Concrete Equipment

While this is a common practice, it is not recommended because while water promotes cream, it will weaken the surface of the slab and promote cracking or flaking.

Power trowels are not used for exterior slabs because a different type of concrete is generally used. Because of the drastic changes in temperature an exterior slab must withstand, air is entrained into the concrete. When a power trowel is used to finish air-entrained concrete, flaking generally occurs.

The float blade is designed to "lay down" the concrete, that is, to do the first passes while the concrete is still wet. It has the most surface area of all the blades, and is used at a relatively flat pitch and a slow rate of speed. The float blade is ideal for smoothing and evening out the surface of the slab. This blade also works well to break open the surface, allowing bleed water to evaporate. It does not perform well in the later stages of the finishing process.

The finishing blade is designed to "burn" or finish the slab. It has less surface area than the float blade and is used at a higher rate of speed, when the concrete has set up. The finish blade is ideal for sealing and burning the slab in the final passes. It does not work well on wet concrete.

The combination blade is a cross between the float and finish blades. It is not as large as a float, yet not as small as a finish blade. This blade is designed to do both jobs. It can lay down the wet concrete and then finish the slab. The combination blade does not perform as well as the other two blades at their respective tasks, but does a good enough job at each to meet many jobsite requirements. The combo blade can save a contractor time and money by allowing more time to be spent on the slab and less time changing blades.

A major contributor to flatness is the initial placement and screeding of the concrete. By using a good power screed and then bull-floating and bump-cutting properly, a much flatter floor can be achieved. Laser guided systems can also be used to achieve a super-flat floor. F-specs should be used in measuring the overall flatness and levelness of the slab.

When mixing something like concrete, start with about 2 1/2 gallons of water and 2 bags (80 lbs/each) of pre-mix concrete. Get the mixer in motion and then put in about 2 gallons of water before adding 1 bag of pre-mix concrete. Progressively add the 2nd bag of pre-mix concrete to the mixer and lastly continue to add the 1/2 gallon of water to the ingredients until you have your desired thickness.

Concrete can be chemically altered at the batch plant to affect the rate of setting. On a hot day, a retardant can be added to slow the setting process. During cold weather, concrete blankets may be used to trap heat near the concrete to keep it from freezing.

Random cracking can be caused by many things such as:

- Sub-base is not compacted properly

- Slab is too thin to support load

- No relief cuts or felt in slab

- Poor quality concrete

- Concrete is not properly reinforced with mesh or rebar

Although a manufacturer will say the total volume of their cement mixer or drum capacity is 9 cu. ft., it's more like 6 cubic feet. Cement mixers mix at an angle meaning the drum is not entirely used at once leaving only 2/3 of the cement mixer for actual mixing.

The sub-base should be well compacted to ensure no further settlement will occur. The soil should be well graded so the concrete is evenly placed and a minimum of concrete is needed. Moisture is also an important factor to consider. If the soil is too dry, it will draw the water out of the concrete, causing it to crack. If the soil is too wet, the concrete may not dry evenly or at a quick enough rate. Over-wet soil may also settle after the concrete has cured.

The correct rate is about one inch per second. This gives the air time to rise to the surface. If the vibrator is removed any faster, it will outrun the air bubbles leaving them in the concrete and weakening the mix.

Yes. Consolidation or vibration of concrete assists in both placement and finishing. Vibrating the mixture allows it to become more fluid and easier to place. The finish is improved by consolidation because of the improved uniformity of the aggregate and the surface cream created by the escaping air bubbles.

A high speed/performance power trowel such as a variable drive machine can improve the quality of slab work. It will give you a harder, more resilient surface that is also easier to maintain.