and continue to deteriorate even after the best efforts to keep it dry. In fact, calcium chloride, the most popular and effective of all de-icing chemicals, is commonly used on construction sites for dust control. It is sprinkled onto the dry earth and wets the surface by pulling moisture from the air. Unfortunately, it works equally well at saturating a parking deck.

Remediating the effects of chloride ion attack, freeze-thaw damage, or moisture-driven chemical reactions is both difficult and costly, so preventing any type of water infiltration is a priority. While keeping a garage perfectly dry is an impossible task, through thoughtful product selection, the degree to which moisture can penetrate the parking deck can be limited. For existing parking structures, numerous waterproofing agents that can be applied to the deck’s surface are available.

Penetrating sealers
These liquid-applied treatments, which include silane, siloxane, and silicates, stop water entry by penetrating deep into concrete and forming a barrier that prevents water from entering, limiting chloride ion migration and freeze-thaw damage. These treatments are also vapor-permeable, allowing them to be used at locations where other coatings may be inappropriate, such as slabs-on-grade. Since they are inexpensive and quickly applied, with little or no down-time, penetrating sealers offer a good first line of defense for a parking structure that is in good overall repair. As invisible penetrants working below the surface, these sealers do not affect deck line striping, saving on project duration and cost.

However, these coatings can be short-lived solutions, requiring reapplication every five years or less. They also do not bridge cracks, so they only limit moisture and chloride penetration in intact concrete. Since cracking can be an ongoing process, the ability to bridge new cracks as they form may be important in parking decks that already have evidence of concrete distress.

Methacrylate and epoxy healer/sealers
These coatings both repair cracks and seal pores, so they can be used to restore a deck that has already undergone some deterioration. Low-viscosity methacrylates and epoxies fill the pores in concrete to create a barrier to liquid-water-driven chloride intrusion. They can also be injected or gravity-fed into cracks to structurally heal them. Where desirable, healer/sealers can also limit vapor transmission, although care must be taken not to lock moisture within the deck.

Moderately priced, this class of surface treatments offers a good solution for parking structures starting to show some signs of distress, both to treat deterioration that has already occurred and to prevent continued water-related damage.

Where methacrylate and epoxy healer/sealers fall short is in wet or soiled fractures (to which the materials will not adhere) and moving cracks (which are likely to re-fracture). On parking decks exposed to continuous sunlight, epoxies can degrade quickly under ultraviolet (UV) radiation, so methacrylates should be considered for these areas.

For enclosed parking structures or other areas where fumes might be a problem, offensive odors from methacrylates might prove prohibitive. Unlike the penetrating sealers, healer/sealers are not just ‘coat-and-go;’ surface preparation necessitates shot-blasting, which means increased down time and cost. Also, pavement markings must be reapplied.

Traffic-bearing membranes (elastomeric)
In parking structures with dynamic cracking, shrinkage, or more advanced damage, a traffic-bearing membrane may be the only option to address the ongoing deterioration. Unlike the sealers, these do not penetrate the concrete, but remain on the surface to create a barrier that locks out moisture and chlorides. Most elastomeric membranes have two layers—a base coat that provides the waterproofing protection, and a top coat, which protects the base membrane and provides skid resistance. Together, these yield an attractive, easy-to-clean surface that can give a ‘face lift’ to older, crack-riddled parking decks.

However, a traffic-bearing membrane’s assets are also its downsides. Flexible varieties offer superior crack-bridging, even for moving cracks, but they do not hold up well to abrasion because they are soft and yielding. More rigid varieties, designed to better withstand abrasive forces of heavy traffic, are too stiff to bridge these moving cracks. So while traffic-bearing membranes, as a class of surface treatments may seem to have the ideal combination of properties, in practice no single membrane actually does. Before specifying one of these coatings, the lengthy down-time required for preparation and application, and considerable ongoing maintenance of re-coating or top-coating every five to 10 years, should be considered. Once a traffic-bearing membrane has been installed, it is nearly impossible to return to an uncoated surface in the future.

Applying an impermeable coating to the bottom of an elevated deck traps moisture in the slab, leading to accelerated deterioration.

Applying an impermeable coating to the bottom of an elevated deck traps moisture in the slab, leading to accelerated deterioration.

Unable to evaporate through the coated surface, water entering the slab migrates to the reinforcing steel, leading to corrosion and spalls.

Unable to evaporate through the coated surface, water entering the slab migrates to the reinforcing steel, leading to corrosion and spalls.

Cast-in-place vs. precast
To select the best of the available surface treatments for the parking structure’s characteristics, condition, and situation, designers should consider numerous criteria to determine which products offer the best-performing option for the cost, in terms of both initial investment and long-term maintenance.

Over the years, many different types of parking decks have been developed. For the purpose of investigating surface treatment options, deck types can be simplified into two basic categories: cast-in-place and precast concrete. Usually composed of a single, contiguous, reinforced slab of concrete spanning a concrete or steel frame, cast-in-place decks are constructed onsite.

Due to its nature, concrete shrinks as it cures, which coupled with the external restraint stress from the structure to which it is attached, can lead to crack formation. Cracks are water-borne chlorides’ direct route to reinforcing steel. Once established, these cracks form natural expansion joints that open and close with changing temperature and humidity.

Consequently, protective techniques tend to focus on these moving cracks. If cracks are few and the deck is chloride-free, then routing and sealing, and applying a low-cost sealer, may be appropriate. If the deck is riddled with cracks that cannot be adequately sealed, then elastomeric membranes can begin to look like a good option.

Cast off-site under controlled conditions, precast decks are lifted and welded into place after they have cured and partially dried out. Since the concrete used for this type of construction is typically high-strength and denser than its cast-in-place counterpart, precast decks should rarely experience cracking. However, this manner of construction is favored for fast-track projects, and the end result is rarely defect-free.

As these materials are factory-made and must be lifted into place, precast units do not create a single, contiguous, monolithic structure. Instead, the individual members meet at sealant joints. Extending around each precast unit, these joints add up to miles of sealant that must be maintained and periodically replaced. Even if cracking is not an issue, water migration through failed joints can be just as damaging.

Aside from routine sealant maintenance, surface protection requirements are typically minimal and can usually be addressed with simple low-cost penetrating sealers. Heavily cracked decks may be routed and sealed or treated with rigid epoxies or healer/sealers, since these cracks are typically non-moving. However, the precast deck’s irregular surface does not readily lend itself to flexible membrane-type coatings. The leading edge of each panel quickly becomes a wear point, bumping against automobile tires or catching the tip of a snow plow. Protection techniques that soak into the deck and keep the concrete as the wearing surface are preferred.

Painting the underside of a parking deck may not improve its appearance if moisture causes the coating to bubble and peel.

Painting the underside of a parking deck may not improve its appearance if moisture causes the coating to bubble and peel.

In this case, coating patches were used to repair damage from snowplow blades.

In this case, coating patches were used to
repair damage from snowplow blades.

 

 

 

 

 

 

 

 

 

Concrete quality and condition
Knowing the concrete quality offers insight into the type of deterioration to which it would be most susceptible. This is usually achieved by ordering a petrographic analysis of a test sample. A petrographic analysis is an extraordinarily useful tool in determining what is wrong with concrete or predicting what can go wrong in the future. This analysis can detect most durability issues, so the most appropriate level of protection can be selected.

Chloride content is determined by removing concrete samples from varying depths and analyzing them in a laboratory. If chlorides are moving through the concrete quickly, the deck protection system must be aggressively enhanced to stop further migration. If the chlorides have reached the level of the reinforcement, chances are deterioration has already begun and low-cost sealers are no longer an option. Deck protection that retards water vapor intrusion or effectively inhibits corrosion is now necessary.

Age
While there is nothing inherently wrong with old concrete, the life of a deck does tend to follow a natural progression. Unless design or installation defects are an issue, a new deck can be effectively treated with low-cost sealers that limit the intrusion of chlorides through the concrete. Further along in the life of the deck, a more positive barrier, such as a moderately-priced epoxy sealer, may be necessary to retard moisture entry.

Ultimately, if not properly protected, a deck may require a traffic-bearing membrane to provide the best defense. However, as these membranes are costly and require maintenance and periodic reapplication, waiting to address signs of trouble until there are no other options is not the best course of action. Once a deck has begun to deteriorate, the coating can only retard further deterioration, not stop it.

Evaluating the condition of the concrete slab is an important part of the coating selection process.

Evaluating the condition of the concrete slab is an important part of the coating selection process.

Application of a traffi c-bearing membrane can take several days, and re-coating/top-coating may be required every fi ve to 10 years.

Application of a traffic-bearing membrane can take several days, and re-coating/top-coating may be required every five to 10 years.

 

 

 

 

 

 

 

Exposure
Whatever protection system is employed, it must withstand the rigors of its environment. UV degradation may be a problem for some coatings on a top deck. Epoxies, in particular, have difficulties when exposed to direct sunlight. Soft, flexible membranes may not withstand abrasion in high-traffic garages or on a typical turning radius and will fare poorly against snowplows. For example, a coating that looks ‘like new’ after many years in an apartment garage may not withstand a year at an airport or shopping mall.

The damage that can be inflicted by snow removal should not be underestimated. Many coating warranties require snow removal equipment to have rubber tips; others do not cover snowplow damage outright. Unless the garage management operates its own snow removal equipment, coatings at exposed decks will likely encounter a steel plow blade at some point in their service life. There are coating systems tough enough to repel the steel tips, but these super-rigid coatings do not bridge cracks. The best solution depends on finding the right compromise between rigidity and flexibility for a specific situation.

Location
As the adage goes, location is everything. Knowing which surfaces in a parking structure can accept an impermeable coating and which are best left bare is critical to prolonging the life of a garage.

A coating successfully applied to an elevated deck may have disastrous effects in the same garage when applied to a slab-on-grade. As water levels and humidity change, ground moisture seeps up into the concrete slab. Vapor barriers, often installed under slabs-on-grade, are designed to block this moisture from entering the slab. However, in reality, breaches in the barrier or cracks in the slab can still permit water entry. If an impermeable coating is applied to the top surface of the deck, that moisture becomes trapped between two impenetrable surfaces. Unable to escape, the water sits in the slab, leading to chloride and freeze-thaw degradation. Even without a vapor barrier, moisture in the ground rises within the slab and becomes trapped within the deck. Therefore, leaving the slab-on-grade uncoated is the best course of action.

On an elevated deck, that permeability gradient is reversed. Moisture enters the deck from above and migrates through the slab to the underside, where it evaporates. Even with a waterproofing membrane protecting the top surface, the deck is still susceptible to water entering at cracks, joints, and failed coating sections. Coating the bottom of the deck with an impermeable coating invariably leads to trapped moisture and accelerated deterioration. For this reason, the underside of an elevated deck should be similarly treated to a slab-on-grade and left uncoated.

Inappropriately specifi ed or applied coatings can lead to moisturerelated damage that is as detrimental as it is aesthetically unsightly.

Inappropriately specified or applied coatings can lead to moisture-related damage that is as detrimental as it is
aesthetically unsightly.

Cost
While the saying ‘you get what you pay for,’ can be applied to surface protection as well as anything else, in terms of quality material selection and skilled application, it is also true lower-cost systems are usually lower-maintenance alternatives. If an inexpensive sealer would suffice, installing a traffic-bearing membrane because it is the high-end option may mean investing in a costly system that may not perform any better in that situation. Additionally, once the membrane is in place, it must be maintained and eventually replaced.

Aesthetics
While a simple sealer can help prevent water infiltration, it will not change the parking deck’s appearance. On the other hand, an elastomeric membrane transforms the look of the garage and provides a uniform, fresh-looking surface that is easily cleaned of dirt and stains. For a crack-riddled older garage, this can be a welcome change. In a newer garage, however, the existing concrete surface is likely fine.

Using paint on the underside of a deck to improve its appearance can have problematic effects, since paint is a type of coating. Many of the concrete paints on the market are epoxy-based and relatively impervious to moisture. Even if a vapor-permeable paint is used, successive reapplications increase the coating thickness and so decrease its permeability. Over time, what was once a high-permeability surface can become surprisingly resistant to moisture migration. With the eventuality of peeling paint, spalls, rust, and cracks taken into account, a deck underside painted only for aesthetics begins to lose its appeal, as compared with a simple, uncoated one still intact.

Coating compatibility
Not all surface treatments are compatible. The parking deck protection specified now may limit future options, so both immediate performance goals as well as long-range planning should be considered before committing to a coating. Any applications already in place should also be investigated.

A ‘quick fix’ to get through the winter, for example, might be a less-restrictive sealer that penetrates the slab—rather than one that coats the surface—because various surface treatments can be applied over it in the future. Epoxy healer/sealers can cover such penetrants, and they provide a good base for membrane systems, should one be installed, down the road. However, once a traffic-bearing membrane is installed there is no way to effectively remove it without damaging the slab surface.

Proper application
Even if the right surface treatment is selected for a given project, problems can still result when the application is not executed correctly. Certain coating deterioration issues—such as delamination and blistering—may be avoidable if care is taken in surface preparation and coating techniques.

Before any coating is applied, surface defects must be corrected in order to create a sound substrate for coating application. Any dirt, dust, grease, paint, or other foreign matter should be cleaned, and surrounding areas protected. To prepare concrete for a penetrating sealer, procedures such as power-washing are often used, wherein high-pressure water or steam, sometimes mixed with mild detergents, forces dirt off an exterior surface. Other methods include hand-scrubbing and simple vacuum or broom cleaning. To prepare the deck surface for healer/sealers or traffic-bearing membranes, shot-blasting is required.

It is crucial to wait a minimum of 24 hours following any kind of water washing before applying a coating, in order to allow the deck to dry sufficiently. Cleaned surfaces should be tested for moisture at various sites just prior to application. If excess moisture remains, the coating may trap it inside the parking deck, exacerbating any water-related deterioration. A damp surface can also cause adhesion problems.

In weather conditions such as extreme heat or cold, wind, or rain, the area must be protected and coatings should not be applied. One must also avoid coating in direct heat of sun, as this may result in rapid drying of the material and cause bubbles or wrinkling. It is important to check the specific temperature range recommended by the manufacturer, as these vary from product to product. Also, keep in mind checking the ambient temperature may not be sufficient as surface temperatures may be significantly hotter or colder.

Conclusion
The preferred protection techniques stop deterioration before it begins. If a parking deck is well-maintained from the start, with sealers applied early and cracks promptly addressed, then surface treatment choices can evolve over time as the garage ages and needs change. However, if conditions are such that distress is advanced and progressing rapidly, more immediate and aggressive action must be taken to slow deterioration and minimize its impact.

Before specifying a concrete coating, one should consider the parking deck type, concrete age and quality, and level of exposure to traffic and weather. A surface treatment must not be specified until the garage’s condition has been assessed through investigation, testing, and evaluation. This can help navigate the array of available coatings. The right parking structure protection program should not only protect the deck today, but also anticipate the maintenance needs of tomorrow.

Lawrence E. Keenan, PE, AIA, is the director of engineering with Hoffmann Architects Inc., an architecture and engineering firm specializing in the rehabilitation of building exteriors. He has extensive experience in parking structure rehabilitation, including investigation, repair, and surface treatment consultation. Keenan can be contacted by e-mail at l.keenan@hoffarch.com.

Robert A. Marsoli Jr., EIT, is a project manager at Hoffmann Architects and has developed remediation solutions for a number of parking garages, from design through administration. He also provides preventive treatment consultation services for new construction. Marsoli may be reached at r.marsoli@hoffarch.com.

Bridging the Specification Gap between Divisions 03 and 09: Concrete and floorcovering associations unite

Photo © Michael Marxer (marxerphotography.com). Photo courtesy Mapei

Photo © Michael Marxer (marxerphotography.com). Photo courtesy Mapei

by Ward R. Malisch, PhD, PE, and Bruce A. Suprenant, PhD, PE

Division 03 specifies concrete floor surface flatness requirements to be installed by the concrete contractor. Division 09 specifies the concrete floor surface flatness for the flooring installer that must be met before installing the floorcovering. What does it mean when these requirements are incompatible?

One of the inconsistencies is Division 03 requires the floor flatness to be measured within 72 hours after concrete placement, whereas Division 09 requires the floor flatness to be measured before the floorcovering installation, which may be six to 12 months after the concrete placement. Additionally, Division 03 requires floor flatness to be measured using F-numbers, while Division 09 usually requires floor flatness to be measured as an allowable gap under a 3.1-m (10-ft) straightedge.

Further, Division 03 requires floor flatness not be measured across a construction joint or within 0.6 m (2 ft) of any slab edge, column blockout, or slab penetration. However, Division 09 requires floor flatness to be measured at all these locations. At the same time, Division 09 includes multiple but different floor flatness requirements for carpeting, vinyl, wood, and ceramic tile.

The owner does not want a specification battle; he or she just needs a concrete slab that allows the floorcovering to be installed to achieve a good appearance and obtain the manufacturer’s warranty. Clearly, there must be a cost-effective and efficient solution. Cooperation between the American Society of Concrete Contractors (ASCC) and six associations has led to a solution for bridging the specification gap between Divisions 03 and 09.

Floor fl atness is initially measured within 72 hours after concrete placement using F-numbers to determine contractor’s compliance with Division 03 specifi cations. Flooring installers need a fl oor fl atness metric when they arrive onsite to install fl ooring in compliance with Division 09 specifi cations. However, because concrete fl oor fl atness decreases with time due to curling or defl ection, the initially fl at fl oor placed by the concrete contractor is unlikely to meet the fl oorcovering specifi cation requirements.

Floor flatness is initially measured within 72 hours after concrete placement using F-numbers to determine contractor’s compliance with Division 03 specifications. Flooring installers need a floor flatness metric when they arrive onsite to install flooring in compliance with Division 09 specifications. However, because concrete floor flatness decreases with time due to curling or deflection, the initially fl at floor placed by the concrete contractor is unlikely to meet the floorcovering specification requirements.

The effect of the amount of curling on fl oor fl atness and levelness for a concrete slab with a 4.6-m (15-ft) joint spacing and initially fi nished to a moderately fl at (FF 25), fl at (FF 40), and a very fl at (FF 51) fl oor.

The effect of the amount of curling on floor flatness and levelness for a concrete slab with a 4.6-m (15-ft) joint spacing and initially finished to a moderately fl at (FF 25), fl at (FF 40), and a very fl at (FF 51) floor.

 

 

 

 

 

 

 

 

 

 

 

 

Defining the gap
The gap between floor flatness requirements is illustrated in Figure 1. The concrete contractor produces a floor that meets F-number flatness requirements included in Division 03 and measured shortly after concrete placement. The floorcovering installer arrives onsite far later to start preparation for floor installation. The floor flatness for a concrete slab-on-ground decreases with time due to curling caused by non-uniform concrete drying shrinkage. The floor flatness for an elevated concrete slab decreases with time due to initial deflection caused by the slab’s dead weight and long-term deflection due to creep and shrinkage of the concrete.

The time between the concrete contractor’s work and the flooring installer’s preparation results in a surface change that is the most significant factor in creating the ‘gap.’ Thus, while the specifications may require a suitable concrete surface as placed and finished by the concrete contractor, the resulting changes in surface shape make it unsuitable when the flooring installer arrives onsite. This decrease in flatness often requires flooring installers to do more surface preparation than they originally planned.

It is often impossible to estimate the degree to which floor flatness changes with time, and to determine when the flooring installation might proceed after concrete placement. As will be shown, the gap might be small (e.g. a slight reduction in floor flatness) or significant (e.g. more than a 50 percent reduction in floor flatness based on F-numbers). Thus, it is difficult for flooring installers to decide how much money to put in their bid for surface preparation. It is also difficult for owners to determine how much they need to pay to receive a high-quality final floor finish.

Modeling the effect on fl oor fl atness and levelness of elevated slab defl ection.

Modeling the effect on floor flatness and levelness of elevated slab deflection.

CS_October_2014_HR-16

 

 

 

 

 

 

Why the gap exists
Four factors contribute to the gap between the concrete contractor’s finished floor and the flooring installer’s requirements:

  • changes in floor flatness due to curling and deflection;
  • differences in floor flatness measurement methods;
  • differences in floor flatness measurement locations; and
  • dealing with multiple floorcovering requirements.

Changes in floor flatness due to curling
Concrete slabs built flat do not stay flat. The foreword of American Concrete Institute (ACI) 302.1R-04, Guide for Concrete Floor and Slab Construction, states it is completely normal to expect some amount of curling on every project.

Slab curling is caused primarily by differences in moisture content or temperature between the top and bottom of the slab. The slab edges curl upward when the surface is drier and shrinks more, or is cooler and contracts more than the bottom. Curling is most noticeable at construction joints, but it can also occur at saw-cut joints or random cracks. Curling usually results in part of the slab edges and corners losing contact with the underlying base.

There are many factors that influence the amount of curling for a concrete slab-on-ground.1 One of the most important factors is the relative humidity (RH) of the drying environment for the concrete slab. For instance, a concrete slab-on-ground in New Orleans, Louisiana, at 90 percent RH might undergo differential drying shrinkage gradient from the top to bottom surface of as little as 60 x 10-6 in./in. (or mm/mm). While the same concrete slab in Denver, Colorado, exposed to 30 percent RH would undergo a differential shrinkage gradient from the top to the bottom surface as much as 200 x 10-6 in./in.

The magnitude of the shrinkage gradient is three times larger for the slab in Denver versus New Orleans; thus, we would expect a slab to curl more in the former than the latter. Other factors that influence the amount of curling include:

  • potential drying shrinkage magnitude of the concrete mixture;
  • modulus of subgrade reaction;
  • concrete compressive strength and modulus of elasticity;
  • reinforcement ratio;
  • slab thickness; and
  • joint spacing.

Poor curing is often cited as the culprit when a slab curls. ACI 360R-10, Guide to Design of Slabs-on-Ground, states:

Extended curing only delays curling, it does not reduce curling.

In 2003, one of this article’s co-authors reported on F-number floor surface measurements taken at the same location lines at different times on two projects:2

  • a 150-mm (6-in.) thick, 28-MPa (4000-psi) concrete slab containing 19-mm (¾ in.) maximum aggregate size placed directly over a vapor retarder for a gym floor at the University of Maryland; and
  • a 150-mm (6-in.) thick slab with 28-MPa concrete placed on a compactible granular base with saw-cut joints every 4.6 m (15 ft) for an industrial warehouse in Pennsylvania.

Measurements for the gym floor were made 72 hours after concrete placement, and then again seven months later when the flooring installer arrived onsite. The measurements indicated the floor flatness had decreased by 20 percent during the seven months. Similarly, measurements for the industrial slab were taken with 72 hours after concrete placement and then again 12 months later. The measurements indicated floor flatness decreased by 40 percent during that year—this shows the magnitude of floor flatness changes that must be accounted for in bridging the specification gap between Divisions 03 and 09.

The length of lost contact area as a result of curling is about 20 percent of the joint spacing at each end of the slab. For a 4.6-m (15-ft) joint spacing, the slab curl would be expected to change the profile for about 1 m (3 ft) from each end. It is possible to take F-number readings from floors with differing profiles, download those values into a spreadsheet, then add a known amount of curl, and calculate new F-numbers. This allows a comparison of F-numbers before and after the curl.3 Good agreement with this approach was found when compared to the actual F-number measurements for the gym floor and industrial slab.

Figure 2 gives the analytical results showing the effect of the amount of curling on floor flatness and levelness for a concrete slab with a 4.6-m (15-ft) joint spacing and initially finished to a moderately flat (FF 25), flat (FF 40), and a very flat (FF 51) floor as defined by ACI 117-10, Specification for Tolerances for Concrete Construction and Materials.

The amounts of curling considered were 1.6, 3.2, and 6.4 mm (1/16, 1/8 and ¼ in.). A 1/8-in. curl will decrease the floor flatness from an FF 51 to an FF 35, while that same amount of curl will only decrease the floor flatness with an initial FF of 25 to a final FF of 23. As the table shows, the effect of curling is more pronounced on floors with higher initial floor flatness and levelness values. Thus, specifying and paying for higher floor flatness and levelness values in Division 03 may not prove to be a cost-effective solution.

Changes in floor flatness due to deflection
Division 03 requirements state the floor flatness and levelness of elevated slabs must be measured within 72 hours after concrete placement and while the concrete is still supported by the formwork and shoring. However, as soon as the formwork and shoring is removed, the slab deflects downward due to its dead weight. The deflected slab shape changes the floor flatness and levelness, just as curling does.

The concrete industry treats deflection as two parts:

  • initial deflection due to dead weight of the structural members; and
  • long-term deflection due to concrete creep and shrinkage.

ACI 318-11, Building Code Requirements for Structural Concrete, can be used to estimate the additional long-term deflection at 12 months as about 1.4 times the initial deflection. For example, a concrete flexural member spanning 9.1 m (30 ft) was designed for an initial deflection limit of L/360, where L is the span length in inches. Thus the initial deflection would be 30 x 12/360 = 1 in. (about 25 mm). The additional long-term deflection as estimated in accordance with ACI 318 would be 1.4 x 1 = 1.4 in. (about 36 mm) of additional deflection. If the flooring installer arrives one year after the concrete has been placed, he or she could thus expect to see a slab that has deflected about 2.4 in. (about 60 mm).

The effect on floor flatness and levelness of elevated slab deflection can be modeled in the same fashion as the curling effect was for concrete slabs-on-ground. First, initial F-number profiles were simulated, representing varying floor quality, and then superimposed structural deflection values on the profiles. The deflection was assumed to vary with position along the beam as a sine wave, with the initial deflection equal to L/360, L/480 and L/960–deflection values typically used in building code requirements, where L is the length of the span. The deflections were calculated at (1-ft) increments along the beam and added to the simulated F-number readings at the same increment. A 9.1-m (30-ft) span was assumed and the analytical results of this approach are shown in Figure 3.

The analysis shows that for a stiff structure with an FF 25 value, a deflection of L/960 (3/8 in. for a 30-ft span [about 9.5 mm for 9.1 m]), FF decreases by only four percent. Even for an initial profile representing an FF 30 floor, a deflection of L/960 affects the FF value by only about seven percent. However, for an initial FF value of 50, the L/960 deflection causes about a 24 percent decrease in flatness. As is the case with curling deflection—the higher the initial FF value, the greater the effect of dead-load deflection.

A composite overall flatness of FF 35 is the maximum specified value typically used for elevated slabs (ACI 302.1R-04). Based on the analysis, and at this specified value, a deflection of L/960—which indicates a stiff building—will probably result in an FF reduction no greater than about 10 percent. Unfortunately, the same cannot be said for deflection values of L/480 and L/360, which are common for structural steel framing systems supporting concrete slabs placed on metal decking. Since these slabs deflect much more than slabs in reinforced concrete frame buildings, the effect of deflection on FF can also be expected to be greater.

Differences in floor flatness measurement methods
Division 03 floor flatness and levelness requirements are usually specified with the F-number system and thus are measured in accordance with ASTM E1155, Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers. Division 09 floor flatness is usually specified as a maximum allowable gap measured under a 3.1-m (10-ft) straightedge that rests on two high spots on the concrete surface. It is important to note the straightedge method measures only floor flatness and not levelness. Figure 4 illustrates the two different measurement methods.

If correlations between F-numbers and straightedge gaps are used, it is important to understand the F-number for a given straightedge gap can vary widely. The table in Section 4.5.6 of ACI 117-90 indicates a 3.2-mm (1/8 in.) gap under a 3.1-m (10-ft) unleveled straightedge is roughly equal to an FF 50. However, the standard’s Commentary states:

there is no direct equivalent between F-numbers and straightedge tolerances; the following table does give a rough correlation between the two systems.

Although there is a caution in the ACI 117-90 Commentary, most people use the table because it provides a simple method for comparing the two measurement methods.

The Commentary in ACI 117-10 contains further information on the correlation between the two measurement methods by stating a specified maximum gap of 3.2 mm (1/8 in.) under a 3.1-m (10-ft) straightedge could be equivalent to FF numbers ranging from 38 to 110. The F-numbers are sensitive to the number of 3.2-mm (1/8-in.) gaps, or waves, in the floor. As the number of waves in a 3.1-m (10-ft) length increases, the FF number decreases. This feature of the F-number measuring system enables specifiers to differentiate among floors with the same measured gap but with different numbers of waves.

The two different methods measure significantly different surface properties. Thus, even if concrete contractors satisfy Division 03 F-number requirements, and the floor does not change with time, flooring installers are unlikely to find their gap under the 3.1-m (10-ft) straightedge satisfies the Division 09 requirements. Additionally, there are still specifications with major floor flatness discrepancies—for example, specifying an FF of 20 in Division 03, but then specifying a Division 09 requirement of a 3.2-mm (1/8-in.) maximum gap under a 3.1-m (10-ft) straightedge.

Differences in floor flatness measurement locations
Although the concrete industry lauds F-numbers as a more precise approach to specifying floor flatness, the F-number measuring method does not meet the floorcovering industry’s needs. For instance, according to ASTM E1155 and ACI 117 the measurement should not be taken:

  • across construction joints;
  • within 0.6 m (2 ft) of a penetration; and
  • after 72 hours.

However, to provide the owner with a satisfactory floor finish, the floorcovering must be placed over construction joints and near penetrations on a floor that is certainly older than three days. Figure 5 shows a straightedge being used to measure the flatness directly across a construction joint and at the intersection of a column blockout and the floor slab. F-number measurements do not reflect the flatness variations indicated by the straightedge at these locations.

Although ASTM E1155 includes a procedure for measuring across construction joints, it is rarely used. If the floorcovering industry were to adopt F-numbers, the measuring method and acceptance criteria would have to change so measurements could be made at any location on the floor.

Dealing with multiple floorcovering flatness requirements
Owners and architects often specify multiple floorcovering products for use in facilities such as retail stores. The floor flatness requirement for each of these products can differ greatly. For instance, the Carpet & Rug Institute’s (CRI’s) 2011 Carpet Installation Standard does not have a floor flatness requirement. In contrast, the American National Standards Institute/Tile Council of North America (ANSI/TCNA) A108-2013, Specifications for the Installation of Ceramic Tile, states:

Tiles with all edges shorter than [380 mm] 15 in., shall have a maximum permissible variation of [6.4 mm in 3.1 m] ¼ in. in 10 ft from the required plane, and no more than [1.6 mm] 1/16 in. variation in [300 mm] 12 in. when measured from high points in the surface. For tiles with at least one edge 15 in. or longer, the substrate shall have a maximum permissible variation of [3.2 mm] 1/8 in. in 10 ft from the required plane, and no more than 1/16 in. variation in [610 mm] 24 in. when measured from the high points in the surface.

Floor flatness requirements for the Division 09 finishes vary for each specific floorcovering. Thus, it is possible to be comparing a Division 03 floor flatness specification with multiple Division 09 floor flatness specifications. To get the best price for owners, and meet their schedule, the concrete contractor must place 1400 to 3700 m2 (15,000 to 40,000 sf) of concrete daily. It is not feasible to have the concrete contractor meet separate floor tolerances and finish requirements for every area where a different floorcovering product will be used. Often, the owner has not even made the flooring product choices for different locations before the concrete slab is placed. Thus, Division 09 is unavailable.

Engineers often choose the floor flatness specification in Division 03 with or without input from the architect. The architect needs to give input to balance the needs of the floor flatness requirements for the specified floorcoverings. It might not be economical to just choose the highest floor flatness requirement for Division 09 and put that in Division 03 because, as previously shown, floor flatness decreases with time. Thus, the extra cost passed from the concrete contractor to the owner for achieving a flatter floor may not be of benefit to the flooring installer 12 months later. It may also not be economical to specify the lowest concrete floor flatness needed because that may increase the cost of grinding and patching later.

Flooring installers need to measure fl atness with a straightedge that crosses construction joints, column blockouts, and near penetrations. F-numbers measured in accordance with ASTM E1155 will not yield this information. The top photo shows a carpenter’s level placed across a construction joint and the bottom photo shows a straightedge being used to check fl atness at a column blockout.

Flooring installers need to measure flatness with a straightedge that crosses construction joints, column blockouts, and near penetrations. F-numbers measured in accordance with ASTM E1155 will not yield this information. The top photo shows a carpenter’s level placed across a construction joint and the bottom photo shows a straightedge being used to check flatness at a column blockout.

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Options for closing the gap
There are numerous options for closing the gap between Divisions 03 and 09 floor flatness specifications, but this article focuses on three:

  • design a long-term flat floor;
  • specify higher initial floor flatness; and
  • grind and patch as needed.

The goal is to balance the owner’s cost for producing the desired floorcovering quality by choosing one option or a combination of options.

Design a long-term flat floor
As shown in Figure 6, the goal is to design the floor to stay flat over time. ASCC Position Statement 30, Responsibility for Controlling Slab Curling, indicates both ACI and the Canadian Standards Association (CSA) recognize curling control is the designer’s responsibility. In 2003, when ASCC Position Statement 6, Division 3 versus Division 9 Floor Flatness Tolerances, was first published, there was not enough technical information or design experience for most engineers to design a floor to remain flat until the flooring installer arrived on site.

In 2014, however, some engineers are designing floors that remain flat by using one or more of the following options:

  • limit concrete drying shrinkage;
  • use shrinkage reducing admixtures;
  • lower concrete compressive strength;
  • use more non-prestressed reinforcement (from 0.5 to one percent);
  • use 3 to 4-kg/m3 (5 to 7-lb/cy) macrofibers in the concrete;
  • use shrinkage-compensating concrete; and
  • use post-tensioning.

All these options could be used for concrete slabs-on-ground to control curling, but some will be of limited value when controlling deflection for elevated slabs. Many engineers are not yet comfortable with the risk of designing a flat floor that stays flat, and will avoid this option.

Specify higher initial floor flatness
As shown in Figure 7, the specifier could ask the concrete contractor to produce a higher initial floor flatness with the intent that when the flatness decreases with time, it will still be usable without further remediation for the flooring installer. Most design teams are reluctant to employ this option as they are unsure of how much the floor flatness will decrease with time and when the flooring installer might arrive onsite.

When this strategy is pursued, there is a cost increase to the concrete contractor to provide the higher floor flatness. However, there remains a risk the floor flatness will decrease more than estimated, which means some grind and patch might still be necessary.

Grind and patch as needed
As shown in Figure 8, the concrete floor is designed as economically as possible (while balancing other design and owner concerns), before grinding and patching as needed to achieve the necessary flatness when the flooring installer arrives.

The cost of grinding and patching can add up to more than $100,000 on multi-story buildings. Some owners believe this is an unnecessary expense, but there are options for cost allocation. In other words, some design teams prefer to use an allowance the owner budgets at the start of the project. If the allowance is not needed, the owner keeps the money.

The money could also be spent in designing and constructing to keep the floor flat with time. However, this is a risk when the money is used in designing and building a flat floor that does not stay flat. In that case, the owner will be spending money twice—once for the flat floor option and then more for grind and patch as necessary to achieve a flat floor before floorcovering installation.

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Specifying an allowance to bridge the gap
Since 2003, when ASCC Position Statement 6 was published, the ‘grind-and-patch-as-needed’ option has been used most often. The design team budgets it as an allowance so the owner need not spend the money if the concrete slab-on-ground or the elevated concrete slabs remains as flat as required by the flooring installer. The owner can then decide before flooring installation whether to use the allowance to ensure the desired quality of finished flooring.

The other benefit of this option is its adaptability to the requirements for different floorcoverings. For a concrete slab to receive carpeting, perhaps no preparation would be needed. However, for a concrete slab to receive 460 mm (18-in.) square ceramic thin-set tile, money used for prep work may be well spent.

Notes
1 One of this article’s co-authors—Bruce Suprenant—wrote a two-part article for ACI’s Concrete International in the spring of 2002. See “Why Slabs Curl−Part I: A Look at the Curling Mechanism and the Effect of Moisture and Shrinkage Gradients on the Amount of Curling” and “Why Slabs Curl−Part II: Factors Affecting the Amount of Curling.” (back to top)
2 See Suprenant’s July 2003 Concrete International article, “The Floor Tolerance/Floorcovering Conundrum.” (back to top)
3 See the authors’ Tolerances for Cast-in-Place Concrete Buildings (American Society of Concrete Contractors, 2009). (back to top)

Ward R. Malisch, PhD, PE, is concrete construction specialist for the American Society of Concrete Contractors (ASCC), an Honorary Member of the American Concrete Institute (ACI), and a member of ASTM International. He has been active in the concrete construction industry for more than 50 years, and has received the ASCC Lifetime Achievement Award, the National Ready Mixed Concrete Association’s (NRMCA’s) Richard D. Gaynor Award, and the Silver Hard Hat Award from the Construction Writers Association. Malisch can be reached at wmalisch@ascconline.org.

Bruce A. Suprenant, PhD, PE, is the technical director for the American Society of Concrete Contractors (ASCC) and a Fellow of the American Concrete Institute (ACI). He has taught concrete materials, construction, and structures for 15 years in universities and has been a consultant in that field for 20 years. Suprenant received ACI’s Roger Corbetta Construction Award and has authored or coauthored more than 100 articles and papers, including one that received ACI’s Construction Award in 2011. He can be reached at bsuprenant@ascconline.org.