Lightweight Concrete for High Strength - Expanded Shale & Clay

Chapter 7. Conclusions and Recommendations
7.1. Analytical Investigation
The use of HPLC has the potential to increase the length of simple span AASHTO Type
IV and V sections up to four percent and Bulb-Tee sections up to three percent. However,
AASHTO Type II and III sections do not benefit appreciably from the use of HPLC. The
Modified Bulb-Tee section extended the length of a Standard Bulb-Tee by 10 feet using 8, 10
and 12 ksi HPLC or high-strength NWC. Bulb-Tee (Standard or Modified) sections provided
longer spans at less weight for girders over 105 feet in length when compared to AASHTO
sections. For spans between 125 feet and 155 feet, the use of HPLC can reduce the gross vehicle
weight to less than 150 kips so that a super-load permit is not required for transport of long span
girders in Georgia.
7.2 HPLC Mixes and Properties
HPLC mixes were developed in the laboratory for 8,000 psi and 10,000 psi compressive
strength using slate lightweight ½-in coarse aggregate and normal weight, natural sand. The dry
unit weight of the concretes was approximately 117 and 119 pcf, respectively. A 12,000 psi
design strength mix could not be developed. The strengths of mixes using lightweight fine
aggregate could not be controlled; therefore no recommended mix used lightweight fines.
The laboratory mixes were verified during field production. The field mixes yielded
slightly higher strengths than found in the laboratory. Close monitoring of lightweight aggregate
moisture was required.
The HPLC, both 8,000 psi and 10,000 psi design strength continued to gain strength over
the 100-day test period. Accelerated cured cylinders, which matched curing conditions of
precast beams, showed about 25 percent higher one-day strength than ASTM cured cylinders; at
56 days, the ASTM cured cylinders were about 4% stronger than the accelerated cured cylinders.
An equation was developed and provides a better estimate of the modulus of elasticity for slate
HPLC than previous relations for normal strength concretes.
7-1