Print this page Low-Density Concrete Basics
John Poullain, P.E.
Course Outline
This three-hour online course provides basic guidance for the properties of low-density concrete (LDC) and specified density concrete (SDC). The physical and mechanical properties, construction considerations, advantages, disadvantages and economic considerations of LDC are discussed. Also discussed are the guidelines for mixing, placing, curing, pumping and field controls during construction. Comparisons between the properties of low-density and normal-density concrete and special construction concerns are presented.
This course includes
a multiple-choice quiz at the end, which is designed to enhance the understanding
of the course materials.
At the conclusion of this course, the student will:
- Have an understanding of the physical and mechanical properties of low-density concrete;
- Understand how LDC properties affect the performance and durability for various applications;
- Be familiar with construction considerations and guidelines for usage with typical applications;
- Understand the differences between LDC and NDC, where they differ and to what extent;
- Be familiar with special considerations used for properly mixing LDC and water criteria based on the properties of low-density aggregate material;
- Be aware of the differences between normal-density and low-density concrete for basic mixture proportioning and criteria for mixture components;
- Be aware of special considerations and techniques for placing, curing and pumping LDC mixes;
- Understand the general economic considerations for choosing construction with LDC; and
- Be familiar
with the basic terminology and background for important properties of LDC
and NDC.
Intended Audience
This course is intended for civil engineers, planners and contractors.
Benefit to Attendees
The student will become familiar with the physical and mechanical properties of low-density concrete and how they compare with those for normal-density concrete (NDC). The student will gain an understanding of the special construction precautions that are different from normal density concrete and what factors to consider for placing, curing and pumping LDC. The characteristics and properties that distinguish LDC from NDC are discussed. Included tables summarize the differences in properties and the criteria for mix proportions described in the text.
Course Introduction
Background
The first known use of low-density concrete was by the Romans to allow construction
of lightweight domes. Later applications include the US lightweight concrete
ships constructed between 1917 and 1925 and cast-in-place columns through the
20th century for high rise buildings. More recent times include marine structures
such as prestressed concrete caissons, floating platforms for North Sea oil
fields and oil platforms towed to oil fields for placement on the ocean floor.
Other applications include bridge girders and entire bridge spans. It's been
advantageous to use LDC where long clear spans are desired so the dead loads
of beams cam be reduced which allow lighter supporting structures to be used
and less foundation loads.
Benefits and
Disadvantages
The principal benefit for using LDC is the large mass that can be saved in a
structure while having the same strength as with normal-density concrete. Since
aggregate may be 60-70% of the weight of concrete, concrete densities can be
15-25% lower primarily because of the low-density of the manufactured aggregate.
The benefits of reduced dead loads include the lighter weight of structural
spans, lighter supporting members and reduced foundation requirements. Lighter
loads or more pieces of prestressed members can be transported and truck concrete
mixers can haul larger volumes because of the lighter weight concrete (provided
legal and safety requirements are met).
The American Concrete Institute (ACI) published the "Manual of Concrete Practice" which designates these guidelines pertaining to lightweight concrete:
ACI 211.2 "Standard
Practice for Selecting Proportions for Structural Lightweight Concrete"
ACI 213 "Guide
for Structural Lightweight Aggregate Concrete"
ACI 318 "Building Code Requirements for Structural Concrete" provides
guidance when designing with light density concrete over ranges of 2900 to 5080
psi.
Also noted are
the ASTM publications, ASTM C330 and ASTM C567 that cover lightweight aggregate
and lightweight density respectively for structural lightweight concrete design.
Another benefit of LDC comes with the possible use of lower capacity cranes rather than special heavier lift cranes for especially long beams made with NDC. Longer reaches are made possible with lighter loads, helping to reduce the movement of cranes.
Disadvantages include the higher cost for the manufacturing process of heating shale, clay or slate for the low-density aggregate and the transportation costs from the supplier. LCD requires an additional 15-50 lb/cy of cement to be comparable to normal-density concrete. Certain precautions must be observed to insure the aggregate is adequately saturated. If the aggregate is not adequately saturated, the slump will be decreased as the concrete mixture looses water to the under saturated aggregate resulting in reduced workability and pumping performance.
The ongoing development of low-density concrete subsequently introduced other concrete mixtures that transition between normal and low density called Specified Density Concrete (SDC). For this type of concrete the density of course aggregate is specified for a custom design that optimizes the concrete to improve the strength to density ratios and reduces transportation costs. SDC customizes the coarse aggregate by replacing part of normal course aggregate with LDA rather than total replacement of the coarse aggregate. The SDC density may be in the 1800-2200 g/m3 (112-137 lb/ft3) range.
Cellular Concrete
Other non-structural low-density concrete may weigh as little as 12-15 pounds
per cubic foot and usually not more than 50 lb/ft3. These lightweight concrete
include cellular or foam cell concrete (air voids in a cement-sand mortar) and
aggregate concrete (perlite etc. aggregate). Materials such as perlite, vermiculite
and expanded polystyrene beads are used for aggregate. Cellular concrete incorporates
aqueous foam mixed with cement and light weight aggregate which creates air
cells which can be up to 80% of the total volume. Weights may range from 12-90
lb/ft3 depending on the amounts of compressed air and/or foam discharged into
the mixture. Compression strength may be 100-200 psi and up to 1000 psi.
Some are used for thermal insulation, building blocks and wall panels. Building applications include pre-cast building panels, tilt-up panels, lightweight flat and curved roofs and floor decks. The low thermal conductivity gives a building insulation values but it must be selected to provide sufficient strength for the design application. Since shrinkage may be high special consideration must be given when designing mixes. Cellular concrete is also used to insulate underground steams lines.
Geotechnical uses
for cellular concrete are as stabilized fill over poor soil such as expansive
clay or mixing together with existing soil. This use can reduce the expense
of hauling borrow soil and can make some soils more resistant to freezing and
thawing. In addition to highway and foundation fill cellular may also be used
as backfill beneath concrete slabs to fill voids. It can be pumped since it
is flowable which is especially advantageous where access or working room is
limited. Other uses include abandonment of underground storage tanks, pipelines,
storms sewers and tunnels when it is not feasible to remove them and can be
left in place safely.
All concrete is a construction material with many attributes and disadvantages
and must be carefully designed for proper construction. However because it is
porous, over time it is not very resistant to acidic chemicals. Deicer chlorides
and also chemical reactions within the concrete destroy its limited natural
protection.
Course Content
This course is based on the US Army Corps of Engineers Circulars,
"Properties of Low-Density Concrete", EC 1110-2-6054 (2002 Edition,
9 pages) and also "Construction Considerations for Low-Density Concrete",
EC1110-2-6055 (2002 Edition, 10 pages), PDF files. The course is also based
on the Expanded Shale, Clay & Slate Institute (ESCSI) paper, "Specified
Density Concrete-Transition", (2000 Edition, 5 pages), PDF file.
The links to the course materials are:
Properties of Low-Density Concrete
Construction Considerations for Low-Density Concrete
Specified Density Concrete-Transition
You need to open
or download above documents to study this course.
Course Summary
This course compares
the properties of low-density concrete with normal-density concrete and the
economics of using LDC. Characteristics, advantages, limitations and possible
problems to avoid with careful planning are discussed. Special considerations
for proper mixing, water requirements and for placing, curing and pumping during
construction are discussed. Tables of the mechanical and physical properties
and criteria for the basic mix proportions are shown. Appropriate ASTM and ACI
standards are noted in the text.
Related Links
For additional
technical information related to this subject, please refer to:
http://escsi.com/
Expanded Shale, Clay % Slate Institute (ESCSI) site has information on lightweight
concrete (structural and building materials) and low-density concrete applications,
guide specs, pumping etc.
Quiz
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.
