Concrete is a remarkable product, having been used for thousands of years as a reliable and durable building material for a countless amount of structures. In its most basic form, concrete is a simple combination of cement, water, air, and a granular material (sand and/or rock). However, to maximize the potential of concrete and provide flexibility for its use in a variety of environments and situations, additional materials can be added to the concrete during the mixing process to change its characteristics. These materials are known as chemical admixtures, and their impact on concrete properties will be discussed further below.
Admixtures related to moisture control (called water-reducing admixtures) generally consist of additives that allow for concrete to retain its workability given a water content lower than that of typical concrete. Reducing the amount of water in a concrete mix will generally cause an increase in strength; however, in unmodified concrete (no admixtures), the reduction in water creates a reduction in the concrete’s ability to flow, and it becomes more difficult to work with and form. Standard water-reducing admixtures generally allow for a drop in water content of 7 to 10%. It should be noted that using a water–reducing admixture may affect the curing time of concrete, increasing or decreasing it dependent on the type of chemical used.
Superplasticizers are considered high range water reducing admixtures since they go beyond the water reduction of normal water-reducing admixtures. The polymer admixtures allow the reduction of water by 12% to 30% or more without reducing the workability, retarding the setting effect, or reducing particle suspension of the concrete. However, the effect of these compounds has a limited duration (30-60 minutes) and their addition must be timed carefully to provide the maximum positive effect. New technology is allowing superplasticizers to be added to the concrete mix while the truck is in transit.
Superplasticizers are primarily used to provide a fast-flowing concrete at standard water content. This is useful in applications where there are narrow forms, tight working spaces, or in the event the concrete needs to be pumped a long distance or dropped from a significant height in order to reach its final placement position without significant separation of materials. Superplasticizers act to disperse particles within a concrete mix, improving the flow of the mix while maintaining overall cohesiveness.
Concrete reinforcing bars (commonly referred to as rebar) typically consist of uncoated carbon steel, and as such have a tendency to corrode over time, particularly when exposed to air, water or chloride ions (salt). In order to counter this tendency, corrosion inhibitors may be added to the concrete.
There are two goals behind the addition of corrosion inhibitors, both with the objective of reducing the sensitivity of the steel to environmental factors. The first goal is to extend the period between the installation of the concrete and the onset of the reinforcement corrosion, and the second goal is to reduce the overall amount of corrosion, if it does occur.
There are several types of corrosion inhibitors, each with a different mechanism of operation. Calcium nitrite has been used extensively for many years to mitigate the impact of chloride ion migration in concrete. Chloride ions are introduced into concrete commonly through exposure to sea water or deicing salts, and may make their way to the reinforcing steel. Normally, the surface of the reinforcing steel inside typical concrete is coated with a compound known as ferrous oxide that results from exposure to air. The ferrous oxide is in an unstable state, and will attract nearby chloride ions, which will lead to steel degradation. The calcium nitrite reacts with the surface layer of the steel and converts it to ferric oxide, which is in a passive or neutral state. Encroachment of chloride ions into the concrete will then have little to no effect on the reinforcing steel as they cannot react with stable molecules
Other types of corrosion inhibitors, such as those comprised of amino alcohols, create a coating over the steel that prevents intrusion of chloride ions, as well as disrupting the effects of oxygen and water on the steel.
Concrete strength is often enhanced through the use of compounds consisting of microsilica (also known as silica fume), which when added to the mix, create a significant increase in compressive and flexural (bending) strength in concrete, as well as a reduction in its permeability. Strength can also be enhanced by using superplasticizers or fly ash, both of which are discussed elsewhere in this article.
An example of an application for high strength concrete is in lower-level building columns in tall structures, which are under extremely heavy loads and require concrete compressive strengths and values of up to 20,000 pounds per square inch. For comparison, most standard concrete is mixed to provide compressive strengths of 3,000 to 6,000 pounds per square inch.
The introduction of air into a concrete mix can provide extensive protection against freezing, by creating an internal buffer within the concrete that allows it to contract without immediately resulting in breakage. To create the addition of tiny air bubbles in the concrete, modern mixes include a surfactant (detergent), which is incorporated into the concrete prior to mixing. The mixing process generates the bubbles, most of which remain in the concrete as it hardens and become part of the final structure.
Winter weather also has an effect on concrete placement, as low temperatures may cause a notable reduction in concrete cure times. In these cases, an accelerator can be added to allow the concrete to cure within a normal timeframe and help reduce the possibility of frost damage.
Hot weather has significant effects on standard, non-modified concrete. The elevated temperatures cause the concrete to cure at an increased rate and begin to set up relatively quickly, which results in problems in workability and also results in shrinkage cracking. In order to counter the effects of hot weather, set-reducing or retarding admixtures can be incorporated to slow down the curing process and provide a normal period of workability and uniform cohesiveness of the concrete mix. These set-reducing admixtures are also used in the event that the mixing plant is located a significant distance away from the delivery area. The set-reducers help to prevent premature setting of the concrete inside of the mixing truck.
Fly ash has been used successfully as a concrete admixture for decades. Consisting of residue that results from incomplete combustion, typically as a result of coal processing, fly ash is often considered a “green” admixture that makes use of a material that would normally be considered waste. However, there are some health concerns with Fly Ash since it contains a large amount of heavy metals, not to mention that it is a by-product of coal use, which is an air pollutant. Sustainability rating systems are starting to limit the use of fly ash due to the health and environmental concerns.
Fly ash can be directly substituted for the cement in a concrete mix in percentages ranging from approximately 20 to 35 percent (by weight). Aside from the reduction in environmental waste, fly ash also provides benefits during the construction process and in the finished concrete product. Generally, mixes with fly ash have improved workability and require less water to maintain a given strength requirement. The cured concrete provides enhanced resistance to corrosion of the internal steel reinforcement and also helps provide a barrier against sulfate intrusion in cases where the concrete is cast directly against soil.
Underwater Concrete Placement
The biggest challenge with placing concrete in an underwater environment is preventing washout of the cement before the concrete has had a chance to set. To maintain the consistency and integrity of the mix during underwater placement, anti-washout admixtures are often used in tandem with superplasticizers to increase the viscosity of the concrete mix and minimize the segregation of the materials.
More Information on Chemical Admixtures
This article is a brief overview of how chemical admixtures can be used to change the properties of concrete. If you are interested in a deeper understanding of admixtures, be sure to check out the American Concrete Institute's free article, Chemical Admixtures for Concrete.