To enhance properties of fresh concrete construction experts are widely using admixtures in concreting process . Properties of fresh concrete are very important during construction phase, which are enhanced by admixtures dosing for high performance concrete. By adding required admixture(s) in appropriate quantity and following mixing instructions, we can play with many properties of concrete. For example, using water reducers or plasticizer during mixing of concrete, we can produce a cohesive mix with a low water-cement ratio having adequate workability. Also we may use accelerators or retarders to increase or decrease the setting time of concrete according to construction requirements.
Purpose of admixtures
Admixtures are used to achieve or enhance concrete performance and/or to overcome some constraints during construction. By using proper type of admixture in required quantity, we can modify the properties of fresh concrete according to our requirements. These requirements may be in form of workability, flowability, increase in setting time or air entrainment in concrete. Admixtures may help in reducing cost of construction due to enhanced properties of concrete. Admixtures are great tool for a site engineer during emergency situations. A site engineer can use appropriate admixtures to modify properties of fresh concrete and can save the resources.
Researchers are using admixtures to develop innovative concrete products such as high strength concrete, self-compacting concretes and other high performance concretes.
Types and uses of concrete admixtures
According to the type of materials, admixtures are broadly classified into two categories
- Chemical admixtures
- Mineral admixtures
- Polymer based admixcures
In this article, we will discuss chemical admixtures, for mineral admixtures please click here. Based on use and requirement of fresh concrete properties, chemical admixtures types and classifications are described below.
Water reducers
Water reducing admixtures are used when there is shortage of water (suitable for construction) or water is costly. Normally recommended dosage of water reducer decreases the requirement of water by 5 to 10% with same value of slump. If we use water reducer but amount of water added is not reduced, it will give us more slump value. Due to reduced water quantity, air voids in hardened concrete will be reduced and results in increase of 28 days compressive strength by 10 to 25%.
Plasticizers and Super-plasticizers
Plasticizers and super plasticizers are high range water reducing agents. These are used when high flowability and workability is required. Plasticizers are required to be added in concrete when there is congested reinforcement, concrete section is thin (flow able concrete is required to fill the formwork appropriately), or concrete pump is to be used. Effectiveness of plasticizers is directly proportional to the percentage of cement and fines in concrete.
Retarding Admixtures (Retarders)
As name suggest, retarding admixtures delay setting time of concrete. Retarding agents are widely used during construction in hot weathers to reduce the rate of hydration (rate of hydration is directly proportional to the environment temperature). Retarders are also very useful when there is delay in concreting due to any unavoidable reason.for example, construction site is away from batching plant or access to the site is difficult or pouring in form-work is difficult. for mass concreting, retarders are used to control crack prevention in concrete by reducing heat of hydration and delaying the setting time.
Accelerators
Engineers recommend use of accelerators during concrete construction to reduce the setting time of concrete. Accelerators are used during cold weather, when emergency repair is required, in foundations of water logged areas, for concreting in water bodies and/or when early removal of form-work is required. Accelerator tends to cause increase in shrinkage and decrease in resistance to sulfate attack. therefore sometimes use of accelerator is more harmful rather that being beneficial.
Air Entraining Agents
Air entrainement is provided in concretes to increase resistance against freezing and thawing. Microscopic air voids provide space to the concrete for stress relief during freezing of water particles. These admixtures (air entraining agents) are gas producing chemical which produces tiny microscopic bubbles in fresh concrete. Bleeding in concrete reduces and workability increases, whereas strength of hand concrete decreases with use of these chemicals. Use of air entraining agents, durability of concrete improves but, for every 1% of increase in air content, 4% strength reduces.
Specifications for Concrete admixtures
Various manufacturer produces chemical admixtures with different brand names. All admixtures are used to modify one or more characteristics of concrete should be accompanied by a certificate from manufacturer or an approved Institute (as per country law and practices), stating:
- Exact range and method of application
- Physical and chemical composition
- Positive and negative side effects
- A recommendation on the maximum allowable quantities (as a percentage of the mass of the cement) to achieve the particular requirement.
- Proportioning and mixing
The admixtures shall contain no chlorides. If a retarder and plasticizer are required only a dual-purpose additive shall be used.
Admixtures should be applied strictly in accordance with the manufacturer’s instructions. However, the contractor shall be responsible for any defective concrete that may result from the use of any malpractice.
Applicable codes and standards
American standards
ASTM C494 / C494M – 16
Standard Specification for Chemical Admixtures for Concrete
ACI 212.3R-10
Report on Chemical Admixtures for Concrete
British & European standards
BS EN 480-1:2014
Admixtures for concrete, mortar and grout. Test methods. Reference concrete and reference mortar for testing
BS EN 934-1:2008
Admixtures for concrete, mortar and grout. Common requirements
Australian standards
AS 1478.1-2005
Chemical admixtures for concrete, mortar and grout – Admixtures for concrete
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