Lightweight Concrete for High Strength - Expanded Shale & Clay

Shrinkage after 620 days of drying was approximately 820 µε for the 8,000-psi HPLC
mix and 610 µε for the 10,000-psi HPLC mix. Fifty and ninety percent of the 620-day shrinkage
was reached after approximately 30 and 260 days of drying for both 8,000-psi and 10,000-psi
HPLC.
The 10,000-psi HPLC had a specific creep similar to that of an HPC of the same grade,
but with less cement paste content, and it had significantly less creep than an HPC of the same
grade and similar cement paste content. The shrinkage of the HPLC was about 20% greater than
the HPC after 620 days. Therefore, the HPLC had less creep yet somewhat more shrinkage than
comparable HPC.
Final prestress losses were estimated using AASHTO refined, AASHTO lump sum, PCI,
and ACI-209 methods. All of them overestimated the measured time dependant losses in 8,000-
psi and 10,000-psi AASHTO Type II prestressed girders made with HPLC. This result means
that those methods are conservative for estimating time dependant losses of HPLC. Total losses,
however, were underestimated by the AASHTO lump sum and PCI method by 1.2 and 1.8% for
8,000-psi girders, respectively.
7.8 Recommendations
7.8.1 HPLC Mixes and Properties
Accurately control the absorbed water in the LWA. Determine the percentage of
absorbed moisture above which mix water will not be absorbed during concrete batching. Apply
water by sprinkling or other method to the LWA stockpile for a sufficient length of time prior to
batching such that the absorbed moisture is in excess of the minimum required amount. Test the
LWA as often as is necessary during batching to insure that moisture contents are within the
tolerance of the given mix design.
Use match curing of cylinders to predict initial strength of the concrete for precast
prestressed concrete girder production. For later age testing (28-days and after) ASTM cured
specimens are adequate. The 4 x 8 cylinders provide a reliable measure of compressive strength
and may be used in place of 6 x 12 cylinders.
Use Equation 7.1 to predict the modulus of elasticity for slate HPLC.
7-4