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Concrete Section - Concrete Information

Do It Yourself Cement Finishing Tips:

  • Always order a little more to make up for spillage and uneven subgrade. This will save money if you have to pay for another truck to return and have workers on payroll standing around waiting.
  • Have plenty of help around in case you have a problem and have to get alot of work done in a short period of time as concrete doesn't wait forever.
  • Order your ready mix concrete as soon as possible as you will probably be put on a schedule that is not your own, and first come, first served.
  • Always check with your MUNICIPALITY as to what you have planned. It is cheaper than leaving it out and having to starting over.
  • Make sure your site is well prepared, from truck delivery to final wash out of the equipment. Remember trucks are heavy and can cause damage and must be washed off before they leave the site. A tow bill can be expensive for you.
  • Always talk to a representative of your READY-MIX SUPPLIER to get tips on your specific job and properly specified concrete. It is better to do it right the first time. And cheaper also.
  • Some jobs are better left to the professionals. If you think you are in over your head, call a professional cement or concrete finisher.

Prevent Concrete Cracks:

If you want to put in a patio, driveway or sidewalk, concrete is a smart choice. Concrete typically consists of cement, rock, sand and water. In the fresh, or plastic stage, concrete is fluid. As it hardens, the cement and water begin to shrink, and the stresses created by this shrinking cannot be overcome by the small amount of strength developed by the new concrete.

If you place the concrete on a windy day, the top may start to harden before the bottom, which will cause the concrete to shrink unevenly (plastic shrinkage cracks.) Also, if the ground underneath the concrete is not level, there will be an unequal dragging force while the concrete shrinks, also causing stresses within the new concrete. How do you get concrete with no visible cracks in it? By following a few simple steps before and after you place the concrete, you will have a nice looking slab that will require very little maintenance, and increase the value of your home.

The Subgrade:

Before you place the concrete, make sure your subgrade (ground beneath the concrete) is thoroughly compacted and level. You can apply a layer of cushion sand if you want. This will help achieve a level surface and allow a consistent friction to the shrinking concrete. Four inches of washed sand ought to be plenty for the cushion.

If you use a wire mesh for reinforcement, use panels and not rolls. The rolled wire mesh is extremely difficult to keep in the top half of the concrete, where it HAS to be in order to do its job. You can also use reinforcement bars (rebar) tied together with steel wire, but spacing and size requirements vary based on load and soil conditions, so it is hard to recommend a standard set up for that. If you do use rebar, it is essential that you keep it in the top half of the concrete. You can use stones, broken brick or you can buy plastic chairs that the steel will sit on to keep it in the proper position when you place the concrete.

You can also have the ready mix concrete company supply fibers to the mix. These fibers are usually nylon or polypropylene. They help keep the cracking of the concrete on a micro level instead of a macro level (where you can see the cracks with your naked eye.) Steel reinforcement also helps keep cracking in check.

Contraction Joints:

When you've got your subgrade ready, you have placed a plastic vapor barrier on the subgrade for slabs that will support dwellings, your steel is sitting on your plastic chairs in the proper position, and you now have a few of your closest friends on their way over to help you place the concrete you have coming.

When the concrete arrives, if you don't have a vapor barrier, wet the subgrade without puddling the water so that the water in the concrete will not be absorbed by the dry subgrade, thus causing uneven drying and the dreaded plastic shrinkage cracks. Once the concrete is placed, make sure to protect it from high winds and direct sunlight so the concrete will dry evenly from top to bottom.

You are now ready to perform the most important step in preventing noticeable cracking. Contraction joints are the secret to no cracking! By placing contraction joints that are at least 1/4th the depth of the concrete and on intervals of 25 to 30 times the depth of the concrete (usually easiest with a jointing trowel or tool while the concrete is still fresh), you will almost ensure there will be no visible cracking in your concrete. If your slab is 4 inches thick, the joints must be at least 1 inch deep and placed about every 10 feet. If you cannot use a jointing tool to put the joints in, you can hire a CONCRETE SAWING CONTRACTOR to do this for you.

Make sure he cuts the joints a minimum of 1/4th the slab depth. This jointing method helps the concrete crack at the weakest point. This is why it is so important for the joints to be deep enough. Variations in subgrade levels could cause greater stress in the concrete in an area where the joint isn't deep enough, and the concrete will crack outside the joint. Once your joints are in place, and the concrete has cured for about two weeks, you are ready to seal the joints. This will prevent water from migrating into the subgrade and expanding and contracting, or getting into the joints and freezing, causing the water to expand and breaking out the concrete around the joints. You now have a concrete surface that will serve you well.

 
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