Lightweight Concrete for High Strength - Expanded Shale & Clay

A.7.2 Strength Ceiling
Harmon discussed a strength ceiling for HSLC being in the region of 12,000 to 13,000
psi based on the limiting strength of the slate LWA. Holm also discussed the strength ceiling
with respect to smaller maximum size of aggregate. As the maximum size of aggregate is
reduced, strength increases to some upper limit determined by the strength ceiling. Beyond the
strength ceiling, additional binder does not increase the concrete’s strength.
Bremner and Holm addressed the elastic mismatch between LWA and the high-strength
cement paste matrix. As the HSLC strength increases, the difference between the elasticity
modulus values becomes more pronounced. Under load, the “elastic mismatch” results in
fracture that begins as transverse splitting of the LWA. This splitting action is indirectly
responsible for the strength ceiling of HSLC.
Holm also reported a tensile strength ceiling for LWC. Since the LWA is approximately
one half voids, its tensile strength will be reduced in comparison to NWA. Holm suggested the
LWA tensile strength ceiling might also be responsible for compression strength limitations.
A.7.3 Internal Curing
Internal curing results when absorbed water in the LWA provides an internal reservoir to
enhance cement hydration and extend the curing process. Several authors have commented on
this phenomenon as related to LWC and HSLC. Bremner, Holm and Ries performed a study in
which LWA was added to NWC mixes to provide additional internal cure water in mixes having
strengths from 6,400 psi to 8,700 psi. The internal curing is reported to not only add in strength
gain, but positively impacts durability characteristics by providing a higher quality and denser
matrix.
A.8 Transfer Length
Transfer length (l t ) is defined as the distance required to transfer the effective prestressing
force from the strand to the surrounding concrete. The transfer length is developed when the
pretensioning strands are released by flame cutting or other method from the restraining
abutments. Numerous factors are thought to contribute to determining the transfer length
including size and surface condition of the prestressing strand, concrete strength and modulus of
elasticity at time of strand release, level of prestressing in the strand, and the amount of confining
A-9