Concrete Work

Concrete, both plain and reinforced, is used for a variety of purposes in houses, such as concrete foundations, and basement and garage slabs-on-ground.

READY-MIX CONCRETE

Ready-mix concrete is available in most locations. When ordering ready-mix concrete for footings, interior slabs, and foundation walls, it is important to specify a minimum strength of 2200 psi (15 MPa). For garage and carport floors, exterior steps and driveways, a minimum of 3600 psi (25 MPa) air-entrained concrete must be specified. Air entrainment for these applications must be between 5 to 8 percent. Air-entrainment will produce a concrete that contains a system of minute air bubbles that make the concrete more workable and more easily placed than plain concrete. Most important, when cured, air-entrained concrete is many times more resistant to damage from frost action. It is a must for all exterior concrete work and is recommended for other applications to improve workability and durability. In areas where soils are sulphate reactive, admixtures are recommended to protect the concrete.

ON-SITE MIXING

Avoid adding water to concrete at the construction site to facilitate its placement. Additional water will lower strength, increase permeability and decrease freeze-thaw resistance. If more workability is required, the concrete supplier should be asked to adjust the mix, because the concrete may need a plasticizer to improve workability and placement.

When mixing must be done on site, water and aggregate should be clean and free of organic material or other substances which might damage the concrete. The aggregates should also be well-graded.

The air-entraining admixture should be added strictly according to the manufacturer's recommendations, because too much admixture will decrease the strength of the concrete. Contact the manufacturer's representative, if possible, for advice about the proper proportion for a specific use. Air-entraining admixtures should be used only if the concrete is mixed in a motorized mixer.

For concrete in footings and foundation walls, not more than 4.4 imp. gal. (20 L) of water should be used for each 88 lb. (40 kg) of cement. For other concrete work, not more than 4 imp. gal. (18 L) of water should be added to each 88 lb. (40 kg) sack of cement. These amounts are based on average moisture content in the aggregate.

The proportions of fine and coarse aggregates, cement and water should be properly adjusted to produce a mixture that will work readily into angles and corners without allowing the material to segregate or free water to collect on the surface. The concrete mixes shown in Table 2 are generally considered acceptable. Aggregate used In these mixes must not be larger than one-fifth the distance between vertical forms or one-third the thickness of the flat-work. The slump for mixes from Table 2 must not exceed 6 in. (150 mm) for footings and foundation walls, and 4 In. (100 mm) for slabs-on-ground.

PLACING CONCRETE

Whenever possible, concrete should be placed into the forms continuously in horizontal lifts not exceeding 12 to 18 in. (300 to 450 mm) in depth. Concrete should not be allowed to fall into the forms from a height of more than 5 ft. (1.5 m) as this causes the concrete to segregate. For higher drops, the concrete should be deposited through a suitable vertical pipe. Buggies, wheelbarrows or chutes may be used to move the concrete if all points in the forms are not accessible to ready-mix trucks. The chutes should be metal or metal-lined with round bottoms and sloped with a rise-to-run inclination between 1:2 and 1:3.

The concrete should not be deposited in a pile but should be spread out and leveled by raking or shoveling. Vibrators may be used to consolidate the concrete, but should not be used to assist placement. Concrete can also be placed by pumping, if proper equipment is available.

If it is necessary to interrupt the placing operations, the surface of the concrete placed in the forms should be leveled off and the concrete allowed to partially set. The surface should then be roughened to provide a good bonding surface for the next lift. When work resumes, the surface should be cleaned and slightly dampened prior to placing the concrete. Bonding agents or grout of 1 part of cement to 2 parts sand should be spread about 1/2 in. (12 mm) thick over the roughened surface to provide a good joint between the two lifts. The new lift should be placed immediately after the placement of the grout.

When being placed, the concrete should be uniformly compacted by means of tamping hand tools (puddling sticks) or, preferably, by a vibrator.

When the air temperature is at or below 4i°F (5°C) or when there is a possibility of it falling to that level within 24 hours, concrete operations should, if possible, be suspended. When placing concrete, ensure that its temperature is maintained between 50°F (io°C) and 77°F (25T) while being mixed and placed. It must be maintained at a temperature of not less than 50°F (io°C) for a minimum of 72 hours while curing. To do this, the water to be mixed into the concrete may have to be heated. The concrete should not be placed against frozen soil, and any ice and snow should be removed from the formwork.

CURINC CONCRETE

Curing involves keeping freshly-set concrete moist or preventing it from drying out and shrinking for several days after placing. The cracking of concrete walls and floors can often result from Improper attention to curing. Proper procedures for curing must be followed to ensure concrete is able to achieve its potential strength, water tightness and durability. To aid in the curing process, wall forms should be left in place as long as practical, at least for three days.

The curing of walls should be carried out after the forms are removed for at least another day if the temperature of concrete is kept above 7o°F (21T), and for another three days if the temperature of concrete is kept between so°F (1 o°C) and 70°F (21X).

A good method of curing is to place a soil-soaker hose around the top of the wall allowing water to run down the wall. When water curing cannot be carried out (for example, in cold weather), spray-on curing compounds that inhibit evaporation may be used. If a dampproofing compound is applied to the wall, no further curing of that face is required.

In hot weather, concrete should be protected from rapid drying. During hot dry weather, wood forms should be sprinkled with water while they are in place in order to prevent excessive drying out.

In freezing weather, freshly placed concrete may be protected with a thick layer of straw or other insulating material. In addition, it may be necessary for the concrete to be protected by an enclosure and the space heated with fuel-burning heaters to ensure appropriate temperatures during the curing period.

Concrete slabs-on-ground can be cured by use of water sprays, by covering with burlap kept continuously moist, or by covering with polyethylene sheeting or other means to prevent moisture loss. Unless curing is carried on for about a week after placing the concrete, the exposed surface of the slab may show unsightly, cracking or be otherwise weakened.

Allowing concrete to cure properly is an important step in the construction process. Attention to this step helps to avoid costly problems.

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