Rodney T. Davis, Formerly with Virginia Transportation Research Council
Virginia has recently built several bridges with lightweight concrete beams and decks including the Route 33 bridge over the Mattaponi River. The measurement programs for these structures indicated that the high creep and shrinkage values assumed in the designs were not accurate for the concretes in the precast, prestressed concrete beams. In fact, the creep coefficients and shrinkage strains measured in the lightweight concretes were the same as, or only slightly higher than, those measured in typical normal weight concrete superstructures.
The most important differences between the properties of the lightweight and normal weight concrete used in the precast, prestressed beams were the reduction in modulus of elasticity and tensile strength of the lightweight concrete. The modulus of elasticity of the lightweight concrete used in Virginia’s beams was about 60 percent of that for normal weight concrete. Typical lightweight concrete had a modulus at prestress transfer of about 3100 ksi (21 GPa), and a modulus in service of about 3400 ksi (23 GPa). Tensile strength of the lightweight concrete was about 7/8 that of the equivalent beam or deck normal weight concrete.
Typical mix designs for beam and deck concretes used in modern Virginia bridges are shown in the table.
Typical Normal Weight and Lightweight Concrete Mix Proportions
The above table contains the concrete mix proportions for both normal weight and lightweight concretes used in precast, prestressed concrete beams and cast-in-place concrete decks.
The desired densities and strengths of the lightweight beam and deck concretes were achieved by doing little more than swapping lightweight aggregates for the normal weight coarse aggregates. Also, lightweight aggregate allowed the use of a lower water-cementitious materials (w/cm) ratio in deck concretes without plastic shrinkage cracking or other early age cracking. Ratios of 0.40 to 0.45 are used in Virginia deck mixes, with cementitious materials content in the range of 585 to 650 lb/cu yd (347 to 386 kg/cu m).
The creep of beam concretes under the application of prestressing force plus self-weight was found to be greatly influenced by the curing method, and not the presence of the lightweight aggregate. Virginia beam concretes contain slag cement in which the reactivity is dependent on temperature. Higher curing temperatures, such as a concrete temperature above 160°F (71°C) produced a creep coefficient of 0.25 to 0.50, with all the creep strain occurring by a concrete age of 7 days. Lower curing temperatures such as a concrete temperature of 135°F (157°C) produced a creep coefficient of 1.0 to 1.2, with all the creep strain occurring by a concrete age of 60 days. Lightweight prestressed concrete beams can be cured at the higher temperature to limit camber. Measured creep in all the precast beam concretes after deck placement has been found to be minimal.
Measured shrinkage was slightly higher for the lightweight beam concretes. Normal weight beam concretes shrunk about 350 to 400 millionths over two years, with about 250 millionths of that occurring before the beams are removed from the formwork. Lightweight beam concretes shrunk about 400 to 450 millionths. Unrestrained deck concrete specimens, both lightweight and normal weight, were found to shrink in excess of 450 millionths in Virginia weather. But measured beam strains indicate that little of this shrinkage strain is actually occurring in the bridge decks.
Many bridge decks, and especially the lightweight concrete decks, exhibit very little or no cracking after several years of service. This indicates that a good deck mix design, expert concreting, and proper moist curing can produce a deck that is capable of high creep or relaxation at early ages. Virginia’s experience with lightweight concrete to date has given us confidence to design beams and spliced girders using lightweight concrete, and to expect higher quality decks when using lightweight aggregates.