Ways to Improve Concrete’s Durability and Workability

One of the ways for concrete to adapt to situations such as specific pumping requirements or to place concrete in extreme weather conditions is to add chemical admixtures.  A chemical admixture can be either a natural or manufactured chemical and is added to the concrete along with portland cement, aggregate, and water, and generally make up less than 5% of the total concrete mix. 

The Portland Cement Association states that, “There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers).”   Also the National Ready Mix Concrete Association (NRMCA) explains that chemical admixtures are added to concrete to increase “durability, workability or strength characteristics of a given concrete mixture.”  They can be in a liquid or powder form and are added to the concrete immediately before or during the mixing process. 

The most commonly used chemical admixtures are air-entraining agents, water-reducers, water-reducing retarders and accelerators.  Air-entraining is the process where microscopic air bubbles are deliberately added to the concrete mix.  This is done to increase the concrete’s durability when exposed to repeated cycles of freezing and thawing as well as to improve the concrete’s workability.  Another benefit is that segregation and bleeding is diminished or completely eliminated.  The only drawback is that the concrete experiences a 5% reduction in strength for each 1% of air added. 

In situations where a job demands a slower pour or during very hot temperatures, retardants are used to slow down or delay the hardening process.  It gives crews more time to place and finish the concrete.  Common retardants are lignin, sugars, and citric and tartaric acids. 

To protect concrete from extreme cold or for those times when there is a tight project deadline, accelerators are used to increase the early strength of concrete and to get it to harden faster.  Chemical accelerators are chloride based or non-chloride based and calcium chloride is one type commonly used as it is inexpensive. 

1. cement.org/cement-concrete-basics/concrete-materials/admixtures

2. nrmca.org/aboutconcrete/cips/15p.pdf

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