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.
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.
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.
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.
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.
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 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.
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,
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 email@example.com.
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 firstname.lastname@example.org.
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 email@example.com.