Construction Chemicals – Admixtures for Concrete with emphasis on Superplasticizers |
Abstract: |
This paper describes the purpose of using admixtures in cement concrete. The purpose for using chemical admixtures as listed by ACI Committee 212 both for fresh state and hardened state of concrete is provided. Classification of admixture by source/ origin and by end-use is provided. A separate section on Superplasticizers is incorporated, as Superplasticizers are polymers and have tremendous application as admixture in cement concrete. The effects of the use of Superplasticizers on concrete early strength and loss of workability are highlighted. Compatibility with Portland cement and dosage are also dealt with. Finally a brief note on how to overcome slump problems.
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1.0 Introduction
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Admixtures are added to cement concrete, during its preparation, to alter the properties of fresh concrete or enhance the properties of the hardened concrete. For conditions in India use of retarders, plasticizers and superplasticizers is more appropriate. But it is only recently that admixtures are being widely used in the organised sector. The nonavailability of uniform quality of admixtures has been the chief reason for this. Even though several advancements have been made in concrete technology it still suffers from several drawbacks, viz poor workability, high shrinkage cracks, poor performance against chemicals, high permeability, inadequate protection of steel reinforcement from corrosion, low tensile strength and low fracture toughness. Concrete also presents problems in extreme cold or hot weather conditions. It is now expected that admixtures will help overcome the drawbacks mentioned above.
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2.0 Classification of admixtures by source / origin [4]:
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Admixtures could be broadly classified in three main groups:
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2.1 Natural and Mineral Admixtures: |
The replacement of cement with natural pozzolanic binders is common and in the recent times has assured greater importance due to increased demand of cement and economic, technological and ecological considerations. It can be said that cement replacement material have come to stay. Small amount of inert fillers can be safely added to cement but if the material is pozzulane, large amounts can be added.
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Mineral admixtures can be classified as follows:
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Natural Pozzalane.
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Calcified clay, shale, and other soils.
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Condensed silica fumes.
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Pulverised fuel ash (flyash).
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Rice husk ash.
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Ore beneficiation tailings.
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2.2 By-products and waste: |
They are:
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2.3 Chemical Admixtures |
The purpose of using chemical admixtures is to modify certain properties of concrete. The AIC Committee 212 has listed the following properties:
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In the fresh state with a view to:
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Increase workability.
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Accelerate or retard setting.
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Reduce or prevent settlement or to create slight expansion.
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Modify the rate and / or capacity for bleeding.
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Reduce segregation.
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Improve pumpability.
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Reduce slump loss.
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In the hardened state with a view to:
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Retard or reduce heat evolution during early hardening.
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Accelerate strength development at early ages.
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Increase strength.
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Increase durability.
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Decrease permeability.
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Control expansion due to alkali-aggregate reaction.
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Increase steel-concrete bond.
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Increase bond between existing and new concrete.
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Improve impact resistance and abrasion resistance.
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Inhibit corrosion of steel.
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Produce coloured concrete or mortar.
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3.0 Classification of admixtures by end use [4]: |
As per the AIC Committee chemical admixtures, depending on the types of material consisting them or the characteristic effects of their use, are classified into following groups:
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Air entraining agents.
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Accelerators.
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Retarders.
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Gas-forming agents.
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Alkali-aggregate expansion inhibitors.
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Damp-proofing and permeability-reducing agents.
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Water reducing and workability agents.
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Super plasticizers.
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Grouting agents.
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Corrosion inhibiting agents.
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Bonding admixture.
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Fungicidal admixture.
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Coloring agents.
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Miscellaneous admixtures – damp-proofing agent & surface hardeners.
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4.0 Super plasticizers: |
Super plasticizers (high range reducer) are linear polymers containing sulfonic acid groups attached to the polymer at regular intervals. They were first developed, as water reducers, in Japan & Germany in the early 1960s. Most of the commercial formulations belong to one of the four families [4] :
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Sulfonated melamine-formaldehyde condensates (SMF).
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Sulfonated naphthalene-formaldehyde Condensates (SMF).
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Modified lingosulfonates (MLS).
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Polycarboxylate derivatives.
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The sulfonic acid groups neutralize the charge on the cement particles and causing dispersion. This releases the water tied up in the cement particles agglomerations, which thereby reduces the viscosity of the particle and concrete.
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4.1 The main purpose of using Super plasticizers is to produce following concrete with high slump in the range of 175-225 mm to be used in heavily reinforced structures and the placements where adequate consolidation by vibration cannot be achieved.
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4.2 The other major application is the production of high-strength concrete at water / cement ratio ranging from 0.30 to 0.40. The capability of super plasticizer to reduce water requirements 12-25% without affecting the workability leads to production of high strength concrete and lower per mobility. Compressive strengths greater than 96.5 Mpa at 28 days have to be attained.
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4.3 Another benefit of super plasticizers is enhancement of early concrete strength (50 to 75%). Typically the reduction of water / cement ratio by 20-25% enhanced early strength by 50-75%
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This shows the rapid loss of slump compared to concrete without super plasticizers. Current product of super plasticizers. Current product of super plasticizers. Current product of super plasticizer has an extended slump retention time twice than the conventional one (up to two hours). Another slump retention effort is through re-dosing super plasticizer to the concrete mix. Figure 4 shows the effect of repeated re-dosing of napthalene based super plasticizer to regain slump.
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Re-dosing practice should be explored with trail mix with close similarity of field condition as rapid slump loss occurrence is detected. Other disadvantage of re-dosing are possibility of bleeding and segregation or air entrapment in the mixture.
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4.4 Compatibility:
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The fixing between C3A of portland cement and super plasticizers will result in loss of workability. This occurs if calcium sulphate is not liberated fast enough to react with C3A. Factors accountable to the compatibility between super plasticizers and cement are C3A & C4A content in cement, reactivity of C3A, calcium sulphate content, final form of calcium sulphate in ground cement, molecular chain length of super plasticizers, position of sulfonate group in the chain, the counter ion type (sodium or calcium), the presence of residual sulphates which affects the de-floculation properties. Based on these factors, Neville proposed ideal cement properties for high performance concrete as fineness up 400m2/Kg determined by Blaire method and low C3A content. Incompatibility of cement and superplasticizers can be a result of cement reactivity or poor performance of superplasticizers. Cross testing can be performed to evaluate the problem by applying other admixtures and other cement. Incompatibility can be reduced by adding retarding agents or pozzolanic material.
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4.5 Dosage [5]:
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The effectiveness of super plasticizers is measured by the time taken by fixed quantity of cement paste and particular content of super plasticizers to flow through a standard funnel, known as Mash Flow Cone (Figure 6). Recommended dosage is not to exceed the saturation point (Figure 7) from Mash Flow Test. Excessive dosage may lead to segregation problem.
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Typical dosage of super plasticizer used to increase the workability of concrete ranges from 1-3 litres per cubic meter of concrete, where the liquid admixture has 40% of active material. In reducing the water-cement ratio, higher dosage is used, that is from 5-20 litres per cubic meter of concrete.
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4.6 Overcoming Slump Loss problem [5]:
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The slump loss problem can be overcome by adding the admixture just before the concrete is placed. There are disadvantages in such a procedure. The dosage control may not be adequate, and it requires ancillary equipment such as truck mounted dispensers and dispensers. Adding the admixture at the batch plant, besides dosage control improvement, reduces areas of truck mixer. However new admixtures being marketed can be added to the batch plant and can hold slump above 200 mm for more than 2 hours.
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Reference:
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U.S. Department of Transportation – Federal Highway Division.
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New Chemical Admixtures for Concrete: Effectiveness and Practical Implication by Ravindra Gettu, Joana Roncero & Migual A. Martin, Structural Technology Laboratory, University Politecnica de Catahinga, Barcelona, Spain.
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Properties of Concrete by A M Neville.
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Building Materials in India: 50 Years – A commemorative Volume – Building Materials and Technology Promotion Council.
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