Concrete with carbonate aggregates (such as limestone and dolomite) and lightweight aggregates (either naturally occurring or created by expanding shale, clay, or slag) retains the majority of its compressive strength up to 1200 degrees F. Carbonated concrete can withstand temperatures as high as 3000 degrees F without suffering permanent damage.
As long as you don't get any water inside the concrete, it will not deteriorate at temperatures above 100 degrees Fahrenheit. Above that temperature, moisture in the air will eventually cause the concrete to crack and deteriorate.
The thermal conductivity of concrete is low; therefore, it does a good job of insulating buildings against heat gain or loss. Concrete also does an excellent job of retaining heat, which is why houses built with concrete siding tend to be warmer in the summer and cooler in the winter than those with other types of siding. Concrete blocks are even more effective at retaining heat than plain old concrete.
Concrete's ability to resist heat damage has helped builders create structures that can't be easily destroyed by fire. Fireproof buildings are common today because most building codes require fire-resistant materials for construction. Concrete is a very strong material and can be used to create structures that cannot be broken down by ordinary means. For example, concrete walls have no vital organs like wood does so they do not burn when exposed to fire.
A comprehensive study of the compressive and flexural strengths of various types of portland cement used in concrete mixed, placed, and cured at various temperatures was conducted, and included tests that indicate that there is a temperature during the early life of concrete that is considered optimal in terms of strength at later ages. This "optimum" curing temperature varies for different cements and mixes of cement with other materials such as sand or gravel.
The study found that if concrete is allowed to cure at too low a temperature, it will not have enough strength after one year of age to withstand the loads applied to it by people walking on it or vehicles driving over it. If, however, it is permitted to cure at too high a temperature, it may become quite brittle and susceptible to fracture.
The report concludes that optimum curing conditions provide sufficient heat to allow the hydration of the cement to progress but not so much heat that damage occurs to the concrete.
For ordinary building applications where concrete is placed above ground level and exposed to direct sunlight, it is recommended that it be allowed to cure at ambient temperatures. Curing at colder temperatures will not adversely affect the concrete's durability, but it will take longer for it to harden and reach its maximum strength. Curing at warmer temperatures will tend to shorten the setting time of the concrete, but this benefit is more than offset by potential harm from thermal expansion and contraction while the concrete is undergoing hydration and compression testing.
Concrete's compressive strength may be enhanced by doing the following:
Concrete has a great compressive strength but a very low tensile strength. As a result, it is generally reinforced with materials that have a high tensile strength (often steel). Concrete can be used as the primary reinforcement for structures such as bridges, buildings, and floors, or it can be used in combination with other materials as secondary reinforcement for these structures.
Concrete's low tensile strength makes it an ideal material for use in applications where failure by rupture is common, such as fence posts and light pole bases. It also has good thermal properties and is relatively inexpensive. These characteristics make concrete a popular material for building projects such as homes and commercial buildings.
Concrete's poor fracture resistance limits its application to items such as furniture where complete breakdown of the material is not necessary for safety. However, this property makes concrete useful for creating decorative elements such as sculptures and artworks. Concrete's average density (3,000 kg/m3) means that it is often used as a lightweight material in construction projects, particularly when water resistance is important such as with walkways and driveways.
Concrete's low cost makes it widely used for material handling equipment such as forklifts and pallets. This usage may account for some deaths and injuries caused by concrete during construction projects.
Residential concrete compressive strength needs range from 2500 psi (17 MPa) to 4000 psi (28 MPa) and greater in commercial projects. Certain applications need higher strengths of up to and surpassing 10,000 psi (70 MPa). Concrete that meets these requirements is called high-strength concrete.
Concrete's compressive strength depends on several factors such as the type of aggregate used, the amount of cement, the age of the concrete, among others. Concrete that has a high early strength tends to be stronger overall later when it is exposed to heat from sunlight or vehicles. This means that if you can place your concrete project in the sun or allow it to air-dry, it will be more resistant to cracking over time.
The European standard for residential concrete requires that it have a minimum 28-day compression strength of 2500 psi (17 MPa). This means that if a slab is loaded down by a heavy vehicle, it should be able to resist crushing for at least this long before becoming too weak to protect itself.
In Europe, there is also a requirement that at least 20 percent of the volume of any concrete floor must be left as open space so that humidity cannot cause the wood beams inside buildings to rot. The minimum width of a beam is 4 inches, and its depth is dependent on the load it has to support.