Working with Concrete

This story shows you how to work with concrete.

Concrete is a mixture of sand, coarse aggregate, portland cement, and water. Here are some things to consider if you are mixing your own concrete rather than using premix or ready-mix.

The sand used in concrete should be sharp sand, not builder's sand, which is used in mortar. It should also be clean--free of vegetation and dirt, which will weaken the mix.

The coarse aggregate--gravel or crushed stone--must also be clean and no larger than one-quarter the thickness of the pour. Most residential projects call for concrete with medium-size, 3/4-inch aggregate. The water you mix with the concrete should be clean enough to drink.

Portland cement is composed of clay, lime, and other ingredients that have been heated in a kiln and ground into a fine powder. Choose Type 1 cement.

Additives
Concrete additives modify the concrete in specific ways. They can accelerate or retard the concrete's setup time, prevent deterioration of a surface due to freezing and thawing, or make the concrete mix workable in extreme heat or cold.

Before mixing or ordering concrete, ask your supplier for advice on what additives to use to adjust the concrete to the specific conditions in which you're pouring it.

Reinforcements
Concrete has great compressive strength but little tensile strength--it doesn't stretch or bend well, so it cracks easily. Steel reinforcing bars, rebar, or wire mesh are used to add tensile strength. Rebar comes in 20-foot lengths; 1/2-inch bar is adequate for most home projects. If your design has curves, you can bend rebar by hand. Have a helper stand on a board placed over the bar. Slip a pipe over one end of the bar to where you want the bend, then lift up on the pipe. There's also a tool designed to cut and bend rebar. Rent one if you have a lot of rebar to install.

 
Reinforcing a Concrete Slab

After you have installed the forms and braced them, lay reinforcing wire mesh in the excavation, leaving about 1 inch between the forms and the ends of the wire mesh. Support the mesh on dobies or wire bolsters (made for this purpose) or small chunks of concrete, tying the supports to the corners of the mesh with soft wire. The dobies or bolsters should approximately center the mesh in the thickness of the slab.

Coat the Forms

Paint the forms with a commercial release agent or vegetable oil. This will make the forms easier to remove after the concrete has cured.

Mixing Concrete in a Wheelbarrow: Step 1

If you're mixing concrete in a wheelbarrow, get a large one--a lot of concrete will spill from a small wheelbarrow as you mix it. Pour the contents of a premix bag into the wheelbarrow (never mix a partial bag) or measure in the dry ingredients with a shovel. Mix the dry mix together with a hoe, then mound it into the center of the wheelbarrow and make a depression in it.

Mixing Concrete in a Wheelbarrow: Step 2

Add about one-half of the total water into the depression and work the mix into the water with a hoe. Then work the mix back and forth the length of the wheelbarrow with the hoe, scraping up dry material from the bottom. Add water as necessary, working it into the mix before adding more. When the concrete clings to a trowel turned on edge, it's ready.

Build a Ramp

Mix the concrete near the work site and protect the lawn with a 2x ramp. Start dumping the concrete in the corner farthest from the mixing site. Place each subsequent load against the preceding one so fresh concrete mixes with what's already there.

Power Mixing

If using premix, empty full bags into the mixer. Partial bags often result in an improper mix. If using dry mix, measure the right proportions of cement, sand, and aggregate into the mixer in shovelfuls. Turn the mixer on to mix the dry materials thoroughly. Then add about one-half the prescribed water and mix thoroughly. Continue mixing, adding water a little at a time, until the mix clings to the side of a shovel turned on edge.

Floating the Surface

Floating pushes the aggregates below the surface of the concrete. If you can reach the entire surface from outside the area, use a darby. Hold the darby flat as you move it across the surface in wide arcs. Then tilt it slightly and work in straight pulls. For large jobs, use a bull float. While one or more people float a section, the finisher can work right behind them, rounding the edges with an edger and cutting control joints, as needed.

Cutting Control Joints

To cut straight control joints, tack a straight 2x4 guide to the forms at both ends. Cut control joints every 8 feet by sliding the side of a jointer along the guide. Tip up the leading edge of the tool slightly as you move it back and forth. Make the depth of the control joints about 20 percent of the thickness of the concrete (for example, 3/4 inch on a 4-inch slab).

Curing Concrete

Concrete goes through two hardening processes. The first is when chemical reactions in the mix cause it to set up. The concrete hardens, but not completely. Concrete sets up within a short time after the pour. Curing, the final hardening, takes longer, at least a week for a patio slab. During this time, you must mist the surface periodically with a lawn sprinkler or cover it with plastic or straw to keep it from drying too quickly. Premature drying causes cracking and other weakness.

 

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