Concrete 101: Basics Every Property Manager Should Know
By Brian Adams, PCM Concrete Division Manager | May 15, 2013
Photo from Portland Cement Association.
Concrete is the most widely used construction material in the world. Concrete can be molded into any shape that will then harden into a strong and durable material. Its versatility explains why it is used to construct the most complex buildings, bridges, highways, and dams in addition to simple sidewalks and driveways.
You can color, stamp, or stain concrete to meet the demands of almost any project. In fact, if you pick up any architectural or builder’s magazine today, you’ll see that some of the most awe-inspiring creations are now being built with concrete.
Cement vs. Concrete – What’s the difference?
Although the terms “cement” and “concrete” are often used interchangeably, cement is actually a component of concrete.
Concrete is made up of approximately…
- 6% air
- 11% portland cement
- 41% gravel or crushed stone (coarse aggregate)
- 26% sand (fine aggregate)
- 16% water
“Portland cement” is the generic term for the type of cement used in virtually all concrete.
In its simplest form, concrete is a mixture of paste and aggregates. The paste (portland cement and water) coats the surface of the aggregates. Through a chemical reaction called “hydration,” the paste hardens and gains strength to form the rock-like mass we know as concrete.
So, there is no such thing as a cement sidewalk; the proper term is concrete sidewalk.
What is “curing” of concrete?
Curing is when the concrete sets during the hydration process. Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. The curing period lasts from five to seven days after placement. During this period, it is critical the concrete surface is kept damp in order for the hydration process to occur. Additives called “admixtures” may be used to accelerate the curing (hardening) process in low temperature conditions.
How does temperature affect concrete placement?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation. If the temperature falls too close to freezing, hydration slows to a near standstill. While too much water weakens the concrete causing peeling and chipping, too little water causes the concrete to cure too fast and will result in cracking. When pouring concrete, the best results are achieved in temperatures between 60 and 90 degrees Fahrenheit. That being said, one may use additives and blankets to accommodate for more extreme temperatures.
There are two basic strategies to control cracking for good overall structural behavior.
One method is to provide steel reinforcement in the slab (rebar or wire mesh), which keeps crack size at a minimum. While the concrete is structurally safe, random hairline cracks do typically appear in the exposed surface of the concrete.
The most widely used method is to place contraction/control joints in the concrete surface where the concrete can crack in a straight line. The result is much more aesthetically pleasing. The crack actually takes place below the finished concrete surface. While the concrete has still cracked, which is normal behavior, the concrete surface appears to be unaffected.