Jerry Zemajtis, CTL Group
A major factor contributing to the cracking of concrete bridge decks is the shrinkage of the deck concrete. One test to measure the shrinkage of concrete is AASHTO T 160 (ASTM C 157)—Standard Method of Test for Length Change of Hardened Hydraulic Cement Mortar and Concrete. This article describes the test method for concrete and its implications.
AASHTO T 160 covers the determination of length changes that are produced by causes other than externally applied forces and temperature changes in hardened mortar or concrete exposed to controlled conditions of temperature and humidity. The method is commonly referred to as the “shrinkage test,” even though the measured changes may not be caused by drying shrinkage alone.
The method is useful for comparing different concrete mixtures. In such cases, the specimens must have the same dimensions. Comparing results obtained on specimens of different sizes may be difficult due to the influence of specimen size on length change. If specifications list any limits, the specimen size should also be listed.
The size of the concrete shrinkage specimens depends on the maximum aggregate size. For all aggregate passing a 2-in. (50-mm) sieve, 4×4-in. (100×100-mm) prisms are used. If all aggregate passes a 1-in. sieve (25-mm), 3×3-in. (75×75-mm) prisms are used. In both cases, the prisms are approximately 11.25 in. (285 mm) long. The test method requires an average of three prisms for each test condition.
The specimens are removed from the molds at 23.5 ± 0.5 hours after casting and are placed in lime-saturated water maintained at 73.4 ± 1.0°F (23.0 ± 0.5°C) for a minimum of 30 minutes before the initial length measurement. The initial comparator reading is taken 24.0 ± 0.5 hours after the addition of water to the mix. Then, the specimens are stored in lime-saturated water at 73.4 ± 3.0°F (23.0 ± 1.7°C) for an additional 27 days. The specimen’s age is then 28 days. At that time, a second comparator reading is taken. Thereafter, the specimens are stored either in air or in water (water storage must be specified prior to testing). Water storage requires the specimens to be immersed in lime-saturated water. Air storage requires the air in the room to be maintained at a temperature of 73.4 ± 3.0°F (23.0 ± 1.7°C) with a relative humidity of 50 ± 4%. Other storage conditions may be used as long as they are appropriately documented in the report. During storage, length change measurements are taken at 4 days and 1, 2, 4, 8, 16, 32, and 64 weeks after initial curing. Results are presented as strain verses time.
Because the initial reading is taken before the specimens are immersed in water, any length change that takes place while the specimens are in the water will be included in the reported strains. For this reason, a modified method is sometimes used, in which the length change is calculated from the reading taken at 28 days.
The length change measured on the prisms under constant environmental conditions does not equal the shrinkage that occurs in a bridge deck. Other factors such as deck thickness, internal restraint from reinforcement, external restraint from beams and diaphragms, variable environmental conditions, and deck curing conditions affect the shrinkage of a real deck. The test, however, does provide a means to compare the unrestrained drying shrinkage of different concretes.
EDITOR’S NOTE
This article is the seventh in a series that describes tests for use with HPC. Previous articles appeared in Issue Nos. 36, 37, 39, 40, 42, and 45.
ERRATA
HPC Bridge Views No. 42, Page 2: The Rigolets Pass Bridge redesign used BT-78 girders spaced at 12.6 ft (3.83 m).