CVE 230 Mechanics of Materials

Lab Report #1

March 5, 2012

LAB REPORT #1

TESTING OF FRESH AND HARDENED CONCRETE

Introduction:

Portland cement is the most common type of cement used throughout the world. This kind of cement is hydraulic because it sets and hardens by reacting with water through the process of hydration. This is the reason for its use in many hydraulic applications.

Type I and II are two of the kinds of Portland cement that were used in the following experiment. Type I is general purpose cement and type II is cement used when there is a need for protection

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The relationship between the time allowed to cure and strength of Portland cement were

investigated in this experiment which involved submitting several samples of concrete to

compression tests and calculating their compression strengths from their failure loads. The

purpose of this experiment was to show how concrete strengthens rapidly over the first few

weeks of hardening and demonstrate how the modulus of elasticity is derived from the

physical properties of concrete. The results from this experiment show the significance of the

increasing maximum strength of the concrete over time with respect to the modulus of

elasticity that can be calculated using the ACI formula or directly from a graph of stress

versus strain of the material. The overall results of this experiment are harmed due to an

error in the first compression test, but the results are nevertheless discussed relating to what

was expected from the investigation.

LAB REPORT #1

TESTING OF FRESH AND HARDENED CONCRETE

against moderate sulphate attacks. Some properties of Portland cement include fast settling time, consistency, and a high compressive strength.

Materials and Methods:

Materials used for this experiment include Portland cement type I and II, water, and course aggregates. Batches of fresh concrete were prepared and used to create several 4x8 (in) cylinders and one cylinder of dimensions 6x12 (in). These cylinders were formed for compressive testing on hardened concrete specimens. Both the compression test and the split cylinder test were conducted in this experiment, on the 4x8 cylinders and the 6x12 cylinder respectively.

The batches of concrete were made by using 4.84 pounds of course aggregate, 3.91 pounds of fine aggregate, 1.96 pounds of cement mix, and 0.88 pounds of water for the 4x8 cylinders while 16.36 pounds of course aggregate, 13.2 pounds of fine aggregate, 6.62 pounds of cement mix, and 2.97 pounds of water for the were used for the 6x12 cylinder. The concrete cylinders were shaped by placing the fresh concrete mixtures into cylinder molds in three equal layers and rodding each layer

with a steel, 38

(in) rod. The top of the molds were then struck after filled, until smooth. The slump

test was subsequently performed on a sample of the concrete by filling a cone mold in the same fashion as mentioned previously, removing the mold, and measuring the difference in height from the top of the cement cone.

Compression tests were carried out on two 4x8 cylinders after 7 and 14 days of hardening and a split cylinder test was performed on the 6x12 cylinder after allowing the cylinder to cure for 28 days. The compression tests were conducted by submitting 4x8 cylinders to applied loads simulated by a machine which eventually brought then to failure, the failure loads were recorded and observed. The compressive strengths were then calculated by using the cross sectional area measured from the cylinders. The split cylinder test was conducted just as the compression test, but required the cylinder to be on its side, causing it to split down the middle. The calculations were conducted just as those from the compression tests.

Results and Discussion:

This experiment was conducted in order to show the relationship that exists between the time allowed for fresh concrete to harden and its strength. The results from this experiment indicate that the concrete is much stronger when fresh, which is incorrect (explained further in conclusion). In reality, the relationship between these two variables is that of a positive correlation; the longer the concrete is allowed to cure, the stronger load it can withstand.

When submitted to compression tests, the 4x8 cylinders all failed due to shear failure, while (of course) the splitting test caused the 6x12 cylinder columnar/splitting failure. The failure loads were 62,000 pounds, 24,500 pounds, and 40,000 pounds for the 7, 14, and 28 day tests respectively. The modulus of elasticity was calculated by using the ACI formula after the 28 day test

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LAB REPORT #1

TESTING OF FRESH AND HARDENED CONCRETE

and was determined to be approximately 245,000 psi. Also, the slump test results at the beginning of the experiment were of about a 2 inch difference in the height of the concrete sample.

Data and Results of Compression TestsDays after mixing Diameter (in) Failure Load (lb) f ’c (psi)

7 4 62,000 493314 4 24,500 1949.6528 6 40,000 3182

0 7 14 280

500100015002000250030003500400045005000

Time vs. Compression Strength

Days vs. Strength

Time (Days)

f'c (

psi)

Conclusions:

The compression tests were by far the most essential portion of this experiment. The results from these tests were what determined the relationship between the time and the allowable compressive strength of the concrete. This is the reason for the erroneous shape of the graph provided above and for the overall relationship between these two variables demonstrated by the data of this investigation.

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LAB REPORT #1

TESTING OF FRESH AND HARDENED CONCRETE

An error was made while taking the first compression strength reading, which caused the data to be scattered and the overall shape of the time versus strength graph to be far from ideal. The concrete cylinder used on the seventh day of curing had an increasing pressure that was much higher than planned and used for the other two sets of data. If the rate at which the pressure was increased was kept consistent, then the data would have been more accurate. Therefore, due to this error, the modulus of elasticity cannot be found from a graph of stress versus strain of this data as it could have otherwise. But, the modulus can be calculated for the fourteenth and twenty-eighth days of curing using the ACI formula. In this experiment, only the twenty-eighth day’s modulus of elasticity was calculated which was that of approximately 245,000 psi.

References:

Portland Cement Concrete Materials Manual, April 1989(U.S. Department of Transportation, Federal Highway Administration.)

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