Concrete Admixtures are main important role in constructions. It is used to improve concrete behavior under variety conditions. Basically Admixtures are classified into two types as Chemical and Mineral Admixtures. during concrete mixing, admixtures are added to concrete batch.
Types of Concrete Admixtures, Functions, Advantages
Concrete Admixtures are improve concrete strength, quality and manageability, acceleration and setting time. Behavior of concrete and Properties could be altered to get specific results. Simple stated, A Material other than water, Aggregates or cement that is used as an ingredient of concrete or mortar to control setting and early harderning workability. Concrete Admixtures are generally used to alter the concrete properties. The use of admixtures offers certain beneficial effects to concrete like improved workability, acceleration or retardation of setting time, reduce water cement ratio and many more. include improving workability, strength material loads bearing capacity, increasing or decreasing cure setting time, and increasing concrete strength.
Admixtures can also be used for aesthetic reasons, such as to change the color of the cement. Admixtures can be used to reduce the cost of building with concrete, or to ensure certain required properties or quality of the cured concrete. Ordinary concrete may not give us the required quality performance such as durability, strength, workability and finishing quality of concrete. concrete admixtures are used to modify or improve the ordinary concrete properties as well. Plasticizing of fresh concrete mixtures or to control the setting time of concrete. Admixtures helps, its broadly catagorised as superplasticizers, accelerators, retarders, water reducers and air entraining admixtures.
Types of Admixtures of Concrete
Chemical admixtures – Accelerators, Retarders, Water-reducing agents, Super plasticizers, Air entraining agents etc.
Mineral admixtures – Fly-ash Blast-furnace slag, Silica fume and Rice husk Ash etc
Functions of Admixtures
To Enhance workability and give more trasportation and place time
To reduce water content in concrete
To Control the shrinkage cracks.
To cause expansion of concrete and automatic prestressing of steel
To impart specific property of concrete or to create favourable condition during placing.
To ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition.
It Can reduce the water content needed to reach a required slump
Improving the curing of the concrete and hardness
Increase bond between existing and new concrete.
Type of Chemical Admixtures:
- Air entrainers
- Water reducers
- Set retarders
- Set accelerators
- Specialty admixtures
Type A: Water-reducing admixtures
Type B: Retarding admixtures
Type C: Accelerating admixtures
Type D: Water-reducing and retarding admixtures
Type E: Water-reducing and accelerating admixtures
Type F: Water-reducing, high range admixtures
Type G: Water-reducing, high range, and retarding admixtures
Air Entering Admixtures
Air Entering admixtures are used to fill type microscopic air bubbles in concrete set. Air Entering type admixtures improves durability of concrete. It increasing resistance power to damage from freeze thaw cycles as well as from deicers. air-entrainment provides improved workability during placement and has superior water tightness. The main goal of this admixture is to increase the resistance against degration and cohesion. It aslo improves the workability of fresh concrete without changing the setting rate of hardening. frozen water expands into these air bubbles rather than cracking the concrete. They improve the workability and durability of the mix.
Water Reducing Admixtures Plasticizers:
These type of admixtures can be used for achieve high stength by decreasing water cement ration at same workability as an admixture free mix and by these admixture, we can observe increase the workability so as to ease placing in accessible locations. The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts. Water reducers are mostly used in hot weather concreting and to aid pumping. Water reducer plasticizers are hygroscopic powder, which can entrain air into concrete. asticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.
SuperPlasticizers: Type of chemical admixture, it added to reduce the water requirement by 15 to 20% Without affecting the workability leading high strength and dense concrete level. Superplasticizers are liner polymers containing sulfonic acid groups attached to the polymer at regular intervals. The commercial formulation can be sulfonated melamine–formaldehyde conden- sates, sulfonated naphthalene formaldehyde condensates, and modified lignosulfonates, polycar- boxylate derivatives. Superplasticizers give each cement particle a negative charge, which causes them to repel each other providing a more thorough distribution. Improves the cohesiveness and reduces segregation and bleeding
- Sulphonated melamine formaldehyde condensates (SMF)
- Sulphonated naphthalene formaldehyde condensates (SNF)
- Polycarboxylate ether superplasticizers (PCE)
Shrinkage Reducing Admixtures: These type of concrete Admixtures are added to concrete during initial mixing. it reduce early and long-term drying shrinkage. High workability concrete containing superplasticizer can be made with a high freeze-thaw resistance, but air content must be increased relative to concrete without superplasticizer.
Mineral Admixtures of Concrete Types:
Cementitious Admixtures of Concrete
Pozzolanic Admixtures of Concrete
Ground Granulated Blast Furnace Slag (GGBFS)
Rice Husk Ash
Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete and in some cases reduce the material cost of concrete. This refines the pore structure, reduces the permeability and improves the durability of concrete structures. Pozzolans are used to improve the workability and quality of concrete. The use of flyash in concrete makes the mix economical, and improves the workability, reduces segregation, bleeding and reduced heat of hydration but also provides ecological benefits. Fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO). Toxic constituents include arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium.
Admixtures are used in concrete to enhance the performance of the mix in various ways. Generally added before or during the mixing process, admixtures can increase the strength the mix, accelerate or slow down the curing process among other benefits.