Lecture 8-Basic Charcteristics of Concrete

Lecture 8-Basic Charcteristics of Concrete - Lecture Eight...

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Lecture Eight Basic Characteristics of Concrete
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Basic makeup of concrete: Concrete is a mixture of sand, gravel, crushed rock, or other aggregates (which occupy most of the volume) held together by a hardened paste of cement and water (which is the binding agent).
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Manufacturing process for cement: 1) Limestone is crushed. 2) The crushed limestone is mixed in clay. 3) The mixture is further pulverized, either wet, or dry. 4) The resultant is burned in a kiln for 4 hours at 3000 degrees Fahrenheit. 5) Gypsum is added, and the clinker (unfinished cement product) pulverized.
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Principles of Portland Cement concrete: 1) Strength depends on the water-cement ratio in the initial mix. Water is kept to a minimum for maximum hardness. 2) Water increases workability, allowing the proportion of aggregate to cement to be increased, thereby reducing cost. 3) The ideal amount of cement is just enough to coat each aggregate particle completely. 4) Fine aggregates require more coating material, and so the total cost of the concrete increases as the size of the aggregate decreases. 5) Shrinkage due to evaporation is proportional to the amount of cement paste; the higher the proportion of cement, the more shrinkage.
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Hardening of Concrete: · Concrete becomes hard and strong through the process of hydration. · Hydration occurs when elements from the cement combine chemically with water to form molecules or crystals. As such, it is necessary that water be present in the concrete until the reaction reaches equilibrium. Approximate time periods for the different stages of hydration of modern cement: 1) Initial set (after 45 min): The concrete is ready to be troweled. 2) Final set (after about 10 hrs): The concrete is hard enough for the forms to be removed. 3) Full hydration (after about 7 days): The concrete is said to be cured. After, there is no need to insure the presence of water in the concrete. 4) Full strength (after about 28 days): Evaporation is essentially complete, and the concrete can accept full loading. Preparation process- Use of Water:
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This note was uploaded on 06/18/2009 for the course CE xxx taught by Professor Tuken during the Spring '09 term at Middle East Technical University.

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Lecture 8-Basic Charcteristics of Concrete - Lecture Eight...

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