How to Make a Large Concrete Slab

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Concrete provides strength and structural integrity when formed and poured correctly. But as the area of the slab increases, so does the necessity for additional workers and/or power equipment in order to manipulate the semi-solid concrete before it hardens. In addition, weather is a consideration when pouring concrete. As the temperature rises and humidity decreases, the concrete hardens more quickly. There are some important guidelines to observe if you're planning to pour a large slab.

Things You'll Need

  • Concrete Dimensional lumber Carpentry hand tools Laser level Rebar Rebar chairs Power screed Bull floats (or a power float) Power trowel Shovels (one for each worker) Concrete pulls (one for each worker) 4 or 5 assistants
  • Prepare the area of the pour by leveling the ground and using dimensional lumber to build the form that will contain the wet concrete. A working knowledge of carpentry skills and concrete formation is imperative when pouring any large slab.

  • Level the top of the dimensional lumber that will hold the concrete so your slab will have a gentle slope. If you're pouring a slab for indoor use, this is not necessary. An outdoor slab must have a grade of not less than ¼-inch per every 4 linear feet. The grade should slope away from foundations or buildings. For instance, if your slab is 32 feet wide, one side should be at least 2 inches lower than the other side for adequate drainage.

  • Obtain power concrete equipment for a large pour or opt to divide the pour into smaller segments. Every part of a concrete pour becomes more difficult as you add square footage. It takes two workers to screed a slab 10-foot wide, with a manual screed. But when the width of the slab reaches 20 feet, it's nearly impossible because the additional span results in immense resistance from the heavy wet concrete. Power concrete equipment is available from construction rental stores.

  • Line the bottom of the pour with a minimum of 1-inch of sand and place a single layer of rebar, spaced on 2-foot centers in the floor of the pour. Elevate the rebar with rebar chairs to ensure that the steel reinforcement will rest in the middle of concrete slab.

  • Position your workers before the concrete truck arrives. Since you're pouring a large slab, every worker must be ready to perform his job quickly. One worker should direct the truck driver while another worker controls the concrete chute. The rest of the workers will level the concrete with shovels and pulls and run the power screed.

  • Begin at one end of the slab and have the truck driver dump the concrete while the worker operating the chute moves it back and forth to spread the concrete evenly. Immediately, the other workers will begin leveling the wet concrete as the truck and the chute operator move forward.

  • Start the power screed as soon as the workers progress approximately 10 feet. One worker will operate the screed, moving forward slowly, following the other workers down the slab. The power screed spans the width of the pour and it vibrates the surface of the concrete, encouraging the larger particles to settle downwards and water to rise to the top.

  • Continue with this method for the entire distance of the slab. Once the concrete begins to thicken, it's time to float the surface. A bull float with an extension is expandable to approximately 12 feet. Two workers can float the surface from opposite sides unless the slab is larger than 24 feet, in which case, a power float is necessary. Renting a power float will add substantially to the cost of your project.

  • Trowel the edges of the slab as soon as possible after floating begins. Hand trowels are sufficient here since the worker can make his way around the edge of the form. Fine-tune the surface of the large slab with a power trowel as soon as you can place a large sheet of plywood on the top without sinking. Standing upon plywood spreads your weight over a greater surface area, allowing you to begin troweling sooner.

Tips & Warnings

  • Make sure your workers know their jobs. Once you begin, the pour must be continuous until the end of the slab and you have only a small window of opportunity before the concrete hardens. Consider sectioning the slab into smaller pours if you are short on help or knowledge. Most large slab pours require concrete professionals with power equipment and trained concrete workers.
  • Hot temperatures make concrete set quickly. The truck driver can add water to the concrete to lengthen the setting time but as you add water, you reduce the integrity of the finished product. Make sure all workers wear protective footwear, gloves and eye wear when pouring concrete. Wet concrete contains chemicals that will leach the moisture from your skin and injure your skin and eyes.

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  • Photo Credit Photo, curtesy of Wikimedia Commons
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