CONSTRUCTION CHEMICALS FOR CONCRETE DURABILITY
The construction chemicals or the chemical admixtures are the ingredients in concrete, other than Portland cement, mineral admixtures, aggregate and water. They are added to concrete immediately before or during mixing. The addition of chemical admixtures modifies the properties of concrete, both in fresh and hardened state.
The effective use of admixtures depends upon the appropriate method of batching and concreting. The chemical admixtures are mostly supplied in ready-to-use liquid form (fraction of solids and density are specified) and are added to concrete at RMC unit or/and jobsite. The admixtures, such as pigments, expansive agents and pumping aids are used in extremely small quantities and usually batched by hand from pre-measured containers.
The effectiveness of an admixture depends on several factors, such as type and amount of cement, water content, mixing time, slump and temperature.
The admixtures are classified according to their function. There are five distinct classes of admixtures: (i) air-entraining, (ii) water-reducing, (iii) retarding, (iv) accelerating and (v) plasticisers (high range water reducing agents (HRWRA) or suerplasticisers). All other varieties of admixtures fall into specialty category, whose functions include corrosion inhibition, shrinkage reduction, alkali-silica reactivity reduction, workability enhancement, bonding, damp proofing and cooling.
The use of air-entraining admixtures (AEA) is not common in
The water-reducing admixtures usually reduce the required water content for a concrete mixture by about 5-10%. Consequently, concrete containing water-reducing admixture needs less water (lower w/c ratio) to reach required slump, than untreated concrete. This usually indicates that higher strength concrete can be produced without increasing the amount of cement.
The retarding admixtures, which slow the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting. High temperature often causes an increased rate of hardening, which makes placing and finishing difficult. The retarders maintain concrete workability during placement and delay the initial set. Most retarders also function as water reducers and may entrain some air in concrete.
The accelerating admixtures increase the rate of early strength development, reduce the time for curing and protection and speedup the start of finishing operations. The accelerating admixtures are useful to modify the properties of concrete in cold weather.
The superplasticisers, also known as high-range water reducing agents (HRWRA), reduce the water content of concrete by 10-40%. They are added to concrete under two circumstances, (i) to reduce the water-to-cement ratio at the required slump or (ii) to obtain increased slump at the given water-to-cement ratio. The effect of superplasticisers lasts for 30-60 minutes, depending upon the type and the dosage. After that period, the loss of workability is generally rapid. In view of the slump loss, many times it is preferred to add superplastcisers to concrete at the jobsite. The biggest advantage of using superplasicisers is the reduction in water content of concrete. That leads to increased strength and durability of concrete. The use of superplasticisers in concrete offers the following advantages:
(i) Easy pumpability
(ii) Reduced time to achieve stripping strengths for precast products
(iii) Improved pigment dispersion
(iv) Faster placing with fewer site personnel
(v) Optimised cement content
(vi) Increased compressive and flexural strength
(vii) Lower permeability, thus increased water tightness and lower absorption.
(viii) Increased resistance to weathering
(ix) Stronger bond between concrete and reinforcement
(x) Less volume change for wetting and drying
(xi) Reduced shrinkage and cracking
(xii) Improved concrete durability
The commonly used superplasticisers are:
v MLS: Modified Ligno-sulphonates (water reduction up to 10 %)
v SMF: Sulphonated Melamine Formaldehyde Condensate (water reduction up to 25 %)
v SNF: Sulphonated Naphthalene Formaldehyde Condensate (water reduction up to 25 %)
A new generation of superplasticisers belonging to polycarboxylate polymer family, which recently entered the market, namely PCE or the polycarboxylic ethers and acrylic polymers or AP, reduce the water content up to 40 %. They also improve the slump retention characteristics of concrete, thus allowing trouble-free transportation and placement of concrete under difficult conditions at the job site or RMC unit.
The water reduction or slump increase in concrete with the addition of superplasticiser is on account of better dispersion of cement particles in fresh concrete mix. In common superplasticisers (MLS, SMF, SNF), this dispersion effect is caused by the electrostatic repulsion between cement particles negatively charged by the adsorption of superplasticiser on the surface. Whereas in the advanced superplasticisers (PCE), this is caused by the steric repulsion created by the long polymer chains of superplasticiser, in addition the electrostatic repulsion.
The superplasticiser is added to concrete in small quantity (up to 2 % m/m of cement). The specification of superplasticiser used in concrete should conform to the provisions of IS: 9103 and its application to the provisions of IS: 456-2000. The international standards on superplasticisers are ASTM C 494 and EN 934.
The cement-superplasticiser compatibility is defined by the rate at which superplasticised mix consumes water during the first moments after mixing. The compatibility of superplasticiser with cement should be tested before using. The simple empirical test, namely Marsh cone flow time test is commonly performed to measure the compatibility.
Question:
Do you think construction chemicals make concrete durable ?
One thing is the most important durability..The use of concrete in construction is widely regarded as economic..durable..structurally sound..
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