What exactly is concrete?
In simplistic terms, 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. As the excess water in the mix evaporates, it leaves capillaries throughout the slab much like a sponge.
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 and 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?
All topsoil should be removed. A smooth, granular (stone or gravel) sub-base should be installed and compacted so that the slab has a uniform thickness.
The sub-base should be properly graded so that water flows away from any structures. As a rule of thumb, 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 (6x6 #10 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.
The genetic component of concrete.
There are many different mixes of concrete for different applications. In our market, most of the mixes involve adding more Portland cement which affects the strength of the concrete. We typically use a 6 ½ bag (of Portland) mix that has a compressive strength rating of 3500 psi for sidewalks and 4000 psi for driveways. 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 request an air entrained concrete mix. This entails adding an air-entraining agent, a sulfactant, 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.
Is it true that concrete shrinks?
Yes. As it cures, concrete shrinks roughly ½” for every 100 linear feet.
Doesn’t this shrinkage cause the concrete to crack?
Yes. Control joints should be put in the concrete to provide a place for stress relief (read 'cracking'). Control joints should be 2 to 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. Thus a 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 abuts against another fixed surface and their interaction might cause cracking. Typically a ½" to ¼" thick premolded 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 and proper workmanship 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 cracks 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 such as Sikaflex 1CSL or Dow Corning 890-SL 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 seven 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?
This is a personal preference item. 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 item here is that the sealant must still allow the concrete to breathe to allow moisture from the soil underneath to evaporate.
Typically sealants last about two years. You should also check to make sure that the sealant doesn't discolor the concrete in an objectionable manner.
Why is exposed aggregate concrete more expensive than standard concrete?
Two reasons. The first is that a special pea gravel or larger smooth gravel is used in the mix.
The second is that 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 such as ours where we periodically have 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 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.
The Portland Concrete Association recommends the use of sand or cinder chips during this first season.