An admixture is a substance that is added to a batch before or during mixing and is used as an ingredient to change the qualities of a cementitious mixture, whether it is freshly mixed, setting, or hardened. It is not water, aggregates, hydraulic cementitious material, or fiber reinforcement.
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A chemical admixture is typically further described as a liquid, suspension, or water-soluble solid that is nonpozzolanic (requires no calcium hydroxide to react).
While set-controlling admixtures are employed when concrete is laid and completed at temperatures below ideal, water-reducing admixtures enhance the plastic (wet) and hardened qualities of concrete.
Both support sound concreting techniques when applied correctly. Additionally, both admixtures must adhere to ASTM C 494’s specifications.
Water-Reducing Admixtures
In essence, water reducers lower the amount of water needed to mix concrete to achieve a specific slump, which can lead to a lower water-cementitious ratio (w/c ratio) and higher strengths and more durable concrete.
It has been determined that lowering the w/c ratio of concrete is the most important factor in producing high-quality, long-lasting concrete; however, in order to save money or the heat of hydration for mass concrete pours, the cement content may occasionally be lowered while keeping the original w/c ratio to lower costs or the heat of hydration.
Water-reducing admixtures also improve the flowability of the concrete and reduce segregation, which is why they are frequently used in concrete pumping applications.
Admixtures that reduce water usually fall into three categories: low-, medium-, and high-range. These categories are determined by the admixture’s water reduction range. The water decrease percentage is in relation to the initial mix water needed to achieve a specific slump.
Despite its commonalities, each water reducer has a specific use for which it is most suitable. Three categories of water-reducing admixtures exist, along with their ranges of water reduction and main applications.
The chemistry will determine how they affect air entrainment. A table outlining how admixtures and other variables affect air entrainment may also be found in Part 1 of this series.
How Water-Reducing Admixtures Work?
When cement and water come into contact, the cement particles’ surface electrical charges attract one another, causing flocculation, or particle clustering. This procedure absorbs a significant amount of the water, which results in a cohesive mix and less slump.
In essence, water-reducing admixtures neutralize solid particle surface charges and make all surfaces carry like charges. Better dispersion and less flocculation of the cement particles are made possible by the fact that particles with similar charges oppose one another. Additionally, they make the paste less viscous, which makes it slump more.
Depending on the manufacturer and product, more components may potentially be incorporated. Certain water-reducing admixtures react with accelerators or retarders or have side effects. We’ll talk about this later.
Effects on Concrete
The main purpose of water-reducing admixtures is to increase the strength of concrete by lowering its water-cementitious content. In certain situations, they can be applied to improve the concrete’s slump or workability, making placing simpler.
To raise the slump beyond the range that standard water reducers can provide without causing the extreme retardation that has been known to happen, mid-range water-reducing admixtures were created.
Superplasticizers, also known as high-range water reducers, were created for concrete applications requiring great performance and strength.
Concrete with a 3-inch droop can be made to have a 9-inch slump using superplasticizers without losing strength or running the risk of segregation. Utilizing a superplasticizer can be advantageous for many precasters, particularly due to its enhanced creation of high early strength.
All water-reducing admixtures improve cement dispersion, which leads to an increase in strength development. This enables more thorough cement hydration by increasing the cement particles’ accessible surface area.
Dosage
The weight of cement is used to administer water reducers; this is commonly expressed as fluid ounces per hundred pounds of cement (fl. oz./cwt). With the mix water, the majority of low- and mid-range water-reducing admixtures are dosed upfront.
Before placement, high-range water-reducing admixtures are typically introduced on the job site. For help with water-reducing additive dosage and applications, get in touch with your admixture provider.
Set-controlling Admixtures
Set-controlling admixtures change how quickly the cement hydrates and, consequently, how quickly the paste sets (stiffens).
Interestingly, they might also have an impact on the paste’s ability to harden or develop strength after setting. Accelerating and retarding admixtures are examples of set-controlling admixtures.
Retarding Admixtures
The hydration process is slowed down by these admixtures. They might also shorten the cement’s setting time. There are two types of retarding admixtures: ordinary and extended-set.
When lengthy travel hours are anticipated or, in an emergency, when placement is postponed, regular concrete placement tools, generally known as simply “retarders,” are utilized in hot areas. In order to avoid cold joints, they are also frequently utilized for large concrete pours.
Admixtures known as “extended-set control” are used to postpone hydration for several hours or even days. These are typically admixture systems with two components. The first ingredient, a stabilizer or retarder, postpones the concrete’s setting time.
The accelerator, also known as an activator, is the second part that gets past the retarder.After the activator is added, the concrete usually reaches its initial set in a few hours.
Additionally, retarding admixtures are utilized as surface retarders in the precast sector. When creating exposed aggregate architectural precast, surface retarders are utilized. Usually, they are rolled or sprayed onto the forms.
After that, the concrete is poured into the shapes. In order to create an exposed aggregate finish, the retarded cement is water blasted or brushed off the concrete’s surface the next day.
Accelerators
The cement hydrates more quickly thanks to these admixtures. In particular, they speed up C3S’s rate of hydration, which boosts its early strength. Accelerators come in two varieties: normal and rapid.
Rapid accelerators are used in shotcreting applications, to repair concrete damaged by hydrostatic pressure, or where extremely quick setting is needed. They can set concrete in a matter of minutes. Usually, precast concrete applications don’t use them.
Although they are not antifreezing admixtures, standard or regular accelerators are utilized to expedite building in cold-weather concreting circumstances. It has been demonstrated that accelerators only lower the freezing point by about 3 F.
Dosage
The dosage of set-controlling admixtures is fl. oz/cwt. Many are dosed in the mix water up front. Dosing later after mixing has started, however, can boost effectiveness. For optimal results, once more, speak with your admixture provider.
Mixtures
Certain admixture chemistries offer a mix of effects, including acceleration or retardation together with water reduction. This has several benefits, such as lowering the quantity of admixtures that need to be kept on hand and added to the concrete, decreasing admixture incompatibility, and saving money.
Less flexibility and restricted application when an accelerating or retarding effect is not sought are drawbacks.
Advantages of Using These Admixtures Together
When applied properly together, water-reducing and set-controlling admixtures offer numerous advantages.
Water-reducing agents will enhance the strength and durability of concrete by reducing the water content of the mix, while set-controlling admixture ensures that the mix is still workable for a specified time period.
These combined effects can be of great interest in large scale pours, pours where environmental conditions may impose challenges, or in cases where high surface finish quality is desired.
Factors Influencing Choice of Admixtures
Some of the factors influencing the choice of admixtures for a given project are type of concrete being mixed, environmental conditions including temperature and humidity, and finally, the desired performance characteristics such as strength, workability, and durability.
Some other considerations like specifics of concrete pour such as mass pours or precasting will also influence decision making.
Challenges and Precautions
While the benefits from using both water-reducing and set-controlling admixture can be significant, implementing them might present certain problems and precautions.
These may include problems in determining proper dosage amounts and combining incompatible admixtures that lead to corresponding unexpected results.
These could be reduced strength, high shrinkage, or segregation. Recommendations of the manufacturer must be followed besides making trial batches to prove compatibility and the expected results.
Future Trends in Admixture Technology
Amidst future trends envisaged within the admixture technology field, there will be growing incentives to producing admixtures that are environmentally friendly, so as to use recycled materials, while innovations tend to improve the nature of concrete.
Researches should go into minimizing and repairing on environmental impacts as a result of mixing concrete that will entail some new developments in admixtures technology.
Conclusion
To lower their water-to-cementitious ratio, make concrete installation easier, and boost early-age strengths, all precasters should use a water-reducing additive.
Use an accelerating or retarding admixture when necessary to make up for temperatures that are higher than 45 to 65 degrees Fahrenheit.
More details on appropriate concrete techniques for hot and cold weather can be found in the NPCA’s Quality Control Manual for Precast Concrete Plants. This is particularly helpful when precasting outside.
FAQS
What are water-reducing admixtures?
Water-reducing admixtures are chemical substances that reduce the water requirement for achieving a given degree of slump of concrete. This results in lowering the water-cementitious material ratio, improving the strength and durability of the concrete.
Why is a water-retarding admixture used?
Water-retarding admixtures slow the hydration of the cement, thus improving the working time of the concrete. This plays a very important role when it comes straight to hot-weather concreting, if long transportation times are involved, or for large pours in which the concrete must be workable for a much longer period of time.
What are the different types of water reducers?
Water-reducing admixtures belong to three categories based on their capacity to reduce the quantity of water required to attain a specific slump: low-range, medium-range, and high-range (superplasticizers).
What is a high-range water-reducing admixture?
High-range water-reducing admixture, also commonly called superplasticizers, are substances to drastically lessen the content of water added to a concrete mix; therefore, it may reach a high slump without losing its strength. These prove especially helpful in high-performance applications and precasting applications where early strength gain is important.