Admixtures are natural or artificial additional materials that are added to concrete mixes either during or just before the mixing. They change the concrete’s properties in the hardened or plastic state.
Functions Of Concrete Admixtures
Types of concrete admixtures perform different functions including:
- Speed up the strength development rate in the earlier stages. Calcium chloride is one of the most commonly used accelerators.
- To slow down the setting time when pumping concrete over long distances.
- To increase workability without having to change the content of the water. Fly ash such as pozzolana is commonly used.
- To maximise strength.
- To increase resistance to thawing and freezing. Vinyl resin is the air-entrainment admixture used for these purposes.
- To elevate water tightness.
- To lower heat evolution.
- To lower water capillary flow.
- To lower the rate of segregation and bleeding.
Types Of Concrete Admixtures
There are two main types of concrete admixtures:
- Mineral admixtures
- Chemical admixtures
#1 Mineral Admixtures
Here is a list of the mineral admixture types:
- Silica fume
- Fly ash
- Rice husk ash
- Granulated ground blast-furnace slag
#2 Chemical Admixtures
Chemical admixtures are available in different types including”
– Accelerating Admixture
These substances increase the hydration rate of hydraulic cement, decrease setting times, or increase the strength development rate.
– Retarding Admixture
These substances slow down how quickly cement paste sets.
– Water-Reducing Admixture
These substances either increase the workability of concrete or mortar that has been freshly mixed without having to increase the cement-water ratio or maintain workability when it comes to reduced cement-water ratio.
– Air-Entraining Admixture
These substances allow air to “entrap/Cd” in tiny bubbles inside the concrete or mortar during mixing. This increases the workability of the concrete or mortar and its resistance to thawing and freezing.
These substances have extremely high workability along with a significant decrease in the water content according to the given workability. HRWRA (high range water reducing agent) is also known as a “superplasticizer”.
Water-reducing admixtures are used in 3 ways:
– When adding an admixture with a decrease in cement-water ratio, concrete with similar workability and increased compressive strength is obtained compared to the concrete products without an admixture. A 0.5% concentration according to the weight of the cement is capable of increasing the 28-days compressive-strength by up to 30% with a 20% reduction in the cement-water ratio.
– When adding admixture without decreasing the cement-water ratio, concrete that has grater workability and the “same” compressive strength is easier to obtain. For example, a 0.5% concentration of the admixture according to the weight of the cement usually increases the slump from 25mm to 100mm.
– When adding an admixture, concrete that features the same compressive strength and workability is achieved at a lowered cement content. For example, a 0.5% concentration of the admixture according to the cement weight, may require 15% less cement.
The concrete admixtures that fall under the chemical admixtures assist with reducing costs of construction, changing the properties of the set or hardened concrete, which maintains the concrete quality during transporting, mixing, placing, and curing. Certain types of concrete admixtures can also be used to deal with certain emergencies that can arise during concrete or mortar operations. Most admixtures are designed to enhance the overall performance of the end product.
Each of the concrete admixtures is made up of different materials and individual properties. It is recommended to contact an expert before choosing one. The costs of admixtures vary according to the type and quantity of the product being used. This is all added to the metre/cubic yard cost of the concrete.