15-11-2012, 03:31 PM
ENGINEERING PROPERTIES OF STRUCTURAL LIGHTWEIGHT CONCRETE
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SUMMARY
This paper discusses the unique physical characteristics of rotary kiln expanded slate
lightweight aggregate for producing high performance and high strength lightweight
concrete. The compressive strength, elastic modulus, splitting tensile strength, specific creep,
and other properties of lightweight concrete are significantly affected by the structural
properties of the lightweight aggregate used. Concrete production, transportation, pumping
and placing are also affected. Raw materials and rotary kiln processing is discussed. Data
from academic and laboratory studies is presented as well as data from actual projects such as
the Raftsundet Bridge in Norway and the Hibernia Offshore Oil Platform Gravity Base
Structure.
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.
Along with the deposition of the volcanic ash was an occasional ash (debris) flow or
gravitational mud-type flow into the same deposition basin. Additional layers, consisting of
volcanic tuff with high calcite concentrations, formed within the system. Subsequent
millions of years of geologic forces caused the alternating layers of material to fold and fault,
causing disorder to the once ordered, layered system. Along with this disorder came diabase
dike rock intrusion of Triassic-Jurassic age (about 180-220 million years ago), which caused
additional rock structures of vertical emplacement that further complicated the system.
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.
ENGINEERING PROPERTIES OF STRUCTURAL LIGHTWEIGHT CONCRETE
The use of lightweight aggregate to reduce concrete densities is a well-established procedure
where properties such as increased fire resistance, ease of handling and transportation, or
reduced structure dead load is desired. Lightweight concrete with compressive strengths of
up to 5,000 psi (34.5 MPa) has been used in commercial construction routinely since the
early 1930’s. During the last two decades, however, much higher strengths have been
specified.
Aggregates normally constitute 60–70% of a concrete mix, and the physical characteristics of
the aggregate will, therefore, have a pronounced influence on the physical property of the
concrete [5]. High strength concrete relies more heavily on the quality of the aggregate than
does low or even medium strength concrete. The function of the aggregate, to a large extent,
is to act as an inexpensive filler material. The cement paste matrix takes up most of the load
imposed on the concrete. As design loads approach and exceed the strength limits of the
cement paste matrix, the load carrying capacity of the aggregate and the interplay between
aggregate and cement paste become the limiting factors in strength development. For all
practical purposes, this limit appears to be in the region of 15,000 to 16,000 psi (100 to 110
MPa) for concrete using normal weight aggregate, approximately 80% of this for concrete
using expanded slate lightweight aggregate and probably less than 70% of this for concrete
using expanded clay or shale aggregate. This limitation is more pronounced for lightweight
concrete because the mechanical characteristics of the lightweight aggregate are more similar
to those of the cement paste matrix than to the normal weight aggregate, and variations in
aggregate quality will be more directly reflected in the concrete characteristics.
Creep and Shrinkage of Lightweight Concrete
In 1989-1990 a study was conducted at North Carolina State University to determine the
shrinkage and creep potential of concrete made with expanded slate aggregate [10]. As part
of this study, elastic modulus and other standard plastic concrete characteristics were also
determined. Four separate concrete mixes were produced for this study. One mix was a
conventional concrete using normal weight coarse aggregate for comparison. Two mixes
were “standard lightweight” mixes, and the fourth mix was a relatively high strength mix.
The author (Leming) noted that values of creep and shrinkage strain for these lightweight
concretes were low compared to national (United States) averages. He also noted that values
of the elastic modulus were significantly higher than predicted by American Concrete
Institute equations based on compressive strength and unit weight.