Lightweight Concrete for High Strength - Expanded Shale & Clay

Normal-Weight Concrete (NWC) - Concrete having a unit weight of approximately 145 lb/ft 3 .
High-Performance Concrete (HPC) – Concrete having a strength over 6,000 psi and/or having
durability characteristics in excess of minimally accepted standards.
High-Strength Lightweight Concrete (HSLC) - Concrete meeting the conditions of
lightweight concrete and having a compressive strength greater than 6,000 psi. This is the same
as high performance lightweight concrete (HPLC).
A.3 Production of Slate Lightweight Aggregate
The expanded slate LWA used in this research is produced from argillite slate found in
the foothills region of North Carolina east of Charlotte. Found in a geologic formation known as
the “Tillery Formation,” the slate is a thinly laminated, fine-grained siltstone composed of clastic
(transported) rock fragments. The slate was from rock fragments of volcanic ash origin, which
were deposited in a water environment and later solidified into solid rock. Subsequent burial and
tectonic pressure changed the rock into argillite slate. Currently, the Tillery Formation is the
only source of slate used to produce expanded slate LWA.
Expanded slate LWA is produced using a rotary kiln approximately 11 feet in diameter
and 160 feet in length. The rotary kiln is constructed on a slight incline on large bearings. The
inside of the kiln is lined with insulation and a refractory material to protect the thick outer steel
casing. Raw slate enters the kiln through pre-heaters that slowly heat the rock. Once in the kiln,
the raw slate slowly tumbles within as it moves down the slight incline towards the “burn zone”
where the raw material ultimately reaches temperatures in excess of 2200 degrees Fahrenheit.
The kilns are normally heated by injecting coal dust or natural gas at the low end. In the “burn
zone” the slate becomes sufficiently plastic for small gas pockets within the slate to expand
forming masses of small, unconnected cells. As the expanded slate cools, the cells remain
providing the material its low unit weight and absorption characteristics. The expanded material
called clinker, exits the kiln and is either air or water cooled depending on the manufacturing
process and is then crushed to the desired gradations.
A.4 Use of LWC for Prestressed Bridge Applications
The following comments describe many of the known uses of LWC and in some cases
HSLC in bridge applications. Many of the applications were not in pretensioned bridge girders,
but demonstrate the feasibility of LWC and HSLC in prestressed construction.
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