6 FOCAL PROPERTIES OF CONCRETE IN FRESH AND HARDENED STATE

1. Strength:

Concrete is strong in compression, but relatively weak in tension and bending. It takes a lot of force to crush concrete, but very little force to pull it apart or cause bending cracks.


Compressive strength is primarily determined by the amount of cement used, but is also affected by the water to cement ratio, as well as proper mixing and placement, and the suitability and degree of hydration and curing.

Tensile strength usually ranges from 7 or 8% of compressive strength in high strength blends to 11 or 12% in low strength blends.

Both tensile and flexural strength can be increased by adding steel or fiber reinforcement.

The required compressive strength is based on an analysis of the loads that will be applied and the soil conditions at the project site. Actual compressive strength is verified by laboratory testing of samples using standardized equipment and procedures. On commercial projects, numerous samples are tested during construction to verify that the concrete being placed actually has the specified strength.

Lab testing is not often required in residential work, except perhaps on large high-end projects or on projects with difficult sites where special foundation designs make the strength of concrete critical.

A concrete that is stronger than necessary for its intended use is not economical and a concrete that is not strong enough can be dangerous.

2. Durability:

Durability can be defined as the ability to maintain satisfactory performance over an extended lifetime. Satisfactory performance is related to the intended use.

Concrete that is walked on or driven on must be wear-resistant so that it does not wear away.

Concrete exposed to the exterior of a building must be weather resistant so that it does not deteriorate from repeated freezing and thawing.

The durability of concrete exposed to repeated freeze-thaw cycles can be significantly increased by air entrapment.

Concrete in which steel reinforcement is embedded must withstand excessive moisture absorption to protect the metal from corrosion.

Natural wear and weathering will cause some change in the appearance of concrete over time, but in general durability also includes the preservation of both aesthetic and functional characteristics.

Just as concrete mix designs can be modified to produce a variety of strengths, appropriate concrete ingredients, mix ratios and finishes can and should be modified based on the durability required.

3. Volume Stability:

All materials expand and contract with changes in temperature, and porous materials such as concrete also expand and contract with changes in moisture content. Cement-based products such as concrete, masonry and stucco experience initial shrinkage as the cement hydrates and excess mixing water evaporates.

This initial contraction is permanent and is in addition to the reversible expansion and contraction caused by subsequent temperature or moisture changes.

Excessive shrinkage can crack concrete. Moisture can penetrate through the cracks and a vicious cycle of decay can begin.

Shrinkage cracks can be mitigated to some extent by steel or fiber reinforcement, and the location and weatherability of shrinkage cracks can be controlled through the use of control joints that divide the concrete into smaller panels or sections.

However, the mixture design and proportions of the ingredients also have an effect on the potential for shrinkage cracking.

The higher the cement content, the greater the tendency for shrinkage cracks during the curing and curing of the concrete.

4. Processability:

Workability is the relative ease with which a fresh concrete mix can be processed, placed, compacted and finished without separation or separation of the ingredients.

Good workability is required to produce concrete that is both economical and high-quality.

Fresh concrete is workable if it can be formed, compacted and finished to its final shape and texture with minimal effort and without separation of the ingredients.

Concrete with poor workability does not flow smoothly into shapes and does not properly encase reinforcing steel and embedded items, and it is difficult to compact and finish.

However, depending on the application, a mix with good workability for one element type or size may be too stiff or hard for another, so the term is relative.



Each mixture must be suitable for its intended use, striking a balance between the required fluidity, strength and economy.

The workability is related to the consistency and coherence of the mixture and is influenced by the cement content, additives, water content and adjuvants.

5. Consistency:

Consistency is the aspect of workability related to the flow properties of fresh concrete.

It is an indication of the fluidity or wetness of a mixture and is measured by the slump test. Fresh concrete is placed in a metal cone. When the cone is removed, the concrete will collapse a certain amount, depending on how fluid it is. A wet, soft mix will sink in more than a drier, stiffer one.

A high-slump concrete is one that is very fluid, and a low-slump concrete is drier and stiffer.

A heavily slumped mixture can cause excessive bleeding, shrinkage, cracking and dusting of the cured concrete surface.

There is a certain range of consistency that is suitable for any type of work. Workability is maximum in medium consistency concrete with slump between 3 and 6 inches. Both very dry (low draft) and very wet (high draft) mixtures are less processable.

6. Consistency:

Consistency is the element of workability that indicates whether a mixture is hard, sticky or plastic.

Plasticity is a desirable property in concrete, indicating that a mixture can form and hold a shape when formed.

A hard mix has no plasticity and the ingredients may have a tendency to separate.

Hardness can be caused by too much or too little mixing water (mixes with a high or low content), a shortage of cement (lean mixes), or a deficiency of fine aggregate particles.

Hardness can also be caused by an excess of coarse, angular, flat or elongated aggregate particles. Hard mixes can sometimes be improved by entrainment of air or by increasing the level of fine aggregates or cement, but the overall mix must be adjusted to maintain the correct proportion of all ingredients.

A sticky mix can have a high cement content (fat blends) or large amounts of rock dust, fine sand or similar fine materials (over-sanded mixtures). Sticky mixes do not segregate easily, but because they require a lot of water to achieve even minimal workability, tacky mixes often develop excessive shrinkage cracks.

A plastic mix is cohesive without being sticky or hard, and the ingredients do not separate easily unless the concrete is treated incorrectly.

PCE_Editor

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