Concrete 28-Day Design Experiment No. 9 CE 307L-01
Team Shh Nathaniel R. Gant
___________________
Bryant Ahn
___________________
Mike Lewis
___________________
Date Performed: November 1, 2001 Date Submitted: November 8, 2011 Submitted to: MAJ Idewu
Virginia Military Institute Department of Civil and Environmental Engineering
Help Received: None
Introduction Concrete is an important material used for several structural uses throughout the world. It is mainly used to construct beams and columns that support buildings, houses, bridges, and roads. Not all concrete is the same. Each type of concrete is design specifically. Concrete is made of water, cement, and aggregate, each with its own specific proportion to the mix. The lab was an end to a 28 day test of a designed batch mix. First, the mix requirements were designed for required strengths. The required strengths were used to calculate the amount of ingredients that were needed to make the designed batch. Enough ingredients were mixed to form four 4x8 cylindrical specimens, one 6x12 cylindrical specimen, and a beam. After seven days of curing, two of the 4x8 cylindrical specimens were tested for compressive strength. The main purpose of this lab was to finish testing the remaining specimens that cured for a total of twenty-eight days and compared the compressive data as well as discuss the concrete production process.
Figure 1: Picture of concrete specimens before compressive strength testing
Results
Calculated Mix Table 1: Team Shh Mix Requirement Data
Team Shh Mix Requirements Coarse
Fine
Cement
Water
lb
lb
lb
lb
Air Entrainment lbs
104.025
63.47
36.39
16.03
N/A
Sample Calculations (Strength R)
Fcr= 3,500 psi + 1,200 psi = 4,700 psi
(Water to Cement Ratio)
w/c= w/c= 0.42
(Cement Weight)
Water Wt. / (w/c) = 275/.42 = 65.5 lb/yd3
Actual Mix Table 2: Class Used Mix Data
Mix Weights Used for 1.5 ft3
Air Slump Entrainment lbs inches
Air Content %
Coarse
Fine
lb
lb
lb
lb
Team Latinos
104.0
73.0
32.0
16.67
N/A
1.0
2.37
Team Shh
104.025
63.47
36.39
16.42
N/A
2.5
2.00
Team Super Troopers
104.0
60.9
47.5
19.44
N/A
0.5
3.25
Group Name
Cement Water
Batch Tests
3
Slump (in.)
2.5 2 1.5 1 0.5 0 Team Latinos
Team Shh
Team Super Troopers
Figure 2: Class Slump Test Data graphed
3.5
Air Content (%)
3 2.5 2 1.5 1 0.5 0 Team Latinos
Team Shh
Team Super Troopers
Figure 3: Class Air Content Data graphed
Strength Testing’s Results Table 3: Class’s compressive strength testing results.
Design Strength
7 Day Test (4x8)
28 Day Test (4x8)
28 Day Test (6x12)
28 Day Test (Beam)
Group Name
psi
Trial 1 psi
Trial 2 psi
Trial 1 psi
Trial 2 psi
psi
psi
Team Latinos Team Shh
4700
3978
3283
4587
4453
4726
726
4700
4581
4490
5278
4184
4870
753
Team Super Troopers
4200
2988
2081
1393
3082
3735
523
Figure 4: Compression test on 4x8 specimen
Figure 5: Compression test on 6x12 specimen
Figure 6: Tensile test on beam
Group Name
Team Latinos Team Shh Team Super Troopers
Average Stress (7 Days)
Average Stress (28 Days)
Total Average Stress
psi
psi
psi
3630.5
4588.7
4205.4
10.52
4535.5 2534.5
4777.3 2736.7
4680.6
0.41
2655.8
36.77
Percent Error
Table 4: Average values for the class and percent error.
Figure 8: Picture of the 4x8 cylinder loaded to failure.
Figure 9: Picture of the 6x12 cylinder after failure.
Discussion Overall, our concrete batch turned out successful using the absolute volume method. Out of all of our initial mix required weights, we only used an extra .39 lbs. of water. The batch had a slump of 2.5� and it had an air content of 2%. All of our texted values fell within standards. The type of coarse aggregate we used was #57 stone, and
we used 104.025 lbs. of it. We used 63.47 lbs. of sand for our fine aggregate, 16.42 lbs. of water, and 36.39 lbs. of cement. One thing we noticed was that some of our aggregate was wet when we dug it from the stockpile. Because of that, we used less water since the voids taken together were already filled with water. Less water was used as a precaution because it is easier to put water in rather than take water out. Compared to the rest of the class’s data, our batch mix had the greatest slump, which was 2.5”, and the least air content percentage, which was 2%. Based off the fact that or air content was the least, it is logical to assume that our concrete will yield the most stress because it has the least air voids. In other words, our rock will be harder to break because it has less empty spaces, which allows most of the rock mass to withstand the affects of the compression or tension. Each group used about the same amount of weight for the coarse aggregate, which was about 104 lbs. Once the teams began to add water and fine aggregates, the data began to vary more. A few reasons for this may have been due to experimental errors or the mix requirements were not calculated correctly. This means that each of the team’s concrete will have varying strength as well as weaknesses. Due to a lack of admixtures, the concrete was not strong as it could have been. It the initial calculations, admixtures were taken into account; however, they were not included into the production. After we made the concrete batch, we placed the cylinders and the beam into a curing room. It stayed there, curing, for approximately seven days and twenty-eight days before we took two cylinders out to test the strength of the concrete. The test we conducted was the unconfined compressive strength test in which two large plates squeezed the concrete cylinder at a rate between 20-40 psi/s until the concrete could only hold 50% its strength. Each cylinder was tested separately and the average value between all the cylinders equaled the maximum stress the concrete had. For the seven-day test, the max stress for our team averaged out around 4588.7 psi; for the twenty-eight day test, the average stress was 4777.3 psi. The total average maximum stress, which included the seven-day test and twenty-eight day test, for our team was 4680.6 psi, which was only 19.4 psi less than the strength that we designed for the batch. Our concrete had a percent error of .41%. For the indirect tensile test, our
concrete beam had a max tensile stress of 753 psi. This value reveals that concrete does not work well with tensile forces. If our concrete beam were reinforced with re-bar, it would have been able to resist much more force that is tensile. Since our concrete’s strength increased as the curing time increased, it is logical to infer that the strength would been even stronger if the concrete cured for 56 days. However, as curing times resumes, the strength of the concrete will reach a maximum stress that will remain constant from then on. By adding admixtures, the maximum strength could be obtained quicker which would have lowered the curing time. From the data, it shows that our team, Team Shh, had produced the best concrete because our percent error was lower than the other groups. It shows that we put a lot of effort into measuring the ingredients as well as mixing the concrete and placing it into the containers for curing. One thing we noticed was that the other group did not rod their mixers into the containers. This was a problem because it left air voids inside the container, which weakened the concrete. Nevertheless, we did an excellent job making our concrete because one of the teams that had the same requirements as us had concrete that was more the 500-psi weaker than our concrete. This shows the importance of the concrete making procedure because all the ingredients could have been simply thrown into the mixer; however, my team continuously scraped the sides to get all the sand and cement mixed in especially the stuff stuck in the back. Once we mixed everything, well our slump got better and better. In addition, our group continuously prodded the mixer as it was being poured into the containers. Prodding allowed more mix to be added into the container, which meant the material would be stronger when it solidified. One last thing to point out is that the exact water requirement was not used and that had an effect on the strength of our concrete. Since more water was put in, theoretically, the water–cement ratio increased, so the strength decreased a little and the workability increased a little.