Concrete is perhaps the most widely used construction material in the world today — there's an entire trade show dedicated to it. From floors to tunnels to skyscrapers, concrete is a necessity in getting the job done. Most contractors find themselves pouring concrete on a typical job. And most rental companies offer concrete working equipment for rent. But along with the expanding role of this popular material comes many questions about its practical use. The following are the most commonly asked questions concerning the use of concrete on today's jobsite.
Q: Will consolidation really improve my concrete placement and finishing and why?
A: 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.
Q: What's the correct rate of speed to remove the vibrator from the concrete?
A: 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.
Q: What condition should the soil or subbase be in and why?
A: The subbase 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.
Q: How should I avoid the problems caused by extreme temperatures such as cold or hot weather?
A: 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.
Q: How do I know what is the best mix to order?
A: Most ready mix companies have the expertise to provide the correct mix. Describe the application to your supplier, and they will be able to provide the best mix for the job.
Q: When I remove wall forms, the concrete often has air pockets and a spotty appearance. How do I avoid this problem?
A: Use a vibrator to consolidate the concrete and remove air. This will improve the appearance of the wall and add strength and durability as well. This problem can also occur from too much release on the form. A thick coating can trap air bubbles.
Q: When is the right time to begin power floating with a trowel and how do I get started?
A: A rule of thumb for starting your first pass with a power trowel is when you leave only a 1/8-inch deep footprint on the surface. Many variables affect this decision including the size and weight of the machine, the rate the concrete is drying, the speed and efficiency of the operator and the specifications of the particular slab.
The first pass should be with float blades, combo blades or float disks. The blades should almost be flat with a slight pitch. Consecutive passes should be made perpendicular to the previous one. This ensures a flatter floor. As the floor sets, the pitch of the blades should increase.
Q: What's better, a float pan or float blades?
A: Float pans can achieve a flatter floor than float blades. The drawback is that pans can be difficult to control, especially on a walk-behind machine. The increased friction of the pans can fatigue an operator quickly.
Q: Can you explain the different blade styles; float, finish, combination and the advantages and disadvantages of each?
A: 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.
Q: Will high-speed power trowels improve my finishing work?
A: 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.
Q: How can I achieve really flat floors?
A: 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.
Q: I've noticed some random cracking on the slabs I've finished. What am I doing wrong?
A: Random cracking can be caused by many things such as:
- Subbase 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
Q: Can I spray a little water on the slab to smooth the surface during finishing?
A: 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.
Q: Can I use a power trowel on an exterior slab?
A: 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.
