Guidelines to select cement for concrete

GUIDELINES TO SELECT CEMENT FOR CONCRETE

J D Bapat

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

CONSTRUCTION CHEMICALS FOR CONCRETE DURABILITY

Dr J D Bapat

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 India. The AEA stabilises the microscopic air bubbles in concrete, which provide freeze-thaw resistance and improve resistance to deicer salt scaling.

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 ?

Using recycled concrete in construction

USING RECYCLED CONCRETE IN CONSTRUCTION

Dr J D Bapat

The concrete structures which have rendered economic service life are fully / partially demolished for repair or rebuilding. In such cases, large quantity of concrete rubble is required to be disposed off. The recycling of such concrete rubble as an aggregate in new construction is an economic and eco-friendly option. This practice is encouraged and prevalent in many countries. According to one report from the Federal Highway Administration (USA), concrete with recycled concrete aggregate (RCA) performs as good as the concrete with natural coarse aggregate (NCA). The reports on the comparison of strength properties of concrete with NCA and that from RCA show that RCA gives at least two-third of the mechanical properties, namely compressive strength and the elastic modulus in comparison to that with NCA. The plants for production of RCA are similar to those engaged in the production of crushed stone aggregate.

The recycling of concrete is a relatively simple process. It involves breaking, removing and crushing existing concrete into a material with specific size and quality. For example, unprocessed RCA with 50 mm or 37.5 mm size is useful for pavement sub-base. The ACI 555 (2001) gives more information on the subject. The processing of RCA involves removal of reinforcing steel and other embedded materials, contaminants like asphalt, soil, glass, gypsum boards, sealants, plaster, paper, wood and roofing materials. The processed RCA can be used in (a) pavements, shoulders, median barriers, sidewalks, curbs and gutters, bridge foundations, (b) structural grade concrete, (d) soil-cement pavement bases, (d) lean concrete and (e) bituminous concrete.

The crushing characteristics of RCA are similar to those of natural rock and are not significantly affected by the grade and quality of the original concrete. It can be batched, mixed, transported, placed and compacted in the same manner as in conventional concrete.

The use of RCA as coarse aggregate is common but using it as fine aggregate needs more care. The substitution of fine aggregate up to 10-20 % by the RCA is beneficial; optimal rate may be decided by trials.

The water absorption of RCA has been found to be higher due to higher porosity. Therefore it is recommended that RCA be batched in pre-wetted and close- to-saturated surface-dry condition. In order to achieve the same workability, slump and water-to-cement ratio, the paste content (i.e. the amount of cementitious) or the amount of superplasticiser may have to be increased.

It is generally accepted that, when natural sand is used, up to 30 % of NCA can be replaced with RCA, without significantly affecting the mechanical properties of concrete; replacing higher amounts may result in increased drying shrinkage.

With the growing construction activity, the shortage of aggregate is growing, especially in cities. At the same time, concrete waste such as rejected concrete elements and rubble from demolished concrete structures, is also growing at a fast rate, raising ecological and environmental threats. The RCA is a viable alternative to using natural aggregate in concrete, especially in situations where natural aggregate is not locally available or transported over a long distance. Although the processing cost of recycled aggregate is higher in some cases, the situation will change as the natural aggregate becomes scarce.

Question:

Do you think recycling the buildings waste in concrete is proper and effective utilisation of waste ?

Suppliers of construction chemicals in India

SUPPLIERS OF CONSTRUCTION CHEMICALS IN INDIA

Dr J D Bapat

The following is the list of suppliers of construction chemicals in India. They are organic compounds used as chemical admixtures in concrete. When added as superplasticisers, these chemicals improve water-to-cement ratio or workability of concrete.

• Altret Performance Chemicals Gujarat Pvt. Ltd.
• Aqua Seal Construction Chemicals
• Atul Ltd.
• BASF Construction Chemicals (India) Pvt. Ltd.
• Beck India Ltd.
• Chemistik
• Choksey Chemicals Pvt. Ltd.
• Chowgule Koster India Construction Chemicals Pvt. Ltd.
• Cico Group- India
• Concare Chemicals Pvt. Ltd.
• Dr Fixit
• Excel Admixtures Pvt. Ltd.
• Fibrex Construction Chemicals
• Fairmate Chemicals Pvt. Ltd.
• Fosroc Chemicals (India) Pvt. Ltd.
• Greenshield India Construction Chemicals Pvt. Ltd.
• Kamsons Chemicals Pvt. Ltd.
• MC-Bauchemie (India) Pvt. Ltd.
• Perma Construction Aids Pvt. Ltd.
• Pinc Group
• Polygon Chemicals Pvt. Ltd.
• Riddhi Chemicals
• Roffe Chemicals
• Samrock Murexin
• STP Construction Chemicals
• Taj Hitech
• Tytan Organics
• Uclid technology Ltd.
• Sika India Pvt. Ltd.

Question:

What do users expect from the suppliers of construction chemicals ?

Books on concrete technology

Books on concrete technology

Dr J D Bapat

(1) Concrete Technology(3^rd Edition), Author: Gambhir M L, Publisher: Tata Mcgraw Hill Publishing Co Ltd

(2) Concrete Technology Theory & Practice, Author: Shetty M S, Publisher: S Chand & Company Ltd

(3) Concrete Technology, Author: Neville A M, Brooks J J, Publisher: Pearson Education Limited

(4) Concrete Technology, Author: Neville A M, Publisher: Pearson Education India

(5) Laboratory Manual on Concrete Technology, Author: Hemant Sood, Mittal L N, Kulkarni P D, Publisher: Cbs Publishers & Distributors

(6) Concrete Technology, Author: Santhakumar A R, Publisher: Oxford University Press, N Delhi

(7) Textbook Of Concrete Technology, Author: Kulkarni P D, Ghosh R K, Phull Y R, Publisher: New Age International (p) Ltd

(8) Concrete: Microstructure Properties & Matter (CD), Author: Mehta P K, Publisher: Tata Mcgraw Hill Publishing Co Ltd

(9) Precast Concrete: Materials, Manufacture, Properties And Usage, Author: Maurice Levitt, Publisher: Taylor & Francis

(10) Reinforced Concrete Design, Author: Sawhney, Publisher: Macmillan India

(11) Concrete Formwork Systems, Author: Hanna Awad S, Publisher: Marcel Dekker

(12) Design Of Concrete Structures, Author: Arthur H Nilson, David Darwin, Charles W Dolan, Publisher: Tata Mcgraw Hill Publishing Co Ltd

(13) Concrete Technology, Quantity Surveying and Valuation, Author: Gupta and Subhash Chander, Publisher: Jain Publishers, New Delhi

(14) Manual Of Ready-mixed Concrete, Author: Spon, Publisher: Spon E and F

(15) Concrete Admixtures Handbook, Author: V S Ramachandran, Publisher: William Andrew Publishing

(16) Pozzolanic and Cementitious Materials, Author: V Mohan Malhotra, Povindar K Mehta, Publisher: Taylor & Francis

(17) Fly Ash in Concrete: Properties and Performance, Author: Karlhans Wesche, Publisher: Taylor & Francis

(18) Fly Ash in Concrete: Production, Properties and Uses, Author: Ramesh C Joshi, Lohtia, R P, Rajinder P, Publisher: Taylor & Francis

(19) Cement-Based Composites: Materials, Mechanical Properties and Performance, Author: Andrzej Marek Brandt, Publisher: Taylor & Francis

(20) Modern Concrete Materials: Binders, Additions and Admixtures, Author: Ravindra K Dhir, Thomas D Dyer, Publisher: Thomas Telford

(21)Concrete Materials: Properties, Specifications and Testing, Author: Sandor Popovics, Publisher: William Andrew Inc.

(22) Waste Materials Used in Concrete Manufacturing, Author: Satish Chandra, Publisher: William Andrew Inc.

(23) Innovations and Developments in Concrete Materials and Construction, Author: Ravindra K Dhir, P C Hewlett, Laszlo J Csetenyi, Publisher: Thomas Telford

(24) Mineral admixtures in cement and concrete, Editor: Shondeep L Sarkar, Publisher: ABI Books Pvt Ltd, New Delhi

(25) Durability of Concrete Structures: Investigation, Repair, Protection, Author: Geoffrey Mays, Publisher: Taylor & Francis

(26) High Performance Concrete: From Material to Structure, Author: Yves Malier, Publisher: Taylor & Francis

(27) Concrete Technology for a Sustainable Development in the 21st Century, Author: Odd E Gjłrv, Koji Sakai, Publisher: Taylor & Francis

(28) Mineral admixtures in cement and concrete, Editor: S N Ghosh, S L Sarkar, S Harsh, Publisher: ABI Books, Pvt Ltd, New Delhi

(29) Concrete and its chemical behavior, Author: M. S. Eglinton, Publisher: Thomas Telford Ltd

(30) Fundamentals of High-Performance Concrete, Author: Edward G Nawy, Publisher: John Wiley and Sons

(31) High-Performance Concrete, Author: Pierre-Claude Aïtcin, Publisher: Taylor & Francis

Question:

Write a paragraph on the best book you read on the concrete technology