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Objective 18.02

Objective 18.02. Explain the procedure for mixing concrete. Mixing Concrete. Quality is determined by: Slump: the consistency or wetness of the mixture Stronger concrete requires less slump If to much slump add more aggregates such as sand or gravel. Mixing Concrete. Quality (Continued)

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Objective 18.02

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  1. Objective 18.02 • Explain the procedure for mixing concrete

  2. Mixing Concrete • Quality is determined by: • Slump: the consistency or wetness of the mixture • Stronger concrete requires less slump • If to much slump add more aggregates such as sand or gravel

  3. Mixing Concrete • Quality (Continued) • Uniformity: thoroughly mixing all ingredients so that the “glue” which is wet cement will be around all sand and gravel particles (aggregates) • Workability: the ease with which concrete can be placed and shaped

  4. Mixing Concrete • Quality (Continued) • Ratio is expressed as a three-digit number • 1-2-3 • 1= parts of cement • 2 = parts of fine aggregates (sand) • 3 = large aggregates (gravel) • Parts maybe shovel full, pounds, cubic foot • Less sand, gravel and cement are used for the same amount of water to make concrete stronger

  5. Mixing Concrete • Hand mixing • May be done in a wheelbarrow or mixing box using a shovel or hoe • Add sand and then cement and mix thoroughly • Then add and mix gravel • Add water last and mix

  6. Mixing Concrete • Machine mixings uses a concrete or mortar mixer • First add about 10% of water needed so that ingredients won’t stick to mixer • Add gravel • Add cement • Add sand • Add remainder of water – adding too much water will weaken concrete • Mix about two minutes

  7. The Reaction… • Cement + water starts a chemical process called hydration • Causes mixture to harden • Concrete does not “dry”!!!!! • It cures through the hydration process • Freshly poured concrete should be kept damp and cool to ensure that this process goes slowly and completely • Usually reaches maximum strength in 28 days • Some say 1 year • Some say Hoover Dam is still curing.

  8. Cracking • It is concrete’s nature to crack because ground is moving below • Need to put in break joints to direct the cracks • 4-10 feet apart • Expansion joints: use soft material should be ever 10 to 20 feet

  9. Clean Up • Do not dump excess concrete mix on the site • Will change soil properties • Will lower soil pH • Will change soil texture • Buried concrete can prevent plant growth

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