Properties of Hardened Concrete
Fully cured, hardened concrete must be strong enough to withstand the structural and service loads which will be applied to it and must be durable enough to withstand the environmental exposure for which it is intended.
When concrete is made with high-quality materials and is properly proportioned, mixed, handled, placed, and finished, it is one of the strongest and most durable of building materials.
When we refer to concrete strength, we are generally talking about compressive strength which is measured in pounds per square inch (psi). 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 (Figure 1).
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, placing, 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.
Structural engineers establish required compressive strengths for various building elements based on an analysis of the loads which 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.
For most residential projects, required concrete strength will be in the range of 20 MPa to 30 MPa, depending on the intended use (Table below). A concrete that is stronger than necessary for its intended use is not economical, and one that is not strong enough can be dangerous.
The primary factors affecting concrete compressive strength are the cement content, the ratio of water to cement, and the adequacy and extent of hydration and curing.
|Construction element||Compressive strength Mpa|
|Basement and foundation walls slabs||20 t0 25|
|Driveways, garage slab||20 t0 25|
|Reinforced concrete beams, slabs, patios, sidewalks and steps||20 to 25|
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.
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.
In cold climates, exterior concrete is exposed to repeated freeze-thaw cycles which can potentially be very damaging. Freeze-thaw deterioration, in fact, is one of the most serious threats to concrete durability, but resistance to damage can be significantly increased by air entrainment.
A network of fine voids formed by air entrained cement or an air-entraining admixture absorbs the expansive force of freezing water to prevent the hardened concrete from fracturing or scaling over repeated cycles of winter freezing and thawing.
Air entrainment improves the durability of horizontal elements such as sidewalks, driveways, patios, and steps, which are most frequently exposed to rainwater, melting snow, and deicing salts. For vertical elements, which are less often saturated with rain and in mild climates where freeze-thaw cycles are infrequent, air entrainment adds little value to hardened concrete but still may be used to increase the workability of fresh concrete.
Air entrainment is sometimes credited with increasing the water tightness of concrete, but this is probably because the increased workability of the mix is conducive to better placement, consolidation, and finishing.
Another important aspect of concrete durability is 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. In addition to reversible thermal expansion and contraction, cement-based products such as concrete, concrete masonry, and stucco experience initial shrinkage as the cement hydrates and excess construction water evaporates.
This initial shrink- age 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 of shrinkage cracks can be controlled through the use of special joints that 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.