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Mixing and Pouring Concrete

Mixing and Pouring Concrete. Proper mixing has been accomplished when:. A. Every particle of sand and stone is coated with the cement paste. B. The sand and stone are evenly distributed throughout the mass. C. The whole mixture is of uniform consistency.

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Mixing and Pouring Concrete

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  1. Mixing and Pouring Concrete

  2. Proper mixing has been accomplished when: • A. Every particle of sand and stone is coated with the cement paste. • B. The sand and stone are evenly distributed throughout the mass. • C. The whole mixture is of uniform consistency. • D. The whole mixture is uniform in color, with no visible streaks of cement or sand running through it.

  3. Machine Mixing • A. Determine the capacity of the mixer and do not overload it. • B. Determine the moisture content of the sand and determine the amount of additional water needed. • C. Measure water needed for each batch mixed. • D. Mark waterline on the bucket so that the same amount of water may be measured quickly for future batches. • E. Determine the amount of cement to use for each batch. Mark the container so that cement for future batches may be measured quickly. • F. Start the mixer. • G. Put the measured amount of water into mixers. • H. Put part of the gravel into the mixer. • I. Put the measured amount of cement into the mixer. • J. Add sand and gravel until desired consistency is obtained. Do not add additional water. Additional water will decrease the strength of the concrete. • K. Run mixer for two minutes after all ingredients have been added. • L. Dump concrete out of the mixer.

  4. Hand Mixing • A. Spread out evenly on the platform the measured quantity of sand for one-batch of concrete. • B. Empty a bag of cement on the sand and distribute it evenly over the pile. • C. Turn the cement and sand over with square-pointed shovels enough times to mix the two together thoroughly, making a uniform colored mixture which does not show streaks of unmixed sand and cement running through it. At least three turnings are necessary. • D. Spread the mixture out evenly on the platform. • E. Spread the measured quantity of pebbles or coarse aggregate over the cement-sand mixture. • F. Turn the coarse aggregates, cement, and sand at least three times, until the pebbles are distributed uniformly throughout the mixture. • G. Form a hollow or depression in the center of the mixture. • H. Pour into the depression the exact quantity of water recommended per sack of cement for the desired mix. Remember that the ratio of water to cement is an important factor in determining the quality of concrete. • I. Turn the materials toward the center of the hollow with the shovels until the whole mixture is of uniform consistency, color, and plasticity or workableness.

  5. One-half sack batch • A typical small concrete mixer is the 2 sack batch size. This means that it will hold the amount of concrete that can be mixed using half a sack of cement. The following steps are suggested in making a 1/2- sack of 1:2 1/4:3 concrete mixture, six gallons of water per sack of cement. • A. Mark a water pail that is to be used as a water measure. If the sand is wet, 5 gallons of water will be used per sack of cement to obtain the 6-gallon mix, as previously indicated. For the 2 sack batch, mark the pail at the 2 2 gallon level. • B. Divide a sack of cement equally in two 5-gallon pails. Mark one pail at the 1/2-sack level. • C. Using the marked cement pail, count the number of shovels of sand required to fill the pail to 2 1/4 times the capacity to the cement line. • D. Count the number of shovels of gravel required to fill the pail to 3 times the capacity of the cement line. •  E. Start the mixer and pour 2 2 gallons of water. • F. Put in 2 or 3 shovelsful (or more) of gravel, followed by the measured amount (1/2 bag) of cement. The gravel helps

  6. One-half sack batch • F. Put in 2 or 3 shovelsful (or more) of gravel, followed by the measured amount (1/2 bag) of cement. The gravel helps prevent the cement from forming balls when water is added or from sticking to the sides of the mixer. • G. Place the counted number of shovelfuls of sand in the mixer. Then add to the remaining shovelfuls of gravel. • H. Allow the mixer to run for 1 to 2 minutes after all ingredients have been added to secure thorough mixing. • I. Observe the consistency of the mix carefully as it is dumped from the mixer to see that it is mushy and workable. • J. After this first trial batch has been made, amounts of aggregate can be changed for succeeding batches if necessary to change the consistency of the mix. The amounts of cement and water, however, should remain the same.

  7. Ready-mix • A. Industry standard for truck mixers. 1. 1. Each batch of concrete must be mixed no less than 70 and no more than 100 revolutions of the drum. • 2. Agitating speed is usually about 2 to 6 rpm. • 3. Mixing speed is generally about 8 to 12 rpm. • 4. Concrete is to be delivered and unloaded within 1 2 hours or before the drum has revolved 300 times after the introduction of water to the cement and aggregate.

  8. Ready-mix • B. Ready mix concrete is manufactured by the following three methods of mixing. • 1. Central-mixed concrete is mixed completely in a stationary mixer and is delivered either in a truck agitator, a truck mixer operating at agitating speed, or a special non agitating truck. • 2. Shrink-mixed concrete is mixed partially in a stationary mixer and the mixing is completed in a truck mixer. • 3. Truck-mixed concrete is mixed completely in a truck mixer.

  9. Ready-mix • C. Ordering ready-mix concrete. • 1. Amount of structure dimensions. • 2. Cement content (bags per cubic yard) • 3. A water-cement ratio • 4. Strength of concrete needed (PSI) • 5. Type of aggregate • 6. Size of aggregate7. Add mixtures (type and amount) • 8. Time of delivery

  10. Clean-up equipment • A. Clean all tools that come in contact with concrete. • B. Wash any mud or dirt from the frame of the mixer. • C. Rinse out the interior of the drum thoroughly. • D. Clean any hardened concrete from tools or equipment by scraping and wire brushing to remove. • E. Hammering on a mixer to loosen concrete is not advisable because this will damage the drum or the mixer. • F. Always clean the mixer as soon as possible after use. • G. A weak vinegar solution (10percent acetic acid) is sometimes used to clean the cement film off the exterior of mixers. Areas cleaned in this manner should be relaxed to protect the mixers.

  11. ORDERING READY-MIXED CONCRETE • SPECIFY: • AMOUNT OR STRUCTURE DIMENSIONS • CEMENT CONTENT (BAGS PER CUBIC YARD) • WATER CONTENT (GALLONS PER BAG), WATER-CEMENT RATIO OR STRENGTH. • TYPE OF AGGREGATE • SIZE OF AGGREGATE • SLUMP • ADD MIXTURES (TYPE AND AMOUNT) • TIME OF DELIVERY

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