GUIDELINES TO SELECT CEMENT FOR CONCRETE
Concrete is used as a material to build varieties of structures under different surrounding conditions. Here are some guidelines to select appropriate cement or binding material for concrete used for different structural applications.
| Sl No | Application | Comments |
| 1 | Conventional structural | Select cement for economy |
| 2 | Concrete in non-aggressive environment | Any cement as per the Standards. Blends of OPC with GGBS or FA or alternatively PPC or PSC, as per the Standards, have been extensively and successfully used in non-aggressive environment |
| 3 | Large placements where temperature rise, due to heat of reaction, is to be kept as low as possible | Best results are likely to be achieved with cements with mineral admixtures contents in excess of 50% GGBS or 25% FA. |
| 4 | Structural precast | Choice of cement for precast elements will depend mainly on strength requirements at early ages. High early strengths, without steam curing, will be achieved most economically with cements of strength Grade 43 and higher and with low mineral admixture content. Cements with higher mineral admixture content are better suited to steam curing. Where there is no requirement for rapid strength gain, the choice of cement should be based on economy |
| 5 | Precast bricks, blocks and pavers | Provided the elements have sufficient strength to allow handling at an early age, typically the day after casting, the choice of cement should be based on economy. |
| 6 | High-performance or High-strength concrete | Strength Grade 43 or higher may be used. The inclusion of about 8% silica fume is common practice in this application. Other mineral admixtures may also be used for durability or economic benefits. Superplasticiser is an essential ingredient in high-performance concrete. The compatibility of the cement/binder material and the Superplasticiser is important and needs to be checked. |
| 7 | Reinforced concrete in marine environment | Research and long-term field experience shows that best results in marine environment are obtained, using concrete with cement replacement > 50% by GGBS or FA around 30% |
| 8 | Concrete with alkali-reactive aggregates | Cement replacement should be > 40% GGBS or > 20% FA, for alkali-reactive aggregates |
| 9 | Concrete exposed to sulphate attack | OPC with 5-8% C3A possesses good resistance towards sulphate attack. Using high levels of GGBS as cement replacement (above 50%) improves sulphate resistance |
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 ?
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SHRINKAGE COMPENSATING CONCRETE
Shrinkage compensating concrete can be produced using expansive cement or expansive constituents.
The expansive cement, also known as sulfoaluminate cement or modified Portland cement, comes in the category of hydraulic cements that expand slightly during the early hardening period after setting. They meet the requirements of ASTM C 845 in which the group of expansive cement varieties is designated as Type E-1. Expansive cement is used to make shrinkage-compensating concrete that is used to (a) compensate for volume decrease due to drying shrinkage, (b) induce tensile stress in reinforcement and (c) improve dimensional stability of post-tensioned concrete structures. One of the major advantages of using expansive cement is in the control and reduction of drying-shrinkage cracks. In recent years, shrinkage-compensating concrete has been of particular interest in bridge deck construction, where crack development must be minimized. It is also used to minimize cracks in the concrete slabs and structures.
Three types of expansive cement are defined in ASTM C 845.
* Type K: Contains anhydrous calcium aluminate along with calcium sulphate and free lime in Portland cement
* Type M: Contains calcium aluminate along with calcium sulfate in Portland cement
* Type S: Contains tricalcium aluminate along with excess of calcium sulfate in Portland cement
The Type K is more commonly used
Concrete is generally mixed with more water than what is needed to hydrate the cement; that is done to obtain desired workability. After the hydration, i.e. during the process of concrete hardening, remaining water evaporates, causing the concrete to shrink. The amount of shrinkage due to drying depends on the characteristics of the constituent materials, mixture proportions and placing methods. When drying in pavements or structural members is restrained by sub-grade friction, reinforcement or other parts of the structure, drying shrinkage will induce tensile stresses. The tensile stress due to drying shrinkage usually exceeds the concrete tensile strength, which results in cracking. When expansive cement is used in concrete it induces stress, in opposite direction, large enough to compensate for drying shrinkage stress and minimize cracking. When the magnitude of expansion is small but usually adequate to offset the tensile stress (about 0.2 to 0.7 MPa) from restrained drying shrinkage, the concrete is known as Shrinkage Compensating Concrete.
The expansive cement can also be used to produce self-stressing concrete. That is done by restraining the expansion to induce a compressive stress high enough to result in a significant residual compression in the concrete after drying shrinkage has occurred.
Shrinkage-compensating concrete can be produced by using an expansive agent provided that an adequate wet curing, for at least one week, is carried out after demolding. A synergistic effect on the shrinkage reduction is observed, when the shrinkage reducing admixture and the expansive agent are used together.
Some water-reducing admixtures may be incompatible with expansive cement. Fly ash and other pozzolans may affect expansion and may also influence strength development and other physical properties. The air-entraining admixtures are as effective with shrinkage-compensating concrete as with Portland Cement in improving freeze-thaw durability.
The structural properties, namely tensile, flexural, and compressive strength of shrinkage compensating concrete are comparable to those of Portland cement concrete (PCC). Some superplasticisers, when added, do not require concrete to be wet cured, it is claimed.
This ACI Standard Practice, ACI 223, gives details on the use of shrinkage-compensating concrete in structures (reinforced and post-tensioned slabs, both on grade and elevated) and pavements. The recommendations cover proportioning, mixing, placing, finishing, curing, and testing of concrete. The scope of this standard practice is limited to shrinkage-compensating concrete made with expansive cements. The recommendations of this standard practice are not applicable to self-stressing expansive cement concretes proportioned to produce a prestressed concrete structure for load-bearing purposes. Procedures for proportioning, handling, and curing of self-stressing concretes are often radically different from procedures for shrinkage-compensating concretes used to compensate for normal drying shrinkage.
Bibliography:
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Question:
Give three possible applications of shrinkage compensating concrete
