In the construction industry, property inspectors encounter a variety of different concrete mixtures. Some of these are prepared by general contractors, while others are specifically designed to meet a client’s specific needs. Depending on the mixture, various admixtures are added to modify its properties in order to achieve the desired result.
Some admixtures alter the physical properties of fresh concrete, while others change its chemical properties in order to improve durability or control shrinkage cracking. Other admixtures alter set time or workability, or reduce water demand. Still, others may be used to reduce the amount of bleeding in concrete or prevent slumping and segregation. All of these admixtures serve a unique purpose and can be beneficial in a particular situation, but it is important to understand the properties of each one in order to use them effectively.
Water reducing admixtures increase the workability of mortar or concrete without changing the water-cement ratio, or maintain the same workability at reduced water-cement ratios. Examples include lignosulphonate and hydroxylated carboxylic acid salts.
Super plasticizing admixtures are the same as water reducing admixtures, but they allow for a larger decrease in the water content for a given workability, and therefore are sometimes referred to as high range water reducers (HRWR). This type of admixture only lasts a few minutes, so it is used at the job site to overcome slump loss and maintain concrete operation.
Accelerating admixtures are used to speed up the hydration of cement and concrete, which allows for higher strengths at early ages. They can also be used to reduce the need for curing, which helps to accelerate project completion and save time and money.