360R-06 Design of Slabs-on-Ground
—Compressive strength and shrinkage
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- 13.6—Compressive strength and abrasion resistance
| 13.5—Compressive strength and shrinkage
In the competitive concrete supply market, increases of 1-day, 3-day, and 28-day compressive strengths are often obtained at the expense of increased shrinkage. More cement and more water per cubic yard (cubic meter) (not necessarily a higher w/c), a higher shrinkage cement, or a water reducer that increases shrinkage are the typical means for increasing compressive strength. The main reason for controlling compressive strength (and therefore modulus of rupture) is to ensure that the unreinforced slab thickness is sufficient to transmit loads to the subgrade. A 60-day, 90-day, or longer strength, rather than a 28-day strength, might be considered for designing slab thickness. This would assume that the design loads would not be applied during the first 60 or 90 days. Instead of using a high design strength to minimize slab thickness, designers might consider other alternatives, such as adding conventional reinforcement or post-tensioning. For another example, quadrupling the slab contact area of equiva- lently stiff base plates beneath post loads (8 x 8 in. [200 x 200 mm] plates instead of 4 x 4 in. [100 x 100 mm] plates) could decrease the required slab thickness by more than 1 in. (25 mm). 13.6—Compressive strength and abrasion resistance Abrasion resistance is a function of the w/cm (and compressive strength) at the top surface of the concrete. The cylinders or cubes tested to measure compressive strength are not a measure of surface abrasion resistance. The upper parts of slabs have a higher water content than the lower portion because of the gravity effect on concrete material before set takes place. Pawlowski et al. (1975) report that compressive strengths are always higher in the lower half of floors, and shrinkage is always higher in the upper half. The finishing process, primarily the type and quality of the troweling operation, significantly affects the abrasion resistance at the top surface. When concrete cannot resist the expected abrasive action, special metallic or mineral aggregate shake-on hardeners may be used to improve surface abrasion of floors placed in a single lift. A separate floor topping with low w/cm can be used to improve abrasion resistance. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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