Admixtures for Concrete

Concrete admixtures

A mixture is " a cement material other than water , the hydraulic aggregate and which is used as an ingredient of concrete or mortar and is added to the mixture , immediately before or during mixing . " A functional addition is added by manufacturer of cement mix cement plant , not at work . There are four categories of additives :

Aerator - added to improve the resistance of concrete to freezing .

Chemical additives used to control - the setting and hardening of concrete or reduce the need for concrete water .

Mineral admixtures - fine solid particles are used to improve workability , durability and provide additional cementitious properties .

Various additives - all other additives not in the above categories .

** The additives should not be used if similar results can be achieved by following good design practices and concrete.

When using additives , the following precautions should be taken into account:

 1 ) mixture must comply with the relevant specifications, ASTM

2 ) following the manufacturer's instructions for dosage

, 3) ensure that reliable procedures for the assay mixture

, 4) whereas the effects of the mixture on other properties of the concrete .

Air entrainment additives

A major disadvantage of concrete is its susceptibility to damage by single or multiple cycles of freeze-thaw when it is in a full or close state - saturated. Without some concrete mixture can be used on the sidewalks , dams , foundations and other important applications. However, concrete can become resistant gel using air-entraining admixtures . The air entrained concrete is regularly U.S. and northern Canada.

Trailed Air System - Void - The air volume for optimum protection from frost is about 9 % by volume of the mixture. Much easier to measure in the field is the amount of air as a percentage of the volume of concrete. The air content must be

in the range of 4-8 % for a good protection against freezing. Air is sucked in naturally cement paste , therefore , of the air voids entraining additives air increases by 3 to 4% by volume of the concrete . The entraining air causes the water to mix the foam obtained in millions of tiny air bubbles that are evenly spaced in the dough. These small holes are not visible to the naked eye but are microscopically observable . The hole spacing is a critical measure of the mixing efficiency . In addition, small gaps are not filled with water easily, even when the concrete is saturated .

Materials entrained air - Good coach composed of air promote the formation of small stable bubbles during agitation. The surfactants are concentrated in the water - air interface , reducing the surface tension of water which allows bubbles . Mixture A typical dose is from 0.0005 to 0.05 % active ingredient by weight of cement - often require pre- dilution prior to administration.

Air entrained Test Agents - One way to test the effectiveness of the drive of air bubbles is effective in a polished section of concrete under a microscope and calculating the separation factor . A proof is a performance specification routine wherein a concrete mix is ​​tested on freezing and rapid thawing repeated .

Factors affecting air entrainment - Increasing doses of the mixture increase the amount of air and separation factors below . Finely ground cements are less drag air around them coarsely. Therefore , the addition of fines in the mixture the content of the air is reduced. The use of additives can affect other air-entraining agents active surface potential . Cement concrete low dragging more air mixtures rich . In addition, under low air drag concrete mix that make a big w E / C / c ratios.

Mixing and consolidation - Air Training occurs during mixing.There are several factors that can affect the content of the air type of mixer , the mixing speed , the amount of concrete, mixing time, consolidation, temperature and depression .

Effects on the properties of concrete - Air training increases maneuverability

fresh concrete. Small bubbles in the fine aggregates and cement acting to reduce interactions between the solid aggregates. Improving the ease of work led to the use of mixtures of air entrainment , even when the freeze-thaw is not a problem . In general, air entrainment produce a uniform concrete compacted . Air entrained concrete is generally 10 to 20% stronger than the non- air-entrained mixture. The excess air reduces the strength and reduce the resistance to freezing and thawing.

chemical additives

Such mixtures includes all soluble chemicals that affect the setting time and reduce water requirements of concrete mixtures . They are classified as follows :

Type A water-reducing admixtures

Lowering the water required for a given regulation . ASTM classifies as a mixture in water requirement is reduced by reduction of 5 % in water . Under this specification , many air-entraining admixtures are also classified as water reducer . Most water-reducing admixtures reduce water requirements by 5-10%. Latest additives called "flow" of achieving reductions of 15-30 % of water needs. Reducing water needs, while maintaining cement contents effectively reduces the ratio w / c with an accompanying gain strength.

The main reaction of all water reducers adjuvants is in the solid - water interface . In general , the solid particles have a residual charge on the surface , which can be positive or negative . This causes the particles together traps are collected and attract water . Water reducers neutralize the surface charge so that all surfaces have a uniform load equal sign. Adjuvants Particles repel each other now instead of attracting each other . Water is free to reduce the viscosity of the paste and improve operability . Most conventional water reducing additives also act as retarding admixtures decision .

Reducing water needs using reducing admixtures of water , increasing compressive strength up to 25% higher than expected from a mixture equivalent to a reduction of the E / C obtained . This can be attributed to a greater uniformity throughout the structure of the cement paste .

Superplasticizers additives - a linear polymer that reduces water requirements by 15-30% . They are used to produce wet concrete with very high depressions (7-9 inches) and high strength concrete with w / c ratios ranging from 0.3 to 0.4. ASTM refers to additives such as " upper water reducing admixtures . " Added If equivalent to those normally used water reducing admixtures results are similar doses (5 to 10 % ) . However , at higher doses , increases the water reduction . Effect of this mixture is that side effects undesirable such as air and behind the drive are very small. concrete high strength can be obtained with respect to W / C below 0.4 ( incomplete hydration) . Superplasticizer can change the Type I cement for increased resistance in type III. the lower cement content reduces the rate of heat generation that accompanies it. Moreover , lower w / c ratios lead to greater durability , less creep and dry shrinkage.

Type B. retarding admixtures

Additives which extend concrete plasticity . Helpful in counteracting the effects of temperature , eliminate cold joints and reduce cracking form associated with diversion. Retardants essentially increase the latent stage in the process of hydration of C3S . However, subsequent hydration steps 3 and 4 will be faster. Too much of a delay the reaction will not proceed beyond step 2 resulting in a cement that will never be resolved. Delay excess concrete has helped many a concrete truck driver out of trouble . Adding sugar to concrete or soft drinks which may be installed on the truck back to a usable form.

The effective retarder depends on the amount of C3a in the concrete. Retarder is removed from the solution by the reaction of C3A , at least , it is available to slow the hydration of C3S . Less retarder is removed if its addition to the fresh concrete is delayed. Although this mixture extending setting time, a side effect is the loss of workability . Set - retardant additives have been reported to increase resistance to the final compression . Shrinkage and creep drying rate is increased, but the most recent values ​​are not affected.

Type C. Set- Accelerating Additives

There are two types of configurations accelerators , which accelerate the normal setting and the development of concrete strength and providing rapid adjustment by means not related to the normal hydration. There are many applications of shotcrete , eliminating leaks under pressure , fast emergency repairs , or when a rapid strength development is necessary.

Conventional accelerators increase the rate of hydration of the C3S shortening the rest period and the increased hydration rate in steps 3 and 4 .

Set- accelerators generally do not affect the inclusion of air. However, the processing time is reduced and additional water -reducing admixture or water may be necessary to control the workability . Gain early strength may be noted, however , the ultimate strength is reduced. Effects on drying shrinkage and creep are similar to those resulting from the whole retardant additives .

Certain mixtures of acceleration parameters have a component chloride will cause corrosion of the reinforcement. As mentioned earlier in the course, chloride should never be used in any application preload . Other additives require higher doses and more expensive . Another solution is to use Type III cement and concrete with a higher cement to obtain an initial resistance levels .

Type D. reducing water and retarding admixtures

E. type water reducing and accelerating admixtures

mineral additives

Inorganic additives are used to improve the workability and durability , and for the hardening of the concrete . This can be achieved by introducing the finely ground mineral , generally fall into three categories:

Materials of low reactivity - Improves handling of deficient concrete fines. In general , cement and pozzolanic materials preferred due to a further increase in strength and durability .

Cementitious materials - materials that have hydraulic reactions were yours, as the hydraulic limestone and blast furnace slag . Mix the most common in this category .

Pozzolanic material - a material which reacts with the calcium hydroxide ( CH ) to form CSH . The reaction improves handling and reduces the heat of hydration while causing more impermeable concrete . This reaction is similar to the hydration of C2S . Type I cement can be converted into cement type IV with a mixture of pozzolan . Therefore , the cement of type IV is rare once manufactured . Cement low initial resistance is obtained similar to the type IV .

various additives

The use of these additives is less than the overall amount used in any particular types discussed so far .

Additives Adhesives - Adhesives between old and new concrete or concrete and other materials .

Corrosion inhibitors - generally , an accelerator mixture that is non- corrosive to the rebar .

Sealant mixture - avoid rain penetration into the porous concrete, provide hydrophobic characteristics .

Additives producer extension - Other ordinary cement expansive cement .

Grouting Additives - A variety of additives for mortar cement that prevent bleeding and segregation , strengthen cohesion and water retention during pumping and increase the time of the system.