Properties of Hardened Concrete
Concrete is strong in compression but relatively weak in tension and bending. It takes a great deal of force to crush concrete, but very little force to pull it apart or cause bending cracks.
Compressive strength is determined primarily by the amount of cement used, but is also affected by the ratio of water to cement, as well as proper mixing and placing, and the adequacy and extent of hydration and curing.
Tensile strength usually ranges from 7 or 8 % of compressive strength in high-strength mixes to 11 or 12% in low-strength mixes.
Both tensile strength and flexural bending strength can be increased by adding steel or fiber reinforcement.
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 testing samples in a laboratory using standardized equipment and procedures. On commercial projects, numerous samples are tested throughout construction to verify that the concrete being put into place actually has the specified strength.
Laboratory 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 concrete strength critical.
A concrete that is stronger than necessary for its intended use is not economical, and one that is not strong enough can be dangerous.
Durability might be defined as the ability to maintain satisfactory performance over an extended service life. Satisfactory performance is related to intended use.
Concrete that will be walked or driven on must be abrasion resistant so that it doesn’t wear away.
Concrete that will be exposed on the outside of a building must be weather resistant so that it doesn’t deteriorate from repeated freezing and thawing.
The durability of concrete exposed to repeated freeze-thaw cycles can be significantly increased by air entrainment.
Concrete in which steel reinforcement is embedded must resist excessive moisture absorption in order 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 maintenance of aesthetic as well as functional characteristics.
Just as concrete mix designs can be adjusted to produce a variety of strengths, appropriate concrete ingredients, mix proportions, and finishes can and should be adjusted on the basis of required durability.
3. Volume Stability:
All materials expand and contract with changes in temperature, and porous materials like concrete also expand and contract with changes in moisture content. Cement-based products such as concrete, concrete masonry, and stucco experience initial shrinkage as the cement hydrates and excess mixing water evaporates.
This initial shrinkage is permanent, and is in addition to reversible expansion and contraction caused by later temperature or moisture changes.
Excessive shrinkage can cause concrete to crack. The cracks allow moisture to penetrate, and a vicious cycle of deterioration may begin.
Shrinkage cracking can be restrained to some extent by steel or fiber reinforcement, and the location and weather resistance of shrinkage cracks can be controlled through the use of control joints which divide the concrete into smaller panels or sections.
However, the mix design and ingredient proportions also have an effect on the potential for shrinkage cracking.
The higher the cement content, the greater the tendency for shrinkage cracks to form while the concrete is curing and hardening.
Properties of Fresh Concrete
Workability is the relative ease with which a fresh concrete mix can be handled, placed, compacted, and finished without segregation or separation of the ingredients.
Good workability is required to produce concrete that is both economical and high in quality.
Fresh concrete has good workability if it can be formed, compacted, and finished to its final shape and texture with minimal effort and without segregation of the ingredients.
Concrete with poor workability does not flow smoothly into forms and properly envelop reinforcing steel and embedded items, and it is difficult to compact and finish.
Depending on the application, however, a mix that has good workability for one type or size of element may be too stiff or harsh for another, so the term is relative.
Each mix must be suitable for its intended use, achieving a balance among required fluidity, strength, and economy.
Workability is related to the consistency and cohesiveness of the mix, and is affected by cement content, aggregates, water content, and admixtures.
Consistency is the aspect of workability related to the flow characteristics of fresh concrete.
It is an indication of the fluidity or wetness of a mix and is measured by the slump test. Fresh concrete is placed in a metal cone. When the cone is removed, the concrete slumps a certain amount depending on how fluid it is. A wet, soft mix slumps more than a drier, stiffer one.
A high-slump concrete is one that is very fluid, and a low-slump concrete is drier and more stiff.
A high-slump mix may cause excessive bleeding, shrinkage, cracking, and dusting of the hardened concrete surface.
There is a certain range of consistency that is appropriate for each type of work. Workability is at a maximum in concrete of medium consistency with a slump between 3 and 6 in. Both very dry (low-slump) and very wet (high-slump) mixes are less workable.
Cohesiveness is the element of workability which indicates whether a mix is harsh, sticky, or plastic.
Plasticity is a desirable property in concrete, indicating that a mix can be moulded and hold a shape when formed.
A harsh mix lacks plasticity and the ingredients may tend to separate.
Harshness can be caused by either an excess or deficiency of mixing water (high- or low-slump mixes), a deficiency of cement (lean mixes), or a deficiency of fine aggregate particles.
Harshness may also be caused by an excess of rough, angular, flat, or elongated aggregate particles. Harsh mixes can sometimes be improved by air entrainment or by increasing the fine aggregate or cement content, but adjustments must be made to the overall mix to maintain the proper proportion of all ingredients.
A sticky mix may have a high cement content (fat mixes) or large amounts of rock dust, fine sand, or similar fine materials (over-sanded mixes). Sticky mixes do not segregate easily, but because they require a lot of water to achieve even minimal workability, sticky mixes often develop excessive shrinkage cracking.
A plastic mix is cohesive without being either sticky or harsh, and the ingredients do not easily segregate unless the concrete is handled improperly.