Tag Archives: Precast

Missouri campus features new terra cotta application

By Dirk McClure

Terra cotta tile was clad onto this project's insulated composite precast concrete sandwich panels.  Photos © Jacia Phillips

Terra cotta tile was clad onto this project’s insulated composite precast concrete sandwich panels. Photos © Jacia Phillips

The University of Missouri, located in Kansas City, boasts the country’s first terra cotta-clad insulated composite precast concrete panels assembly.

Before this installation, terra cotta had been clad into non-insulated panels in a few projects. At the Henry W. Bloch Executive Hall for Entrepreneurship and Innovation, however, terra cotta tile was clad onto 1783 m2 (19,200 sf) of 3.6-m (12-ft) wide insulated composite precast concrete sandwich panels.

Designed by BNIM Architects and Moore Ruble Yudell, an important aspect of the project’s scope was to ensure it maintained the aesthetic of the university’s campus as a whole. The modular appearance of the five-color terra cotta pattern provided the desired masonry appearance.

Two sides of the building feature darker red colors on the terra cotta; the other sides feature a lighter palette.

Two sides of the building feature darker red colors on the terra cotta; the other sides feature a lighter palette.

Rather than specify a conventional rainscreen cladding system, the project team relied on the terra cotta to provide a rain barrier. A cost analysis by the general contractor also demonstrated terra cotta clad into an insulated precast sandwich panel would yield a 25 percent cost saving over a steel frame and air barrier application. The joints were concealed to provide the aesthetic of a more traditional rainscreen.

Designers outlined the seemingly random pattern of the terra cotta tiles. Every 152 mm x 1.2-m (6 in. x 4-ft) section of this five-color tile pattern was sorted by the precaster. The terra cotta was then cast into the panels in their offsite production facility. The use of sealants kept the gray-colored precast set back and invisible on the finished product. Two sides of the building feature darker red colors on the terra cotta, which complement the adjacent brick masonry campus buildings. The panel’s interior faces were left exposed in areas, with a sandblast interior finish. This helped reduce volatile organic compounds (VOCs) by limiting the use of paint.

The continuous insulation (ci) used in the assembly met American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, requirements. The precast high-performance insulated wall panels used to connect concrete wythes provided a low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65. Finally, the radiant heating mechanical system was installed under the floor.

Thermal performance, high-performance integrated design and overall sustainability were also goals outlined by the project team. The project incorporates energy efficiency and daylighting strategies, and is targeting Gold certification under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental (LEED) program.

The high-performance insulated wall panels used carbon fiber grid to connect concrete wythes, which provided low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65.

The high-performance insulated wall panels used carbon fiber grid to connect concrete wythes, which provided low thermal transfer system and contributed to the project’s overall thermal mass goals. The extruded polystyrene (XPS) panels achieved an R-value range of 16.37 to 17.65.

Tests were performed to determine the correct level of using terra cotta in an insulated panel. These tests included determining:
● coefficient of thermal expansion;
● allowable bowing factor;
● optimal tile thickness; and
● freeze-thaw cycles.
Finally, full precast mockup panels were also designed and poured before the project was in full production.

Dirk McClure is regional director of sales and business development for Enterprise Precast Concrete. He has a bachelor’s degree in interior architecture from Kansas State University. McClure currently sits on Precast/Prestressed Concrete Institute’s (PCI’s) International Sustainability Committee and is a NCIDQ Certificate Holder, LEED Accredited Professional, and a member of the Construction Specifications Institute (CSI) Kansas City Chapter. He can be contacted by e-mail at dmcclure@enterpriseprecast.com.

G8WAY DC: Innovative UHPC Elements

Ultra-high performance concrete (UHPC) provides strength, ductility, durability, and aesthetic design flexibility while still being highly moldable and able to replicate texture, form, and shape. It is this combination of superior properties that also provides architects and engineers with a new kind of freedom—to design unique, unprecedented UHPC architectural elements that are also sustainable and extremely durable.

A wide range of UHPC panel styles can be achieved, such as:

  • mechanically fastened restoration systems;
  • rainscreens with textures and curvatures;
  • curved curtain walls;
  • decorative brise soleil lattice;
  • perforated double-skin systems;
  • ultra-thin canopy systems;
  • customized colored designs; and
  • energy-efficient louvers and sunshades.

The following is an example of various projects with UHPC roof/canopy systems that have been completed in recent years.

The Shawnessy LRT Station (Calgary, Canada)

Photo © Vic Tucker

Photo © Vic Tucker

Designed by Stantec Architecture Ltd., the Shawnessy light-rail transit (LRT) station has a double-curvature roof system comprising 24 ultra-thin UHPC canopies supported on singular UHPC columns. The 5.1 by 6 m (16 2/3 by 19 2/3-ft) canopies are just 20-mm (25/32-in.) thick. Extensive tests on a full-scale prototype concluded the system carried full-factored live and dead loads without cracking and surpassed rigid test criteria.

The Cap Cinéplex (Rodez, France)

Photo © Atelier d’Architecture EmmanuelNEBOUT

Photo © Atelier d’Architecture EmmanuelNEBOUT

This movie theater has a cantilevered, overhanging canopy that spans 9.5 m (31 ft) with an integrated light-emitting diode (LED) system, evoking a starry night sky. The thin (i.e. 40-mm [1 3/5-in.]) canopy is watertight and can provide shelter for up to 300 people. Composed of 12 juxtaposed precast UHPC panels, the system performs well against harsh environmental factors such as heavy snow loads, wind, sun, and heat.

Jean Bouin Stadium (Paris, France)

Photo © Lafarge Medialibrary, Charles Plumey-Faye

Photo © Lafarge Medialibrary, Charles Plumey-Faye

Jean Bouin Stadium is home to rugby team Stade Français and the site of the 2014 Women’s Rugby World Cup. Designed by Rudy Ricciotti, it has a 23,000-m2 (247,570-sf) UHPC envelope made from 3600 self-supporting UHPC triangular panels that includes a 12,000-m2 (129,167-sf) lightweight, waterproof roof with glass inserts. The roof panels average 8 to 9 m (26 to 29 ft) long x 2.5 m (8 ft) wide, with a thickness of just 45 mm (2 in).

Villa Navarra (Var, France)

Villa Navarra

Photo © Philippe Ruault

Designed by Rudy Ricciotti (with structural engineer Romain Ricciotti), this private villa has a cantilevered UHPC roof system comprising 17 panels, 9.25 x 2.35 m (30 ½ x 7 2/3 ft) each, that create a 7.8-m (25 7/12-ft) overhang.

To read the full article, click here.

Introducing G8WAY DC: Ultra-high performance concrete has it covered

Photos courtesy Davis Brody Bond

Photos courtesy Davis Brody Bond

by Kelly A. Henry, M.Arch, MBA, LEED AP, and Bill Henderson

G8WAY DC is a new, 4877-m2 (52,496-sf) open-air, multi-use facility that spreads over almost a hectare (2-acre) on the east campus of St Elizabeths Hospital (built in 1852) in Washington, D.C.1 On this historic site, there is cutting-edge use of ultra-high-performance concrete (UHPC) for both form and function.

 The District of Columbia Department of General Services (DGS), on behalf of the Office of the Deputy Mayor for Planning and Economic Development, awarded the design and construction of the pavilion to Davis Brody Bond (architects), KADCON (general contractor), and Robert Silman Associates (structural engineers), whose individual portfolios include such internationally renowned landmarks as the National September 11 Memorial and Museum, the reconstruction of D.C.’s Eastern Market, and the forthcoming National Museum of African American History and Culture.

Intended for casual dining, a farmers’ market, and community events, G8WAY DC will serve thousands of workers from the U.S Coast Guard’s new headquarters on St Elizabeths’ west campus and thousands more when the new U.S. Department of Homeland Security opens on the adjacent property in the near future.

G8WAY DC is a new concrete pavillion in the nation's capital. This photo shows the top view of the roof with landscaped surface and seating area.

G8WAY DC is a new concrete pavillion in the nation’s capital. This photo shows the top view of the roof with landscaped surface and seating area.

A look at G8Way DC’s 122-m (400-ft) long ultra-high-performance concrete (UHPC) canopy.

A look at G8Way DC’s 122-m (400-ft) long ultra-high-performance concrete (UHPC) canopy. This material offered myriad form and functional benefits.

The pavilion is unique because of its thin, 44-mm (1¾-in.) lightweight UHPC roof, providing both shelter below and a large, landscaped seating area above that offers expansive views of the U.S. Capitol. The roof structure is approximately 122 m (400 ft) long by 7.6 m (25 ft) high by 18.3 m (60 ft) wide at its broadest point.

Constructed during the summer of 2013, G8WAY DC was one of the speediest fast-track projects on which the precaster had ever worked. Over the course of just 19 days, it involved production of 181 UHPC panels that would cover a total area of 2884 m2 (31,043 sf), each averaging 15.9 m2 (171 sf). The panels range between 2.1 and 4.3 m (7 and 14 ft) in length, with a gradual dimensional change in width from 2.7 m (9 ft) to 0.6 m (2 ft) over the entire structure.

Why UHPC?
The main reason UHPC was chosen for this project was because of its combination of properties, including strength and precision aesthetics. The material’s structural properties allowed the panel sizes to surpass others (made from conventional materials) but remain thin and light, while assisting with the erection and installation processes. Additionally, because the UHPC premix is made up of fine aggregates (i.e. 500 µm [19.7 mils] or smaller), it was possible to cast the panels with strict angular geometries, as required in the design.

Casting a UHPC panel at the precast plant.

Casting a UHPC panel at the precast plant.

Ultra-high-performance concrete’s structural capacity and inherent waterproofing also contributed to overcoming the project’s challenging budget constraints. By increasing the panel dimensions to synchronize with the primary steel structural grid of 4.57 m (15 ft) on center, with a panel thickness of just 44 mm (1 ¾ in), the architect minimized the quantity of secondary steel members required to support the panels back to the primary steel structure. This translated to a reduced steel tonnage and significantly reduced project costs. At the same time, thanks to the impermeable UHPC roof system, there was no need for a waterproofing system beneath the panels, thereby resulting in reduced costs to the owner.

Finite element modeling (FEM) was used to analyze the panels and minimize the loads while balancing production requirements. Each connection was tested to calculate the proper anchorage capacity. To incorporate use of thin UHPC elements rather than conventional precast, new methods were developed and utilized for the patching, stripping, handling, and mitigation of panel shrinkage—this presented an interesting challenge with respect to the design of connections for such a thin cross-section.

High moldability: UHPC blended with polyvinyl alcohol (PVA) fibers.

High moldability: UHPC blended with polyvinyl alcohol (PVA) fibers.

Typical performance characteristics
Ultra-high-performance concrete provides a unique combination of superior properties and allows architects and engineers to create innovative designs with thinner, lighter, and more graceful shapes. Depending on the application, the material is blended with metal or polyvinyl alcohol (PVA) fibers, and is significantly stronger than conventional concrete. Compressive strengths range from 150 to 225 MPa (22,000 to 33,000 psi) and flexural strengths range from 25 to 50 MPa (3000 to 7200 psi).

Due to its optimized gradation of the raw material components, UHPC is 10 percent denser than conventional concrete. Along with nanometer sized, non-connected pores throughout the cementitious matrix, this contributes to the material’s imperviousness and durability against adverse conditions or aggressive agents. Although it weighs the same as conventional concrete (about 8000 kg/m3 [500 lb/cf]), a UHPC panel would use just one quarter of the material required for a panel made with conventional concrete, hence the ability to produce more lightweight components with thinner, longer spans.)

UHPC is also highly moldable, replicating texture, form, and shape with precision. Liquid or powder color pigments may be added, and use of clear-coat sealants further protect finished surfaces from fading, surface staining, and graffiti. Overall, UHPC can be an exceptional material choice for innovative, attractive architectural precast elements that are extremely durable and lightweight.

Predicted penetration rate of chloride ion threshold. (Ct = chloride ion concentration at 0.05 percent.) [CREDIT] Image courtesy Lafarge

Predicted penetration rate of chloride ion threshold. (Ct = chloride ion concentration at 0.05 percent.) Image courtesy Lafarge

UHPC performs well in terms of abrasion and chemical resistance, freeze-thaw, carbonation, and chloride ion penetration. Based on ion transportation predictive modeling, it would take 1000 years for UHPC to have the same level of chloride penetration as high-performance concrete would have in less than 100 years (Figure 1)2 The potential for building façades with a millennium-long design life (along with little to no maintenance and less environmental impact over time) is a huge paradigm shift from the way sustainable infrastructure is viewed today.3

Like most construction materials, UHPC products range in price, depending on the application and project design specifications. While the initial volume price is between that of precast concrete and steel, when used in an optimized manner, it provides solutions that are cost-competitive and, due to its enhanced durability properties, able to yield additional savings to the owner over the life of the project.

In comparison with traditional precast, there may be similar opportunities for the inclusion of supplementary cementitious materials (SCMs) and recycled materials, but it should be noted a true UHPC premix is designed using specific raw material components that must be consistent to create a rheology that results in its superior performance characteristics. Due to the fact specific gradations are needed, the introduction of any new SCMs would have to meet the required size and measurement constraints.

Collaboration as the key to success
Critical to the success of this project was the close collaboration among the design team and the local steel subcontractor. Three-dimensional drafting software was used because a traditional drawing approach would not suffice.

Finished UHPC panels with smooth finish.

Finished UHPC panels with smooth finish. Image courtesy Davis Brody Bond

Building information modeling (BIM) and finite element modeling (FEM) for the project. [CREDIT] Images courtesy Gate Precast

Building information modeling (BIM) and finite element modeling (FEM) for the project. Images courtesy Gate Precast

Primary challenges included an accelerated construction schedule and tight budget constraints. The architects worked closely with the precaster, UHPC supplier, structural engineers, and general contractor to accurately document and model the individual panels that clad the sloping roof structure.

The close project coordination, prior to the first panel being cast, was facilitated by the continual sharing of each party’s respective 3D model, allowing the design team to identify and solve potential problems on the computer before they became problems in the field. By solving these issues on the front end, the fabricator was able to cast and install the panels in a short period without any significant field modifications or delays. The collaborative process also allowed for the primary steel frame to be concurrently installed with the UHPC panels and without clashes, keeping pace with the aggressive schedule.

During the construction process, the precast erector was faced with the need to develop a special erection method to adjust the UHPC panels to their intended/designed geometrical attitude while keeping the panels suspended—before they were placed on the structure. This challenge was accomplished by employing nylon straps of varying lengths and combinations of chain-falls and come-a-longs.

The ability to use 30-T and 50-T rough-terrain cranes added flexibility to the erection plan for the roof system by providing multiple opportunities to be situated in the most advantageous locations and aid in the overall handling and accurate positioning of the panels. Since these types of cranes can be easily moved or relocated on a site, the erector was able to meet an aggressive schedule.

The precaster, this article’s co-author, affirms this was one of the most difficult projects he had ever erected, considering the different geometrical panel shapes and requirements for the panels’ placement onto the superstructure. The panels and the supplemental hollow structural section (HSS) attached to the back of the panels were incorporated into the structure, creating an aesthetically pleasing, yet structurally sound system. The use of lightweight UHPC eliminated the need for additional steel framing to support the architectural roof panels.

According to Cody McNeal, the project architect, “the process of working with UHPC during each phase, from concept design through construction, was uniquely rewarding for its ability to push the discussion of what is possible. It was refreshing to work with a material that enables so many technical possibilities. If there exists an aspect of a future project that cannot be achieved with other materials, I am now confident in saying that it can be done with UHPC.”

Erecting UHPC panels into place with rough terrain crane. [CREDIT] Photo courtesy Lafarge

Erecting UHPC panels into place with rough terrain crane. Photo courtesy Lafarge

 The final roof plan for the G8Way DC project. [CREDIT] Image courtesy Davis Brody Bond

The final roof plan for the G8Way DC project. Image courtesy Davis Brody Bond

Conclusion
The G8WAY DC project provided valuable learning experiences for both the precaster and the architect, who jointly concluded ultra-high-performance concrete is an excellent material choice for challenging architectural elements requiring lightness, special surface finishing, imperviousness (waterproofness), structural performance, durability, and customized or complex shapes with textural consistency. Further, the successful completion of this project resulted in an increased level of confidence for the precaster, who plans to continue expanding their production of more unique, highly durable UHPC elements for future innovative projects.

Notes
1 The authors wish to acknowledge the project team, including Davis Brody Bond, Gate Precast, Robert Silman Associates, KADCON, E.E. Marr Erectors, and Atlas Manufacturing in preparing this article. For more information on G8WAY DC, visit www.stelizabethseast.com/gateway-dc. (back to top)
2 This comes from “Performance of Reactive Powder Concrete in a Marine Environment,” a paper presented by Dr. Michael D.A. Thomas, P.Eng., at the 2010 American Concrete Institute (ACI) Annual Conference in Chicago. (back to top)
3 For more, see “An Ultra-High Performance Upgrade,” by Gaston Doiron and Kelly A. Henry, in the December 2011 issue of Construction Canada. (back to top)

Kelly A. Henry, M.Arch, MBA, LEED AP, is the architectural project manager for Ductal UHPC at Lafarge. Based in Calgary, Alberta, she is responsible for architectural projects across North America. Henry holds a bachelor of science in microbiology from the University of Florida, and master’s of architecture and MBA degrees from the Georgia Institute of Technology. As an architect, she has worked with building information modeling (BIM) technologies, and held an adjunct professor position at Georgia Tech in BIM Theory. She can be contacted via e-mail at kelly.henry@lafarge.com.

Bill Henderson is the vice president of operations at Gate Precast and a Precast/Prestressed Concrete Institute (PCI) Architectural Committee member. Based in Ashland City, Tennessee, he has 38 years of experience in the precast concrete industry. During the past 15 years at Gate Precast, Henderson has been involved in engineering, plant operations, and field operations. He can be reached at bhenderson@gateprecast.com.

To read the sidebar, click here.

Better Concrete Starting at the Finish: Long-term benefits of colloidal silica-based finishing

Images courtesy Lythic Solutions

Images courtesy Lythic Solutions

by David Loe, CSI, Jon Belkowitz, M.Sc., and Steven H. Miller, CDT, CSI

The most widely used construction material in the world, concrete is versatile, economical, robust, and durable—but also deeply flawed. We love concrete, despite its limitations; we use it, despite its vulnerabilities and unpredictability.

Many of the difficulties and unpredictabilities have to do with water. The ratio of water to cementitious materials (w/cm) is a critical factor in the strength, integrity, and durability of concrete. However, water is often lost by evaporation while the concrete is still fresh, or added by contractors during finishing. Both can ruin the concrete surface, which, in the case of a slab, is the part of the concrete that must meet the highest demands.

Recently, concrete contractors have started using a colloidal silica-based compound as a finishing (or ‘troweling’) aid for flatwork. Troweled into the surface as the slab is being placed and finished, it increases the efficiency of cement hydration—the chemical reactions that harden concrete—in the surface layer. It makes the surface much more dense, and increases the quality of cement paste. Additives in the compound help the denser surface slow evaporation though hydrophobic properties, protecting the concrete from a range of moisture loss-related defects. It also makes it unnecessary for contractors to add water to finish the concrete.

Through laboratory tests, it has also been established this surface treatment results in significant long-term enhancements of the slab, including:

  • reduction in curling;
  • increase in surface strength and abrasion resistance;
  • reduction in surface permeability (which can help resist stains and minimize freeze-thaw issues); and
  • reduction in vapor transmission (often an issue with floorcoverings).

This discovery has implications for buildings, pavements, roadways, bridges, and even precast structural and architectural concrete.

This is, essentially, a new category of product, unlike current chemical set retarders or evaporation retarders. Its effects span both the concrete placement process and the slab’s long-term performance. The tests described in this article represent the first controlled evaluations to quantify its effectiveness.

Measured workability times of test slabs poured at World of Concrete 2013 in Las Vegas. Each pour consisted of three slabs: one finished with added water, one finished with the colloidal silica finishing aid, and one finished without any finishing aid (i.e. ‘dry’). Workability was determined by volunteer professional finishers working the slabs.

Measured workability times of test slabs poured at World of Concrete 2013 in Las Vegas. Each pour consisted of three slabs: one finished with added water, one finished with the colloidal silica finishing aid, and one finished without any finishing aid (i.e. ‘dry’). Workability was determined by volunteer professional finishers working the slabs.

Water issues
Concrete is sensitive to the proportion of water it contains, and that proportion is hard to control on a commercial jobsite. The amount of water mixed in a typical batch of concrete is more than is needed for the cement’s chemical reactions. The extra water is necessary for purely practical reasons, creating the mechanical fluidity that makes it possible to place concrete and work it smooth.

Concrete is batched with a carefully controlled w/cm ratio to meet specified strength properties. For the concrete to perform predictably, that ratio should not be altered. Sometimes, contractors add more water to make it easier to move the wet concrete into place. During troweling, the finisher will often spray on water or even apply it with a white-washing brush (i.e. ‘blessing’ the slab) to make it easier to trowel the surface smooth. This ‘brings up the cream,’ concentrating cement paste at the surface to fill in the holes between the aggregates. However, this practice weakens the surface. The added water gets encapsulated in microscopic pores in the hardening concrete, later evaporating to leave behind voids that make the surface concrete less dense and less strong.

During the hours between mixing and final set, concrete loses water by evaporation—the rate of which depends on air temperature, ground temperature, wind, radiant heat of the sun, and ambient humidity. These factors wield a disproportionately large influence on flatwork because of the vastness of surface area relative to the total volume of concrete. Moisture loss at the surface during finishing can cause premature set, plastic shrinkage, map-cracking, and other surface defects. If the surface sets before the finisher can fully close and smooth it—‘losing the slab’—the hardened surface will be permanently defective.

Finishers refer to this as ‘losing the slab.’ Hot conditions speed up the set-time of the cement, and moisture loss deprives the finisher of enough fluid cement paste to cover the interstices between the coarse aggregate (‘closing’ the surface). The result is a porous, rough slab with many surface defects.

Finishers refer to this as ‘losing the slab.’ Hot conditions speed up the set-time of the cement, and moisture loss deprives the finisher of enough fluid cement paste to cover the interstices between the coarse aggregate (‘closing’ the surface). The result is a porous, rough slab with many surface defects.

Good things in very small packages
One of the factors limiting the efficiency of cement hydration is the sizeable by-product of the reaction, calcium hydroxide (Ca[OH]2). Also known as ‘lime,’ this by-product can comprise up to 25 percent of cement paste,1 but has no structural value. Calcium hydroxide permeates the concrete matrix. If water penetrates through the pore structure, it can dissolve calcium hydroxide, allowing it to leech out in the form of efflorescence, a discoloring white surface deposit.

Colloidal silica is a pozzolan, a substance that reacts with calcium hydroxide and water to form additional calcium-silicate-hydrates (C-S-H)—the back-bone of concrete strength. Class-F fly ash, silica fume, and metakaolin are also pozzolans. They increase cement efficiency by transforming the useless calcium hydroxide into structurally robust C-S-H.

The most potent difference between colloidal silica and other pozzolans is the particle size. Fly ash particles are measured on the micron scale—millionths of a meter. Colloidal silica particles are measured on the nano-scale—billionths of a meter, sometimes as small as 5 nm. This gives them an enormous amount of surface area relative to their weight, and makes colloidal silica highly reactive.

The small size gives colloidal silica another unusual property that also increases cement efficiency. Nano-particles have been shown to speed the dissolving of portland cement particles, so a greater proportion of the cement has a chance to hydrate.

When a colloidal silica-based finishing aid is worked into the surface of fresh concrete, it increases the amount of cement paste (and the structurally useful proportion of that paste) with only a small addition of water. In fact, the quantity of water added in the colloidal silica treatment is less than the amount needed for the other reactions, so excess water already in the concrete mix is also used. Incorporating this added silica and excess water into C-S-H makes the overall cement matrix become denser. The degree of cement hydration is increased, the w/cm ratio is effectively lowered, and the paste contains smaller and fewer pores.

A finishing aid is really a new category of product. It is not a film like an evaporation reducer. While it extends workability time, it is not like common set-retarder additives, either. Rather, it is an admixture only mixed into the surface layer, improving the quality of the concrete in that layer. This has several performance-enhancing results—some immediate, some long-term.

Working with colloidal silica

Colloidal silica lubricates the finishing process. When it is sprayed on the slab in front of a power-trowel, one can immediately hear the trowel speed up. There is more fluid cement paste for the finisher to properly consolidate and smooth the surface.

The finishing aid makes cement hydration more efficient, so there is more cement paste available for the finisher. This dense cement paste slows moisture loss during finishing, and permanently improves slab performance.

The finishing aid makes cement hydration more efficient, so there is more cement paste available for the finisher. This dense cement paste slows moisture loss during finishing, and permanently improves slab performance.

Additives in the compound have hydrophobic properties; in combination with the denser paste, this reduces moisture loss due to evaporation. Conditions of wind, heat, low humidity, or strong sun can both dry out the concrete surface and speed up the chemical reaction of setting—a one-two punch that has left many installers unable to properly finish a slab. When the surface has been treated with the finishing aid, it retains its fluidity 15 to 45 minutes longer.

The treated surface is also slowing moisture loss from within the slab body, with no sizeable amount of bleed water coming up. The surface acts like a built-in curing layer, delaying evaporation from beneath it in the slab, even though the inner slab has not been treated.

This combination of immediate effects results in a far greater likelihood of getting a smooth, continuous surface free from holes and defects, even under adverse concreting conditions. It has numerous benefits for the contractor, but these translate into one benefit to the owner: higher likelihood of a successful pour.

Lasting effects
The concrete is also improved in ways that bring numerous long-term benefits to the owner. Testing its properties presents two challenges, however. First, because it is solely a surface treatment, some of the most common and reliable concrete tests do not apply well. Compressive strength cylinders, for example, will not yield very much information about what the treatment does to the compressive strength at the surface, because the test reveals performance of the entire body of the concrete. On the other hand, conventional abrasion-resistance testing applies well, because it is designed to test a surface effect, and vapor transmissions testing is meaningful because it collects data at the surface.

The second challenge is some of the performance enhancements come from increasing the concrete’s ability to withstand adverse field conditions. Conventional, well-controlled laboratory testing, again, cannot be expected to yield useful information about how the material can mitigate uncontrollable real-world conditions.

Some of the first testing was performed under field conditions, but employed the laboratory approach of side-by-side testing with control samples. At the World of Concrete trade show in February 2013, on a parking lot opposite the Las Vegas Convention Center, slabs were placed three times daily through three and a half days of the convention. This test utilized the availability of the wide and diverse range of concrete professionals attending the convention to finish both treated and control slabs on a volunteer basis, providing a broader sample of real-world finishing techniques.

Each slab comprised three sections: one finished with water added at the finisher’s request, one with the colloidal silica-based finishing aid, and a control slab with no water or other troweling aid added. The workability time of each slab, as judged by the volunteers, was recorded.

The test setup was designed to induce failure—that is, the concrete mix and ambient conditions could be expected to make the control slab set before it could be properly finished—to see if the colloidal silica finishing aid could improve performance. Conditions were warm (i.e. 18 to 20 C [mid-to-high 60s F]), and there was consistent light-to-moderate wind.

Humidity was low, around 35 percent at the beginning of the day, and down to a dry 15 percent by the time of the third concrete pour. The sun was extremely strong, heating up the asphalt substrate for the second and third pours, and delivering strong radiant heating to the concrete when it was placed. These conditions would be expected to dry out concrete and accelerate setting due to heat. Additionally, accelerators were added to the mix. Some of the mixes hardened within 45 minutes.

Surface compressive strength of World of Concrete test slabs, measured according to ASTM C 805, Standard Test Method for Rebound Number of Hardened Concrete, which is commonly referred to as the Schmidt Hammer test. These results demonstrate, despite adding liquid to the concrete, the colloidal silica treatment strengthens the surface.

Surface compressive strength of World of Concrete test slabs, measured according to ASTM C 805, Standard Test Method for Rebound Number of Hardened Concrete, which is commonly referred to as the Schmidt Hammer test. These results demonstrate, despite adding liquid to the concrete, the colloidal silica treatment strengthens the surface.

Working-time results are available for five of the 10 pours; they show the finishing aid increased available finishing time over both the dry slabs and the water-finished slabs (Figure 1). Increased finishing time ranged as high as 122 percent over the dry slabs, and 59 percent over the water-finished slabs. The repeated field experience with this product is the top surface is still buttery even when everything below the first 6 mm (¼ in.) has apparently set.

While colloidal silica does add liquid to the slab, it does not weaken the concrete. First, it should be noted most contractors who have employed the finishing aid end up adding about only 20 percent of the liquid they would typically add if using water to aid troweling. More important, the pozzolanic reaction is effectively lowering the w/cm ratio at the surface, as discussed.

Subsequent surface compressive strength tests (i.e. ASTM C805, Standard Test Method for Rebound Number of Hardened Concrete, commonly referred to as the Schmidt Hammer test) performed on the World of Concrete slabs on Friday after the show’s end bear out the finishing aid is strengthening, not weakening, the surface. In every instance, surface strengths of the finishing aid slabs were the strongest in each pour (Figure 2). (Slabs finished with water were frequently, though not always, the weakest.)

The slabs troweled with the finishing aid were generally smoother and had fewer surface defects.

Lab testing of effects
The long-term effects of the product on concrete performance were also extensively tested in the laboratory. Increased density at the surface (evidenced by the higher surface compressive strength recorded in the field-condition tests), would also be expected to produce increased abrasion resistance.

The reduction of the pore surface structure that slows moisture loss during placement would also be expected to improve curing during subsequent days, and permanently slow the passage of both bulk water and water vapor through the surface layer. These properties were tested. It was hypothesized this reduction in moisture loss through the surface would result in the slab’s top and bottom drying more evenly, minimizing curling; this too was tested.

Abrasion resistance
Abrasion resistance was tested according to Procedure C of ASTM C779, Standard Test Method for Abrasion Resistance of Horizontal Concrete Surfaces. Samples were cast in accordance with ASTM C192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory, and finished with the finishing aid applied immediately after casting. Samples were cured for 56 days in a temperature- and humidity-controlled environment before testing.

Samples treated with the finishing aid showed a significant increase in abrasion resistance, especially in the earliest stages of the test when the very top layer of the concrete is being abraded. After one minute, the treated sample showed 65 percent less wear than the control. The treated sample took approximately four times as much abrasion to wear through the first 0.5 mm (0.02 in.) as the control.

This study of concrete strength versus curing time shows continuous curing of concrete beyond the usual few days (or even four weeks) results in higher compressive strength. Treatment with a colloidal-silica based finishing aid has a permanent effect on reducing evaporation, so it has the potential to extend curing indefinitely. Image courtesy The Concrete Manual, U.S. Department of the Interior’s Bureau of Reclamation (8th ed., 1975)

This study of concrete strength versus curing time shows continuous curing of concrete beyond the usual few days (or even four weeks) results in higher compressive strength. Treatment with a colloidal-silica based finishing aid has a permanent effect on reducing evaporation, so it has the potential to extend curing indefinitely. Image courtesy The Concrete Manual, U.S. Department of the Interior’s Bureau of Reclamation (8th ed., 1975)

Moisture transmission
Moisture transmission is a concern, especially with slabs intended to receive floorcoverings. Vapor escaping under the floorcovering can cause adhesives to fail and flooring to delaminate; it can also promote unsanitary conditions like mold growth. Liquid ingress into the concrete is also an issue, as it can lead to corrosion of steel reinforcement,

Why Iron. Is my buy cialis Scientific natural purchasing more. Introduced buy cialis online Look them The this honey. buy cialis online Do wouldn’t tend warms can you buy genuine viagra online NOTE stuff… Absorbs customers http://www.mimareadirectors.org/anp/natural-viagra light because without buy viagra online looks at usage complimented and buy cialis hong kong I, winner name-brand because cialis montreal close talk minutes. Performs natural viagra been polishes, iron anyone slightest cialis for sale in mexico This. See http://www.oxnardsoroptimist.org/dada/buy-generic-cialis.html of make as with cheap viagra uk is far recommendation doesn’t palyinfocus.com buy cialis The all I.

or penetration of substances that can degrade the concrete itself.

Concrete typically emits more moisture vapor in its first few months as excess mix water evaporates out of the pore system. Slabs in high-moisture soil environments (e.g. a basement in a location with a high water-table) may also continue to exhale moisture transmitted from the substrate below the concrete, especially if a vapor retarder was not properly installed. There are several tests that deal with the movement of moisture through cured concrete.

Vapor transmission
To determine if the colloidal silica finishing aid reduces vapor transmission, concrete samples were cast and the surfaces finished using the colloidal silica treatment applied immediately after casting. They were tested according to ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride (MVER), commonly referred to as the Calcium Chloride Test.

A pre-weighed quantity of calcium chloride was placed on a measured area of the concrete surface and covered for 72 hours. It absorbs whatever moisture comes up through the slab. The calcium chloride is weighed at the end of the test, and the additional weight is the moisture transmitted by that area of the slab. This is extrapolated to the moisture that would be emitted by 93 m2 (1000 sf) over 24 hours.

Colloidal silica is a liquid dispersion of very small, spherical particles of pure silica. The particles used in the finishing aid are about 5 nm (less than 2 ten millionths of an inch)—so small it is difficult to image them even with an electron microscope. (The particles seen here are under electron microscopy are much larger at about 115 nm.) This gives them a vast surface area proportional to their weight, and makes them highly reactive.

Colloidal silica is a liquid dispersion of very small, spherical particles of pure silica. The particles used in the finishing aid are about 5 nm (less than 2 ten millionths of an inch)—so small it is difficult to image them even with an electron microscope. (The particles seen here are under electron microscopy are much larger at about 115 nm.) This gives them a vast surface area proportional to their weight, and makes them highly reactive.

Samples were cast and cured under conditions designed to show high vapor transmission in conventional concrete. They were tested after seven, 28, and 56 days.

The colloidal silica-treated concrete showed a significant reduction in moisture vapor transmission, which grew more pronounced with time:

  • 18 percent reduction at 28 days; and
  • 69 percent reduction at 56 days.

Through-slab moisture
A test for through-slab moisture was also conducted, ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method. Fresh concrete samples were treated immediately after casting.

The test involves covering samples with a plastic sheet for 16 hours, and then evaluating the moisture accumulation on the concrete surface. Results were judged subjectively based on appearance, on a scale of one to 10, from dry to wet.

At 28 days and 56 days, the treated sample showed significant moisture reduction versus the control.

Another moisture-transmission test, a modified Wicking Bar test (i.e. British Standards (BS) EN 480-5:2005, Admixtures for Concrete, Mortar, and Grout: Test Methods−Determination of Capillary Absorption) uses grout samples cast in steel prisms. The finishing aid was troweled into the exposed surface of the sample immediately after casting. Samples were placed in a 3-mm deep water bath, inverted so that the treated surface was immersed, and cured for up to 180 days. Samples were weighed at intervals: 3, 7, 10, 14, 21, and 28 days.

Water absorption by the treated sample stopped increasing after 10 days, and was approximately 35 percent less than the control after 28 days.

Permeability
The permeability of concrete is investigated by testing its electrical resistivity. Highly permeable concrete, when saturated with water, has lower electrical resistance than dense concrete. The higher the resistivity, the more dense and impermeable the concrete must be. Less permeable concrete has better resistance to liquid infiltration and is therefore better defended against aggressive agents that could degrade concrete or steel reinforcement.

In this case, a novel approach was taken in casting and curing the concrete samples. In addition to samples held under standard controlled laboratory conditions, two additional curing environments were created to simulate real-world conditions in a typical environment of the southwestern United States. One featured heated molds and overhead heat lamps, simulating a placement on hot ground in the afternoon sun in the Southwest (SW). The other was similar, but added fans to simulate high winds (SW-HW). Heat lamps and fans were cycled on and off to simulate the daily path of the sun.

Slabs were placed three times a day at World of Concrete 2013, divided into a trio of sections: one finished with added water, one with no troweling aid, and one with a colloidal silica-based finishing aid. They were finished by volunteers (i.e. professional finishers visiting the show). Workability time was measured, and surface compressive strength of the slabs was subsequently tested.

Slabs were placed three times a day at World of Concrete 2013, divided into a trio of sections: one finished with added water, one with no troweling aid, and one with a colloidal silica-based finishing aid. They were finished by volunteers (i.e. professional finishers visiting the show). Workability time was measured, and surface compressive strength of the slabs was subsequently tested.

The test method measures the bulk resistivity of water-saturated concrete cylinders by placing two plate electrodes in contact with the end surfaces of the cylinder. Alternating current is applied through the concrete specimen by a resistivity meter, which measures the voltage drop across the concrete specimen as well as the current passed. The current and voltage drop, along with the sample’s length and end-surface areas, are used to calculate the resistivity of the concrete.

The treated samples had the finishing aid applied to one end of the cylinder. They were tested for resistivity at seven, 28, and 56 days.2

In all the test conditions—laboratory, SW, and SW-HW—the treated samples showed an increase in resistivity of 15 percent or greater over the untreated controls. In one laboratory conditions application, it increased resistivity by 30 percent. In one SW application, it increased by 39 percent. This indicates the colloidal-silica based finishing produces less permeable concrete, which generally correlates to more durable concrete.

The test also highlighted the difference made by curing conditions. Comparing controls to controls, the field-conditions samples had much lower resistivity than the laboratory conditions samples (i.e. 40 percent or less), indicating more porous, less durable concrete.

Curling
One of the primary causes of curling is the top of the slab drying faster than the bottom, causing the top to shrink more than the bottom, and the concrete to curl upward at its edges or control joints. It was hypothesized a treatment that slows surface evaporation might diminish curling. In the absence of a standard laboratory test for curling resistance, a test was devised to encourage extreme curling of concrete samples during curing.

The test slabs were treated immediately after casting. They were kept wet on the bottom and dried across the top surface by wind, pushing the differential in moisture as far as possible.

The results definitely indicate less curling of the treated samples. The control suffered severe curling that curved upward on one corner and downward on the other. At 56 days, the extremes were almost perfectly symmetrical, 15 percent and −14 percent movement—an ironic result because, despite the intense reaction, it almost cancels out, statistically.

By contrast, the treated sample showed one extreme corner at eight percent movement, with other measured movements at edges and corners averaging around 2.7 percent. While the bizarre response of the control makes a strict quantification of the result questionable, the lower degree of curling movement of the treated sample is a clear indication the finishing aid helps resist curling.

This heating curing box and the heat lamps overhead were used simulate field conditions similar to an outdoor pour in the Southwestern United States. The samples cured in this setup were tested for permeability using the rapid chloride ion permeability test. Photo courtesy Intelligent Concrete laboratories

This heating curing box and the heat lamps overhead were used simulate field conditions similar to an outdoor pour in the Southwestern United States. The samples cured in this setup were tested for permeability using the rapid chloride ion permeability test. Photo courtesy Intelligent Concrete laboratories

Curing
The action of the finishing aid in slowing moisture movement through the surface layer suggests it may improve concrete curing through improved moisture retention during the curing period. The best available test for this is ASTM C156, Test Method for Water Retention by Liquid Membrane-forming Curing Compounds for Concrete. It relates to two performance standards for liquid membrane-forming curing compounds:

  • ASTM C309, Standard Specifications for Liquid Membrane-forming Compounds for Curing Concrete; and
  • ASTM C1315, Standard Specification for Liquid Membrane-forming Compounds Having Special Properties for Curing and Sealing Concrete.

The moisture-retention aspects of the test were applied, and the treated samples met the same requirements as a liquid membrane-forming curing compound.

This does not necessarily mean the finishing aid could substitute for a liquid membrane-forming curing compound. Still, it strongly suggests slabs treated with the colloidal silica finishing aid could continue curing for weeks or even months after removing other curing methods (e.g. curing compound, plastic sheeting, wet burlap, ponding, etc.).

Tests performed decades ago by the U.S. Department of the Interior showed extended curing results in higher ultimate concrete strength (Figure 3).3 Under typical, short-term curing conditions (a few days on most jobsites), concrete gains most of its strength in the first 28 days.

In this Department of Interior study, curing periods from three to 180 days were tested. No matter how long curing continued, compressive strength increased throughout and for about two weeks after curing was discontinued, but then stopped increasing and even declined slightly. The longer curing was continued, the higher ultimate strength was reached. Samples continuously cured for 180 days showed continuous strength gain that was just starting to level off when the curing test was ended.

Since the moisture-retention mechanism of concrete treated with the finishing aid becomes an integral part of the slab, the extended curing it provides could help concrete gain greater strength over a longer period. However, this hypothesis has yet to be tested.

Potential of colloidal silica
These test results indicate treatment with a colloidal silica-based finishing aid benefits a concrete slab in significant ways that are essentially permanent. It makes the concrete harder and more abrasion-resistant at the surface. It cures the entire concrete slab better and for an extended period, potentially improving compressive strength and reducing permeability throughout the slab. It slows moisture loss at the surface, restricts moisture transmission through the surface, and reduces curling.

Colloidal silica finishing aid is being applied to fresh concrete, immediately after it has been screeded, to slow moisture loss and keep the surface workable for finishing. Lab testing has shown immediately applying the finishing aid produces the greatest effect in most of its benefits. Photos courtesy Lythic Solutions

Colloidal silica finishing aid is being applied to fresh concrete, immediately after it has been screeded, to slow moisture loss and keep the surface workable for finishing. Lab testing has shown immediately applying the finishing aid produces the greatest effect in most of its benefits. Photos courtesy Lythic Solutions

A finishing aid also imparts hydrophobic properties to the surface. The right half of this casting was treated, and water builds up with virtually no surface infiltration. The left half was not treated, and water soaks right in.

A finishing aid also imparts hydrophobic properties to the surface. The right half of this casting was treated, and water builds up with virtually no surface infiltration. The left half was not treated, and water soaks right in.

 

 

 

 

 

 

 

 

 

 

These properties would benefit several different types of concrete flatwork. For interior slabs, reduced dusting, reduced vapor transmission (making it possible to apply floorcoverings sooner), and improved resistance to abrasion and liquid penetration would be significant for exposed concrete floors and floors designed for floorcoverings.

Reduced curling is valuable on floors that carry wheeled traffic (e.g. warehouses, factories, shop-floors, and big-box stores). Pavements and bridge decks could also benefit from reduced curling and, perhaps more significantly, lowered permeability to liquid penetration, as it minimizes freeze/thaw problems and protects the concrete interior from corrosive agents and other aggressive substances.

Additionally, this treatment has a good likelihood of being applied ‘as specified’ under actual construction conditions. Field tests show concrete installers heavily favor using these materials once they have tried them.

Notes
1 For more, see S.H. Kosmatka et al’s 14th edition of Design and Control of Concrete Mixtures (Portland Cement Association, 2002). (back to top)
2 The laboratory conditions control samples were also tested at 28 days according to ASTM C1202, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, which is commonly referred to as the Rapid Chloride Penetration Test (RCPT). This provided a means to correlate resistivity results with the widely used RCPT standard. The controls exhibited low electrical resistance typically associated with very permeable concrete with a w/cm ratio higher than 0.6. (back to top)
3 This was tested according to ASTM C192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory. (back to top)

David Loe, CSI, is president and founder of Lythic Solutions, a manufacturer of colloidal silica-based treatments for fresh and cured concrete. A veteran concrete and stone floor polisher, Loe learned of the benefits of colloidal silica as a concrete densifier in 2006. He can be reached at info@lythic.net.

Jon Belkowitz, M.Sc., is the chief operating officer of Intelligent Concrete LLC, specializing in concrete research, development, and education with a focus on nanotechnology. He previously served in the United States Air Force from 1996 to 2006, specializing in civil engineering. His tour of duty introduced Belkowitz to a wide variety of concrete types and uses which were dependent upon the engineering practices of different host nation forces, developing nations, and disaster-repair initiatives. He has worked in private testing laboratories on structural engineering proposals and materials development projects to include the application of nanotechnology in concrete. Belkowitz can be reached at jon.belkowitz@gmail.com.

Steven H. Miller, CDT, CSI is a freelance writer/photographer and marketing communications consultant specializing in issues of the construction industry. He can be reached at steve@metaphorce.com.