Lightweight Concrete for High Strength - Expanded Shale & Clay

60 ksi. The alternate design procedure listed in ACI-318 Section 11.4.2.2 for predicting shear
strength produced some unconservative predictions for concrete compression strengths over
10,000 psi. The method for predicting interface shear strength was conservative for slate HPLC.
The current AASHTO LRFD (1998) specification provided a conservative prediction of
ultimate shear strength for slate HPLC.
7.6 Development Length
An evaluation of current code provisions using the twelve HPLC girder development
length tests in this study and eight normal weight HPC girder tests from Dill (2000) showed the
current AASHTO and ACI provisions to be conservative. The code equations overestimated
development lengths by 19 percent for both HPLC and normal weight HPC and never
underestimated them. Use of the current code provisions for design of development length for
slate HPLC and 0.6-inch diameter strand was conservative. Based on the concrete strength range
addressed in this research project, modification of the current code specifications for
development length was not necessary for HPLC or for 0.6-inch diameter strands.
Test results showed conclusively that shear cracking in the transfer length region across
the bottom strands did not induce significant strand slip if stirrup density was doubled over the
current AASHTO specified density in the region.
The addition of silica fume into the mix design at a 10% replacement rate by weight of
cementitious materials gave indications of reducing development length.
There was no indication throughout this analysis that need existed to differentiate
between slate HPLC and normal weight HPC. With regard to development length, for concrete
compressive strengths over 8,000 psi, the prediction of development length was the same for
both slate lightweight and normal weight concrete.
7.7 Long Term Performance of HPLC
Creep of 8,000-psi HPLC after 620 days under load was close to 1,650 µε for 40% of
initial strength and approximately 2,000 µε for 60% of initial strength. Creep of 10,000-psi
HPLC after 620 days under load was 1,160 µε for 40% of initial strength and 1,500 µε for 60%
of initial strength. Fifty and ninety percent of the 620-day creep was reached after approximately
16 and 250 days of loading, regardless the type of HPLC.
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