Role of Admixture in Precast Concrete

 With shifting global markets, increased customer expectations, and increasing labour costs, the precast concrete industry is constantly innovating. Productivity, quality, and bottom-line profits are always on the minds of producers. Additionally, the heightened awareness of environmental issues, coupled with a greater emphasis on project life-cycles, has meant more demand for environmentally friendly construction practices, especially regarding the reduction of waste, greenhouse gas emissions, and energy consumption.

It is no surprise that concrete is one of the major construction materials and the most abundant material produced around the world. This has driven the development of new concrete technologies both in general and specifically for precastconcrete.

As a result, a number of concrete types have been developed, including fibre reinforced concrete, polymer concrete, Ferro cement, sulphur concrete, lightweight aggregate concrete, autoclaved cellular concrete, high-density concrete, ready-mixed concrete, self-compacting concrete, roller compacted concrete, high strength concrete, super high-strength concrete, high-performance concrete, high-volume fly ash concrete, self-curing concrete, floating concrete, and smart concrete.

What are concrete admixtures?

The purpose of concrete admixtures is to improve certain properties of fresh or hardened concrete, such as workability, durability, or early and final strength. These can be natural or manufactured chemical additives added during concrete mixing to enhance the above-mentioned properties.

Concrete admixtures are no new discovery and have a history as old as the concrete or concrete technology. The use of concrete admixture in mortar and concrete can be traced back to Roman Empire, which is renowned for its elaborate constructions and architecture.

The Romans discovered that certain material allow greater workability in cementitious mixtures. Thereby starting the tradition of mixing the admixtures into the concrete before its use.

The recent times however like every other technology has witnessed development and improvement and so has the concrete technology. Now the concrete admixtures are used to enhance the quality of concrete but also to make it suitable for different conditions.

Concrete admixtures are now used to modify or improve the ordinary concrete properties to benefit situations like mass concreting, concreting at narrow spaces etc. They are also used at remote places where water is not available easily or where there is a necessity of removing the formwork early, or for the concrete work near coastal area, as such.

 

Chemical admixtures and their importance

Chemical admixtures are added to concrete to achieve or amplify its properties. The admixtures are added typically immediately before or during the mixing process to the concrete, in addition to cement, water and aggregate. As we understand that different chemical admixtures have different capabilities and utility, they are segmented in few categories.

Chemical Admixtures are classified as:

·         Air Entertainers

Air entertainers help in distributing tiny air bubbles in the concrete. These admixtures are capable of reducing the bleeding and segregation of fresh concrete. Further they are also capable of improving the resistance of concrete to severe frost action consequently reducing the damage during freeze-thaw cycles. This in turn increases the durability of concrete. Further these concrete admixtures also help to reduce the density of concrete.

(b) Water Reducers:

As the name suggests the water reducer admixtures are added to reduce the quantity of mixing water, required to produce concrete of a desired slump. These admixtures are used to reduce water-cement ratio, reduce the cement content and increase slump to obtain specific strength of concrete as needed in the projects.

·         Set Retarders:

The retarders are admixtures that help in slowing down or delaying the chemical process of hydration. The purpose of such requirement is to increase the time so that concrete remains plastic and workable for a longer duration. These concrete admixtures are widely used when the concrete mixtures have to be is carried out at high temperature or to a different place from its mixing.

·         Set Accelerators:

Accelerators are the opposite of the retarders and are added to concrete to increase the rate of early strength in concrete. These admixtures permit earlier removal of formwork and reduces the required period of curing. Among the admixtures of this category the Calcium chloride is the most common accelerator. These categories of concrete admixtures are especially advantageous for modifying the concrete properties in cold weather.

·         Corrosion Inhibitors:

Corrosion inhibitors are the admixtures used to reduce the corrosion in steel and steel bars in concrete. These can reduce the maintenance cost of structure. Interestingly they can reduce maintenance cost of RCC structures throughout a typical service life of 30 to 40 years.

So to summarise from above here is why admixtures are important in precast concrete:

·         It reduces water content in concrete and maintains the water cement ratio as low as possible.

·         It increases the compressive strength.

·         Enhances the durability parameter of concrete

·         Aggravates the workability and provides longer transportation and placing time.

·         To impart specific property of concrete.

·         Creates favourable condition during placing.

·         High quality admixtures are capable of controlling the shrinkage cracks.

·         Ensures the quality of concrete in adverse weather condition.

·         Modify the rate or capacity for bleeding.

·         Improve pumpability and decrease permeability

·         Increase bond between new concrete and pre-existing if any.

Mapei manufactures globally proven solutions and products for the demanding production and repair of diverse precast concrete elements including hollow-core planks, concrete pipes, burial vaults, septic tanks and double tees, among others. As one of the global names and 85-year-old legacy Mapei ensures but one thing the quality of product while providing variety to suit the different site requirements.