Types of Chemical Admixtures for Concrete
Unlike mineral admixtures, which may be introduced as blended cements, chemical admixtures are typically added during the mixing process of concrete production. Depending upon the requirement, different types of chemical admixtures are manufactured such as air-entraining admixtures, accelerating admixture, water reducing admixture etc. To a civil engineer, description of admixtures according to their function is more useful than based on their chemical compositions.
1. Air-Entraining Admixture
Chemical admixture was first used in 1930s for entraining air into concrete to increase its frost resistance. The fine air bubbles with a close spacing provide partial relief as the liquid phase in concrete progressively freezes.
Dodson (1990) clarified that air-entraining admixtures (AEA) do not generate air in the concrete. Their function is to stabilize the air present within the void system of the mixture and water as well air infolded and mechanically enveloped during mixing. Even without an AEA, concrete contains some air, which is often referred to as “entrapped” air. These air voids are typically 1 mm or more in diameter and irregular in shape. They often collect at the paste-aggregate interface. Entrained air bubbles are mainly within the paste with diameters typically between 10 mm and 1 mm. They are spherical in shape at close spacing. The spacing factor, which is the maximum distance in the cement paste from the periphery of an air void, is usually in the range of 0.10 to 0.20 mm. The commonly recommended air content is 5 to 6% in the compacted concrete. Air bubbles promote workability but their presence reduces the strength of concrete. These factors are taken into consideration in the design of concrete mixtures.
2. Accelerating Admixture
The use of accelerating admixtures is common during cold-weather concreting, as the rate of hydration of cement is decreased by lower temperatures. Their function is to increase the rate of hydration, thereby speeding up the setting time and early strength development.
In the past, Calcium Chloride has been the most commonly used for this purpose. However, in recent years, the effect of chloride on the corrosion resistance of embedded steel reinforcement and prestressed tendons has been recognized. This has resulted in limiting the total chloride content in concrete at levels that is exceeded by the normal addition of calcium chloride as accelerating admixture. Currently, non-chloride accelerating admixtures are available, e.g., calcium nitrite (also a corrosion inhibitor). The use of calcium nitrite leads to a better strength gain at later ages than calcium chloride. However, this may not be of importance in practice as moist curing on site is limited to early ages only.
3. Water Reducing Admixture
Although water reducing admixture and retarding admixtures have listed differently, it is more typical to use both at the same time. This is also due to the fact that the two are available in the typical materials used in their formulation, e.g., salts of lignosulfonic acids. In particular, both functions are useful in the case of hot-weather concreting.
The amount of mixing water in a typical concrete mixture is more than that needed for full hydration of the cement used. The excess water is intended to promote workability. However, when water is added to cement, there is a tendency for the cement particles to cluster together, forming into flocs. Some of the mixing water is trapped within the flocs and not available to contribute to the fluidity of the mixture.
Water reducing and retarding admixtures are surfactants (i.e. capable of reducing the surface tension of liquid) and are adsorbed onto the surface of cement particles when added to the mixture. This induces a charge on to the cement particles thereby preventing their flocculation. The water so released improves the workability and the increase in surface of cement particles available for early hydration.
Water reducing admixtures provide the following potential applications:
(a) The simple addition of a dosage of the admixture to a plain concrete mixture increases its workability with only a small increase in the strength of the concrete — improving workability or plasticising action.
(b) By adding a dosage of the admixture, the mixture has the same degree of workability at lower water content and hence strength is increased if cement content remains the same — improving strength or water reducing.
(c) By adding a dosage of the admixture, the mixture may have the same degree of workability and strength by reducing both water content and cement content to retain its original water/cement ratio — saving cement. The cost of cement saved is generally more than the cost of admixture used — saving cost.
The effectiveness of a given dosage of water reducing and retarding admixture is reduced when the cement has a higher amount of Alkalis or Tricalcium Aluminate. A higher fineness of the cement also has the same influence due to its larger surface area in adsorbing the admixture. Over a period of time, such variations in properties may occur even if it is supplied from the same cement manufacturer.
The first generation of commercial water reducing admixtures, e.g., salts of lignosulfonic acids, provides about 10 to 15% reduction in water.
The second generation of water reducing admixtures enables about 15 to 20% in water reduction, e.g., sulfonated naphthalene formaldehyde, and is also called high range water reducing admixtures or superplasticisers.
In recent years, the third generation of water reducing admixtures, e.g., carboxylate copolymers, has even higher water reducing capability as they enable the production of self-compacting concrete (no mechanical compaction required during concrete placing).
4. Retarding Admixture
Retarding admixtures delay setting but not rate of strength development, except early strength when long retardation is provided. The effect of set retardation is assessed in terms of the time to develop a given degree of stiffness as indicated by the penetration resistance of the concrete. This is determined on the mortar fraction wet-sieved from a concrete mixture (ASTM C 403/ IS-8142). The elapsed time after the initial contact of cement and water to reach a penetration resistance of 0.5 MPa (500 psi) is referred to as the initial setting time of concrete. The time to reach a penetration resistance of 27.6 MPa (4000 psi) is referred to as the final setting time. Although these are arbitrary limits selected for the purpose of testing chemical admixtures with specified mix proportions, they are approximately related to observed behavior in corresponding concretes as follows:
(a) Penetration resistance at 0.5 MPa (BS 5075 only) — limiting time for placing with initial workability.
(b) Penetration resistance at 3.5 MPa (BS 5075 and ASTM C 403) — limiting time for vibrating concrete without formation of cold joint (ASTM C 403 — initial set).
(c) Penetration resistance at 27.6 MPa (ASTM C 403 only) — final set or when compressive strength of standard 150mm diameter cylinder is about 0.7 MPa (100 psi).
When the above is applied to concrete mixtures in construction, their indicated significance should be taken as indicative only. The penetration resistance at an elapsed time after initial contact of water and cement is dependent on its initial stiffness (a physical factor involving mixture proportions of the mortar fraction, with or with chemical admixtures) and the change in stiffness due to cement hydration (chemical factor including the retarding effect of admixtures, if used). For example, a plain concrete with a higher water/cement ratio takes a longer time to reach the same penetration resistance than one at lower water/cement ratio even though the former tends to have a faster rate of reaction as the cement particles are more dispersed. Similarly, the test method (ASTM C 403) does not permit the use of a directly mixed mortar to simulate the mortar fraction of the concrete as this may lead to an increase in the setting times.