Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21554 ~~
BACKGROUND OF THE INVENTION
The invention relates to a method and apparatus for determining expansive
volumetric changes in curable materials. In particular, the invention relates
to a
method and apparatus for accurate on-site determination of expansive
volumetric
~~ changes in relatively large aggregate size, hardened, shrinkage-compensated
concrete.
An example of shrinkage-compensated concrete is defined by America Concrete
Institute Committee ACI 223. The test is operative without delay from the time
of a
concrete pour and employs a cylindrical container and a strain gauge for
measuring
hoop stress.
1o II Portland cement used in concrete mixes results in a material that has
about
0.04%-0.08% shrinkage over its lifetime. This results in surface cracking over
time.
Shrinkage-compensated concrete is a high performance material which, by means
of
expansive additives, compensates for normal shrinkage. This material is useful
in
applications where a high quality, crack free surface is required. In order to
assure
~~ that the shrinkage-compensated concrete has a desired expansion
characteristic, a test
is needed to measure such expansion.
A currently available test for determining expansion of shrinkage-compensated
concrete is set forth in ASTM designation C878. The test employs an elongated
mold
for casting a test specimen and a restraining cage consisting of a threaded
steel rod
~~ and spaced apart end plates secured to the rod. The restraining cage is
placed in the
mold and the material to be tested is compacted into the mold between the
plates.
After the material initially sets up or cures (six hours minimum), it is
demolded and
length measurements of the rod are taken at specific times over a period of
seven
days. At present, expansions in a range of 0.03 and 0.07 percent are
considered to
be suitable for reduction in ultimate shrinkage cracking in shrinkage-
compensated
concrete.
~~ Although the C878 expansion test is the currently acceptable standard, it
is
difficult and costly to implement. For example, the test requires a skilled
laboratory
technician to perform it properly. The test procedure requires that the
specimen be
demolded not less than six hours after casting. In a field construction
context, this
means that the laboratory technician is operating on an overtime basis, or two
shifts
to ~~ are required. Further, demolding fresh, weak, elongated concrete bars is
a process
requiring great delicacy and skill. It is not unusual for the molded sample to
break
during the demolding process. Also, there is a considerable delay between the
time
that the sample is cast and the first significant expansion measurement is
taken, for
example, an initial expansion measurement can only be taken after the sample
is
~~ demolded. Consequently, bar expansions that occur after the concrete sets
but before
the first significant measurement may be made are not detected by the
procedure.
The C878 test also places some limitation on the maximum size of coarse
aggregate which may be employed in the samples. The mold in the ASTM test is
3"
X 3" X 10" and the rod in the restraining cage is disposed centrally of the
mold,
~~ limiting the size of the opening available for concrete entry and
placement. This
makes it difficult to fabricate samples with concrete containing large
aggregate. In
- 2 -
~1 ~~~ 71
fact, the ASTM standard has a caution regarding the use of concrete containing
coarse aggregate with a maximum size greater than about one inch.
Finally, measurement of length changes in the ASTM C878 test sample is time
consuming and must be done in a laboratory by a skilled technician. Thus,
there is a
~~ considerable expense involved.
It is therefore desirable to have available a method and apparatus for
performing
an on-site concrete expansion test. It is also desirable to have such a method
and
apparatus which requires less skill and time to perform as the currently
available
method. Further, it is desirable to have a method and apparatus for testing
curable
~~ mixtures which provide instantly available information and which is not
limited to the
aggregate sizes as is the C878 test.
SUMMARY OF THE INVENTION
The present invention is based upon the discovery that measurements of
expansive volumetric changes in hardenable curable concrete samples may be
~~ performed by utilization of an apparatus which comprises a container having
an open
top and a cylindrical sidewall for receiving the curable sample therein. A
strain gauge
is secured to the sidewall of the container and is disposed to sense
circumferential
expansion with the container as the volume of the sample changes. When
energized
by an electrical source, the strain gauge produces an output which may be
correlated
~~ with the corresponding expansion and contraction of the sample.
- 3 -
CA 02155471 1999-11-16
In a method for measuring volumetric changes in hardened concrete samples,
the apparatus above described is utilized for sensing such volumetric changes
shortly
after casting without demolding the sample.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a fragmentary schematic illustration of a testing apparatus for
measuring volumetric changes in a curable concrete sample;
Figs. 2A-2D are comparative plots of expansion over time of test samples
performed according to a known method and the method of the invention; and
Fig. 3 is a plot of data collected by both methods illustrating the
correlation of
to II the methods.
DESCRIPTION OF THE INVENTION
Fig. 1 illustrates an apparatus 10 for testing expansive
volumetric changes, of a hardenable sample 11, such as shrinkage
compensated concrete. The apparatus 10 comprises a container 12
in the form of a right regular cylinder having a closed bottom 14
an uninterrupted upstanding cylindrical sidewall 16 and an open
top 18. The cylinder 12 has at least one strain gauge 20 attach
to the exterior of the upstanding sidewall 16 by a suitable
adhesive, as illustrated. The staain gauge is electrically
connected to a meter or detector 22. Changes in the length of th
strain gauge 20 cause a corresponding change in the meter output.
- 4 -
2155~17I
Measurements are made by coupling the strain gauge 20 directly to the detector
22. If desired, measurements may be taken manually and periodically recorded.
Alternatively, a digital data logging or pen recording device 24 may be
coupled to the
output of the detector 22. Additionally, a switch 26 having one or more inputs
28,
~~ may be coupled between the gauge 20 and the detector 22. If desired,
multiple
readings from other samples may be selectively taken by coupling the outputs
of a
plurality of containers 12 to corresponding ones of the multiple inputs 28 of
the
switch 26. The switch 26 may then be used to electronically or manually scan
the
inputs 28.
~~ The detector 22 illustrated in Fig. 1, may be implemented by means of a
bridge
circuit such as a Wheatstone strain bridge having a constant voltage supply.
The bridge may be P350A strain bridge manufactured by Vashay Instruments
of Measurements Group, Inc. Such a device was used to collect the data
referred to
in Figs. 2A-2D and Fig. 3. The bridge had a switch to increase the range of
the
~~ instrument and a dummy gauge was used for all the measurements. A more
advanced
type of digital bridge, for example, a Vashay 3500 provides strain
measurements
equaling those of the P350A. The Vashay 3500 is somewhat easier to use. Both
instruments may be utilized in the so-called quarter bridge mode. The dummy
gauge
is preferred when temperature excursions are anticipated. The quarter bridge
mode
~~ is somewhat easier to implement. Another useful strain bridge is a Baldwin
SR4 strain
bridge which is excited by a constant voltage AC source.
- 5 -
2155471
Strain gauges of various types are well known. For example, strain gauges may
be formed of grids of thin wire bonded to a paper substrate. When these
devices are
glued to a structural member, strains in the member are translated into a
changing
resistance in the gauge. Another type of gauge is a foil gauge prepared by
sputtering
~~ a grid of vacuum deposited metal on a foil substrate. Other gauges employ a
semiconductor material. In the present invention, a CEA-060-250-UN-350 foil
gauge
was used to take the measurements hereinafter discussed.
Referring again to Fig. 1, the container 12 has an overall height H and a
diameter D. The sample 1 1 is poured into the container 12 to a sample height
HS.
~~ Curing fluid 32 is deposited or poured over the sample 1 1 to a height H~.
The sample
1 1 comprises various components including cement 33 and aggregate 34.
According
to the invention, the maximum aggregate size is not limited to one inch as in
the
described prior arrangements, because the interior of the container 12 is
unobstructed.
For example, in an exemplary embodiment, the container 12 has a diameter D
which
~~ is about 6 3/4 inches and an overall height of about 8 1 /4 inches. The
sample height
HS is about 7 inches and the curing fluid height H~ is about 1 inch.
In accordance with the invention, the sample of concrete 1 1 is placed in the
container 12 to within about one inch of the top 18. In practice, the concrete
1 1 is
compacted in three layers, each of which is rodded 25 times using a standard
~~ concrete testing placement technique. Immediately after the concrete is
compacted
and finished, the curing water 32 is poured over the concrete 1 1 to a depth
of about
- 6 -
21 ~~~ ~1
1 inch. The filled pail is placed where it will not be disturbed, and where
its
surrounding temperature will remain approximately constant while measurements
of
concrete expansion are made.
The initial concrete sample 11 is relatively stiff and begins to harden in the
~~ container 12 soon after placement therein. Upon hardening, expansion of the
sample
1 1 is transferred laterally to the container 12. This expansion is sensed by
the strain
gauge.
The present invention has a number of advantages over the previously described
ASTM standard test. For example, the test may be performed by personnel with
little
to ~~ training. Also, according to the invention, the sample 11 need not be
demolded to
make measurements. Indeed, data recording begins as soon as the material is
properly
placed in the container 12 and begins to harden. Thus, there is no delay
before useful
data may be acquired, and it is unlikely that data will be lost due to
destruction of the
sample.
~~ Also, as previously noted, samples containing large maximum sizes of coarse
aggregate may be tested. This is important because these materials minimize
concrete
drying shrinkage. In the prior method, placement of the restraining cage
limits the
maximum aggregate size to about one inch. In the present invention, the
maximum
aggregate size may be increased up to about 1 /3 of the container diameter. In
the
~~ arrangement illustrated, the maximum aggregate size is about 2".
2155471
Finally, the present invention allows the test to be conducted on the
construction site without the need for transferring the samples to a
laboratory. Thus,
the job supervisor has physical possession and control of the samples. Also,
current
expansion information which may be utilized to tailor how the job is completed
is on-
II site.
Figs. 2A-2D are comparative plots illustrating test data obtained utilizing
the
standard method and the method according to the present invention. As can be
readily appreciated, there is a correlation between the data obtained by both
methods.
Some of the data obtained in accordance with the standard method, however,
appears
to ~~ to be erratic. This may be due to the difficulty associated with making
standard
measurements.
As illustrated in Figs. 2A-2D, the initial expansions of the concrete filled
container occur at a faster rate than those associated with the standard
method.
However, the faster rate may to some degree be caused by the plotting
convention,
~~ because no compensation is made in the graphs for the delay of several
hours in
reading the standard samples. Generally, the maximum allowable expansion is
limited
to an amount, e.g., 0.15% where concrete deterioration occurs. In general,
acceptable expansion occurs in a range of about 0.02% and about 0.1 %. A
particularly useful range of expansions is about 0.03% and 0.07%. The data
from
~~ both tests generally intersect at an expansion of about 0.03% which is near
the lower
end of the range and is generally considered to be optimum for floor slabs.
_ g _
2154 7I
Consequently, the measurement results achieved by both methods appear to be
about
the same in the range of optimum expansion. When the expansions are greater
than
0.03%, the standard method indicates expansions that are greater than those
produced by the present invention. Conversely, for expansions less than 0.03%,
the
~~ standard method is usually, but only slightly, greater than those indicated
by the
present invention.
Fig. 3 is a comparison of the standard method as a percent of expansion versus
the expansion as a percent in accordance with the present invention. The data
is a
random compilation of data points taken from both methods. The data were taken
to ~~ either at the ultimate expansion or at a four day cutoff point. The
clustering of the
data indicates that the standard method and the method in accordance with the
present invention correlate well in the optimum range. As can be appreciated
from
Fig. 3, there does not seem to be a straight line relationship between data
points taken
by the standard method and that of the present invention. The curved dashed
line
~~ appears to provide a better fit for the data points. This suggests that a
more
complicated relationship between the test procedures may exist. However, as
noted
above, in the lower expansion ranges, the methods correlate well.
Differences between data obtained by the standard method and the present
invention are probably affected by the fact that the container 12 has a
greater
~~ stiffness than the single threaded rod of the standard method. Further, the
container
12 confines the sample 1 1 more completely than the restraining cage of the
standard
_ g _
2I ~~~ ~
test. For example, the bar in the restraining cage secures the concrete only
along one
axis, whereas the container 12 restrains the sample radially in 360°.
Also, the sample
11 in the container 12 is farther from the source of curing water 32 which is
deposited on the top of the sample. The concrete in the comparatively thin bar
in the
~~ standard method is immersed completely in the curing water. The lack of
curing water
at the depth where the strain gauge is located appears to limit expansion more
quickly,
while the molded bars in the standard method continue to expand. The exposure
to
curing water on the top and the radial restriction of the sample in the
container 12 are
probably closer to conditions inside a typical concrete slab than the
conditions which
II occur in the standard test. The data therefore indicates that the method
can be used
to closely approximate the standard test values, especially when concrete
expansions
are near the optimum of about 0.03%.
The simplicity of the present invention, the capacity of the container to
accommodate concrete with large size coarse aggregate, the ease of recording
strain
~~ gauge data and the availability of data at the test site provide great
advantages over
the prior method. Most importantly, however, is the elimination of much human
error
caused by the difficulty of implementing the prior arrangements.
In an exemplary embodiment, the container 12 is a 26 gauge steel cylindrical
one gallon pail, (approximately 8 1 /4"H X 6 3/4"D) having a welded seam,
~~ manufactured by Central Can and available through Palmer Supplies Co.,
Cleveland,
Ohio. Extruded cans, if available, may also be used. The strain gauge is a CEA-
06-
- to -
~1 ~~4 71
250uw-120. An SB-10 switch balance may be employed for connecting up to 10
pails to a P-3500 strain indicator with 326DFV wire, these components are
manufactured by and available from Measurements Group, Inc. of Raleigh, North
Carolina. The strain gauge may be secured to the container 12 by a
cyanoacrylate
~~ adhesive sold under the name M-Bond 200 by Micro-Measurements and available
from
Measurements Group, Inc.
While there have been described what are at present considered to be the
preferred embodiments of the present invention, it will be apparent to those
skilled in
the art that various changes and modifications may be made therein without
departing
1o ~~ from the invention, and it is intended in the appended claims to cover
such changes
and modifications as fall within the spirit and scope of the invention.
- 11 -
