29-03-2014, 03:50 PM
Guide for Structural Lightweight Aggregate Concrete
Guide for Structural Lightweight.pdf (Size: 769.52 KB / Downloads: 688)
Historical Development
Early development through World War II -Prior to
1917, S. J. Hayde developed a rotary kiln process for heat
expansion of shales and clays to form hard, lightweight
material which served as aggregates in making concrete of
substantial strength and lower weight. At about the same
time, F. J. Straub pioneered in the use of bituminous coal
cinders as an aggregate for manufacture of concrete ma-
sonry units which attained high production volume follow-
ing World War I, and which are still being manufactured
today. Commercial production of expanded slag began in
1928; and in 1948, the first structural quality sintered shale
lightweight aggregate was produced using a coal-bearing
shale in eastern Pennsylvania. Pumice aggregate has been
used in Europe for centuries. Also it has been used in the
western part of the United States where deposits are readily
available.
Objective of the guide
The objective of the Guide for Structural Lightweight
Aggregate Concrete is to provide the best practices of pre-
paring and applying structural lightweight aggregate con-
crete. Using such practices, structures may be designed and
their performance predicted with the same high degree of
accuracy, and with the customary factors of safety, that is
attained for normal weight structural concrete.
Economy of structural lightweight concrete
The use of lightweight aggregate concrete in a structure is
usually predicated on lower overall costs. While lightweight
concrete may cost more per cubic yard than normal weight
concrete, the structure may cost less as a result of reduced
dead weight and lower foundation costs. This is the basic
reason, in most cases, for using structural lightweight con-
crete. Economy then depends on attaining a proper balance
among cost of concrete per volume, unit weight, and struc-
tural properties. Normal weight concrete may be the least in
cost per cubic yard, but will be heavier, resulting in greater
dead loads, increased sizes in many sections, and therefore
may require more concrete and reinforcing steel. Concrete
in which the aggregate is entirely lightweight will usually be
costlier per cubic yard, but will be the lightest, resulting in
reduced dead loads, reduced section dimensions,
Production of aggregates
Raw materials used in commercial production of struc-
tural lightweight aggregates are generally (a) suitable natu-
ral deposits of shales, clays, or slates; or (b) by-products of
other industries, such as iron blast furnace slags. Repara-
tion of raw materials can range from negligible to extensive
prior to treatment to produce expansion. In many cases
crushing to suitable sizes is the only prerequisite. In the
cases of finely divided materials such as silty and laminar
clays, and fly ash, the raw material may need to be ag-
glomerated with water, or possibly require addition of sup-
plementary binder, fuel, gas-forming or fluxing agents,
prior to heating.
Several different methods are used to produce structural
lightweight aggregates, and the aggregates produced may
vary widely in their characteristics. Any single description
will seldom apply fully to any raw material or process. A
generalized description follows for the several principal pro-
cesses used.
Method of presenting data
In the past, properties of lightweight concrete have been
compared with those of normal weight concrete, and usually
the comparison standard has been a single normal weight
material. With several million cubic yards of structural
lightweight concrete being placed each year, a comparison
of properties is usually no longer considered necessary.
With numerous recognized structural lightweight aggregates
available, it is as difficult to furnish absolute property values
as it is for normal weight concretes made from various
aggregate sources. For this reason, the data on various struc-
tural properties are presented as the reasonable conservative
values to be expected in relationship to some fixed property
such as compressive strength, unit weight, or in the case of
fire resistance, slab thickness.
Specifications
Lightweight concrete may be specified and proportioned
on the basis of laboratory trial batches or on field experience
with the materials to be employed. Most structural light-
weight aggregate suppliers have mix proportioning informa-
tion available for their material, and many producers
provide field control and technical service to assure that the
quality of concrete specified will be used.
The average strength requirements for lightweight con-
crete do not differ from those for normal weight concretes
for the same degree of field control.