Cement Concrete is by far the most widely used construction material today. The versatility and the mouldability of this material, its high compressive strength, and the discovery of the reinforcing and prestressing techniques which helped to make up for its low tensile strength have contributed largely to its widespread use.

Cement:

Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminium, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Lime and silica make up about 85% of the mass. Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.

Chemical Composition and Hydration of Cement.

Oxide Composition of Portlant Cement

• Portland cement is composed of four major oxides: lime (CaO), silica (SiO2 ), alumina (Al2 O3 ), and iron ( Fe2 O3 ).
• Also Portland cement contains small amount of magnesia (MgO), alkalies (Na2 O and K2 O), and sulfuric anhydrite (SO3 )

Major Compounds of Portland Cement (Bogue’s Compound Composition)

Hydration of cement

When Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical reaction with water is called “hydration”. Each one of these reactions occurs at a different time and rate. Together, the results of these reactions determine how Portland cement hardens and gains strength.

Cement hydration

• Hydration starts as soon as the cement and water are mixed.
• The rate of hydration and the heat liberated by the reaction of each compound is different.
• Each compound produces different products when it hydrates.
• Tricalcium silicate (C3 S). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of C3 S will exhibit higher early strength.
• Tricalcium aluminate (C3 A). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately and contributes somewhat to early strength.
• Gypsum is added to Portland cement to retard C3 A hydration. Without gypsum, C3 A hydration would cause Portland cement to set almost immediately after adding water.
• Dicalcium silicate (C2 S). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.
• Tetracalcium aluminoferrite (C4 AF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to C4 AF.

Heat of Hydration

• The heat of hydration is the heat generated when water and Portland cement react. Heat of hydration is most influenced by the proportion of C3 S and C3 A in the cement, but is also influenced by water-cement ratio, fineness and curing temperature. As each one of these factors is increased, heat of hydration increases.
• For usual range of Portland cements, about one-half of the total heat is liberated between 1 and 3 days, about three-quarters in 7 days, and nearly 90 percent in 6 months.
• The heat of hydration depends on the chemical composition of cement.

Concrete

Fresh Concrete

There are two sets of criteria that we must consider when making concrete;

• Long-term requirements of hardened concrete, such as, strength, durability, and volume stability,
• Short-term requirements, like workability.

However, these two requirements are not necessarily complementary.

For fresh concrete to be acceptable, it should:

• Be easily mixed and transported.
• Be uniform throughout a given batch and between batches
• Be of a consistency so that it can fill completely the forms for which it was designed.
• Have the ability to be compacted without excessive loss of energy.
• Not segregate during placing and consolidation.
• Have good finishing characteristics
• The first 48 hours are very important for the performance of the concrete structure. It controls the long-term behaviour, influence fc, Ec, creep, and durability.

Various properties which are to be considered during green state are:

• Workability
• Slump Loss
• Segregation/Bleeding
• Plastic Shrinkage
• Time of Set
• Temperature
• Workability: Definition Effort required to obtain a concrete mix with minimum of segregation and without changing the water cement ratio. It is not a fundamental property of concrete.
• Workability is often defined in terms of the amount of mechanical energy, or work, required to fully compact concrete without segregation. This is important since the final strength is a function of compaction.

Factors Affecting Workability:

• Water Content of the Mix -- This is the single most important factor governing workability of concrete. A group of particles requires a certain amount of water. Water is absorbed on the particle surface, in the volumes between particles, and provides “lubrication” to help the particles move past one another more easily. Therefore, finer particles, necessary for plastic behavior, require more water.
• Influence of Aggregate Mix Proportions – Increasing the proportion of aggregates relative to the cement will decrease the workability of the concrete. Also, any additional fines will require more cement in the mix. An “oversanded” mix will be permeable and less economical.
• Aggregate Properties -- The ratio of coarse/fine aggregate is not the only factor affecting workability. The gradation and particle size of sands are important. Shape and texture of aggregate will also affect workability. Aggregate which is porous will absorb more water leaving less to provide workability. It is important to distinguish between total water content, which includes absorbed water, and free water which is available for improving workability.
• Time and Temperature -- In general, increase in temperature will cause an increase in the rate of hydration leading to evaporation of water and hence decrease in workability.
• Cement Characteristics -- Cement characteristics are less important than aggregate properties in determining workability. However, the increased fineness of higher grade cements will result in rapid hydration and increased water requirements.
• Admixtures -- In general, air-entraining, water-reducing, and set-retarding admixtures will all improve workability. However, some chemical admixtures will react differently with cements and aggregates and may result in reduced workability.

Segregation and Bleeding:

• Segregation refers to a separation of the components of fresh concrete, resulting in a non-uniform mix. This can be seen as a separation of coarse aggregate from the mortar, caused from either the settling of heavy aggregate to the bottom or the separation of the aggregate from the mix due to improper placement. Some factors that increase segregation are:
• Larger maximum particle size (25mm) and proportion of the larger particles.
• High specific gravity of coarse aggregate.
• Decrease in the amount of fine particles.
• Particle shape and texture.
• Water/cement ratio.
• Good handling and placement techniques are most important in prevention of segregation.
• Bleeding is defined as the appearance of water on the surface of concrete after it has consolidated but before it is set. Since mixing water is the lightest component of the concrete, this is a special form of segregation. Bleeding is generally the result of aggregates settling into the mix and releasing their mixing water. Some bleeding is normal for good concrete. However, if bleeding becomes too localized, channels will form resulting in “craters”. The upper layers will become too rich in cement with a high w/c ratio causing a weak, porous structure. Salt may crystalize on the surface which will affect bonding with additional lifts of concrete. This formation should always be removed by brushing and washing the surface. Also, water pockets may form under large aggregates and reinforcing bars reducing the bond.

Bleeding may be reduced by:

• Increasing cement fineness.
• Increasing the rate of hydration(higher grade of cement or admixtures)
• Using air-entraining admixtures.
• Reducing the water content.
• Workability, a term applied to many concrete properties, can be adequately measured by three characteristics:
• Compactibility, the ease with which the concrete can be compacted and air void removed (Normal concrete with slump values in the range of 25 to 175mm)
• Mobility, ease with which concrete can flow into forms and around reinforcement (Flowing concrete such as RMC and SCC) and
• Stability, ability for concrete to remain stable and homogeneous during handling and vibration without excessive segregation (Roller Compacted Concrete)

Will be Continued in Issue 6..

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References:
National Building Code of India 2005




Published by:
Task Force for Quality Assurance in Public Constructions
GOVERNMENT OF KARNATAKA ISBN




Important Note:
This reference manual is intended for the use of Government Engineers.




Disclaimer:
Every effort has been made to avoid errors or omissions in this publication. In spite of this, some errors might have crept in. Any mistake, error or discrepancy noted may be brought to our notice which shall be taken care of in the next edition. It is notified that neither the publisher nor the authors will be responsible for any damage or loss of action to any one, or any kind, in any manner, therefrom.