ENGINEERING PROPERTIES OF STRUCTURAL
LIGHTWEIGHT CONCRETE


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INTRODUCTION

Structural lightweight aggregate concrete is an important and versatile material in modern
construction. It has many and varied applications including multistory building frames and
floors, bridges, offshore oil platforms, and prestressed or precast elements of all types. Many
architects, engineers, and contractors recognize the inherent economies and advantages
offered by this material, as evidenced by the many impressive lightweight concrete structures
found today throughout the world [1]. Structural lightweight aggregate concrete solves
weight and durability problems in buildings and exposed structures. Lightweight concrete has
strengths comparable to normal weight concrete, yet is typically 25% to 35% lighter.
Structural lightweight concrete offers design flexibility and substantial cost savings by
providing: less dead load, improved seismic structural response, longer spans, better fire
ratings, thinner sections, decreased story height, smaller size structural members, less
reinforcing steel, and lower foundation costs. Lightweight concrete precast elements offer
reduced transportation and placement costs [2].

RAW MATERIALS

Currently, the foothills region of North Carolina, east of Charlotte, is the only source of slate
that is being used as a raw material for rotary kiln expanded slate lightweight aggregate. This
argillite slate is found in a geologic formation known as the Tillery Formation. It is a thinly
laminated, gray, fine-grained siltstone, composed of clastic (transported) rock fragments.
The geologic history of the Tillery Formation began 550 million years ago in the Cambrian
Period, approximately 330 million years before dinosaurs. Rock fragments of volcanic ash
origin were deposited in a water environment (sedimentation) and later solidified into solid
rock (lithification). Consequent burial and tectonic pressure then changed (metamorphosed)
the rock into argillite slate.

ROTARY KILN PROCESS

Expanded slate aggregate is produced by the rotary kiln method. This discussion describes
one specific lightweight aggregate manufacturing plant. Other rotary kiln process facilities
are similar, but may have variations from the process described herein.
The rotary kiln is a long tube that rotates on large bearings. Typical kilns are approximately
11 feet (3.4 meters) in diameter and 160 feet (49 meters) long constructed on a slight incline.
The kiln is lined with insulation and refractory materials. Raw slate is fed from the storage
silos into patented pre-heaters that allow the rock to heat up at a moderate rate. It then enters
the upper end of the kiln where it slowly revolves and moves toward the "burn zone" near the
lower end of the kiln. The "burn zone" reaches temperatures in excess of 2200° F (1200° C).
The plant being described uses high BTU, low sulfur coal for its heat source.