In This Lab We Created Fresh Concrete Using Two Different Water Cement Ratios and Did Tests

Danny Hendrickson

Fresh Concrete Properties

Discussion:

In this lab we created fresh concrete using two different water cement ratios and did tests on each batch to determine unit weight, slump, and volumetric air content. These tests are very important in determining the strength of the hardened concrete and are essential any time concrete is made for a construction project.

The slump test is a good measure of strength and workability. It is performed by filling a cone with fresh concrete, lifting the cone straight up, and measuring the difference between the initial height of concrete and the final height. For the water/cement (W/C) ratio of 0.4 our slump was 1.75inches and for the W/C of 0.7 the slump was 7 5/8 inches. This shows that by adding the 2lbs of water to our mix we were able to change its strength dramatically. Sources of error for this test could come from vibrating the cone as it is being lifted which cause the concrete to fall more than it would have normally. There also could have been error in accepting a false test. If the concrete slumps all to one side then the test must be done again because that would mean that the aggregate was too segregated.

The unit weight test helps you figure out how much your concrete is going to weight per cubic length. It is performed by finding the dimensions and weight of the given container, filling the container with fresh concrete and striking off the excess concrete, and then weighing the container and concrete to find the weight of the concrete. For the W/C of 0.4 we found the unit weight to equal 148.9338lb/ft3 and for the W/C of 0.7 the unit weight was 148.0876lb/ft3. The mass of the concrete changed from 35.2lb to 35lb when we added the extra 2lb of water which shows how the lighter water took the place of the heaver cement and aggregates. Sources of error in this test could be because the top of the container was not leveled properly or because there might have been excess concrete left on the side of the container when it was weighed.

The volumetric air content helps you figure out how much air is going to be trapped in your concrete per unit length. This is important to know when calculating your concrete mix so you can decide if you want to use any admixtures to try and remove the air and also to calculate your actual strength of concrete. The test was performed by filling a container of known dimensions with concrete and striking off the top. We then attached the top section of the meter to the bowl and filled the apparatus with water until it reached the zero mark. The whole apparatus is then agitated until the meter stops going down which means that all the air has been removed from the sample. For the W/C of 0.4 we found the air content to be 3.25 inches and we did not perform the test for the W/C

of 0.7. Sources of error in this test could come from not filling the water all the way to the zero mark, allowing concrete or water to spill out during the test, or not filling the container all the way to the top with concrete.