How do you expand an existing two-lane bridge into a three-lane bridge? To widen the Interstate 84 crossing over the Hudson River from 30 feet to 39 feet, the New York State Department of Transportation used structural lightweight concrete. Because the concrete used in the new deck weighed 35 pounds per cubic foot less than the concrete that was removed from the old deck, less than 20 percent of the steel support framing needed strengthening. And this is not unusual. Replacing an old bridge deck made of normal weight concrete with a new deck made of lightweight concrete will often permit the deck to be widened with minimal modifications to the substructure.
The most important consideration in proportioning any type of concrete for bridge decking should be durability. Assurance of other properties must always come second. This is as true for lightweight concrete as it is for normal weight concrete. However, because the lower modulus of elasticity of lightweight aggregate is closer to the modulus of elasticity of concrete mortar, lightweight concrete may be more durable that normal weight concrete. Even so, lightweight concrete must be air entrained to provide resistance to freezing and thawing and to salt scaling. High air contents can reduce concrete strength, but the air content levels recommended here for lightweight concrete will not significantly reduce strength if a sufficient amount of cement is used and slump is kept less than 4 inches. Dry lightweight aggregate will naturally absorb more water than wet lightweight aggregate. To minimize the slump loss caused by absorption, the moisture content of the aggregate should be at least 75 percent of the total moisture content that the aggregate can absorb during 24 hours of presoaking. Moisture content and unit weight of the aggregate can be checked by following standard test methods described in ASTM C 566 and ASTM C 29, respectively.