What's the difference between cement and concrete?
Cement (sometime referred to as Portland cement or hydraulic cement) is one component of concrete. Cement is to concrete what flour is to a loaf of bread. Concrete is basically a mixture of 2 components: aggregates and paste. The paste, comprised of cement and water, binds the aggregates (sand and stone or gravel) into a rock like mass as the paste hardens because of the chemical reaction of the cement and water. This reaction is called hydration.
What is Portland cement?
Portland cement is a manufactured product made in several steps. First we carefully blend limestone with shale or clay and iron to a desired chemical composition. We then grind it to a fine powder called raw meal. We feed the raw meal to a rotary kiln that is fired by coal, natural gas or another high energy fuel. The temperatures in the kiln reach about 2,800Â° F. At these temperatures the raw meal undergoes a chemical reaction and forms a new product called clinker. The clinker is mixed with about 5% gypsum and ground to a fine powder called portland cement.
How do I make a small batch of concrete for various jobs around the house?
A good rule of thumb for small batches of concrete is: 1 part portland cement, 2 parts sand, 3 parts gravel and just enough water to make it workable (not soupy). Be careful to protect your skin from the wet concrete as it is a strong base that will cause alkali burns. If you get a small amount on your skin, wash it off with soap and water
What makes concrete crack?
Concrete "shrinks" slightly as it hardens. A normal shrinkage rate is approximately 1/8" per 100 linear feet. This shrinkage is caused by loss of excess water from the mix. Obviously, the "wetter" the mix, the higher the shrinkage rate. Control joints should be placed in the concrete at intervals equal to 2.5 times (in feet) the thickness of the slab. For example, a slab 4" thick should have control joints every 10 feet.
Why should fresh concrete be properly cured?
The surface of freshly placed concrete should be kept moist for at least 7 days. Contrary to popular belief, concrete should not be allowed to "dry out." If the concrete is allowed to "dry out," the ultimate strength gain will be considerably less than its designed strength. Proper curing will also minimize the potential of cracking. The simplest method of moist curing is wetting the surface of the concrete, then covering it with polyethylene.
What causes scaling and surface shrinkage cracks?
These defects are generally a result of improper finishing of the concrete. As discussed earlier, the prime factor affecting concrete strength is water/cement ratio. If excess water is added to the surface of the concrete during placement and finishing, the water/cement ratio on the surface may be drastically increased. This condition greatly reduces the strength of the concrete on the surface. Unfortunately, this is where the wear takes place.
I've heard the term slump pertaining to concrete. What is slump?
The term slump simply refers to the consistency of the concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more water used in producing the concrete, the wetter (or higher) the slump will be. Again, excessive water causes a dramatic loss of strength. Applications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps can be attained through the use of water reducing admixtures (chemical additives).
How long does concrete continue to gain strength after it is placed?
Concrete's most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years!) As long as moisture is present to continue the chemical process of hydration.
What is the setting time of Cement Concrete?
The Setting time of Cement is the time required by concrete to harden and set itself according to the formwork in which it has been placed. The amount of time taken by concrete to set is 7 hours to 10 hours depending on the conditions under which the concrete is placed in the formwork. But this does not mean that the formwork may be removed after 7 hours the initial setting of concrete does not impart enough strength for it to withstand its own self load so the formwork should be removed only after 7 to 10 days of proper curing.
Does the colour of cement have any bearing on its strength?
No, the colour of cement does not affect the strength of cement in anyway. It just gives a proper finish to the concrete. There is a general misconception that the cement that is darker in colour has greater strength; this is not true and the cement that is lighter in colour not only has the same strength but also has a more pleasing finish than the cement that is darker in colour.
What effect does the water/cement ratio have on the strength of concrete?
The water cement ratio is very important with regards to the strength of concrete. The strength of concrete is inversely proportional to the water cement ratio, i.e. with the increase in water content the compressive strength of concrete decreases and vice versa.