Concrete Admixtures are elements added to concrete mix before or during mixing to improve various properties in the fresh or hardened form, in addition to the three main elements of cement concrete (cement, aggregate and water). The most commonly changed properties due to the use of admixtures in concrete are rate of hydration or setting time, workability, dispersion and air entrainment.
Many admixtures have an effect on multiple properties of concrete, and while some may be beneficial, others may be detrimental. An additive could only be employed after a thorough investigation of its effects on concrete. Some level of supervision is essential to ensure the proper amount of mixing.
Functions of Admixtures in Concrete
There are many functions due to which admixtures are added to concrete mix to enhance its properties. Some of them are mentioned below.
- Main function of admixture is to improve workability of concrete.
- To produce concrete of fungicidal, germicidal and insecticidal properties.
- Produce non-skid concrete surfaces to make it slip-resistant.
- To reduce heat of evolution to prevent concrete from damaging.
- Increases durability of concrete.
- Resistance to freezing and thawing action.
- It control’s expansion caused by aggregate-alkali reaction.
- It decreases capillary flow of water to make it impermeable.
- To increase the penetration and pumpability of concrete.
- It also reduces segregation in grouts, strengthen the bond between old and new concrete surfaces and that between steel reinforcement and concrete.
- To accelerate the initial set of concrete. It generally speeds up the rate of development of strength at early ages.
- To retard the initial setting of concrete.
- Increase the strength of concrete.
- Inhibit corrosion of concrete.
- It increases resistance to chemical attack.
- To produce colored and cellular concrete admixtures are used.
Types of Admixtures
Admixtures for concrete can be classified into various category types such as:
- Accelerators
- Water Proofing Admixtures
- Retarders
- Bleeding Agents
- Air Entraining Agents
- Workability Agents
- Bonding Admixtures
- Plasticizers
- Superplasticizers
- Puzzolanas
- Grouting Agents
- Algicides
- Fungicides
- Expansion Producing Admixtures
- Coloring Agents
Accelerators
These are admixtures that speed up the hardening of concrete or the development of early strength. This admixture has no effect on the setting time in any way. However, in practice, the setting time of concrete is shortened.
Accelerators are extensively used in civil engineering. Plugging leaks in swimming pools, water tanks, and pipelines, as well as emergency highway, bridge, airport runway, and tunnel repairs are just a few of the applications where this admixture is used.
Some of the common examples of substances which are used as accelerators in concrete are:
- Alkali carbonates
- Aluminates and silicates
- Aluminium chloride
- Calcium formate
- Calcium chloride
- Sodium chloride
- Sulphates with an exception of calcuim sulphate
- Potassium hydroxide
- Sodium hydroxide
- Para formaldehyde, etc.
Concrete Water Proofing Admixtures
Water proofing admixtures seek to minimize water penetration by reducing surface tension in capillary pores in hydrated cement paste, which pulls in water via capillary suction.
Water proofing admixtures work in a variety of ways, but their main purpose is to make concrete hydrophobic, which means it repels water due to a greater contact angle between the capillary walls and the water.
These admixtures are categorized into water repellent and pore filling agents. Some of the common examples of these water proofing admixtures are:
- Water Repellent Materials :
- soda
- potash soaps
- fat
- wax
- coal tar residue
- calcium soaps
- resin
- vegetable oil
- Pore Filling Materials :
- alkaline silicates
- silicate of soda
- aluminium and zinc sulphate
- aluminium and calcium chlorides
Retarders
Retarders are primarily used as a setting control for large structural units to ensure that the concrete remains workable during the placement process.
It’s also useful to counteract for the rapid setting induced by the ambient temperature, which happens a lot in hot weather.
Under freezing and thawing conditions, retarders have also been found to boost compressive strength.
Some of the common examples of retarding admixtures for concrete are:
- Hydrocarboxylic acids
- Sugar
- Sodium hexametaphosphate
- Ferrous and ferric chlorides
- Ammonium
- Starch
- Cellulose
- Lignosulphonates
- Calcium sulphate
- Acid and their salts
- Carbohydrates
Bleeding Agents Used in Concrete
Bleeding agents are normally used to reduce the bleeding effect of concrete. Paraffin wax (0.2–0.75% by mass of cement) or air entrainment admixtures in concrete are used to prevent bleeding. The latter is more efficient, but it necessitates a high level of control.
Air-entraining Agents
Entrained air is the air that is purposely injected into cement during its manufacture or during the production of concrete.
Air entrainment improves the workability and weather resilience of concrete. Bleeding, segregation, and laitance are all reduced as well.
This type of additive is used to make concrete more frost resistant.
These agents are further classified into 2 sub-categories.
- a. Dispersing Agents
Surface active compounds that impart electrostatic charges to cement particles are known as dispersing agents. As a result of mixing this admixtures, cement particles reject one other, preventing coagulation.
A little amount of air is also entrained in the concrete, which improves its workability. Moreover, the strength is reduced by using dispersing agents. Calcium lignosulphonate is the most prevalent dispersing agent.
- b. Surface Active Agents
Surface active agents, also known as air entraining agents, lower surface tension. A 5% increase in air may result in a 0.07 rise in compacting factor and a 12 to 50 mm increase in slump.
Besides some of the other materials which can be used as air-entraining admixtures are:
- Animal or vegetable fats.
- Sulphated organic compounds.
- Natural wood resins.
Workability Agents
These substances are used to make concrete more workable. This is accomplished by increasing the volume of paste and, as a result, the cohesiveness of the paste. It should be noted that if used in excess, the amount of water must be increased as well, which will result in concrete strength loss and cracking.
Some of the common examples of workability agents are:
- Bentonite
- Kaolin
- Diatomaceous earth
- Chalk
- Lime
Bonding Admixtures
These are polymer emulsions that help fresh concrete attach to hardened concrete. This property qualifies it mainly for repair work. Bonding admixtures are somewhat expensive because they attempt to improve the tensile and flexural strength, durability, and bonding qualities of concrete.
Some of the common used bonding admixtures in concrete are:
- Synthetic rubber.
- Synthetic latex emulsions.
- Polyvinyl chloride.
Plasticizers
Plasticizers are organic or inorganic chemicals that allow for a reduction in water content for a given workability or a higher workability at the same water content. Concrete’s strength and durability are strengthened as a result of the lower water content.
They are primarily used to improve the workability of concrete so that it may be placed in inaccessible regions with ease. Moreover, to achieve the higher strength by decreasing the w/c ratio.
Some of the organic compounds commonly used as plasticizers are:
- Carboxylic acid & their salts
- Hydroxy Carboxylic acids
- Lignosulphonates Salts (sodium & calcium)
Superplasticizers
Superplasticizers are primarily used to create higher strength concrete with a w/c ratio of 0.23–0.3, as well as “flowing” concrete with a high slump flow in the region of 500 to 600 mm, known as self-compacting concrete.
They’re used to make flowing concrete in instances where it needs to be placed in difficult-to-reach places, such as floor or pavement slabs, or when it needs to be placed or poured quickly.
They are upgraded versions of plasticizers that interact with cement particles both physically and chemically.
Some of the examples of superplasticizers which are commonly used are:
- Naphthalene sulphonate formaldehyde condensates
- Modified lignosulphonate
- Sulphonated melamine formaldehyde condensates (synthetic polymers).
- Mixture of saccharates and acid amines.
Puzzolanas
Puzzolanas are siliceous materials that are inert on their own but react with lime in the presence of water to generate cementitious compounds.
Puzzolanas react with free lime in cement to improve concrete’s durability and slow the rate of hardening, which is the main drawback to its use.
The common materials which can be used as puzzolanic admixture are:
- Blast furnace slag.
- Lime.
- Burnt clay.
- Fly ash.
Grouting Agents
As the grouting is done for different purposed, the grouting agents required for satisfying the perfect grout also varies. Grouting agents are generally classified according to their purpose.
Algicides & Fungicides
Some organisms, such as bacteria, fungi, or insects, can harm concrete by corroding steel and colouring the surface. Because bacteria thrive on the rough surface of concrete, surface cleaning is inefficient, necessitating the addition of poisonous admixtures to the mix. Fungicidal and algicide admixtures are the best treatments for them.
Some of the common examples of algicides and fungicides are:
- Sodium pentachlorophenate.
- Tributyl tin acetate.
- Arsenic.
- Tin.
- Mercury Compounds.
Coloring Agent
A new trend of concrete colouring has emerged. You’ve probably seen colourful patios or slabs before. Concrete colour gives your construction project a nice look. Concrete colouring is not only a gorgeous design option, but it’s also cost-effective and works with both new and old concrete.
Some of the common coloring agents used in the process are:
- Red oxide (red).
- Ferrous oxide (black).
- Raw umber (brown).
- Chromium oxide (green).
Although the various concrete admixtures presented above have numerous benefits, adequate care is required to reap the full benefits of admixtures.