Finish a Large Span of Concrete
Finishing large areas and wide spans of concrete takes different techniques than smaller slabs, but if with the right preparation, it can be done.
Steps
- to fit with a handsaw.]]Set your forms. These need to be set to the correct grade, braced securely, and aligned correctly.
- Determine the width you are able to place and screed off in a single pass. Normally, the straight-edge for "screeding", or levelling off the plastic concrete, should be less than {{safesubst:#invoke:convert|convert}}, unless you have lots of people to "pull" the excess concrete from in front as you progress down the length of the placement. {{safesubst:#invoke:convert|convert}} is a more reasonable width for 2 persons working the screed board, and one person raking the higher concrete in front of the screed.
- Set a "floating form" or a "pipe screed" along the length of the pour. This is a guide to keep the screed at the correct height so that the concrete finishes off flat. It can be a 2x2 board, or a {{safesubst:#invoke:convert|convert}} metal pipe set on top of stakes, that can be removed from the concrete after the first section is placed. With 2X2 lumber, simply cut 2X2 stakes long enough to be driven snugly into the soil beneath the form. The pipe screed can be screwed to small metal or wood stakes in a similar fashion, using a string line stretched tightly across the outside form boards to keep it at the correct elevation.
- Leave a "lane" in your slab area to back the concrete truck so that the discharge chute will be able to swing sufficiently to place the concrete near the screed board as you work down the form.
- Place {{safesubst:#invoke:convert|convert}} plywood or 2x12 lumber along the path spaced for the concrete truck wheels to travel on if the subgrade is not packed sufficiently to support this heavy load without creating ruts, or damaging reinforcing bars or wire.
- Lay out any control joints along the outside forms and the intermediate floating forms. You will need to cut the joints into the slab in sections if the process of placing the concrete takes long enough so that the material begins setting up before you are finished placing your concrete. This will usually apply only to exposed concrete like Build-a-Concrete-Driveway and patios, interior slabs that are covered with carpet or other flooring may not require control joints.
- Install any reinforcing and/or moisture barrier.
- Figure the concrete. Measure-a-Room the width and length, and multiply these measurements to get the area, then multiply the area times the decimal measurement of depth in the same measuring system, either feet or meters. If using meters, this will give you the cubic volume in meters, if you use feet, divide the result by 27 and you will have the volume in cubic yards, the unit by which ready mixed concrete is sold in the USA.
- Get your help together. You will need at least 3 able persons to assist with the placement of your concrete. This includes 2 persons to pull the screed board, and one to operate the concrete shoot, placing the material in a uniform depth ahead of the screed.
- Check the Read-a-Weather-Map forecast for the day you plan your concrete placement. Inclement weather can make it almost impossible to finish the concrete correctly. Temperatures below 40 degrees (F) are generally considered too cold for this operation, and by no means should the concrete be placed if there is a possibility of the mix freezing before it cures. Rain, especially heavy downpours can wash away the cement paste leaving nothing but gravel and sand on the concrete's surface.
- Gather up the necessary tools. For a simply Make-a-Straw-Broom or sack finished driveway, you will need an edger, jointer, bullfloat with {{safesubst:#invoke:convert|convert}} of handles, magnesium or wood hand floats, a straight edge board, shovels, and 1 or 2 "come-alongs". A come-along is like a wide flat hoe with a wooden or fiberglass handle, used to rake and pull the plastic concrete ahead of the screed board to the correct height. For hard trowelled Finish-Concrete concrete, it would probably benefit you to rent a power trowel, but these are heavy and powerful machines, so use common sense where this applies.
- Order your concrete. Large "pours", or placements, may take hours and several truckloads of material, so schedule accordingly, allowing yourself time to complete the job before dark.
- Back your concrete truck down the path you have prepared, or down the outside of the forms if the width allows. Normally, a concrete truck's discharge chute will reach about {{safesubst:#invoke:convert|convert}} from the nearest wheel, so it is possible to place a {{safesubst:#invoke:convert|convert}} wide slab from the outside of the forms.
- Check the "slump" of your concrete. If you are doing a non-specification placement, get the driver to adjust the slump to about {{safesubst:#invoke:convert|convert}} by adding water. Keep in mind that the more water you add, the less structural strength the finished concrete will have, due to chemical processes in the hydration of the materials, but trying to make a large placement with inexperienced people is difficult enough with "wet" concrete.
- Begin placing your slab at the farthest, or back side. Have the driver of the truck pull forward slowly while he discharges to keep from piling up large quantities of concrete that has to be pulled down the form. The discharging concrete should not accumulate more than a few feet ahead of the screed board at any time.
- Pull the screed. It often helps to tilt the screed board back about 10 to 15 degrees as you pull it, and some finishers prefer to "saw" the board back and forth across the concrete to close up small voids in the surface.
- Rake the "high" concrete down in front of the screed board. Because the material is plastic, if you allow it to build up in front of your board, it will flow underneath it, leaving the product high behind the screed.
- Cut any joints using the screed board as a straight edge as you work past them. This is done with a concrete jointer, an inexpensive wood handled tool available from hardware stores and home improvement warehouses.
- Place and screed the concrete to the end of your slab. If you have left a path for the truck to back down through your pour, you will have to stop the truck long enough to quickly set the end form in place and stake it down.
- Back the truck back down the slab on the next section of your placement. Pull out the pipe or floating screeds. If the weather is fairly cool, or you have placed the concrete too plastic, it may sag somewhat along the edges, but it should maintain grade close enough to the edge to "wet screed", or use the concrete to hold the straight edge to grade. If will not hold grade, you will have to leave the floating screed in place until the placement is complete, then walk in the wet concrete and pull it out, filling the little trench or depression in with shovels full of concrete as you go.
- Begin placing concrete again at the "far" end of the pour. The person holding the straight edge on top of the adjacent wet concrete will need to just allow it to "float" or skim across the top of that material.
- Bull float the concrete before it begins to set. This may mean having an extra person working behind the crew who is doing the placement. This is especially true in hot, dry Know-Your-Local-Weather, or on dry, porous material which allows the concrete to begin to set much more quickly. The bull float is pushed across the surface of the wet concrete with the front edge tilted up, (by holding the handle down) and pulled back with the back edge tilted up (by holding the handle high). Tilt the float just enough going each way so that the leading, or forward to travel direction edge doesn't dig into the wet concrete. Spraying a mist of water on the concrete's surface will help ease the float across it.
- Work the edges of your slab with an edger, and clean up the joints and any "cat faces" (rough spots) with your magnesium float by working off of knee boards when the concrete will support your weight.
- Finish the slab by either dragging a wide, soft bristled push broom across it, or trowel with the power trowel.
- Cure-Concrete with plastic sheeting if the temperature range and humidity require it. If the slab is not going to be exposed to high temperatures, hot sun, or dry winds, curing is optional. Many people will argue this point, but without engineering specifications and design requirements, there is no absolute "requirement" for curing the concrete.
- Clean up your tools, find a recliner, and hope the Tylenol will help you out of bed the next day. If you are a weekend warrior type concrete finisher, you may think twice about contracting your next large pour.
Tips
- For large placements where it is impossible to get the chute across the slab, you may use a plasticizer like "Super P", which will cause the concrete to behave like a liquid without affecting the strength. This will enable you to "chute" the concrete fairly long distances, and make leveling the surface somewhat easier.
- Give yourself enough time, and find enough help before taking on this task.
- Ask about adding retarder, if placing in very hot conditions, to slow the setting time of your material. Adding an accelerator, like calcium carbonate, in cold weather may decrease the waiting time for finishing your concrete, but follow the concrete suppliers suggestions if you do use it.
- For very wide exterior slabs like Repair-a-Sinking-Stone-Patio and driveways, slope or crown your concrete to allow for water to run off. A general rule of thumb is about {{safesubst:#invoke:convert|convert}} or more per {{safesubst:#invoke:convert|convert}}.
- Order your concrete on a slump you can work with, usually {{safesubst:#invoke:convert|convert}}. Normally, the concrete company will adjust the design mix so that your required structural strength will be met at this Cement to Water ratio.
Warnings
- Concrete can cause skin irritation, especially if it gets in your boots or other places where it can act abrasively. Wear rubber boots and gloves to protect your hands and feet.
- Be cautious around concrete trucks, especially when backing them. The driver can only see you if you can see his mirrors...and if he is watching.
- Wear safety glasses to avoid getting the material in your Protect-Your-Eyes.
Things You'll Need
- Safety glasses.
- Rubber boots.
- Tools mentioned in the article, including bull float with handles, come alongs, straight edge, hand floats, edger, jointer, shovels.
