Some Factors Influencing Shrinkage of Concrete Pavements
F. N . Hveem and Bailey Tremper
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Portland cement concrete pavements tend to become rougher with time. To a degree the development of roughness is initiated by curling of the ends of the slabs. In California, at least, slabs curl upward more than downward because shrinkage due to drying is more pronounced in the upper part of the slab. When curled upward, the slabs are not supported uniformly by the subgrade and the ends deflect more under load. Under heavy traffic, cracks and faulting develop and the pavement becomes progressively rougher. The integrity and smoothness of the pavement can be prolonged if the character-istic shrinkage of concrete can be reduced. Both portland cement and aggregates affect the amount of shrinkage. Some of the factors contributing to the influence of portland cement are known, although not adequately restricted in standard specifications. The contribution of clay in aggregates to shrinkage has not received the attention it warrants and national specifications do not guard adequately against excessive amounts of clay. The California Division of Highways has developed simple, short field tests that evaluate both the quantity and activity of clay contained in aggregates. These are known as the "sand equivalent" and "sedimentation" tests. Data are presented to show that a high degree of correlation exists between the results of these tests and the drying shrinkage of mortar and concrete. With suitable specification limits, these tests are effective in securing important reductions in drying shrinkage.