360R-06 Design of Slabs-on-Ground
R-54 ACI COMMITTEE REPORT
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- 13.11—Relation between curing and curling
- 13.12—Warping stresses in relation to joint spacing
| 360R-54 ACI COMMITTEE REPORT
Provided that the subgrade is smooth with a low coefficient of friction, as detailed in Section 13.8 , then thickened edges should not be a significant linear shrinkage restraint; however, curling stresses would be increased somewhat. 13.11—Relation between curing and curling Because curling and drying shrinkage are both a function of potentially free water in the concrete at the time of concrete set, curing methods that retain water in the concrete will delay shrinkage and curling of enclosed slabs-on-ground. Child and Kapernick (1958) found that curing did not decrease curling in a study of concrete pavements where test slabs were cured 7 days under wet burlap, then ponded until loading tests for the flat (uncurled) slabs were completed. After the loading tests were completed on the flat slabs, usually within 5 to 6 weeks, the water was removed, the slabs were permitted to dry from the top, and the load tests were repeated on the curled slabs. The curl could be reduced by adding water to the surface, especially with hot water, but after the water was removed, the slabs curled again to the same vertical deflection as before the water was applied. Water curing can saturate the base and subgrade, creating a reservoir of water that may eventually transmit through the slab. This is also discussed in Chapter 3 . All curing methods have limited life spans when the concrete’s top surface is exposed to wear. Thus, curing does not have the same effect as long-term high ambient relative humidity. Extended curing only delays curling; it does not reduce curling. 13.12—Warping stresses in relation to joint spacing Several sources (Kelley 1939; Leonards and Harr 1959; Walker and Holland 1999) have shown that the warping stress increases as the slab length increases only up to a certain slab length. The slab lengths at which these warping stresses reach a maximum are referred to as critical slab lengths, and are measured diagonally corner to corner. Critical lengths, in feet (meters), are shown below for slabs 4 to 10 in. (100 to 250 mm) thick and temperature gradients T of 20, 30, and 40 °F (11, 17, and 22 °C). A modulus of subgrade reaction k of 100 lb/in. 3 (27 kPa/mm) and a modulus of elasticity E of 3 × 10 6 psi (21,000 MPa) were used in determining these values. Computer studies indicate that these lengths increase mostly with slab thickness and temperature gradient, and only slightly with changes in modulus of elasticity and modulus of subgrade reaction. Figure 13.4 shows both defor- mation and warping stress curves for three highway slabs with lengths less than, equal to, and greater than the critical slab length. Warping stress does not increase as slab length increases beyond the critical length because vertical deformation does not increase. PCA (Spears and Panarese 1983) states that there will be a marked loss of effectiveness of aggregate interlock at sawcut contraction joints if the joints are too far apart. Positive load transfer using dowels or plates should be provided where joints are expected to open more than 0.035 in. (0.9 mm) for slabs subjected to wheel traffic (refer to Section 5.2 for additional information). Slabs may be more economical if sawcut contraction joint spacing is increased beyond lengths noted previously by using distributed reinforcement designed for crack width control, but not less than 0.50% of the cross-sectional area. The lowest floor and fork lift truck maintenance cost may well be achieved with the least number and length of joints, as long as curling is not sufficient to cause cracking or joint spalling. Increased joint spacings larger than the critical slab length will not increase warping stresses. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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