Materials selection for concrete overlays : the final report Page: 67
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burlap to prevent any further evaporation of water. After 24 hours, they were removed from the
plastic molds, labeled, and transferred to the moist room until the time of testing, which is kept at
72F and at 100% humidity.
The compressive strength tests were performed using a Forney universal load machine.
The cylinders were loaded at an average rate of 30,000 lbf/min. Each cylinder was properly
centered in the testing machine and neoprene pads were replaced at regular required intervals.
4.4.2 Flexural Strength
The flexural strengths were determined in accordance with ASTM C78 - 10 "Standard
Test Method for Flexural Strength of Concrete." For each mixture, six 4-in. by 4-in. by 14-in.
beams were made. 7-day and 28-day strengths were measured.
The beams were cured for 24 hours in the mixing room. For the first hour, they were
cured uncovered. After an hour, they were covered with wet burlap to prevent any further
evaporation of water. After 24 hours, they were removed from the steel molds, labeled, and
transferred to the moist room until the time of testing. The flexural strength tests were performed
using a third-point set-up in a Forney machine. The beams were loaded at an average rate of
6,000 lbf/min.
4.4.3 Length Change
Length change, due to drying shrinkage, was measured in accordance with ASTM C157
- 08 "Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and
Concrete." The specimens were 3-in. by 3-in. by 11.25-in. beams with Humboldt pins on each
end face.
The sides of the molds were oiled to facilitate removal. Oil was applied before putting
the pins at the end faces to ensure that the concrete would bond to the pins. When placing the
pins, care was exercised to prevent the pins from getting coated with oil. The equipment for
measuring the changes in length was manufactured by Humboldt and had a resolution of 0.0001
in.
Measurements of drying shrinkage were taken at the following stages:
* Casting: soon after mixing, the concrete was poured into specimen beams. The
beams were cured for 24 hours in the fog room.
* Demolding: after 24 hours, the specimens were carefully demolded and transferred
from the fog room to limewater. Measurements were taken after the specimens had
been submerged for 30 minutes (1st reading). The lime water in which the
specimens were submerged to prevent leaching of cement paste constituents was
made of approximately three cups of lime powder for each 5-gallon bucket of water
or until supersaturated.
* Transferring: the specimens were left submerged in the lime water for six days.
One week after the specimens were cast, they were transferred to the environmental
chamber with a relative humidity of 50% and a temperature at around 750F (2nd
reading).
* Measurements: Length change measurements were taken at 4, 7, 14, 28, 56, and
112 days after the specimens were taken outside of the lime water.67
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Kim, Dong H.; Fowler, David W.; Ferron, Raissa P.; Trevino, Manuel M. & Whitney, David P. Materials selection for concrete overlays : the final report, report, July 2012; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth303706/m1/87/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.