Lightweight Concrete for High Strength - Expanded Shale & Clay

showed a silica fume blended cement to provide superior strength results compared to a beliterich
cement. In a study including high-strength NWC and HSLC, Leming reported that silica
fume was particularly effective in increasing the compressive strength of any concrete. Mor
reported the dramatically improved bond strength between LWC and reinforcing steel based on
the use of silica fume, an occurrence that was verified during this research.
A.5.5 Moisture Content
Control of moisture in HSLC is critical. In order to produce concrete having the water to
cementitious materials ratio desired, the exact absorbed (moisture within the pore structure of the
LWA) and adsorbed (moisture on the outside surface of the LWA – free moisture) moisture of
the aggregate must be known. The amount of absorbed moisture directly impacts the specific
gravity of the LWA as proportioned and mixed. Smeplass made this point very clear by stating
“The consequence of this observation may be that the mix water absorption of LWA must be
determined in the actual initial moisture condition, as under concrete production, and as often as
necessary to detect variation.”
Based on the porosity of LWA, the determination of absorbed moisture content and
specific gravity must be handled differently than with normal weight aggregate. Holm and
Valsangkar suggested a 1-day soak of LWA prior to determining specific gravity using a
pycnometer. After soaking the aggregate for 24 hours, the additional water absorbed in the
pycnometer during specific gravity testing is minimal. Based on the then determined absorbed
moisture content, the dry specific gravity could be determined by dividing the 24-hour soak
specific gravity by (1 + absorbed moisture content).
A.5.6 Mix Proportioning
Mix proportioning of LWC is covered in ACI 211.2-91. The current guide does not
cover mix designs for concrete in excess of 6,000 psi strength.
A.6 Field Production of HSLC
Comments on field production of HSLC were limited. Holm and Bremner (2000)
suggested four basic principles including well-proportioned, workable mixtures that use a
minimum amount of water, equipment capable of expeditiously moving the concrete, proper
consolidation in the forms, and quality workmanship in finishing. They suggested that the
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