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Concrete Report

Andrew MotyckaASM 2241

Lab #112-1-12

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The purpose of this report is to show compressive strengths of concrete with different mix designs and curing methods. Three different mixes were made with water to cement ratios of .4, .55, and .7. The curing methods used were a wet cure and a dry cure. These mixes were broken at 7, 14, 21, and 28 day intervals using a hydraulic breaker.

These results are important to construction as it would give construction managers ideas of the respective strengths of concrete with these water to cement ratios at different intervals. This is important as it shows when each concrete mix can reach load bearing strengths. It also shows the differences between dry curing and wet curing in the strength of the concrete. These two factors combined are laid out in a spread sheet for easy reference. This makes the decision process considerably easier and more accurate when choosing a mix design. The results of this report can also affect which curing method is used in construction.

Methods and Materials

The overall mix quantity was determined by the overall volume to fill sixteen, 2x4 inch test cylinders, plus 15% to account for waste. As a whole, both lab groups filled 96 cylinders. These labs were broken into groups, based on water to cement ratios, which then mixed different mix designs. Each group filled 16 cores. Portland cement type 1 was used. This is a normal Portland cement suited for general purpose applications. Three different water to cement ratios were used: .4, .55, and .7. Water to cement ratio is a ratio that shows a direct correlation between water and cement in the mix. Test cylinder core sizes were 2”x4”, and two different curing methods were used, wet and dry. The wet cure consisted of submerging in water while the dry cure was just letting the cylinders sit out in the air in the shop. These cylinders were cured up until the point that they were broken in the compressive test procedure. The compression testing was done by a machine that is driven by a hydraulic ram. The machine is a Humboldt model number CM-2500-DIR. The machine records the amount of down pressure, in pounds, that is required to break the concrete core. Calculations must then be done to convert the pounds into pounds per square inch.

For each water to cement ratio, 16 cylinders were cast. These cylinders were 2”x4”. Also an additional 15% was added to the volume to account for waste. The area was found of one cylinder by finding the volume of a cylindrical solid. To do this, use the equation, V=(pi)(r^2)(height). So the equation would be: V=(pi)(1^2)(4) which equals 12.566 cubic inches. Multiply this by 1.15 to account for the waste and you get 14.45 cubic inches. Take this product times 16 to account for the 16 cylinders, and get a total volume of 231.22 cubic inches. This must then be converted to cubic yards. To do this take the 231.22 cubic inches and divide by 1728 cubic inches to obtain 0.1338 cubic feet. Then divide this product by 27 cubic feet to get 0.00496 cubic yards of concrete required per each respective water to cement ratio mix.

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The materials used were fine sand with a very low moisture content and 3/8” limestone used for the gravel. A fineness modulus that was very low was used, around 2.5. This was because fine sand was used and was experimentally found. The slump goal was ¾” using a 2x4 inch slump cone. Tap water out of a sink in the shop was used. This is potable water. Slump measures the workability of the concrete. The higher the slump, the more workable it is.

To get the measured quantities of each ingredient (sand, gravel, cement, and water) a mix design out of Concrete and Concrete Masonry by David L. Ahrens was used. This mix design was based off of a 3/8” maximum aggregate size and varied slightly for each water to cement ratio. First, start with the .40 water to cement ratio mix. The mix design calls for 3% air entrapment, 385 lb. of water per cubic yard of concrete, 965 lb. of cement per cubic yard of concrete, 1240 lb. of sand per cubic yard of concrete, and 1260 lb. of gravel per cubic yard of concrete. Utilizing these quantities and the cubic yards of concrete required to fill the 16 cylinders, the required amount of each ingredient in the mix can be found. To find pounds of water, take the 385 lb. per cubic yard of concrete that the mix calls for and multiply it by 0.00496 cubic yards required to fill the cylinders and 1.9096 lb. of water to add will be found. Do the same for cement and 4.7864 lb. of cement to add will be found. Repeating the calculation for sand and gravel 6.15 lb. and 6.25 lb. to add is found.

The .55 water to cement ratio mix varies slightly from the .4 mix ingredient wise. The amount of water and coarse aggregate remains the same; however the amount of cement and fine aggregate varies. To find the amount of cement, take 700 lb. of cement per cubic foot of concrete that the mix calls for, and multiply it by the .00496 cubic yards of total concrete. This leads to 3.472 lb. of cement to add. The mix also calls for 1460 lb. of fine aggregate per cubic yard of concrete. Again, take the 1460 lb. times the amount of cubic yards needed, .00496, and 7.24 lb. of fine aggregate to add is found.

The .7 mix also requires the same amount of water and coarse aggregate as the other mixes. Multiply 550 lb. of cement per cubic yard required by the mix times .00496 cubic yards to obtain a number of 2.73 lb. of cement to add. Then take 1590 lb. of fine aggregate per cubic yard of concrete times .00496 cubic yards to get pounds of fine aggregate to add. This ends up being 7.89 lb. of fine aggregate required.

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To mix the concrete, an electric bucket mixer was used. The dry ingredients were thoroughly mixed first and the water was added last. The ingredients were mixed until a uniform consistency of concrete was seen. The concrete was then slump tested. A slump cone with dimensions of 2 x 4 inches and a 3/8 inch rod were used to do the slump testing. The slump cone was placed on a damp piece of plywood to ensure that water was not pulled out of the mix by the wood. Approximately 1/3rd of the cone was filled with the mix and then it was rodded 25 times. Care was taken to make sure not to strike the piece of plywood while rodding. Another 1/3rd of the cone was filled and rodded again 25 times. The remaining 1/3rd of the cone was then filled and rodded 25 times also. During the rodding, care was taken to make sure not to penetrate the previously rodded layers. If the concrete dropped below the top of the cone on the final 25 roddings, then concrete was added to fill the cone again and the remaining amount of rod strokes were completed. Once the rodding was finished, the top of the cone was screeded off with the rod in a rolling, sweeping motion. The cone was then lifted off the mix in a time interval between 3 and 5 seconds. The cone was then flipped upside down beside the concrete, and the rod was placed on top of the cone, extending over the concrete. The distance between the rod and the top of the concrete was then measured. A desired slump for this small of a cone was ¾ inches. If the slump was too low, especially for the .4 mix, then superplasticizer was added to make it more workable.

The 2”x4” cylinders were being prepared while the mixing was taking place. To prepare these cylinders they were first tightened down. While being tightened down, care was taken to ensure that the ends of the cylinder were flush. If not, the ends of the core would be uneven and slanted, causing for an inaccurate compression test. The inner walls of the cylinder were then coated with a form-release compound to ensure that the concrete would separate from the cylinder easily when the concrete has hardened. It is also important to note that the cylinders were placed on a plywood board during curing. This board also had to be coated with form release compound to ensure that the cores would separate from the board without compromising structural integrity. Also this ensured that the board did not absorb water from the concrete cores which is crucial to continuing the hydration process.

Cores were filled by shoveling concrete into the cores with a small pointed trowel

and then rodding thoroughly to consolidate the mix inside the core. If this rodding was

not fully and correctly done then honeycombing on the sides of the cores formed. Also,

the bottom of the concrete core would be uneven, which will give an inaccurate

reading when breaking.

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Half of the cores for each respective water to cement ratio were wet cured by submerging them completely in water. The other half of the cores were dry cured in the open air. The wet cure ensures that the concrete has enough water so that the hydration process continues throughout curing. The dry cure limits the hydration process to the water available inside the mix.

Two wet cured cores and two dry cured cores of each water to cement ratio were broken at intervals of 7, 14, 21, and 28 days. The breaking machine gave a number in pounds of force required to break the core. To convert pounds to pounds per square inch, one must divide the pounds by the area of the core. In this case (2”x4” core), you would divide the pounds by pi. These numbers were then recorded each week. Each cylinder was broken by placing it on a platform inside the machine. An operator would then pull a handle which makes the hydraulic ram move towards the cylinder. As the hydraulic ram contacts the surface of the cylinder, multiple people observed the cylinder until a crack was found. The down force reading was then taken and converted to pounds per square inch. Note: Test machine was not calibrated.

Results

The first graph shown is wet and dry compressive strengths versus day for the .40 water to cement ratio concrete. Figure 1 shows that the initial 7 day and final 28 day compressive strengths are higher for the wet cure. The dry cure rises slightly above the wet cure between the 14 and 21 day intervals. Overall the compressive strengths are very similar until the 28 day mark. Here, the wet cure is approximately 1000 psi higher than that of the dry cure.

Figure 1.

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This next graph shows wet and dry compressive strengths versus day for the .55 water to cement ratio concrete. Figure 2 shows that the compressive strengths are approximately the same at the initial 7 day test. After the 7 day test the compressive strength of the wet cure rises significantly over the dry cure and remains approximately between 500-800 psi throughout the testing period.

Figure 2.

Figure 3 in this report is wet and dry compressive strengths versus day for the .70 water to cement ratio. The data in this graph that the wet cure starts out head and sshoulders above the dry cure for the initial 7 day test; approximately 1500 psi higher. After the 7 day test the dry cure slowly starts to rise and is even to the wet cure at the 21 day interval. The dry cure even slightly rises above the wet cure at the 28 day mark.

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Figure 3.

Figure 4 is the average wet and dry compressive strengths versus day for all water to cement ratios. This graph shows that the wet cure is higher in compressive strength than the dry cure for each day that testing was done. The dry cure actually breaks even with the wet cure at the 21 day interval, but then the wet cure rises above it again after that.

Figure 4.

The final graph in this report is the 28 day compressive strengths for both the wet and dry cures of each water to cement ratio. This graph shows that for each water to cement ratio, the wet cures are higher than the dry cures. The .40 water to cement ratio concrete boasts the highest compressive strengths for both wet cure and dry cure. The .7

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water to cement ratio concrete is the second highest in both wet and dry cures, and the .55 water to cement ratio concrete holds the lowest strengths for both wet and dry cures.

Discussion

There are a couple of outliers in this report. The .55 water to cement ratio mix should be the second highest in overall compressive strength. However, the results of this report show that the .7 water to cement ratio mix is stronger. There could be several reasons for this error. The ingredients could not have been mixed properly, there could have been errors in the testing procedure, or the cores could have been improperly prepared causing uneven cores or honeycombing.

The dry cure for the .7 water to cement ratio mix rises above the wet cure at the 21 day interval. The most reasonable explanation to this outlier is that our testing procedure was flawed. It is very plausible that the hydraulic ram on the testing machine was not stopped immediately at the first sign of failure when testing the wet cure. This would allow for a higher compressive strength to be recorded.

The rest of the results are very consistent with knowledge learned throughout previous courses. The fact that several different groups prepared the mixes that were averaged together for this report, allows room for human error. Also, different groups broke the cores in the testing machine, which could cause even more human error.

Conclusion

Figure 5.

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The most obvious conclusion that can be drawn from this report is that a wet cure creates a higher compressive strength overall than a dry cure will. This coincides with common knowledge in that since the concrete is able to continue the hydration process in a wet cure, that this should be the case.

Another conclusion that can be drawn is that the .40 water to cement ratio is the highest strength of concrete that was tested. The lower the water to cement ratio, the higher the compressive strength should be. The outlier of the .55 water to cement ratio having a lower compressive strength than the .7 water to cement ratio has already been addressed above.

A third conclusion that can be made from the results of this report, is that compressive strengths of all concrete types tested, along with all curing types, rises significantly in the first 7 days, and then levels out over time after that. The total average 7 day strengths were 81% of that of the average 28 day strengths. In all cases the compressive strengths jumped at least 3000 psi within the first seven days. The curve is much more gradual after this initial jump.

Citations

Ahrens, Donald L.. Concrete and Concrete Masonry. Minneapolis, Minnesota: Hobar Publications, 2005. Paperback.

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Addendum

The tool I found, which was invented in 2012, is the EZ Screed. This is a handle operated, hand held screed tool. It allows you to stand up in an upright position and screed smaller surfaces such as walkways, driveways, and smaller slabs. All you have to do is gently pull the screed, and the surface strikes off the excess without causing strain on the operator. This tool screeds concrete without rolling or vibrating it, which helps to maintain the structural integrity of the mix. The tool is light, weighing in at 6 pounds, and can easily screed slumps of over 4 inches. One person can screed a surface with this tool, eliminating the need for two guys and a 2x4. The screed surface comes in variations between 3 and 7 feet. This tool would be very applicable for smaller, residential concrete construction. It would not be reasonable to use this for commercial construction, as larger, laser screeds are used in this application.