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Exterior Concrete

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The Basics
Concrete is a mixture of Portland cement, water and aggregate. The strength of the concrete is determined largely by the amount of cement in the mixture. Other factors that affect the strength of the concrete are the type of aggregate, the slump of the mixture, and the absence of organic particles in the mixture.

Slump- The slump of concrete is a measurement of the amount of water in the mix. To test the slump concrete is put into a cone shape container and then dumped out. The more water the more the cone shape slumps, the weaker the concrete.

Aggregate- Sand and gravel make up the aggregate in concrete. The strongest concrete has a mixture of particles from sand to -3/4" gravel.

Cement- The cement content is usually described as bags per yard. A standard mix is 5 bag mix. Because exterior concrete flatwork is exposed to the elements 6 bag minimum mix is recommended.

Organic Matter- Dirt is a common organic matter that if mixed into concrete will reduce it's strength. That is why only washed aggregate is used in quality concrete.

The Base
The Quality of the concrete job is only as good as the base. The soil (base) that the concrete is placed on must be level drainable and compacted, If not the base will give way causing the concrete to sink and break.
In poor bearing soils (clay, silt and sand mixtures) compacted road base is used to give the base extra bearing strength. Where soil strength is not a concern, pea gravel is often used because it give doesn't need compaction and drains well. The gravel is usually placed to a depth of 4"-6". Ask your concrete contractor what he recommends for your particular situation.

The Process
Forms (usually 2 x 4's) are laid on edge to create the mold. The concrete is laid in place, screeded (leveled), floated, then toweled, and broom finished. During the finishing process joints are placed in the concrete.

Control joints- Control joints are placed no more than 10' apart in both directions to control cracking. Concrete will crack... control joints help the slab to crack in the joint.

Expansion joints- Since concrete expands due to climate changes, expansion joints are used when concrete is placed between immovable objects (a driveway between a sidewalk and the garage floor), or when new concrete is placed against old concrete. Expansions joints are used in these situations and generally every 30-40'.

Reinforcement
Reinforcement in walkways, and driveways helps to hold the concrete together. 6" x 6" wire mesh and rebar are commonly used to reinforce concrete slabs. If the base is properly prepared reinforcement is not crucial to the longevity of a concrete slab. It is interesting to note that steel reinforcement is rarely required in city sidewalks.

Pouring the concrete and the weather
Concrete should never be poured on frozen ground. Concrete can be poured in freezing weather as long as it is protected from freezing. Hydration will help to protect the concrete from freezing for the first couple of nights after pouring. Concrete blankets or a combination of plastic and straw are often used to keep the concrete warm.
Rain Storms can ruin a concrete finish. If the forecast call for rain, it is a good idea to reschedule the pour.

Hydration- Chemical reaction between concrete and water gives off heat as the concrete cures.

Curing
For Concrete to cure properly, it needs to be kept relatively moist for the first 3-4 weeks after pouring. If concrete dries out to quickly it will loose strength. Most concrete contractors will pour the concrete and finish it, but it is up to the homeowner, builder or contractor to protect it from the elements and make sure that it is properly cured. After the concrete has hardened for a day or two place a sprinkler and moisten surface periodically for the next few weeks. Concrete poured in the cooler part of the year or in humid climates will need less watering. Ask your concrete contractor for recommendations.

Notes to owner-builder
Some contractors like to pour the exterior concrete as soon as the foundation is complete. This may sound like a good idea... to "get it out of the way as soon as possible". However, we believe that unless weather will be a problem later on, it is usually best to wait.
Waiting allows the ground a chance to settle where it has been excavated. Laterals still need to be put in, and it takes more preparation to plan that out. Subcontractors with oil leaking vehicles will park on the driveway if it is there. No matter when the driveway is poured it must be protected from traffic for a minimum of 7-10 days (longer when the weather is cold). Place barriers around freshly poured concrete or someone will walk or drive on it.

 

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