Tempo:
For site-batched concrete and concrete for garage floors, a slump test may be used to gauge the consistency and the workability of the concrete. Slump is determined by filling a standard cone-shaped form with concrete and measuring the amount of settlement or slump that the concrete exhibits when the cone is removed.
READY-MIX CONCRETE Ready-mix concrete is available in most urban areas. Manufactured in plants to established mix designs, the quality of ready-mix concrete can be tailored to meet strength, durability, and workability requirements of a particular application. Workability is attained by the ready-mix provider meeting the water-cement ratios set out in CSA Standard A23.1-09: Concrete materials and methods of concrete construction.
ON-SITE MIXING When mixing must be done on site, the water and aggregate should be clean and free of organic material or other substances that might damage the concrete. The aggregates should also be well-graded, in other words, with the correct proportion and size of fine and course aggregates. The air-entraining admixtures must be added according to manufacturer’s recommendations— adding too much will decrease concrete strength. Air-entraining admixtures should be used only in concrete mixed in a motorized mixer. On-site mixing can use premixed bags of cement and aggregate. In such cases, follow the manufacturer’s instructions to obtain the desired strength and durability. If site-mixed concrete is to be proportioned on site, determine the correct ratios of fine and coarse aggregates, cement and
water to produce a mixture that will work readily into angles and corners without causing free water to collect on the surface. The concrete mixes in Table 2 (p. 264) are considered acceptable for site-mixed concrete if the water to cement ratio is: 0.70 for walls, columns, fireplaces, chimneys, footings, foundation walls, and grade beams and piers; 0.65 for floors other than those in garages and carports; and 0.45 for garage and carport floors and exterior steps. Aggregate used in these mixes must not be larger than one-fifth the distance between vertical forms or one-third the thickness of the flatwork.
PLACING CONCRETE Place concrete into forms continuously in horizontal lifts not exceeding 1.2 m (4 ft.) in depth. Do not allow concrete to fall into the forms from a height of more than 2.4 m (8 ft.) as this causes the concrete to segregate. For higher drops, use a pipe to deposit the concrete. Buggies, wheelbarrows, chutes or pumping can be used to move the concrete to locations 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. Do not deposit the concrete in a pile. Spread and level it by raking or shovelling. Use vibrators to consolidate the concrete but not to move it horizontally. Compact the concrete uniformly by means of tamping hand tools (puddling sticks) or, preferably, by a vibrator. If it is necessary to interrupt concrete placement, the surface of the concrete placed in the forms should be levelled. If partial setting has started by the time concrete placement is ready to resume, roughen and dampen the surface to
READY-MIX CONCRETE Ready-mix concrete is available in most urban areas. Manufactured in plants to established mix designs, the quality of ready-mix concrete can be tailored to meet strength, durability, and workability requirements of a particular application. Workability is attained by the ready-mix provider meeting the water-cement ratios set out in CSA Standard A23.1-09: Concrete materials and methods of concrete construction.
ON-SITE MIXING When mixing must be done on site, the water and aggregate should be clean and free of organic material or other substances that might damage the concrete. The aggregates should also be well-graded, in other words, with the correct proportion and size of fine and course aggregates. The air-entraining admixtures must be added according to manufacturer’s recommendations— adding too much will decrease concrete strength. Air-entraining admixtures should be used only in concrete mixed in a motorized mixer. On-site mixing can use premixed bags of cement and aggregate. In such cases, follow the manufacturer’s instructions to obtain the desired strength and durability. If site-mixed concrete is to be proportioned on site, determine the correct ratios of fine and coarse aggregates, cement and
water to produce a mixture that will work readily into angles and corners without causing free water to collect on the surface. The concrete mixes in Table 2 (p. 264) are considered acceptable for site-mixed concrete if the water to cement ratio is: 0.70 for walls, columns, fireplaces, chimneys, footings, foundation walls, and grade beams and piers; 0.65 for floors other than those in garages and carports; and 0.45 for garage and carport floors and exterior steps. Aggregate used in these mixes must not be larger than one-fifth the distance between vertical forms or one-third the thickness of the flatwork.
PLACING CONCRETE Place concrete into forms continuously in horizontal lifts not exceeding 1.2 m (4 ft.) in depth. Do not allow concrete to fall into the forms from a height of more than 2.4 m (8 ft.) as this causes the concrete to segregate. For higher drops, use a pipe to deposit the concrete. Buggies, wheelbarrows, chutes or pumping can be used to move the concrete to locations 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. Do not deposit the concrete in a pile. Spread and level it by raking or shovelling. Use vibrators to consolidate the concrete but not to move it horizontally. Compact the concrete uniformly by means of tamping hand tools (puddling sticks) or, preferably, by a vibrator. If it is necessary to interrupt concrete placement, the surface of the concrete placed in the forms should be levelled. If partial setting has started by the time concrete placement is ready to resume, roughen and dampen the surface to
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