Concrete surfaces can flake or spall for one or more of the following reasons:
In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.
So, there is no such thing as a cement sidewalk, or a cement mixer; the proper terms are concrete sidewalk and concrete mixer.
Materials that contain appropriate amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.
The kiln resembles a large horizontal pipe with a diameter of 10 to 15 feet (3 to 4.1 meters) and a length of 300 feet (90 meters) or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures that range between 2700 and 3000 Fahrenheit (1480 and 1650 Celsius). This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton the comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. A small amount of gypsum is added during the grinding process to control the cement's set or rate of hardening.
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period-from five to seven days after placement for conventional concrete-the concrete surface needs to be kept moist to permit the hydration process. new concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50 Fahrenheit (10 Celsius) during the curing period.
Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced through the use of air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete, but may be varied as required by special conditions.
Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's:
A minimum cement content of 6 bags per cubic yard of concrete, A maximum water content of 6 gallons per bag of cement, A curing period (keeping concrete moist) a minimum of 6 days, and An air content of 6 percent (if concrete will be subject to freezing and thawing)
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete, not including test methods unique to organizations like the Army Corps of Engineers, the Federal Highway Administration, and state departments of transportation.
Slump, air content, unit weight and compressive strength tests are the most common tests. Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can't flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation. Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing.
Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure. Unit weight measures the weight of a known volume of fresh concrete. Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at proscribed intervals as they harden. According to Building Code Requirements for Reinforced Concrete (ACI 318), as long as no single test is more than 500 psi below the design strength and the average of three consecutive tests equals or exceed the design strength then the concrete is acceptable. If the strength tests don't meet these criteria, steps must be taken to raise the average.