Tag Archives: Concrete

Mix Design Fundamentals: Considerations for concrete for slabs-on-ground

Photo © BigStockPhoto/Theerapol Pongkangsananan

Photo © BigStockPhoto/Theerapol Pongkangsananan

by Paul Potts

“Concrete cracks and nothing can be done about it” is a common refrain when the material cracks unexpectedly. However, it is too often an excuse when poor design or improper placement has resulted in excessive random cracking. The real problem is too much mix design water, a lack of welded wire reinforcement, insufficient aggregate, or inadequate curing methods.

Grade-level concrete for hospitals, schools, and commercial buildings is something that can be walked on, supports vehicle traffic and other moderate loads, and provides a hard surface for floorcoverings. Interior concrete should not randomly crack or curl excessively to the point grinding is required before the floor finish can be installed. Similarly, exterior concrete should not randomly crack or deteriorate prematurely from freeze-thaw cycles. Using less water and more aggregate, making the right choices about reinforcement, and ensuring proper contraction jointing will improve the outcome.

This polished concrete floor was designed with synthetic fiber reinforcement but without welded wire reinforcement (WWR). Photos courtesy Paul Pott

This polished concrete floor was designed with synthetic fiber reinforcement but without welded wire reinforcement (WWR). Photos courtesy Paul Pott

It is a well-established belief among some architects, structural engineers, and contractors that strength is the defining characteristic predicting the quality of concrete. Another common erroneous assumption is synthetic fiber reinforcement can replace welded wire reinforcement (WWR) to make concrete less disposed to long-term shrink cracking. Others believe more, not less, cement is better, or the main impact of adding more aggregate to the mix involves making it more expensive. None of this is true.

Clearing incorrect info
It is not uncommon to base the specification for concrete on strength and slump. Unfortunately, strength does not get tested until several days after hardening; and slump, which has a strong relationship to placement, is only loosely related to the most desirable qualities of hardened concrete.

In this author’s first years as construction administrator, he was taught slump was an indication of water content, but there is not direct relationship between the two. As the Portland Cement Association’s (PCA’s) Design and Control of Concrete Mixtures points out, slump tests are simply measures of consistency—in other words, the ability of fresh concrete to flow.

The desirable qualities of slab-on-grade concrete are resistance to curling and shrinkage cracking, along with finishability, flatness, strength, and durability. In northern latitudes, ‘durability’ particularly means resistance to freeze-thaw cycles. To better ensure these qualities, the designer must consider total water content, size and quantity of well-graded aggregate, type of reinforcement, and timeliness of applying curing and cutting construction joints as controlling factors. The factors in designing concrete with these qualities are:

  • low water-to-cement (w/c) ratio;
  • minimum total cement content needed (with the understanding more cement requires more water);
  • size and weight of aggregate in a yard of concrete;
  • entrained air in exterior concrete; and
  • presence of WWR.

According to the PCA handbook, almost every quality of concrete will be improved by reducing the total water content in the batch. The w/c ratio is the weight of water divided by the weight of cement. To minimize total water content, designers must start by specifying a low water-to-cement ratio (e.g. 0.45 w/c ratios for interior concrete and either a 0.45 or 0.40 for exterior concrete). To further minimize total water, they should opt for the lowest practicable cement content (e.g. 5-1/2 sack). At the same w/c ratio, the less cement in a batch of concrete, the less total water needed.

According to Chapter 9 (“Designing and Proportioning Normal Concrete Mixture”) of PCA’s Design and Control of Concrete Mixtures:

For any particular set of materials and conditions of curing, the quality of hardened concrete is determined by the amount of water used in relation to the amount of cement. The less water used, the better the quality of the concrete—provided it can be consolidated properly. 

Concrete with a water to cement (w/c) ratio of 0.45 and below is stiff when it comes off the truck—otherwise, it may have been tampered with.

Concrete with a water to cement (w/c) ratio of 0.45 and below is stiff when it comes off the truck—otherwise, it may have been tampered with.

 

Using more—rather than less—large aggregate (e.g. 900 to 1000 kg [2000 to 2200 lb]) reduces the total cement and water needed, improves resistance to cracking and curling, and increases the strength at the same time. This is also economical, as aggregate is cheaper than cement.

It is important to keep in mind there is a limit to how low one can go with cement and water because there must be enough water (or substitute) to allow concrete to flow into place. Further, there must be enough cement to promote the finisher’s work. The effect low water has on finishing work can be counteracted with water-reducing admixtures (WRAs).

Water content, wire reinforcement, and cracking
Excess water (i.e. more than required for hydration) improves concrete flow, making the material more economical to place. However, water not hydrated by the cement eventually becomes ‘bleed water’ and evaporates. The evaporation reduces the total volume of the slab, causing the slab to shrink in overall dimensions. More excess water available for evaporation means more shrinkage and more potential for long-term shrink cracking.

When the concrete’s shrinking volume is restrained by a pipe, masonry corner, steel column, or the drag of an uneven subgrade, there may be stresses beyond the material’s tensile strength to resist—as a result, shrink cracks develop. (Tensile strength is only 10 per cent of compressive strength.)

Welded wire reinforcement buttresses the tensile strength of concrete by distributing the stresses over a wider area where it can be better controlled. Some architects and engineers believe synthetic fiber mesh can be substituted for welded wire in concrete for the same crack-resisting properties. However, synthetic fiber mesh is only effective at preventing plastic shrink cracking—hairline cracks that develop before final finishing.

To clear up confusion about the purpose of synthetic fibers, PCA published the following statement on its website:

Plastic fibers should not be expected to replace wire mesh in a slab-on-ground. However, although not affecting joint spacing, plastic fibers are used to reduce plastic shrinkage cracking.1

While finishing is not any more difficult with low w/c concrete, placement, and screeding require an ambitious crew.

While finishing is not any more difficult with low w/c concrete, placement, and screeding require an ambitious crew.

Mix design
‘Strength’ is the reciprocal of the w/c ratio—the lower the ratio, the higher the concrete strength. While it may be prudent for legal reasons to specify a minimum strength, it is just as well as to develop the mix design for slab-on-ground applications by specifying the maximum sack content, water-cement ratio, and aggregate quantity/size. Strength follows the water-to-cement ratio as surely as day follows night.

A w/c ratio of 0.045 and 5-1/2 sack content with 900-kg (2000-lb) of large aggregate with welded wire reinforcement is a good standby mix design for an interior slab-on-grade. The strength will fall around 30 N/mm2 (4500 psi). A w/c ratio of 0.040 and 5-1/2 sack specification with 900 to 1000 kg (2000 to 2200 lb) of aggregate and six per cent air with welded wire reinforcement is a durable exterior mix design, but this concrete is stiff and may need a water-reducer to aid placement. The strength will be around 35 N/mm2 (5000 psi).

Pop-outs in exterior concrete are the result of waterlogged soft stone or chert in concrete mixtures that, owing to their lighter density and porousness, absorb water and float up near the surface during finishing operations, then explode during the first freeze cycle. Most concrete mix designs limit soft stone and chert to less than one percent (if permitting it at all). However, even this one percent can leave an awful-looking mess on the surface of a new driveway or sidewalk. To avoid pop-outs in exterior concrete, one should use 21AA crushed limestone aggregate where it is available. When limestone is unavailable, the soft stone and chert in natural stone aggregate should be limited to less than one percent.

Contraction joints versus random cracking
Contraction joints in slab-on-ground designs are for aesthetic purposes—without contraction joints, cracks would occur randomly. Contraction jointing limits the cracking to where it is least objectionable and, in the case of thin-set terrazzo, epoxy, and urethane floorcoverings, at locations that can be coordinated with joints in the surface materials. Still, random cracking can be preferable in some scenarios.

For example, the appearance of carpet, vinyl, rubber, and linoleum can be improved by allowing the concrete to randomly crack instead of adding contraction joints with their inherent curling problems. To control the width of random cracking, one should start with a low water-to-cement ratio mix and require WWR. Random cracks in concrete are free of curling, and a low w/c ratio combined with welded reinforcement minimizes the width of cracks.

In a random crack system, attention must be paid to corridor intersections that may need contraction joints to prevent excessively wide cracks from developing where the long corridor runs and restraints at corners can produce quite large random cracks, regardless of the mix design and WWR. Concrete slabs for wood floors on sleepers or rubber cushions can be designed the same way.

It is not an option to allow random cracking under hard tile, epoxy floorcoverings, and thin-set terrazzo bonded to the concrete, because any cracks in the substrate will telegraph through to the surface of the bonded floorcovering (Figure 1). Concrete under bonded floorcoverings must be reinforced with WWR or a mat of steel reinforcement to reduce shrinkage cracking. Such components lower the risk of shrinkage cracking by restraining contraction during set time and reducing the width of cracks once the concrete sets. While some structural engineers may not see the need for WWR in non-load-bearing slabs-on-grade, its importance for reducing curling and cracking must not be overlooked.

telegraphed crack

This shrink crack telegraphed through the thin-set terrazzo and became the subject of litigation and costly court-ordered repairs. (A core sample can be seen at right/below.)

This shrink crack telegraphed through the thin-set terrazzo and became the subject of litigation and costly court-ordered repairs. (A core sample can be seen above.)

Vapor barrier and curing considerations
A vapor barrier should be placed under all slabs-on-ground that will receive floorcovering. Properly consolidated concrete is waterproof, but not vapor-proof. If a vapor barrier is not used, any moisture under the slab migrates upward by capillary action through the concrete to eventually degrade the floorcovering adhesive. Most floorcovering manufacturers consider the lack of vapor barrier a defective concrete installation—many void their warranties on that basis.

While some specifiers recommend a blotter layer of granular material between the vapor barrier and the slab, in practice this has met a lot of resistance, principally because of the difficulty of keeping the blotter layer intact while completing the mechanical and electrical underground and preparing the concrete pour.

According to Chapter 9 in PCA’s Floors on Ground handbook:

Other specifiers believe that no blotter layer is needed and that concrete should be placed directly on the vapor retarder. The idea is that concrete slabs should be cured from both the top and the bottom. A granular layer between the vapor retarder and the concrete creates a potential water reservoir that could cause moisture problems at a later date. 

A properly installed vapor barrier directly under the slab has other benefits beyond reducing the transmission of moisture vapor. First, the polyethylene film acts as a slip-sheet between the underside of the concrete and the sub-base; this reduces drag on the slab and decreases shrinkage cracking. Further, when combined with timely surface curing, a polyethylene vapor barrier under the slab is an asset, retaining moisture in the concrete to improve hydration during setting. Whenever a vapor barrier under the slab is used, the water-to-cement ratio should be 0.45 or less.

It is especially important to start curing as soon as practical after finishing operations are complete. Where concrete is placed directly on a vapor barrier, a double application of cure-only compound should be applied at right angles to each other. In cases where the floorcovering requires an adhesive, cure-only compounds should be used rather than cure-and-seal products—sealing the concrete interferes with the bond between the concrete and the floorcovering or topping.

Low water-to-cement concrete is harder to move around and requires considerably more handwork to get it in place. Placement and spreading of low water-to-cement ratio concrete can be improved by adding water-reducing admixtures (WRAs)—however, excessive use can cause more drying shrinkage. These are rarely required by the specification, but are included in the specification as an alternate product that may be used.

Low water-to-cement concrete is harder to move around and requires considerably more handwork to get it in place. Placement and spreading of low water-to-cement ratio concrete can be improved by adding water-reducing admixtures (WRAs)—however, excessive use can cause more drying shrinkage. These are rarely required by the specification, but are included in the specification as an alternate product that may be used.

Aggregate
Larger-size and greater quantities of coarse aggregates work in several ways to reduce cracking and curling, and improve the economy of concrete. Coarse aggregates, less than 50 mm (2 in.) in size, cost about half as much as the cement in concrete. The larger aggregates leave less room that must be filled with cement paste so they reduce the total water required in the mix. To avoid pop-outs in exterior concrete, the designer should specify crushed limestone aggregate where it is available.

More and larger aggregates reduce shrinkage cracking, and reduce curling. The weight and dimension of larger aggregates put a drag on the movement of the materials within concrete during the shrinkage stage, and reduce overall contraction thereby reducing shrinkage cracking. Greater quantities of coarse aggregate magnify these benefits—900 kg (1984 lb) of 32 mm (1 ¼ in.) coarse aggregate is a good starting point for specifying the aggregate in slab-on-ground concrete.

Conclusion
Concrete designed with low water-to-cement ratios and the maximum size and content of large aggregate, reinforced against shrinkage cracking with properly spaced contraction joints will have fewer random cracks and less curling. To that end, the purpose of this article is two-fold:

  • encourage the concrete mix designer to think in terms of the low w/c ratios and aggregate content instead of strength and slump as the starting point for the concrete mix design; and
  • encourage the use of WWR where crack mitigation is important, cautioning against misapplication of synthetic fiber as countermeasure with regard to long-term shrink cracking.

Notes
1 Visit www.cement.org/tech/faq_fibers.asp. (back to top)

Paul Potts has worked as construction administrator for Kingscott Associates, a school design/construction firm. A licensed MasterSpec writer and a consultant to the construction industry in Michigan, he is currently working on the reorganization of the Michigan State University Cabinet Shop and finishing construction administration work on the New Community Building and Pavilion project for the City of Potterville. Potts can be reached at paulpotts1@comcast.net.

ASTM celebrates concrete centennial

Concrete Finishers

ASTM International’s concrete-focused committee has worked to improve the material’s use and durability for a century. Photo © BigStockPhoto/FrenchToast

During last month’s round of standards development meetings in Toronto, ASTM International celebrated the 100th anniversary of the group that became Committee C09 on Concrete and Concrete Aggregates.

Since 1914, when a small group gathered to work on methods for making and testing field specimens, C09 has grown to more than 1400 members from 62 countries, maintaining a portfolio of more than 175 standards. Its 50 subcommittees focus on aspects ranging from self-consolidating concrete and chemical admixtures to supplementary cementitious materials (SCMs) and pervious assemblies.

C09 has developed global standards in construction, industrial, transportation, defense, utility, and residential sectors, but the group says its first standard remains one of its most important. ASTM C94/C94M, Specification for Ready-mixed Concrete, was first approved in 1933, but has kept pace with technology changes to present day.

“Looking at C09’s book of standards doesn’t tell the complete story of the committee’s success and accomplishments over the past century,” said committee member Richard Szeczy (president of Texas Aggregates and Concrete Association).

lobo_colin_2014 (2)

Colin Lobo, PhD, F.ASTM, received the ASTM International Award of Merit for Service to Concrete Committee. Photo courtesy ASTM International

“To produce the defining concrete industry documents stakeholders around the world rely on every day has taken countless hours of dedicated effort and cooperation from thousands of international experts,” he continued. “Over the years, C09 has embodied everything that is great about the ASTM process. That itself is truly worth celebrating.”

In related ASTM news, Colin Lobo, PhD, has received the organization’s Award of Merit for Service to Concrete Committee (along with Fellowship). Chair of the ASTM Cement and Concrete Laboratory (CCRL) executive group and senior vice president of engineering for National Ready Mixed Concrete Association (NRMCA), Lobo was lauded for his contributions to specifications for concrete materials, test methods for fresh and hardened concrete, data evaluation, and laboratory assessment.

Insulating concrete forms manufacturers unite as an association

Picture1

A new industry group will promote insulating concrete forms (ICFs).

Photo courtesy Logix ICFs

Four insulating concrete form (ICF) companies have cemented a deal to form a new industry group.

The Council of ICF Industries (CICFI) seeks to promote the construction assembly, which comprises dry-stacked formwork for reinforced concrete, usually made with a rigid thermal insulation that stays in place as a permanent interior and exterior substrate for structural walls, floors, and roofs.

The association’s inaugural chair, Andy Lennox, told The Construction Specifier CICFI will seek strategic alliances with related groups and delve into industry-level technical research to raise awareness about the sustainable attributes of these assemblies.

“Overall, construction professionals do not have an appreciation for the speed of construction that ICFs bring to the table, especially for larger commercial structures,” he said. “Many design professionals are also unaware of the size, scope, and range of buildings that have been successfully constructed with ICFs.”

CICFI’s initial membership includes Logix Insulated Concrete Forms Ltd., Nudura Corporation, Quad-Lock Building Systems Ltd., and Superform Products Ltd. According to Lennox, these companies cumulatively represent the majority of the ICF products manufactured in North America.

“Now that our association is officially up and going the implementation and execution begins,” Lennox said. “This association is long overdue, we are excited to get started and we look forward to additional ICF manufacturers joining us as we move forward.”

How Thin is Too Thin?

Evaluating slab thickness in reinforced concrete flat-plate construction
by Dimitri Papagiannakis, PE

Typical flat-plate construction.
Photos courtesy SGH

Reinforced concrete flat-plate construction is popular among mid- and high-rise residential construction projects. It provides a great deal of flexibility in the placement of the structure’s vertical load-carrying elements (i.e. columns and walls) without sacrificing the efficiency of the floor framing—as could potentially be the case with steel or masonry.

In the project’s early stages, structural engineers are often asked by architects and owners how thin the slabs in a flat-plate system can be. The question is usually motivated by a desire to achieve taller floor-to-ceiling heights, which can be an important selling feature to end users. There are building code provisions that address minimum slab thickness as a function of the span length and span condition (e.g. continuous versus discontinuous, etc.). There are also practical and economic factors that often influence the design of concrete flat-plate slabs.

The design of reinforced concrete structures is governed by American Concrete Institute (ACI) 318, Building Code Requirements for Structural Concrete, which provides minimum thicknesses for one- and two-way slabs supporting structural and/or nonstructural building elements. These are intended to limit deflections that may result in serviceability issues with the structure or that may damage architectural building elements.

The prescriptive minimum thicknesses are a function of the span length, continuity conditions, and end restraints of the slab; they are intended to provide a slab section that conforms to code-prescribed deflection limits without the need for the engineer to perform detailed deflection calculations. However, the code also permits the design engineer to specify thinner slabs when calculations are performed showing short- and long-term deflections will not have an adverse effect on structural or nonstructural elements attached to or supported by the slab.

Clusters of mechanical/plumbing penetrations through flat-slab. Slab design must be checked for required additional reinforcement at penetrations.

Clusters of mechanical/plumbing penetrations through flat-slab. Slab design must be checked for required additional reinforcement at penetrations.

Pros of a thinner slab
There are several benefits to specifying thinner slabs from a structural perspective. One obvious advantage is less concrete is required. Consequently, a reduction in concrete also decreases the gravity loads on the vertical load-carrying elements. This will usually result in smaller columns with less reinforcement, and thus a savings in material costs.

A reduction in building mass also has a direct effect on the seismic loads to which a building is subjected. The seismic base shear of a building structure is directly proportional to its seismic weight—a reduction in the seismic weight of a building generally results in proportional decrease in the seismic-load demands to the lateral-load-resisting elements of the building structure, and thus a more cost-effective design. Additionally, reduced building loads may also yield a less-expensive foundation design depending on the proposed system.

Plumbing sleeves placed near columns.  This requires careful review of slab shear capacity.

Plumbing sleeves placed near columns. This requires careful review of slab shear capacity.

Cons of a thinner slab
Depending on the horizontal spans that must be achieved, minimum slab reinforcement may not provide enough strength to support code-prescribed loads. Therefore, additional reinforcement may be required within the slab, negating some of the aforementioned material cost savings.

Thinner concrete sections are also susceptible to punching shear failures and must be carefully evaluated. Under certain circumstances, the avoidance of the punching shear limit state can preclude the use of smaller column cross-sections. The potential for overstressing the slab at the slab/column interface is further exacerbated by the use of slab-column moment frames often employed as part of the lateral-load-resisting system (where permitted by code). The magnitudes of the unbalanced moments and shear stresses at the slab-column connections are highest at the moment-frame locations, and may require use of thickened drop-panels at the columns to resist the applied loads. Alternatively, shear studs may be placed at the column heads to provide the required strength, or larger beam sections may be used around the perimeter to develop moment-frame action in lieu of the slab. These options result in added labor and additional cost for the project.

Flat-plate construction requires a great deal of coordination between the structural system and the mechanical, electrical, and plumbing (MEP) components. Slab penetrations for vertical mechanical and plumbing risers must be evaluated for potential additional required reinforcement. Riser penetrations located around columns must also be carefully coordinated and evaluated, as they can have a significant impact on the punching shear and flexural stresses near the columns, and may require additional flexural or shear reinforcement.

Electrical/plumbing conduit placed within slab.  Coordination is required to avoid over-congestion of conduit (such as shown here) and delays associated with modifying conduit locations in the field.

Electrical/plumbing conduit placed within slab. Coordination is required to avoid over-congestion of conduit (such as shown here) and delays associated with modifying conduit locations in the field.

Electrical conduit is also typically placed within the slab, at mid-height. Sufficient cover must be provided around the conduit and between the conduit and slab reinforcement. The conduit diameter and spacing must be kept within certain limits to prevent it from degrading the slab’s strength or becoming the focus of shrinkage stress cracks. Design and coordination of these items becomes more challenging—and potentially more expensive—as the slab’s thickness, and thus the space within which to fit the components, is reduced.

For thinner flat plate slabs, the increased surface area-to-volume ratio makes it more susceptible to early drying due to a reduction in the heat of hydration (i.e. the reduced concrete mass retains less heat—a key component to the curing process). Higher drying rates increase the likelihood of early-age cracking and, in turn, the slab’s deflections.

This reduction in heat of hydration also becomes a factor in cold-weather conditions, where the freshly poured concrete may be more susceptible to freezing due to lower concrete temperatures than would otherwise be present to help protect the slab. Thinner slabs are also more prone to early-age cracking from the shoring and re-shoring loads typical of rapid construction cycles.

Conclusion
Selecting the most appropriate slab thickness is a critical aspect of a reinforced concrete flat-plate project. Modern engineering methods and the availability of finite-element software provide useful tools for quick and efficient evaluation of flat-plate systems.

The design engineer should assess the feasibility of reducing the slab thickness beyond the prescriptive limits provided by the code, and should communicate to the owner and design team the implications of doing so (e.g. additional reinforcement, connection detailing requirements, coordination issues, etc.). As mentioned, there are numerous pros and cons to reducing design slab thickness, and each must be evaluated to arrive at the most appropriate conclusion.

DimitriPDimitri Papagiannakis, PE, joined Simpson Gumpertz & Heger (SGH) in 2011 with nearly a decade of structural engineering experience. A registered professional engineer in New York and New Jersey, his work includes design of new building structures and subdivisions, as well as renovations, alterations, repairs, and investigations of existing buildings. He can be reached at dpapagiannakis@sgh.com.

Concrete Moisture Mitigation to Help Floors

Hopps, Emily_36801FAILURES
Emily R. Hopps

Concrete floor slabs contain excess moisture that can damage many types of floor finishes. To address this problem, manufacturers have developed products aimed at mitigating the moisture in concrete. However, not all these products are suited to their intended purpose. Continue reading