admixtures

 

Different admixtures can be added to a concrete mix to control the set time of the concrete. There are several kinds of admixtures, such as, accelerating admixtures, retarding admixtures, air entraining admixtures, and water-reducing admixtures. Admixtures reduce the cost of concrete construction, achieve desried proprties in concrete, and maintain the quality of concrete during the mixing, transporting, and curing stages. The effectiveness of an admixture depends on its type, water content, shpe, mixing time, and amount. The chart below shows several concrete admixtures, their composition, and their uses. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Air-entraining admixtures add air bubbles to concrete in order to increase the concrete's durability to freeze/thaw conditions. Retarding admixtures delay the rate of setting in concrete, so that the concrete can be easily worked and placed nicely before it dries. Coloring admixtures can even be added to change the color of concrete for aesthetic purposes. 

 

aggregates

 

 

 

 

 

 

 

 

 

 

 

 

 

Aggregates are granular materials such as sand, gravel, or crushed stone that are an essential material in concrete.  Some examples of aggregates are shown in the image above. These aggregates account for 60-75% of the total volume of concrete. There are fine and coarse aggregates.  The selection of aggregates is an important process because they influence the properties of the concrete. Aggregates are chosen by their durability, shape, texture, weights, number of voids, and surface moisture. 

 

Precast concrete

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In this building method, concrete is cast and cured at a different location than where it will be used. After precast concrete members are made, they are brought to a construction site and erected. Since they are already made, it takes less time on site to erect them because they do not have to be cured. Since these members are made in a factory rather than on site, they have a higher quality. However, precast concrete has its limitations. It needs to be transported to the site, so the size and weight of the members has to be considered. Precast concrete cannot be used for two-way structural systems. Basically, it is a good method for making small, identical members in bulk, using the same mold. 

 

 

cast-in-place concrete

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In this method, the mix is deposited and formed at the place where the finished concrete is desired. The concrete is brought to the construction site by concrete-transporting mixer trucks, and then poured into molds. This building method is beneficial when forming unique forms and large spans.  However, since the concrete is placed on site, a team needs to be ready to place the concrete and finish it quickly. Weather conditions also need to be considered.