Editor’s Note: This is the final article in a year-long series explaining common raw materials used in precast.
With their ability to support stable, cohesive concrete mixes, viscosity-modifying admixtures offer incredible utility in the precast industry.
By Kayla Hanson, P.E.
What do concrete, shampoo and toothpaste have in common? They are all classified as non-Newtonian substances, meaning each material’s viscosity – the resistance to deformation or how “thick” it is – varies depending on applied stresses. In other words, mixing, pouring or otherwise applying stresses to these substances can cause them to be more or less flowable.
Because fresh concrete is continuously exposed to applied stresses throughout its plastic state – including during mixing, transport, placement, consolidation and finishing – its viscosity, and the ease with which it flows, may be affected. Other fresh concrete characteristics may also be impacted. Fresh concrete’s consistency, homogeneity, stability and cohesiveness all respond to stresses applied to the mix throughout the production process.
Self-consolidating concrete is more sensitive to these applied stresses than other mix designs. Due to its highly flowable nature and unique mix proportions, SCC is more susceptible to segregation – where the water and fines are inclined to separate from the rest of the mix – particularly during transport and placement. Viscosity-modifying admixtures can be used to combat this natural tendency and ensure a stable, cohesive mix from the time the concrete is discharged from the mixer until it sets.
What are VMAs?
Viscosity-modifying admixtures (VMAs) are additives used to stabilize fresh concrete’s homogeneity and consistency.
“VMAs are used for stabilization of the mix, but they do so by modifying the rheology,” said Terry Harris, technical service director of GCP Applied Technologies.
VMAs increase the mix water’s viscosity through a variety of mechanisms. By doing so, it impacts fresh concrete’s rheological properties – how its flow characteristics change under applied stress. This includes plastic viscosity and yield stress.
ASTM C494, “Standard Specification for Chemical Admixtures for Concrete” categorizes VMAs as Type S admixtures. Type S admixtures affect specific aspects of concrete’s performance or behavior and fall outside of the classifications of water-reducers, retarders or accelerators that are also outlined in ASTM C494.
VMAs are comprised of a wide range of chemistries. Some VMAs consist of fine, inorganic materials like colloidal silica while others are based on larger, more complex synthetic polymers like hydrophobically modified ethoxylated urethane (HEUR).1 Most VMAs, however, consist of polymers – chains of large molecules containing many repeating units – made of polyethylene oxides, cellulose ethers, alginates, natural and synthetic gums, polyacrylamides or polyvinyl alcohol.2
Impact on rheological properties
According to ACI 212.3R-16, “Report on Chemical Admixtures for Concrete,” plastic viscosity is defined as the property of a material that resists change in the shape or arrangement of its elements during flow. A tangible example of viscosity differences is shown by comparing the flowability of traditional wet-cast concrete to that of SCC. Traditional wet-cast is more viscous or “thicker” and has limited flowability, whereas SCC is less viscous and highly flowable.
The most significant contributor to SCC’s flowability is the use of water-reducing admixtures. VMA use, in contrast, increases fresh concrete’s viscosity, making it slightly less flowable. Despite causing the slight reduction in flowability, which may be seen initially as a drawback, VMAs’ impact on viscosity results in more cohesive and stable concrete mixes that retain homogeneity and consistency.
Besides the obvious increase in plastic viscosity, VMAs can affect other rheological properties of fresh concrete as well. Fresh concrete’s yield stress – the amount of force required to cause the plastic concrete to flow – is another rheological property that can be impacted by VMAs. The impact of VMAs on yield stress can range from none to significant, depending on the type of VMA. VMAs that cause a significant increase in yield stress result in plastic concrete that requires significantly more force for the concrete to flow. Using a VMA that increases viscosity without impacting yield stress tends to be preferred for SCC applications.2
Some non-Newtonian substances, like concrete, shampoo and toothpaste, also exhibit time-dependent thixotropic properties, which means they become less viscous when subjected to applied stress. Thixotropic non-Newtonian substances exhibit a progressive decrease in viscosity with time under constant shear stress. This behavior is also referred to as shear thinning. In the case of SCC treated with VMAs, when the mix begins to flow, the force of the flow causes the VMA molecules to align in the same direction as the flow. This action allows the paste to lubricate the aggregates, reduce the tendency of aggregate interlocking, reduce internal friction and enhance flowability.2
How VMAs work
VMAs are generally categorized as one of two types: thickening or binding. Thickening VMAs rely on the addition of large polymer molecules to the paste and increase viscosity through molecular obstruction. The thickened paste results in increased cohesion. Conversely, binding VMAs work by chemically combining with water molecules in the paste and producing a gel. The gel inhibits changes in viscosity caused by applied stresses throughout normal production practices, promoting thixotropic behavior.3
With most types of VMAs, including those that are diutan gum-based, when concrete is at rest, the VMA molecules form a network, as shown in Figure 2a. This network increases the fresh concrete’s viscosity and inhibits segregation, preventing fines and mix water from separating from the bulk concrete. Under applied stresses, such as mixing, transport, placement and finishing, the VMA polymer chains align in parallel, as shown in Figure 2b. This disrupts the network of cement paste and aggregates and enhances the fresh concrete’s flowability and thixotropy. When the concrete returns to rest, the network of VMA molecules is rebuilt, again resulting in increased viscosity and reduced likelihood for segregation, as shown in Figure 2c.1
The impact of VMAs on fresh concrete properties and behavior is most significantly dictated by the source materials from which the VMAs are developed. However, even when sourced from some of the same base materials, VMA chemistry, dosage rate, performance and impacts on fresh concrete vary from one supplier to the next.
“They are not interchangeable,” said Ara Jeknavorian, Ph.D., consultant at Jeknavorian Consulting Services and veteran research fellow and chemical admixture developer. “They are highly dependent upon source materials, and thus have different potencies. Welan gum is about 1/3 the potency of diutan gum, but diutan gum is available at a higher price.”
VMA use in precast concrete is especially prevalent for SCC applications, but precasters can also turn to VMAs in traditional wet-cast applications to enhance stability and retain consistency and slump over time. VMAs can also be used in dry-cast applications to resist segregation and improve surface finish, compaction and moisture retention. VMAs offer a wide range of benefits across other types of concreting applications, as shown in Table 1.
Mix design considerations
VMAs are primarily specified to increase concrete’s plastic viscosity, improve cohesion, and enhance homogeneity and consistency. However, VMAs can offer other benefits. In some cases, VMA use may allow for a reduction in the fines content that would otherwise be used to improve fresh concrete’s cohesion.
VMAs can also help accommodate challenging aggregates – like gap-graded aggregates or those with very rough textures or elongated shapes – which may otherwise decrease fresh concrete’s flowability.
“A less obvious benefit from VMAs is their lubricating effect, whereby water and fines are more homogeneously maintained throughout the mix, thus coating rough aggregates surfaces and thereby reducing frictional forces,” Jeknavorian said.
Additionally, VMAs tend to make SCC mixes more robust by increasing segregation resistance in response to small changes in mix water content.
Fresh concrete considerations
When VMAs are used in accordance with supplier recommendations, precasters should not expect changes in plastic concrete’s air content, density or temperature. Cement hydration rates, setting time and compressive strengths should also remain unaffected.
However, because VMAs impact fresh concrete’s viscosity, and in some cases its yield stress, VMAs on their own are likely to reduce slump of traditional wet-cast concrete and reduce slump flow or the spread of SCC.
“VMAs will tighten up the spread a little,” explained Andrew Pearson, concrete business unit district manager with Sika Group. “With all other factors held constant, if Mix A has no VMA and a 26-inch spread and Mix B has the VMA, we might measure a 24-inch spread with Mix B. We might also use a water-reducer to counteract the reduced SCC spread.”
Additionally, VMAs’ impact on fresh concrete’s viscosity, consistency and stability also tend to reduce bleed water and the visibility of flow lines.
Tips and troubleshooting
As with any change in raw materials or mix design, incorporating VMAs requires careful attention to detail, plenty of trial batches, diligent tracking of fresh and hardened concrete test results, and ample patience.
Experimenting with trial batches is key to success, particularly when determining the proper dosage rate for your VMAs.
“The most common mistake is making big jumps, shooting for the moon right away and starting off with a high dosage rate,” Pearson said. “Instead, make one minor change and see what impact it has. Start at one ounce per hundredweight.
“Some VMAs are more potent than others.”
Because the dosage rate of VMAs can be as low as one ounce per hundredweight, a slight variation in dose could have a significant impact on the concrete mix.
“This low dosage rate amplifies the need for accurate and routine equipment calibrations,” Jeknavorian advised.
Although VMAs vary by supplier, they are usually batched into the mixer with the water, or at the beginning of the batch. Because VMAs are used in such small doses, incorporating them with mix water helps ensure consistent, thorough distribution.
Window of effectiveness
Unlike other types of admixtures, the window of effectiveness and functionality for VMAs is open.
“The material tends to be active from the time of batching all the way through initial set,” Jeknavorian said.
This attribute is critical to ensuring that the mix’s consistency and homogeneity are retained and all raw materials – including heavy aggregates – remain in suspension until the concrete hardens.
Despite their far-reaching benefits and variety of applications, VMAs should not be mistaken for an all-purpose solution or a quick fix for a batching error.
“The addition of a VMA to a concrete batch that has segregated may reduce or eliminate the segregation, but if the segregation was due to excessive water, the strength and durability of the concrete will be affected,” Harris explained.
“One should never use a VMA to correct a mis-batch,” he said. “You can try to fool the rheological properties, but the concrete will know something is wrong.”
Precasters are encouraged to rely on their admixture suppliers for support when implementing VMAs and for troubleshooting guidance.
Pushing the boundaries of precast
VMAs are yet another novel advancement in the concrete industry. They allow precasters to design mixes that continue to push boundaries – achieving very high flowability, self-consolidation, great strengths, homogeneity, consistency and cohesion – all while defying their natural inclination to segregate. With proper procedures in place and careful attention to detail, VMAs can provide reliable, effective precast solutions.
Kayla Hanson, P.E., is NPCA’s director of technical services.
1 “Viscosity Modifying Admixtures: A Handy Chemical Admixture for the Concrete Producer.” 2016. Ara Jeknavorian, Ph. D.
2 “Design and Control of Concrete Mixtures,” 16th Edition. 2016. Page 229.
3 “Viscosity and Rheology-Modifying Admixtures for Concrete.” 2017. G. Terry Harris, Sr.