Concrete Basics

Getting a concrete slab poured seems to be a fairly straight forward simple operation. Call several concrete contractors for bids, take the bid you like and give the Ok for the job. In a day or two the contractor shows up, sets the forms, adds some rebar or wire mesh. Then the ready mix truck backs up and dumps a lot of liquid concrete, the contractors smoothes it out and in a few hours you have a new slab.

Good to go…right? Well maybe. It all depends. Here are some basics tips on getting a good concrete job.

The ideal concrete pouring day would be about 50 degrees and cloudy with high humidity. In my estimation that day would happen at concrete pouring time about once every ten years. Otherwise it is either to hot, to cold, to dry…it is never just right.

The building goes on. You can’t wait for a just right day. It is up to your trusted concrete contractor to know what to do and when to do it. But it helps if you know a little something about the properties of concrete. A little extra knowledge helps in choosing a contractor and it helps to know when it is Ok to pour and when it’s not.

A basic concrete mix used for home building slabs, driveways and sidewalks should be mixed at about 6 bags of cement per 1 cubic yard of concrete. This yields design strength of 4000 pounds per square inch. This mix ratio is considered the minimum for concrete exposed to freezing temperatures.

Concrete recipe

• Cement -about 11 percent. This can vary in different building situations. More cement yields a stronger but more expensive mix

• Coarse aggregate -washed gravel or crushed stone about ¾ inch in average size. This stone can be smaller or larger depending upon the concrete application.

• Fine aggregate -clean fine sand free of organic materials

• Water -about 16 percent. To much water weakens concrete

• Air -large pockets of air can be trapped in concrete poured into deep forms, such as forms used for basement walls. This concrete should be vibrated mechanically as it is poured to force the large pockets of air to the surface. Very small pockets of air are actually a good thing, as these very small pockets help keep concrete form cracking under stress and during freeze thaw cycles. There is actually a chemical that can be added to concrete to create these small air pockets. This process is called: Air entrained concrete. You always should ask for air entrained concrete mix.

Weather conditions are not always favorable for optimum concrete curing, but most situations can be overcome with proper planning and technique.

• Hot temperatures and low humidity may cause excessive evaporation of water on the surface and may need to be covered with burlap, plastic or sprayed with a special evaporation control agent to slow the loss of water.

• Concrete should not be poured when it is raining or a threat of rain. If it starts to rain to concrete should be covered, excessive water will wash the surface cement away and leave a rough weak surface.

• Concrete should not be poured onto frozen ground or left uncovered in freezing temperatures. Concrete can be poured in cold weather with proper technique and planning.

Pouring concrete must be a coordinated operation. The needed amount (cubic yards) of mix should be ordered before hand and delivered as needed. If more than one truckload is needed the second truck should be waiting before the first is emptied. Concrete begins to set or cure as soon as it stops being stirred. If you let it set and then pour in a new batch an undesirable weak cold joint will be produced. A cold joint is especially bad in a basement wall.

Search the net for concrete calculator to help you figure the needed volume of ready-mix.

Concrete should not be dumped from the truck any faster than it can be spread and worked into place. Each batch should be placed against the last and worked into each other ether by vibrating or manually stirring.

Working and finishing slabs begins with screeding or striking off, this is accomplished with a long straight edge. This operation begins the leveling and contouring process. Bullfoating using a large wide float with a long handle removes the high and low places and works the larger aggregates away from the immediate surface. Very large areas may floated with a machine.

Wait until the poured surface water is all absorbed before starting to trowel finish the surface. Dip the trowel into water to clean it and add a slight about of water to the concrete surface.

Slab jointing is necessary to control cracks that always occur. Jointing creates a weak place in the concrete so the crack, when it occurs, will happened in a controlled non-random manner. Jointing can be done before the concrete sets or sawn with a concrete saw after it sets. It is a fact of life concrete will crack, so don’t be alarmed when it happens.

Concrete can be final finished with a variety of actual surface finishes. Slabs for floors should be troweled to a smooth finish. Whereas driveways and sidewalks should be given a broom finish to produce a slip resistant surface. Steel towel finished concrete can be very slick when wet.

Special Note Loaded concrete ready-mix trucks are very heavy and will sink into seemingly solid ground and will crack a normal driveway into smithereens. If you are having the truck back into your yard be very careful. If the truck sinks a bulldozer may be needed to pull it out.