Concrete Cores and Beam Preparation
Purpose of this activity is to prepare concrete core and beam
specimens in accordance with ASTM C31. During this lab activity
4’X8” cylinders will be molded to compare relative compressive
strength of different aggregate sizes. Concrete beams will be
molded to test for flexural strength. After molding cylinders
and beams the specimens will cure for a full 28 days at which
time they will be tested for there compressive strength and
flexural modulus.
A slump test is prepared in accordance with (ASTM C 143). it It
has been determined that a slump of less than ½” may not be
adequately plastic and concretes having slumps greater than 9”
may not be adequately cohesive. In adequate amount of water will
not allow full hydration of the cement and excessive amounts of
water will dilute the cement. Both extremes will significantly
weaken the strength of concrete. Under laboratory conditions
with strict control of all concrete materials, the slump is
generally found to increase proportionally with the water content
of a given concrete mixture, and thus to be inversely related to
the concrete strength.
Three sets of 4” X 8” concrete test cylinders will be prepared by
the class. Each set of concrete cylinders will be molded using
three different water to cement ratios determined by a slump
test. After 28 days testing of these cylinders should indicate a
change in compressive strength as the water to cement ratio
changes.
1. Three cylinders will be molded out of a standard Quikrete
mix. With a slump of 3/4” to 1-1/2”.
(approx. 32-34 oz. water)
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2. Three cylinders will be molded out of a standard Quikrete
mix. With a slump of 2” to 3”. ( approx. 48 to 50 oz. water)
3. Three cylinders will be molded out of a standard Quikrete
mix. With a slump of 6” to 8”. (approx. 64 to 68 oz. water)
4. Each cylinder shall be properly labeled on its lid stating,
class section number and mix or slump and oz. of water.
Each plastic cylinder mold will be filled ½ full then rodded 25
times with a rounded end rod. The cylinder will then be filled to
the top and rodded an additional 25 times. Using a rod or trowel,
the cylinders should be leveled off and made smooth. Cylinders
should then be capped with a plastic lid and labeled. This process
will be repeated for all nine cylinders. They then will b e placed
on a plywood panel and stored for 28 days.
Design mixes will be mixed by hand in a wheel barrow:
1.
2.
3.
4.
5.
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Measure our six quarts of Quikrete for each batch of
three cylinders.
Start by adding the above recommended water.
Mix thoroughly with shovel or hoe.
Perform Slump test and add additional water to achieve
desired slump.
Record the actual total amount of water added to the
mix on the lid and class section number.
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Procedures for ASTM C-143 (Slump Test for Hydraulic Cement)
1.
Start the test within 5 min. after obtaining the final portion
of the mixed concrete sample.
2.
Dampen the mold (inside) and place on the dampened base
plate.
3.
Hold the mold firmly in place during the filling and rodding
operation (by the operator standing on the two foot pieces).
5.
Fill the mold in three layers, each approximately one-third
the volume of the mold.
5.
Rod each layer with 25 strokes of the tamping rod. During
filling and rodding the top layer, heap the concrete above the
mold before rodding is started.
6.
Strike off the surface by a screeding and a rolling motion
of the tamping rod
7.
Remove the mold immediately by raising it in a vertical
direction. (Steps 2 through 7 should be completed in less than
2.5 minutes).
8.
Place the empty mold (inverted) adjacent to the concrete
sample and measure the vertical difference between the top of
the mold and the displaced original center of the sample. This is
the slump.
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The concrete beams will be 6”X6”X20” in length. They will be
molded using a standard Quikrete mix with a slump of between
2” and 3”.
Mix up 3- 60lb. bags of Quickrete in a wheel barrow.
Add 6 to 6-1/2 quarts of water and mix thoroughly. Perform a
slump test. The desired slump should be between 2” to 3”. If
slump is less than 2” add a small amount of water and re-mix.
Check slump again. If mix is more than 3” add a small amount of
Quickrete and re-mix. Check slump. If in the desired range fill
beams as directed below.
1. One beam will be molded with no reinforcing bar.
2. One beam will be molded with a #3 reinforcing bar located
centrally through the center of the beam.
3. One beam will be molded with a #3 reinforcing bar located
below the neutral axis of the beam.
All beam molds will be filled half full then rodded 25 times. The
mold will then be filled completely and then rodded an additional
25 times. Using a trowel or bar, level the top surface even with
the top of the mold. Place molds in storage to cure for 28 days.
Be sure to rinse and clean all tools and equipment before
returning to storage.
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