What exactly is concrete?
Concrete is a mixture of Portland cement, a fine aggregate (sand), a course aggregate (gravel), water and several chemical additives. When water mixes with the cement, a process known as "hydration" takes place that binds the whole mixture together. In its finished state, concrete appears to be a solid slab, but is actually quite porous (full of tiny holes or openings). As the excess water in the mix evaporates, it leaves capillaries throughout the slab, similar to a sponge.
The genetic component of concrete
There are many different mixes of concrete for different applications. Most mixes involve adding more Portland cement, which affects the strength of the concrete. Concrete is sold by the cubic yard.
Concrete does not require much water to achieve maximum strength. Too much water makes it easier to install, but greatly reduces strength. The 'slump' of a mix is a measure of consistency of freshly mixed concrete, measured in inches. It is the distance that the mix falls when a conical mold (slump cone) is lifted from a test specimen. Increasing the slump is typically done by increasing the water in the mix. We normally use a slump of '3' to '4' for sidewalks and driveways.
We also use an air entrained concrete mix. This involves adding an air-entraining agent, a surfactant, which creates tiny air bubbles in the mix that help reduce or absorb stresses from freeze/thaw cycles.
Concrete should be in place within 90 minutes of its loading at the concrete plant.
Can I pour concrete in all weather?
Extreme temperatures make it difficult for the hydration process to take place. When it is too close to freezing, the hydration slows to a standstill and the concrete will not cure or gain strength. Typically, the ground should be at least 50 degrees and rising. On the other extreme, when it is very hot, too much water is lost by evaporation and care must be taken to keep the concrete wet.
What kind of sub-base should I have?
John’s Asphalt removes all topsoil. A smooth, granular (stone or gravel) sub-base will be installed and compacted so that the slab has a uniform thickness. The sub-base is properly graded so that water flows away from any structures. As a general rule, the base should slope 1/4" for every linear foot to provide proper drainage.
How thick should the slab be?
We recommend a 4” thickness for sidewalks and 5” to 6” for driveways, depending on the type of vehicular traffic. Aprons are generally 7" to 8" thick.
Should I use reinforcement in the concrete?
Yes. Steel reinforcement (welded wire mesh), can help prevent cracks over the life of the slab. The wire mesh should be elevated to the center of the slab as it is poured. Additionally, fiber can be added to the concrete to help minimize cracking and reduce permeability.
Is it true that concrete shrinks?
Yes. As it cures, concrete shrinks roughly 1/2” for every 100 linear feet.
Doesn’t this shrinkage cause the concrete to crack?
Yes. Control joints will be put in the concrete to provide a place for stress relief (similar to drive way cracking precautions). Control joints will be 2 - 3 times (in feet) the thickness of the slab in inches, i.e. a 4" slab should have control joints every 8 to 12 feet.
Control joints should be ¼ the thickness of the slab. As an example, 4" sidewalk should have 1" deep control joints. They should either be scribed into the surface with appropriate concrete finishing tools or saw cut within 8 to 12 hours of the pour.
Isolation joints are areas where the new concrete slab borders another fixed surface and their interaction might cause cracking. Typically a 1/2" to 1/4" thick pre-molded joint filler is used in these situations.
Does all concrete crack?
All concrete will crack to some extent. The key is to use the correct mix combined with the proper professional application to minimize or control the cracking.
Primary causes of cracking are stress induced by shrinkage of the concrete as it cures or stress occurring from a poor sub-grade or due to use.
How do I address cracks that appear in my concrete?
We do not recommend addressing hairline crack(s) until they become 1/8" to 1/4". At that point, the crack can be ground out with a grinder and caulked with a self-leveling concrete caulk. The caulk should be tack free within 2 hours and cures fully within one to two weeks.
What finish should the concrete have?
We recommend a simple ‘broom finish’ to provide traction and a clean aesthetic look.
How long does it take for concrete to cure?
Concrete actually gets stronger as it gets older, the curing process continuing for years. The hydration process happens rapidly at first and then slows down and so the standard has become that all concretes are rated at their 28-day strength. For practical purposes, we recommend that car traffic can generally enter onto concrete slabs after three days and truck traffic after 7 days.
Do you recommend a curing compound?
Yes. It is very important to maintain proper moisture levels in new concrete in its early stages of hydration and protect it from the sun and wind.
Do you recommend the use of concrete sealants?
That is a personal preference. After the concrete has had a chance to cure properly (at least 28 days), a sealant can be applied to prevent moisture from getting into the slab. The key is that the sealant must still allow the concrete to breathe, in order for moisture from the soil underneath to evaporate. Typically, sealants last about 2 years. Also, confirm that the sealant won’t discolor the concrete in an objectionable manner.
Why is exposed aggregate concrete more expensive than standard concrete?
There are two main reasons.
First, a special pea gravel or larger smooth gravel is used in the mix.
Second, the installation process becomes a 'two step' process. Once the concrete is poured and finished, a special surface retarder is sprayed over the entire area. This allows the center of the concrete slab to cure while the top surface stays more malleable. The next morning, we return again and hose off with water the top layer of concrete.
What does the term “spalling” mean?
In regions where there are severe freeze thaw cycles, water can get into the concrete, freeze and cause small pieces of the concrete to chip off or spall. Sealants can help protect against spalling.
Why shouldn’t I use salt on my new concrete?
During a new concrete's first winter, it is very susceptible to water damage. During freezing conditions, salt melts ice and allows the water to penetrate into the mass of the concrete. When the water freezes again, it expands as much as 9%, causing the surface of the concrete to spall off in small chips.
John’s Asphalt recommends the use of sand or cinder chips during this first season.