Unlike mineral admixtures, which can be offered as mixed cement, chemical admixtures are typically added during the mixing process of concrete production. Depending on the requirement, different types of chemical mixtures are manufactured such as air-penetration mixture, accelerated mixing, water-reducing mixture, etc. For a civil engineer, a description of mixtures based on their chemical composition is more useful according to their function.
1. air intake mixture
Chemical admixtures were first used in the 1930s to allow air to penetrate concrete to increase its frost resistance. Fine air bubbles with close spacing provide partial relief from progressively solidifying as the liquid phase in the concrete.
Dodson (1990) explained that air-permeable mixtures (AEA) Do not generate air in the concrete. Their function is to cover the air present within the void system of the mixture and the water as well as the air inside and mechanically during mixing. without any AEAConcrete contains some air, often called “trapped” air. These wind gusts are usually 1 mm or more in diameter and are irregular in shape. They are often aggregated on the paste-aggregate interface. The penetrating air bubbles are mainly within the paste, usually between 10 mm and 1 mm in diameter. At close distance, their shape is spherical. vacancy factor, which is the maximum distance in cement paste from the circumference of an air void, is usually in the range of 0.10 to 0.20 mm, In general The recommended air content in compacted concrete is 5 to 6%, Air bubbles promote workability but their presence reduces the strength of concrete. These factors are taken into account in the design of concrete mix.
2. quick mix
The use of quick mixing is common during concreting in cold weather, because the rate of hydration of cement decreases with low temperature. their job is increase the rate of hydration, which led Accelerate setting time and initial power development,
last, calcium chloride 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 results in limiting the total chloride content in concrete to levels that exceed the normal addition of calcium chloride because of accelerated mixing. in present, Non-chloride accelerator mixtures are available, for example, calcium nitrite. (also a corrosion inhibitor). The use of calcium nitrite yields better strength at a later age than calcium chloride. However, this may not be of importance in practice as moist curing at the site is restricted to a young age only.
3. water softener
Although water reducing mixtures and retarding mixtures are listed separately, it is more typical to use both at the same time. This is also due to the fact that both are available in the specific materials used in their manufacture, for example, the salts of lignosulfonic acid, In particular, both are useful in the case of hot weather concreting,
The amount of water added to a typical concrete mix exceeds the amount required for complete hydration of the cement used. The purpose of the extra water is to promote practicality. However, when water is added to cement, there is a tendency for the cement particles to clump together. Some of the water in the mixture becomes trapped in the flakes and is not available to contribute to the fluidity of the mixture.
Water reducing and retarding mixtures are surfactants (ie capable of lowering the surface tension of the liquid) and are adsorbed to the surface of the cement particles when added to the mixture. This induces a charge on the cement particles preventing their flocculation. The water thus released improves performance and increases the surface of the cement particles available for initial hydration.
Water reducing mixtures offer the following potential applications:
(a) Simple addition of a single dose of admixture to a plain concrete mix enhances its efficiency only a. with small increase in strength of concrete Improving workability or plasticizing action.
(b) By adding a single dose of the mixture, the mixture contains Same degree of working at low water content And hence the strength increases if the amount of cement remains the same – Improving power or reducing water.
(C) By adding a single dose of the mixture, the mixture can be Equal degree of workability and strength by reducing both water content and cement content To maintain its original water/cement ratio – saving cement. Cement cost saved The cost of a commonly used mixture is more than – saving cost.
When the amount of A in cement is high, the effectiveness of a given dose of water reducing and retarding mixture decreasesLakalis or tcalcium aluminate, The high fineness of cement due to its large surface area in adsorbing the mixture also has a similar effect. Over time, there may be such changes in properties, even if it is supplied from the same cement manufacturer.
first generation Commercial water reducing mixtures, for example, salts of lignosulfonic acid, provide about 10 to 15% decrease in weater
second generation Enables about water-reducing mixtures 15 to 20% water loss, like, Sulfonated Naphthalene Formaldehyde, and also called high-grade water-reducing mixture or superplasticizer,
In recent years, third generation water-reducing mixtures, e.g., Carboxylate Copolymer, also have greater water reducing capacity as they enable the production of self-compacting concrete (No mechanical compaction is required during the laying of concrete).
4. retard mix
Except for early strength when late retardation is provided, retarders delay mixture setting but not the rate of strength development. The effect of set retardation is evaluated in terms of the time a certain degree of stiffness develops, as represented by penetration resistance of concrete, It is determined on the mortar fraction of a wet sieve from a concrete mix (ASTM C 403/IS-8142). The time elapsed after the initial contact of cement and water is . to reach the penetration resistance of 0.5 MPa (500 psi) is known as Initial setting time of concrete, time to reach the entry resistance of 27.6 MPa (4000 psi) is known as last setting time, Although these are arbitrary limits chosen for the purpose of testing chemical mixtures with specified mixing ratios, they are closely related to the behavior observed in compatible concretes:
(a) Penetration resistance at 0.5 MPa (BS 5075 only) – Limited time to keep up with initial workability.
(b) Penetration resistance at 3.5 MPa (BS 5075 and ASTM C 403) – Limiting the time for concrete to vibrate without forming a cold joint (ASTM C 403 – Initial set).
(C) Penetration resistance at 27.6 MPa (ASTM C 403 only) – final set or when the compressive strength of a standard 150 mm diameter cylinder is approximately 0.7 MPa (100 psi).
When the above are applied to concrete mixes in construction, their stated importance should be taken as indicators only. Penetration resistance over an elapsed time after the initial contact of water and cement in its initial hardness (a physical factor involving the mixing ratio of the mortar fraction with or without a chemical mixture) and in hardness due to cement hydration (chemical factor) dependent on change. including the diluting effect of the mixture, if used). For example, a plain concrete with a higher water/cement ratio takes longer to reach the same penetration resistance than one with a lower water/cement ratio, even though the former is faster due to the greater dispersion of the cement particles. There is a rate. , Similarly, the test method (ASTM C 403) does not permit the use of a directly mixed mortar to simulate the mortar fraction of concrete as this may increase the setting time.