Suggestions On the Use of Concrete Admixtures

Concrete admixtures can improve and regulate the performance of concrete, and in today’s engineering more and more attention, the addition of admixtures to improve the performance of concrete play a role, but the choice of admixtures, adding methods and adaptability will seriously affect its development, should be based on the nature of the concrete itself and the surrounding environment to choose the right admixture.

 

 

Water Reducer

Water reducing agent can maintain the slump of concrete under the condition of maintaining the same, reduce the amount of mixing water to improve the fluidity of concrete mix, or reduce the amount of cement per unit, saving cement. According to the water reducing agent water reducing capacity is divided into ordinary water reducing agent (also known as plasticizer, water reduction rate of not less than 8% ), high efficiency water reducing agent (also known as super plasticizer, water reduction rate of not less than 14% ) and high performance water reducing agent (water reduction rate of not less than 25%).
Among them, high efficiency water reducing agent: powder dosing 0.75% to 1.5%, liquid dosing 1.5% to 2.5%; when using porous aggregate should be mixed with water first, and then add water reducing agent; when the slump is large, should pay attention to the vibrating time is not too long, in order to prevent water secretion and delamination. High-performance water reducing agent: the common dose is 0.4%~1.2%; concrete trial test should be conducted before use to get the best dose; it should not be used in combination with naphthalene high efficiency water reducing agent, and should be used in combination with other admixtures to conduct concrete compatibility test in advance; the sensitivity of slump to water consumption is high, and water consumption must be strictly controlled when using; pay attention to concrete surface maintenance.

Early Strength Agent

Concrete early strength agent can improve the early strength of concrete, and no significant impact on the later strength. The main role of early strength agent is to accelerate the hydration rate of cement, but also has a certain water reduction and enhancement function.
Scope of application: early strength agent does not contain chloride ions, no rust effect on the reinforcement, suitable for all civil and industrial buildings and prestressed reinforced concrete components, mortar, etc.; early strength agent is suitable for construction in low temperature (early winter and early spring season) conditions; its use for silicate cement, the enhancement and modification of slag cement is particularly significant. Instructions for use: the amount of admixture is generally 3% to 6% of the cement; to maintain uniform quality of concrete, the mixing time is extended by 1~2min; if the early strength agent has moisture caking phenomenon, it must be crushed or weathered before use, and its performance remains unchanged.

Retarding Agent

Retarding agent by reducing the cement or gypsum hydration rate and heat of hydration, to extend the hydration hardening time of cement, so that the fresh concrete can remain plastic in a longer period of time, thereby regulating the setting time of fresh concrete. Its dose is 0.1%~0.6% of cement dosage, which is suitable for continuous concrete filling, mass concrete, ready-mixed concrete and pumped concrete under high temperature conditions.

Air-entraining Agent

Air-entraining agent can improve concrete slump, fluidity and plasticity, make concrete bubble structure, small bubble radius, high frost resistance index, commonly used in high durability of concrete structures (such as dams, high-grade highways, ports, etc.), is also a common admixture to improve the durability of concrete pavement. AH-1 dose of 0.01% ~ 0.03%, suitable for general impermeable anti-freeze durability of concrete; AH-2 dose of 0.6% ~ 0.7%, suitable for pumping concrete construction.

Expansion Agent

The expander is mainly used to compensate for the shrinkage of the material during the hardening process and prevent the concrete from cracking. The amount of concrete expansive agent is 10%-12% (in terms of cement), for the requirement of higher impermeability, anti-cracking concrete, can increase the amount of admixture through the test. In the concrete before the initial set should be in accordance with the relevant construction regulations, repeatedly wipe the surface of the concrete to prevent air dry, to avoid settlement caused by cracks, after the final set to start maintenance, maintenance period of not less than 12d. Expansion agent should be tested with local cement before use.

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