Use of waste materials in concrete

1. I
f disposal of a material is an ex-
pense, one naturally thinks of
ways to turn it into something
useful. What better way could
there be to dispose of materials that
are otherwise useless, or nearly so,
than to form them into concrete,
producing a structure or pavement?
Interest has developed in using a
variety of wastes in this way.
The history of fly ash shows that
the effort can be successful. Saw-
dust concrete also has a consider-
able history of usefulness. What
about waste plastics, broken glass,
mine wastes and garbage? Serious
interest has developed in all of
these.
Broken-glass concrete—
decorative or structural?
Colored glass may be used as an
exposed aggregate to impart deep
hues with brilliance. Special colored
glasses are manufactured for the
purpose but waste glass is also
available from member companies
of the Glass Container Manufactur-
ers Institute, who now operate 90
glass container redemption centers.
Exposedaggregate surfaces made
with colored glass aggregate have
permanent color and gloss. They
are most effective in mixes made
with white portland cement. Aggre-
gate should pass the 3
⁄4- or 3
⁄8-inch
sieve depending on the effect de-
sired. Pleasing combinations can be
obtained by using a blend of more
than one color.
A number of techniques are use-
ful. One is to embed the dry aggre-
gate into a freshly troweled surface,
spray it with a retarder and later ex-
pose the aggregate by scrubbing
and washing with water. Another is
to make a special facing mix con-
taining the aggregate and place it in
the mold for a precast panel, the
bottom face of which has previously
been painted with a retarder. The
facing mix is immediately backed
up with a mix containing an ordi-
nary structural aggregate. After
stripping, the face is scrubbed and
washed to expose the aggregate.
A more complicated but some-
times useful method permits verti-
cal casting of panels. The interior of
one vertical surface is painted with
a retarder. Aggregate is prepackedin
the form but a vertical separator
near the painted face, and parallel
to it, makes it possible to use only a
thin layer of glass and to fill most of
the mold with another aggregate.
The separator is pulled upwardlittle
by little as the aggregates are added
so that by the time the form is full
the separator has been completely
removed. Grout is pumped into the
form from the bottom to fill all the
voids. After the concrete has hard-
ened the form is stripped and the
aggregate is exposed by scrubbing
the retarded surface.*
Before using any glass as an ag-
gregate, assurance should be ob-
tained from the supplier that it will
meet the requirements of ASTM C
33 with respect to alkali-aggregate
reaction when tested according to
ASTM C 227. If it does not meet the
specification by itself the supplier
should be able to provide the cor-
rective material such as opal or oth-
er pozzolan that will prevent disrup-
tive expansion and he should give
information about the amounts of
these materials required, based on
laboratory evidence. In many cases
it may be sufficient simply to use a
portland cement whose alkali con-
tent (when expressed as Na20) is
0.60 percent or less but this should
be substantiatedby laboratory data.
The use of glass as ordinary ag-
gregate for structural applications
has not become common although
the pressure to find new applica-
tions for waste materials might at
first seem to make it attractive. Any
glass aggregate to be used for con-
crete must certainly be required to
meet the ASTM requirements just
cited, either by itself or in combina-
tion with some other material. Even
after meeting these requirements it
is likely that it will contribute less to
the desirable properties of concrete
than the aggregates now in com-
mon use.
A comparison has been made of
the effect of replacing a normal
weight aggregate with glass cullet
obtained from broken bottles. The
bottles had been roughly hand sort-
ed according to color before break-
ing and crushing so that the aggre-
gate was almost free of brown glass
but contained perhaps 20 percent
or more of green glass; the remain-
der was clear. Particles were midway
in shape between flat plates and
cubes, with most sharp edges worn
away. Maximum size of particles
was 3
⁄4 inch. Some large particles
were convex-concave.
In one mix the normal weight
sand was replaced by glass of the
same size, in another the coarse ag-
gregate was replaced by coarse glass
and in another both the sand and
coarse aggregate were replaced.
Starting with an 8,000-psi concrete
mix of normal weight sand and
gravel the strength was reduced to
less than 50 percent of the original
when the sand was replaced, to less
than 40 percent when the coarse ag-
gregate was replaced and to less
than 30 percent when both were re-
placed.The resistance to salt scaling
Waste materials in concrete
Can concrete be made
from broken glass?
sawdust? plastics? mine
wastes? garbage frit?
* Details of these techniques and typi-
cal quantities of glass required are giv-
en in “Exploring Color and Texture,”
published by the Portland Cement
Association.

2. in freezing tests was also reduced in
the same way: all replacements
caused reduction in resistance and
the greatest damage occurred when
both sand and coarse aggregate
were replaced.
Broken glass therefore seems to
be valuable for use in exposed ag-
gregate concrete when used proper-
ly, butnot to have significant poten-
tial for use in structural concrete.
Brick from waste products
Despite the conclusion just
reached for structural concrete,
crushed glass is said to have pro-
duced satisfactory strength in
pressed concrete brick. This may be
partly because the glass used was
fine and so did not decrease the
strength as much as if it had been
coarser. It may also mean that the
strength achieved withfine glass ag-
gregate was adequate but that far
more strength might have been ob-
tained if sand had been used in-
stead of glass in a mix that was oth-
erwise the same.
In addition to reclaimed glass a
variety of other waste materials in-
cluding mine tailings, waste
foundry sand, ocean bottom dredg-
ings and garbage frit have been in-
corporated into experimental
pressed concrete brick in a process
that is now available for commercial
use. Many other materials have also
been tested for possible use in this
process, alone or in combination,
because most waste materials are
not available everywhere and some
can be obtained in only a few loca-
tions.
Mine tailings are the granular ma-
terials discarded from mines after
separation from the more valuable
part of the ore. In some places they
have accumulated since the very
beginning of the mining operation
and now constitute large hills.
Waste foundry sand is fine silica
sand from metal foundries that has
become too loaded with spent
binder to be used further in metal
casting. It is sometimes disposed of
as fill.
Garbage frit comes from the slag
produced when unsegregated
garbage is burned in a type of verti-
cal furnace newly introduced in a
few metropolitan areas. They oper-
ate at 3,000 Fahrenheit, a tempera-
ture adequate to oxidize all organic
materials. The slag which contains
the metal, glass, ash and other min-
eral matter that does not form
gaseous combustion products
amounts to about 3 percent of the
original garbage volume. When the
slag is drawn off the hearth it is
quenched to produce frit.
Oyster shells, crushed asbestos-
cement pipe and aggregates have
been formed into brick. Costs of the
aggregates are currently $2.50 per
ton or less.
Mix proportions for the use of
such materials in making pressed
brick have required 4 to 10 percent
portland cement by weight of total
solids. An accelerator is commonly
used. Brick or block is pressed in a
machine that turns out 3,000 to
3,600 bricks per hour. The brick are
cured in stockpiles. It is anticipated
that the brick and block will meet
the specifications in ASTM C 55 and
C 90, respectively, for grades suit-
able for use in exterior walls.
Fly ash aggregate
Expanded slag and cinder aggre-
gates are so well known and so use-
ful in concrete that we seldom think
Bricks and blocks showing various kinds of materials utilized.
Exposed glass aggregate panel. A
single-colored glass was used for the
exposed aggregate, yet a two-color
effect was obtained due to the
difference in size of the translucent
aggregates. A special glass that was
nonreactive with portland cement was
used.
Exposed glass aggregate panel. A
combination of a reflective glass with
a nonreflective natural aggregate was
used.

3. of them any longer primarily as
waste materials. Fly ash used as a
pozzolan is also so commonplace
that it needs no discussion here. But
aggregate made from fly ash is new
enough that notice should be taken
of it.
It is only within the last several
years that fly ash aggregate has
come into commercial production
but now it is available from several
sources. It may be produced in a ro-
tary kiln or on a sintering grate.
Perhaps the main difference be-
tween fly ash aggregate and other
lightweight aggregate is that fly ash
aggregate requires more air entrain-
ing agent. The fine portion of the fly
ash aggregate may also be poorly
graded, with the result that concrete
mixes made with unmodifiedaggre-
gate may be harsh towork. For these
reasons it may be desirable to re-
place a part of the fines with normal
weight sand.
When enough normal weight
sand is included to make a workable
mix the concrete produced is very
similar to other structural light-
weight concretes in unit weight,
compressive strength, modulus of
elasticity, tensile strength, drying
shrinkage, creep and freeze-thaw
resistance.*
Plastics as aggregate?
What about salvaged plastics? Re-
cent announcements have told of
ground polyethylene scrap being
used successfully as a partial re-
placement for sand in concrete.
Compressive strengths equivalent
to those of normal weight concrete
are said to have been achieved with
about a 10 to 15 percent saving in
weight.
More complete studies indicate
that when from 20 to 40 percent of
the sand volume in either normal
weight or structural lightweight
concrete is replaced by this kind of
scrap, the 28-day compressive
strength is lowered significantly.
Furthermore, creep increases
markedly with the substitution of
plastic scrap for a portion of the fine
aggregate. On the other hand, dry-
ing shrinkage is improved some-
what.
Probably the worst problem with
substituting plastic aggregates is
that in the range of summer tem-
peratures (70 to 120 Fahrenheit) the
coefficient of thermal expansion of
concretes is greatly increased. This
means that the plastic aggregate
could easily cause disruption of
concrete in hot weather.
Fractured surfaces of concretes
made with colored plastics can be
highly decorative. This may suggest
their use for special purposes,
though particles of many plastics
are likely to be so weakly bonded
that they easily come out of their
sockets.
Foamed polystyrene pellets have
been successfully used as aggre-
gates to make insulating concretes
with unit weights of 30 to 40 pcf and
strengths of 400 to 650 psi. These
lightweight pellets are made for the
purpose however, and cannot be
considered scrap. Foamed plastics
that are scrap couldundoubtedly be
used in the same way but unless the
particles were rounded the mix
would suffer in workability.
It is not yet known whether non-
foamed plastics other than polyeth-
ylene might modify concrete more
favorably. Nor is it known what kind
of future developments might im-
prove the performance of concrete
made with polyethylene. It does
seem clear that at this stage of the
technology the use in concrete of
plastic aggregate other than foam is
not promising.
Sawdust is well established
Sawdust has been used in con-
crete for at least 30 years, but not
widely. Although seriously limited
by its low compressive strength,
sawdust concrete can be made to
perform well in certain floor and
wall applications. When low struc-
tural strength is not a problem this
lightweight material may answer
the contractor’s need for concrete
with good insulation value and sat-
isfactory resiliency.
When dry, most sawdust concrete
weighs only 30 percent as much as
normal weight concrete, its insulat-
ing properties approximate those of
wood. It can be sawed and drilled as
easily as wood and it will hold nails
and screws. With proper cement-to-
sawdust ratios it is not flammable.
However, the strength of sawdust
concrete when made in the most
commonly used proportion of 1:3 is
only 10 to 20 percent of that of nor-
mal concrete. It is not usable where
high structural strength is required
or where it would be subjected to
heavy traffic and severe abrasive ac-
tion. Its wood content also prohibits
installation of lean mixes in envi-
ronments of excessive moisture.
Cement-to-sawdust ratios in
standard sawdust concrete mixes
are usually from 1:2 to 1:6. However,
strength is drastically reducedas the
percentage of sawdust is increased.
For example, tests made by the
Commonwealth Scientific and In-
dustrial Research Organization
(Australia) show that a mix of 1 ce-
ment: 2 sawdust develops an ulti-
mate compressive strength of 1,100
psi after 7 days of curing, but the
strength of a 1:3 mix is only 500 psi,
and that of a 1:6 mix 110 psi (see
Table I).
The strength of sawdust concrete
can be increased substantially by
adding sand to the mix. A ratio of 1
cement: 11
⁄2 sand: 11
⁄2 sawdust devel-
ops a compressive strength of 1,300
psi. Although a mix of this ratio will
lose some degree of nailability and
sawability, its higher strength and
density make it very usable in floor
applications. The Portland Cement
Association suggests a 1 cement: 3
sawdust mix for floor and other ap-
plications that are subject to only
moderate wear and exposure.
Quantities of cement and sawdust
for various jobrequirements are giv-
en in Table II.
According to tests conducted at
the University of Wisconsin, a 2-
inch thickness of 1:3 sawdust con-
crete has the same insulating value
* A good summary of the proportion-
ing and properties is given in Donald
W. Pfeifer’s “Fly Ash Aggregate Light-
weight Concrete,” published by the
Portland Cement Association.

4. as a 1
⁄4-inch fiber insulating board.
The Portland Cement Association
recommends a 1:4 mix when saw-
dust concrete is tobe used for fill in-
sulation. Mixes that exceed 1:4 ra-
tios have low compressive strength
and low nail-holding power and are
flammable. They also may disinte-
grate in very moist environments.
In general, sawdust concrete is
not recommended for use where
water accumulates or where water is
constantly present. Because of its
wood content the concrete would
absorb and emit large amounts of
moisture causing it to expand and
contract considerably and possibly
to crack and chip.
If an aggregate of low porosity
like ordinary sand is added to the
mix it will reduce the water absorp-
tion, thus making sawdust concrete
applicable in more humid atmos-
pheres. However, it is recommend-
ed that before using it in such appli-
cations mixes be made and tested
to establish the proportions neces-
sary for suitably low absorption.
Experiments with different
species of woods have indicated
that the type of sawdust used in the
mix is of primary influence on the
quality of the concrete. Tests at
Washington State University and
the University of Minnesota indi-
cate the preferred sawdusts to be
spruce or Norway pine. Fair results
may also be obtained from jack
pine, ponderosa pine and aspen
sawdusts.
Not recommended are sawdusts
from red oak, Douglas fir, cotton-
wood, maple, birch or red cedar.
Tests have indicated that sawdusts
from these species produce a con-
crete of extremely low strength.
In some instances a mix contain-
ing several different types of saw-
dust may be satisfactory. Tests of
some mixes containing several saw-
dusts have produced concretes with
a moderate degree of weatherability.
No matter which species of wood
is used, the sawdust should be
passed through at least a 1
⁄4 inch
screen before it is added to the mix.
In addition, presoaking of the saw-
dust for at least 24 hours prior to use
is highly recommended. Presoaking
will dissolve away certain con-
stituents that might otherwise in-
hibit strength development of the
portland cement.
When presoaked sawdust is to be
used it should be allowed to drain
for several minutes before it is
added to the mix. Generally very lit-
tle, if any, additional water will be
required in the mix after the cement
has been added to the presoaked
sawdust.
A second method of mixing is to
combine air-dry sawdust with port-
land cement in proper ratios until a
uniform color is achieved. Then
enough water is added to produce a
mix that can be finished satisfactori-
ly under vigorous tamping and
troweling. It should be remembered
that too much or too little water will
impair the strength of the concrete.
The mix should never be wet
enough to compact itself after
placement.
The best mix will have the con-
sistency of moist earth. (See Table
III). No free moisture will be pre-
sent. A proper sawdust concrete
Table II Approximate Amounts of Materials Required per 100 Sq.
Ft. of Sawdust Concrete
Thickness
of 1:3 Mix 1:4 Mix
concrete, Cement, Sawdust, Cement, Sawdust,
inches bags cu. yd. bags cu. yd.
2 51
⁄2
2
⁄3 41
⁄4
2
⁄3
3 8 1 61
⁄2 1
4 11 11
⁄3 81
⁄2 11
⁄3
5 131
⁄2 12
⁄3 101
⁄2 12
⁄3
Courtesy of Portland Cement Association. (Adapted)
Table III Water Requirement and Yield of Sawdust Concrete
Cement, No. Sawdust, Water-cement ratio Yield,
of 94-lb. bags cu. ft. gal./bag wt. ratio cu. ft.
1 1 3.5 0.31 1.3
1 2 3.9 0.35 1.7
1 4 4.4 0.39 2.2
1 6 5.1 0.45 2.8
Adapted from Constructional Review, February 1960
Table I Properties of Sawdust Concrete
Ult. Comp. Thermal
Cement: Sawdust, Strength, Density Conductivity,
parts by volume 7-day curing, pcf B.T.U./sq. ft./hr./
psi degree F./in.
1:2 1,100 75 2.0—2.4
1:3 500 49 —
1:4 150 41 1.2—1.4
1:6 110 40 1.2 (estimate)
Adapted from Constructional Review, February 1960

5. mix has been achieved when no ce-
ment can be drawn to the surface
by troweling.
Tamping should take place as
quickly as possible after the mix has
been deposited. If a float finish is
desired final floating should be de-
layed until all surface dampness has
evaporated. This delay will prevent
working the water into the paste,
producing a high water-cement ra-
tio at the surface. After hardening,
such a paste will form a weak, easily
flaking surface.
In applications where sawdust
concrete is to be bonded to aregular
concrete base the surface of the
base must be thoroughly cleaned
and dried. A coating of cement slur-
ry should be broomed into the base
before the sawdust concrete is
placed. However, the concrete must
be placed before the slurry has dried
and both should be well troweled
and consolidated.
The procedures for handling,
placement and curing of normal
weight concrete also apply to saw-
dust concrete. The recommended
curing period ranges from 2 to 6
weeks depending upon mix ratios,
the application and weather. The
contractor should make sure that
adequate protection is provided
against premature drying and
against freezing. In warm weather
he should cover the concrete with
polyethylene, damp bags or wet
straw for at least one full week. In
cold weather this protection should
be provided for at least two full
weeks. In any installation it is es-
sential that ample drying time—
usually 2 weeks—be provided after
curing before putting the concrete
into service. Testing of various mix-
es for their suitability to the specif-
ic requirements of the job is highly
advisable.
As a basic construction material
sawdust concrete does indeed have
its functions. It performs well in a
limited number of floor applica-
tions or as a building component
not subject to high structural stress-
es. It has serious limitations that
must be understood before it is put
to use. Within these limitations, the
advantages that sawdust concrete
offers—lightness ofweight, sawabil-
ity, nailability and low thermal con-
ductivity—make it a useful con-
struction material.
Which materials?
Of the materials discussed it is
clear that fly ash aggregate and saw-
dust are well established for use in
concrete. Broken glass is suitable for
some uses but not others. Mine tail-
ings, waste foundry sand, ocean
bottom dredgings and garbage frit
may be useful for some purposes.
Most plastic scrap is not likely to be
useful with the possible exception of
plastic foam.
PUBLICATION#C710372
Copyright © 1971, The Aberdeen Group
All rights reserved