CONCRETE TESTS
SAMPLING OF CONCRETE
Taking concrete samples at the point-of-placement rather than the
point-of-delivery is always
preferable.
Special situations may occur which create difficulties in sampling and
transporting
samples taken from the point-of-placement.
When this occurs, sampling at the point-of-delivery is
satisfactory but only after running correlation tests between the 2 sampling
locations.
(Caution: The correction factors developed between the 2 sampling locations
may vary due to changing conditions such as air and concrete temperature
changes, pumping distances, etc.
Perform correlation tests several times throughout the day.)
If necessary, modify sampling procedures to fit a specific situation.
If the slump test result is outside of the slump range, correct before placing
the concrete in the work. Make corrections as follows:
Slump too low:
Add water in measured amounts to bring the slump within the specified
range.
Record the added water on the Certificate of Compliance.
Slump too high:
Add additional cement by full bag to bring the slump within the
specified range.
Provide cement from the same manufacturer as the original batch.
Record the added bags of cement on the Certificate of Compliance.
After the addition of water, cement, or air-entraining admixture, remix the
batch for 50 revolutions at mixing speed to insure adequate dispersion of the
materials throughout the batch.
Retest to verify compliance with the Specifications. If the concrete is placed
in the work, report both tests on the Weekly Concrete Report with a
notation of the amount of water, cement or air-entraining admixture
added.
The number of revolutions at mixing speed shall not exceed 150 per
Specification. Mix the concrete at agitating speed for all revolutions over
150.
Obtain samples for strength tests by the same procedure as for slump or air
test, except obtain the
sample from the middle half of the load whenever possible.
If the sample is transported to a different location for specimen fabrication,
remix the sample to ensure uniformity. The following
sections provide procedures for sampling from various mixers.
Start slump, temperature, and air content tests within 5 minutes after
obtaining the sample of fresh concrete. Start strength tests within 15 minutes
of obtaining the sample.
SAMPLING FROM STATIONARY MIXERS
Sample the concrete by either passing a container through the discharge
stream of the mixer or by diverting a portion, or the entire discharge
stream into a sampling container.
When a container is passed through the discharge stream, sample the entire
stream to reduce segregation that may occur as the material leaves the
mixer. When sampling by diverting the discharge, diversion of one-half the
discharge is satisfactory provided it is full-depth.
SAMPLING FROM READY-MIX CONCRETE TRUCKS
Normally, take samples during the discharging operation.
Obtain samples by collecting the full width of the discharge stream of the
chute into a sampling container. Control the rate of discharge
from the truck at a satisfactory rate for sampling. Do not reduce the rate
of discharge by closing or reducing the size of the discharge opening of the
truck.
You may need to stop the discharge from
the drum to collect the full width and depth of the discharge stream on the
chute into the sampling container.
Provide a sample of sufficient size to perform the tests without reusing the
same concrete for subsequent tests. Usually samples are not taken from the
first one-quarter or last one-quarter of a cubic meter (cubic
yard) of truck discharge.
Should these or other portions of the discharge indicate improper mixing,
reject the concrete. It is recommended that after sampling; stop further
discharge from the mixer
until the tests are completed.
If the slump and air content meet requirements, complete the discharge. This
delay in placing concrete is extremely important when the test is on the
first load
of concrete of the day.
CONCRETE AGE-STRENGTH & RELATIONSHIP
Concrete strength varies with age. Under continuous favorable conditions,
concrete continues to gain strength indefinitely. Samples of concrete taken
from old pavements and tested in compression indicate higher strengths after
25 years than when the pavement was 1 to 3 years old.
Tests for concrete strength are made shortly after it has been placed (7 to
90 days).
It is always tested for acceptance at fairly early ages with respect
to the concrete life and for this reason the
strength obtained is less than the ultimate strength the concrete will
attain.
Flexural strength and compressive strength develop at different rates within
the concrete.
Except for the first few days, the rate of strength gain is greater in
compression than in tension (flexural).
COMPRESSIVE STRENGTH TESTS
Strength tests are required for one or both of the following purposes:
1. To check the potential strength of the concrete under controlled
conditions against the desired
strength; and
2. To establish a strength-age relationship for the concrete under job
conditions as a control for construction operations or the opening of the
work.
Tests made for the first purpose are referred to as standard tests and those
for the second purpose are referred to as control tests.
For uniform and comparable results, follow a standard and consistent
procedure in making all of
the test specimens whether they are used either for standard or for control
tests.
Twenty-four hours after casting, place the cylinders in water at a
temperature of 15 to 25°C (60 to 80°F) for a period of 12 to 14 days.
You do not need to remove plastic molds when curing cylinders in water
however, remove the caps during the curing period.
Transport the cylinder, in a capped plastic mold to prevent moisture
loss, to the laboratory for further curing and testing.
Adequately protect the specimens to avoid shipping damage. Completely fill
out the Concrete
Cylinder ID card so that the cylinder is positively identified when it
reaches the Office of Materials Laboratory. Number cylinders made on each
contract in consecutive order, beginning with number 1.
Show the cylinder numbers on both the identification card and on
the Weekly Concrete Report for the concrete mix used.
Cylinders that have the suffix “C” after the numbers are considered control
cylinders and are cured the same as the structure.
The standard cylinders are tested at 28 days, unless for special reasons,
tests at different ages are
desired.
Control cylinders should indicate the desired test age. If the test age is
not given, they will be tested at 28 days.
Some of the factors that can result in failures are listed below. Take
precautions so none of these occur on the project.
The first 3 factors do not affect the strength of the concrete in the
structure, but cylinder test results do indicate lower strengths than the
actual structure.
The last 5 conditions affect the quality of the concrete in both the
structure and in the specimens.
1. Improper molding of the specimens may result in honeycombed sections in
specimens.
Honeycombing indicates improper molding methods, segregation of the coarse
aggregate or a
batching error resulting in excessively high rock content of the mix.
2. Towards the end of each construction season, cylinders that broke below
80% of anticipated strength are checked to determine whether casting
procedures were proper.
In many cases the
tops were poorly finished with open texture and projections. Identifications
scribed in the tops are also observed as well as “nibs” on the bottom caused
by striking the bottom of the mold with the rounded, semi-hemispherical rod.
All cause reduced strength.
3. Inadequate curing of the cylinders may result in low strengths. Concrete
subjected to poor curing conditions in the first 48 hours may never develop
its potential strength.
Cylinders that
are exposed to frost, direct rays of the sun on warm days, and to adverse
dry conditions during early age are also affected.
Low temperatures and lack of moisture retard strength gain, yielding 28-day
strength values lower than expected; however, the concrete will
eventually
develop its full strength.
4. Rough handling of cylinders, particularly during early ages, will cause
fractures or planes of weakness in the cylinders. Disturbance of any nature
during the setting period may reduce the
concrete strength.
5. A decrease in the cement content of the mix by either withholding part of
the cement per batch or by increasing the quantity of aggregates per batch
reduces the concrete strength.
During the
proportioning operations, assure that the proper quantities are measured
out. Where sacked cement is furnished, assure the full content of each sack
is placed in the mixer.
6. The use of dirty or contaminated aggregates results in a lower bond
strength between the cement paste and the aggregates, thus producing a
poorer quality cement paste, thereby
reducing concrete strength.
7. Increasing the water content of the mix for easier workability without
increasing the cement content to compensate for the additional water results
in a weakened cement paste.
Use
sufficient water to give the desired consistency; additional water will
shorten the life of the
concrete structure.
Water above the amount in the batch design results in a weaker
cement
paste along with a decrease in strength.
8. Decreasing mix time, either intentional or otherwise, may leave portions
of both sand and gravel that are uncoated with cement paste. Worn mixing
blades can also result in reduced mixing
action.
CASTING CYLINDERS
In making the test specimens, place molds on a level, firm foundation in a
sheltered place where they can remain undisturbed and protected from direct
sunlight and from temperatures below 15°C
(60°F) for at least 24 hours.
If metal molds are used, lightly oil the inside before placing
concrete in them. If a suitable casting site is not available in the
immediate proximity of the work, transport the concrete and cast specimens
to a location where they are kept undisturbed for the initial 24-hour
period.
If sampling has caused segregation, re-mix by hand shoveling prior to
casting the test
cylinder.
Mn/DOT standard cylinder mold size is 100 x 200 mm (4 x 8 in.).
If aggregate has a maximum
size greater than 31.5 mm (1 1/4 in.), use 150 x 300 mm (6 x 12 in.)
molds.
White is Mn/DOT’s standard color for concrete cylinder molds. Mn/DOT also
recommends using
domed lids instead of flat lids.
This is to discourage the practice of stacking cylinder molds
containing plastic concrete on top of one another.
This is to eliminate one possible cause for low cylinder strengths.
White cylinder molds do not attract nearly as much heat from the sun as
the
black ones do.
Standard cylinders must be kept in a protected area, out of the sun and
protected
from temperatures below 15°C (60°F), and undisturbed for the initial 24-hour
period.
Cylinder Casting Procedure
1. Use steel, fiber, or plastic molds.
2. Cast on flat surface.
3. Start casting cylinder within 15 minutes
of taking a representative sample of concrete.
4. Place the concrete in the mold and rod each layer 25 times per layer
in 2 equal layers for 100 x 200 mm (4 x 8 in.) cylinders using a 9.5
mm (3/8 in.) diameter hemispheric-nosed steel rod.
For 150 x 300 mm (6 x 12 in.) cylinders, rod concrete 25 times per layer in
3 equal layers with a 16 mm (5/8 in.) diameter, hemispheric-nosed steel rod.
Uniformly distribute the rodding over
the cylinder area and penetrate slightly into the previous layer when
tamping the second and/or
third layers.
5. Tap the sides of fiber and plastic molds after each layer lightly with
the hand prior to the strike-
off to remove entrapped air bubbles on the sides of the mold. When using
steel molds, tap the
mold lightly with the mallet.
6. After rodding the top layer, strike-off the surface evenly using a wood
float or other suitable
object. Do not use the cap as a finishing tool.
7. When casting cylinders in plastic molds, place the cylinder cap on the
mold immediately after
finishing the cylinder. Place adequate curing medium on other molds. Do not
disturb for 24
hours.
8. Place SP and Field ID on cylinder mold, not on cylinder surface. Do not
mark caps since they
are reused. Do not etch identification into the surface of the cylinders.
When using metal
molds, transfer the ID to the top surface of cylinder and strip metal
molds.
9. Place in curing tank. Cylinders in plastic molds may cure in water tank
with caps off.
10. Fill out Concrete Test Cylinder ID card
11. Protect cylinder from moisture loss and shipping damage. Ship
sufficiently prior to testing
date. Ship cylinders in plastic molds if the tops are sealed.
Protect the cylinder from moisture loss and extreme temperature change after
casting. Control
moisture loss by Placing the specimen in an insulated box containing
moisture
• Covering with polyethylene sheeting or bags
• Covering with a minimum of 4 thicknesses of wet burlap
• Covering with waterproof paper
• Placing caps on plastic cylinder molds
Protect specimens from the direct heat of the sun and from extreme changes
in temperature.
In
cold weather, keep specimens in a heated enclosure and do not allow
specimens to freeze.
Fill out the sample card completely, including the source of concrete when
ready-mix concrete is
used.
Enter the full name of the concrete source each time a card is made out.
FLEXURAL STRENGTH TESTS
These routine tests are usually made only on paving jobs and are tested at
the job site.
Rehabilitation projects requiring early openings may also utilize flexural
tests.
As with all testing practices, the value of the standard flexural test
results depend entirely on
uniform adherence to the standard procedure outlined below. Tests are
normally made at the
standard ages of 7 and 28 days.
High early strength concrete may require additional beams to
verify opening strengths.
Record the test data for all beams on Concrete Test Beam Data (Form 2162)
and submit to the
Mn/DOT Concrete Engineering Unit every week after the 28-day specimens are
tested. Whenever
there is a change in the mix or in the source of any of the materials,
report this data so that only one
set of conditions are on a sheet.
CASTING BEAMS
Carefully make the beams of representative concrete as follows:
1. Obtain enough concrete to cast 7 and 28-day specimens. Casting should
occur within 15
minutes of obtaining the representative concrete sample.
2. Clean and oil molds.
3. Cast on a flat surface.
4. Place concrete and rod 65 times per layer in 2 equal layers with a 16 mm
(5/8 in.) rod. Spade
along edges after each layer. Tap lightly along inside and outside edges
after each layer.
5. Strike off surface with a straight edge and finish with a trowel or wood
float.
6. Impress ID into the surface within 150 mm (6 in.) from outside edge. (If
placed near the third
point, the impressions of the numbers might adversely affect the test
results.)
7. Adequately cure and do not disturb for 24 hours.
8. Place in curing tank.
9. Test beams according to the procedure.
NOTE: Control beams are intended to verify opening strengths and therefore
cure them in a
similar manner as the pavement. In cases of late season paving, you may cure
control beams in
water tanks to avoid freezing and assume there is an equal trade off between
the moist cure of the
sample and the additional heat generated by the slab. In any case, ACI
requires moist curing
beams for at least 24 hours immediately before testing in water saturated
with calcium hydroxide at
23 ± 2°C (73 ± 3°F).
Number the beams in consecutive order for each contract, beginning with
number 1.
Where more
than one project is included in one contract, DO NOT use separate series of
numbers. The
numbers of the beams made each day and the station where they were cast are
entered on the FLEXURAL STRENGTH TESTS .
These routine tests are usually made only on paving jobs and are tested at
the job site.
Rehabilitation projects requiring early openings may also utilize flexural
tests.
As with all testing practices, the value of the standard flexural test
results depend entirely on
uniform adherence to the standard procedure outlined below. Tests are
normally made at the
standard ages of 7 and 28 days. High early strength concrete may require
additional beams to
verify opening strengths.
Record the test data for all beams on Concrete Test Beam Data (Form 2162)
and submit to the
Mn/DOT Concrete Engineering Unit every week after the 28-day specimens are
tested. Whenever
there is a change in the mix or in the source of any of the materials,
report this data so that only one
set of conditions are on a sheet.
CASTING BEAMS
Carefully make the beams of representative concrete as follows:
1. Obtain enough concrete to cast 7 and 28-day specimens. Casting should
occur within 15
minutes of obtaining the representative concrete sample.
2. Clean and oil molds.
3. Cast on a flat surface.
4. Place concrete and rod 65 times per layer in 2 equal layers with a 16 mm
(5/8 in.) rod. Spade
along edges after each layer. Tap lightly along inside and outside edges
after each layer.
5. Strike off surface with a straight edge and finish with a trowel or wood
float.
6. Impress ID into the surface within 150 mm (6 in.) from outside edge. (If
placed near the third
point, the impressions of the numbers might adversely affect the test
results.)
7. Adequately cure and do not disturb for 24 hours.
8. Place in curing tank.
9. Test beams according to the procedure described.
NOTE: Control beams are intended to verify opening strengths and therefore
cure them in a
similar manner as the pavement. In cases of late season paving, you may cure
control beams in
water tanks to avoid freezing and assume there is an equal trade off between
the moist cure of the
sample and the additional heat generated by the slab. In any case, ACI
requires moist curing
beams for at least 24 hours immediately before testing in water saturated
with calcium hydroxide at
23 ± 2°C (73 ± 3°F).
Number the beams in consecutive order for each contract, beginning with
number 1. Where more
than one project is included in one contract, DO NOT use separate series
of numbers. The
numbers of the beams made each day and the station where they were cast
are entered on theAfter casting, the specimens are left uncovered only
until the identification numbers are inscribed
and then covered with wet burlap, impermeable paper, or polyethylene
plastic and left undisturbed
for 24 hours. Protect these test specimens from direct sun and extreme
temperatures.
When 24 hours old, carefully move the specimens in the molds to the
testing locations. Exercise
care in this operation to avoid damage to the beams. DO NOT throw or drop
at any time. Upon
arrival at the testing site, remove the beams from the molds and store the
specimens in saturated
limewater at 15 to 25EC (60 to 80EF) until the 7 or 28-day testing date
has arrived. DO NOT
subject the specimens to water temperatures lower than 15°C (60°F).
TESTING BEAMS FOR FLEXURAL STRENGTH
Nearly all State Departments of
Transportation use third point beam
breakers for flexural testing of concrete
pavement. By using third point beam
breakers, Mn/DOT can share its test
results with other states and adapt the test
results for use with AASHTO pavement
design equations.
A. Before the First Use of the Beam
Breaker After Transportation or
Storage
1. Release all four case latches and lift
the cover straight up.
2. Place the cover at the desired operating
location and set the apparatus squarely
on the top of the cover. See Figure A
5-694.522.
3. Remove any ties from the recorder or
other components.
B. Recording Charts
Third point loading recording charts are used to determine the Modulus of
Rupture for each
specimen. Proper use of the testing machine will apply a rate of loading
between 861 and 1207
kPa (125 and 175 psi) per minute for any beam whose cross section dimensions
are 150 x 150 mm
(6 x 6 in.), ±8 mm (±5/16 in.). Four different spirals labeled A, B, C, and
H are indicated on each
recording chart. See Figure B 5-694.522. The chart in Figure E 5-694.522 is
a guide to use in
determining which spiral to use in testing each specimen. Note the exact
dimensions of the
specimen to determine which spiral to use. The chart is also used to correct
the Modulus of
Rupture values for specimens that do not have an exact 150 x 150 mm (6 x 6
in.) cross section. The
spiral labeled H, is used for loading specimens at a rate of 861 kPa (125
psi) per minute, and is not
used by Mn/DOT.
C. Test Procedures
1. Remove the plastic dust cover.
2. Close the control valve and pump the loading head approximately 6 mm (1/4
in.) until the
piston floats.
3. Open the recorder door and install a recording chart: slip the edge of
the selected chart under the
pen lifter and over the open chart hub. See Figure C 5-694.522. Do not clamp
(in order to
rotate the chart manually for checking).
a. Adjust Zero - Adjust the cam as necessary until the pen traces the zero
circle, turn the cam
with your fingers: clockwise to raise the pen and vice versa. The piston
must float under no
load.
b. Check the friction - Move the pen arm up the chart approximately 25 mm (1
in.) and release
it. It must return to zero without help.
c. Correct pen adjustment occurs when:
• The pen arm assembly is fully inserted and clamped in the pen arm holder.
Check that
the two pen arm fingers are under both rivets and clamped with the
screw.
• The pen point trough is at approximately right angles to the chart both
vertically and
horizontally; flexing the pen arm and holding it close to the pen point
adjust this.
• The pen point just makes contact with the chart. Too much pressure will
cause skipping.
Pressure is adjusted by flexing the pen arm.
• The length of the pen arm radius will trace a minute arc or a pen tracking
arc on the
chart. This length is important because an incorrect pen arm radius will
generate an error
directly proportional to the error in its length. To check this adjustment:
swing the pen
point to the maximum travel; rotate the chart and hand clamp it so that the
pen point rests
precisely on an arc at maximum reading; allow the pen to draw its own arc as
it returns to
the zero circle. Examine the coincidence of two arcs. If the two arcs vary
more than 1
mm (1/32 in.) and are not corrected by adjustments as described in 1 and 2
above, the
recorder needs calibration. You can also test the arc by tracing the
radius
4. With the beam on its side in relation to its position as molded, measure
the width (b) and depth
(d) at the center of the specimen. Take measurements to nearest 0.5 mm (0.02
in.). Use good
outside calipers and a steel machinist’s rule. See Figure D 5-694.522. Using
these
measurements turn to Figure E 5-694.522 to determine which spiral you should
trace.
5. Use the key and wind the chart drive clockwise approximately one full
turn. See Figure F 5-
694.522. Rotate the chart to its starting position, and then clamp the
recording chart.
6. With the beam on its side in relation to its position as molded (same
position as used for taking
the previous measurements), insert the beam from either end of the apparatus
between the tie
rods. Center the beam in relation to the four tie rods and have at least 25
mm (1 in.) of concrete
protrude outside of each of the two cross-head bearing blocks. See Figures G
and H 5-694.522.
7. The chain drive provides a quick method of adjusting the crossheads
synchronized in height so
that the load is applied normal to the beam. To operate, grasp opposite
sides and move
simultaneously clockwise to bring them down into contact with the beam, and
counterclockwise
to raise them. Stop as soon as they touch the beam; further loading is
hydraulic.
8. Close the Control Valve clockwise. Counterclockwise rotation of the pump
handwheel will fill
the pump with oil and clockwise rotation will introduce it into the main
hydraulic system to
apply load. The centerhead is quickly raised to establish initial contact
with the beam by
rapidly spinning the hand wheel clockwise. As soon as contact is established
and the loading
block(s) seated (the recorder pen will show a small load), refill the
pump.
9. Carefully trace the spiral of the rotating chart and load the beam until
failure.
10. Open the control valve (counterclockwise two turns only), to allow the
piston to retract by
gravity, raise the crossheads and remove the beam sections.
11. Unclamp and remove the chart. After the end of the test, the chart drive
will tick away until run
down (clamp loop inverted). NOTE: You can rewind the chart drive at any
time.
12. Fill in all pertinent test result data. Measure the average width and
average depth of the
specimen at the section of failure, and record. If the beam does not measure
exactly 152.5 x
152.5 mm (6 x 6 in.), refer to the chart in Figure E 5-694.522. This chart
is stored in the plastic
holder on the front of the recorder door. Specimens must break in the middle
one-third or 228.5
mm (9 in.) of the beam or they are not acceptable.
CONSISTENCY (SLUMP) TESTS
Check and control the consistency of the concrete during each pour.
According to Specification
2461.4A4a, the slump test is a measure of the consistency of the concrete.
The consistency
therefore is a measure of the water content of the concrete. The water
content controls and affects
the cement content of the concrete. Since the slump test is important, do
not substitute a guess for
an actual test.
The minimum tests required according to the Schedule of Materials Control
may not provide a
sufficient number of tests to assure quality. An individual slump test may
not indicate the true
consistency of the concrete because of unavoidable variations in the
composition of the concrete
and because of variations in the manipulation of the concrete. For this
reason, take several tests to
obtain a true average value. On small pours, one test may not provide
sufficient information to
assure quality.
Enter the results of slump tests on the Weekly Concrete Report (Form 2448).
See Figure B 5-
694.727.
The consistency of concrete mixes, in terms of millimeters (inches) of
slump, is determined by
their relative water contents. Thus, a given change in the water content of
a mix will result in a
corresponding change in slump. The percentage change in water content per
millimeters (inches)
of slump change is not constant over the whole range of consistency. It is
greater at the dry end of
the range and less at the wet end. Only use this procedure when the plant
has held back water from
the design mix and the slump is less than required by the mix design. The
added water to increase
the slump cannot exceed the design water by more than 4% per Specification
2461.3J(2).
Guidelines for the approximate changes in water content, in percent, for
various changes in slump
are shown in Table A 5-694.530.
To illustrate the use of the table, assume the water for a given mix will
produce a 50 mm (2 in.)
slump. If a 100 mm (4 in.) slump is desired with this mix, increase the
water content 7.9%. In
another illustration, the water content is decreased 10.3% to reduce the
slump from 125 to 50 mm (5 to 2 in)
CONSISTENCY (SLUMP) TEST PROCEDURE
Equipment needed:
• Slump cone in clean and good condition
• Smooth, rounded 16.0 mm (5/8 in.) diameter steel rod with a rounded
tip
Consistency (Slump) Test Procedure:
1. Obtain a representative concrete sample. Start test within 5 minutes of
when sample was
taken.
2. Dampen the slump cone and place on a flat, moist, non-absorbent and rigid
surface; hold the
cone firmly in place by standing on the foot pieces.
3. Immediately fill the cone in 3 layers, each layer approximately one-third
the volume of the
mold or about 67 mm (2 5/8 in.) for the first layer and 155 mm (6 1/8 in.)
for the middle layer.
4. Rod each layer with 25 strokes of the tamping rod. Uniformly distribute
the strokes over the
cross-sections of each layer making approximately half of the strokes near
the perimeter, then
progress with vertical strokes spirally toward the center, slightly
penetrating into the
underlying layer. In rodding the top layer, an excess of concrete is
maintained above the top
of the cone. After the top layer is rodded, the surface of the concrete is
struck off even with
the top of the cone.
5. Remove any excess spillage of concrete from around the base of the cone
and lift the cone
clear of the concrete allowing the concrete to settle or slump under its own
weight. Slowly
lift the cone vertically and carefully to secure a proper result, with the
lifting operation taking
approximately 3 to 7 seconds.
6. The amount of slump is measured immediately after the mold is lifted by
placing the rodding
bar across the inverted mold and measuring from the top of the mold to the
displaced original
center of the top of the concrete. Record the slump as measured to the
nearest 5 mm (1/4 in.).
7. If the slump test shows results higher than the maximum of the 25 mm (1
in.) slump range
specified in 2461.3B3, notify the Contractor of the results and test the
next load. When
testing the next load, do not allow the placing of any concrete until the
test shows acceptable
slump.
Notes:
• Specification 2461.4A4a allows an additional 25% above or 50% below the
upper end of the
specified range on an occasional basis. This does not change the slump
range. This is
intended to give the Contractor some time to correct any problems they may
have and get the
slump back within the range as soon as possible. If any load’s Certificate
of Compliance
shows the water used (including any added on site) is greater than mix
design water subject to
2461.3J(2), reject the load and do not place into the work.
• Water reducers are currently more frequently used. According to Mn/DOT
Specifications,
water reducers must have the Engineer’s approval for use. If approval is
granted, investigate
any slump failures to verify if the high slump is due to the water or
admixture. The price
reductions listed in the Schedule of Price Reductions for high slumps,
especially bridge deck
concrete, were originally based on high slumps due to high water. High
slumps caused by
admixture may not warrant the price reduction imposed similar to that caused
by water. In
any case, the slump of the concrete should remain constant.
VERIFICATION OF WATER IN FRESH
CONCRETE BY USE OF A MICROWAVE OVEN
Mn/DOT has incorporated the use of AASHTO TP23-93 to verify the Contractor’s
water in
concrete pavements. This test uses a microwave oven to drive the water out
of fresh concrete.
Mn/DOT has an incentive/disincentive program for the water-cementitious
(w/c) ratio. This
incentive is based on the Contractor’s actual batch weights and is verified
using the microwave.
Knowing the weight of the fresh concrete and the weight of the dried
concrete, the total water
content can be calculated. This total water content is not the same as the
total batch water content
because the total water content includes all absorbed moisture in the
aggregates. The water
content used to determine the w/c ratio is the batch water added by the
Contractor plus any free
moisture; this does not include the absorbed moisture.
There are many variables in this procedure. The most significant are the
absorption and moisture
of the aggregates. For this reason, Mn/DOT will run new absorption tests on
all aggregate sources
before paving begins. The Paving Contractor will also need this information
since they will be
designing their own mixes. Together with the Agency, the Contractor will
have to submit samples
to the Mn/DOT Office of Materials when they decide which aggregates they are
going to use.
Moisture tests and microwave oven testing are performed by the Agency.
Testing rates are found
in the special provisions.
A. Verification of Water in Fresh Concrete Test Procedure
See 5-694.734 for instructions and the worksheet for
performing the microwave oven test procedure.
B. Definition of Lots and Sublots Involving Water/Cementitious Ratios
For determination of water/cementitious ratio incentive/disincentive, a lot
represents one day of
paving. Paving includes integrant curb and gutter and curb and gutter placed
with the same
mixture as paving. A change in mix design requires beginning a new lot.
Changes in mix design
for small quantities such as hand work and high early mixes greater than 356
kg/m3
(600 lbs/yd3
)
cementitious are not included as part of a lot or sublot for
incentive/disincentive determinations.
All samples for testing are taken in a random manner according to the
prescribed sampling rate. A
minimum of 2 tests and a maximum of 4 tests are required per day. The
minimum sublot size is
250 cubic meters (cubic yards). Gradation testing is not required when
production is less than 250
cubic meters (cubic yards) per day.
If less than 3 sublots are produced in a day before the new mix design is
initiated, the sublots shall
be averaged with the previous lot and included as part of that lot. On the
first day of production or
whenever the mix design is changed prior to the production of 3 sublots,
this production is hereby
defined as a lot. If production is less than 3 sublots, the sublots are
included in the next day’s
production. On the last day of paving or on the last day of using a specific
mix design, the
concrete involved shall constitute a separate lot/sublot unless the above
applies regarding less than
3 sublots.
AIR CONTENT TEST PROCEDURE
A. Operation of the Pressure Meter
(Type B Meter)
1. Obtain a representative sample. Begin air content testing
within 5 minutes of obtaining the representative sample.
2. Dampen bowl. On a level surface, fill container in 3 equal
layers, slightly overfilling the last layer.
3. Rod each layer 25 times with a 16 mm (5/8 in.) rounded tip
rod, uniformly distributing strokes.
4. Rod bottom layer throughout its depth without forcibly
striking bottom of container.
5. Rod the middle and top layer throughout their depths and
penetrating 25 mm (1 in.) into the underlying layer.
6. Tap the sides of the container smartly 10 to 15 times with
the mallet after rodding each layer.
7. Strike off concrete level with top of container using the bar
and clean off rim.
8. Clean and moisten inside of cover before clamping to base. Figure A
5-694.541
9. Open both petcocks.
10. Close air valve between air chamber and the bowl.
11. Inject water through petcock until it flows out the other petcock.
12. Continue injecting water into the petcock while jarring and tapping the
meter to insure all air is
expelled.
13. Close air bleeder valve and pump air up to initial pressure line.
14. Allow a few seconds for the compressed air to stabilize.
15. Adjust the gage to the initial pressure.
16. Close both petcocks.
DO NOT TILT THE METER AT ANY TIME.
17. Open air valve between chamber and bowl.
18. Read the air percentage after lightly tapping the gage to stabilize the
dial.
19. Close the air valve and then open petcocks to release pressure before
removing the cover.
20. Calculate air content:
Air Content = ( meter reading − aggregate correction factor if
requred)
21. Properly report (record) the results.
22. Clean up the base, cover, and petcock openings.
When the test indicates an air content outside the limits specified in
Specification 2461.4A4b, run
recheck test immediately. Record the results of the air tests on the Weekly
Concrete Report
UNIT WEIGHT (DENSITY) TEST
The unit weight (density) test is a measure of the weight per cubic meter
(cubic foot) of freshly mixed
concrete. By knowing the unit weight of the concrete, other information can
be determined such as the
concrete yield and water content for microwave oven testing. See 5-694.734
for the unit weight test
procedure.
5-694.550 TEMPERATURE TEST
The Inspector should determine and record the concrete temperature at time
of placement. Unless
the Special Provisions for the Contract provide otherwise, the concrete
temperature requirement is
in the range of 10 to 30°C (50 to 90°F) per Mn/DOT Specification 2461.4A3.
The Certificate of
Compliance provides a space for the concrete temperature and air
temperature.
On most work, take the temperature with issued thermometers that have been
checked for
accuracy. On massive pours in large bridge piers or abutments, special
installations of electrical
thermocouples are sometimes needed to secure data on the rate of temperature
change as produced
by the heat of hydration of the cement. Such data is very important in
determining how long
protective coverings, forms, etc. shall remain in place before exposing the
concrete to atmospheric Conditions.