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The Effect of Ultraviolet Exposure
on Rubberized Concrete
by Melissa Lesmeister '04
Table of Contents
Acknowledgements
Abstract
Introduction
Purpose
Parameters
Testing Program
Concrete Mixture Design
Procedure for Making Specimens
Exposing the Specimens
Compression Testing
Results
Relationship of Stress with W/C Ratio
Relationship of Stress with Rubber Content
Relationship of Stress with Exposure Length
Relationship of Stress, Rubber Content, and Exposure Length
Modulus of Elasticity
Relationship of Modulus of Elasticity with W/C Ratio
Relationship of Modulus of Elasticity with Rubber Content
Relationship of Modulus of Elasticity with Exposure Length
Relationship of Mod. of Elast., Rubber Content, and Exposure Length
Correlation Between Lab and Field Exposure
Conclusions
References
List of Symbols
Acknowledgements
Special thanks to the Union College Department of Civil Engineering for the purchase of the Ultraviolet Crosslinker used to perform this research.
Special thanks again to the Union College Department of Civil Engineering for their support in the presentation of this paper at the Saint Lawrence Section of the American Society for Engineering Education Student Presentation Competition held at West Point in 2002.
Special thanks also to civil engineering Professor Ashraf Ghaly, whose whole-hearted supervision and valuable guidance made this research possible.
Abstract
The endless build up of old rubber tires has been, and continues to be, a serious problem for many states and the federal government. When recyclable materials, such as retired tires, are disposed of in landfills they consume space that could otherwise be used for terminal materials. Terminal materials are those that cannot be recycled or used in new applications. However, recyclable materials can be used in many applications thus serving a meaningful environmental purpose. For example, granulated rubber chips from old tires can be used in concrete manufacture. Rubberized concrete is lightweight and durable. These characteristics make rubberized concrete an excellent material for use in applications where high strength is not crucial and great resistance to cycles of expansion and contraction or freeze and thaw is desired. Building facades are the best example of such structural elements. Weather and environmental conditions continuously impact building facades. Ultra Violet (UV) exposure is one of the natural elements that can damage the granulated rubber chips in the concrete. This research program was structured to examine the effect of UV exposure on the mechanical properties of rubberized concrete. A total of 144 concrete specimens were made and tested in the experimental program. The parameters that varied in this study were the percent of rubber in the mix, the water/cement ratio, and the exposure period. In addition to control mixtures with no rubber content and no UV exposure, three rubber contents, three water/cement ratios, and three exposure periods were used. An Ultra Violet crosslinker (oven) was used to achieve accelerated exposure. The exposure periods were correlated to the real UV exposure in many cities throughout the United States. Exposure periods used in the lab were meant to represent a reasonable life span of real structures. For each mix examined in this study, three specimens were made to ensure consistency and accuracy of the recorded properties. Based on the results of this investigation it could be concluded that, with all other factors being equal, the compressive strength of rubberized concrete decreases with the increase in the rubber content in the mix, the increase in the water/cement ratio, and the increase in the UV exposure period.
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Introduction
Throughout the United States today there is a problem that revolves around the disposal of old rubber tires. There are many ways to dispose of these tires, however most of the options offer problematic drawbacks. One such option is to stockpile the tires, however this causes a fire hazard, as shown through the stockpile fire that occurred recently in California. These tires can also be placed in a dump, or basically piled in a large hole in the ground. However these dumps serve as a great breeding ground for mosquitoes and due to the fact that mosquitoes are responsible for the spread of many diseases, this becomes a dangerous health hazard. The old tires can also be burned for fuel, however this process causes the release of a thick black smoke, which has a negative environmental impact on clean, breathable air. These tires can also be placed into landfills, but the design and upkeep of landfills can be very expensive. Today, there is a vast amount of landfill space taken up by old rubber tires. Due to the fact that old rubber tires can, in fact, be reused in other applications, these tires are taking up space that could be occupied by terminal materials. There is already a severe lack of available landfill space, and therefore, removing and reusing rubber tires from landfills would be a great benefit to the landfills, and therefore the people who use them, along with being a great recycling triumph. One such application that could use old rubber tires is rubberized concrete. Concrete can be made cheaper by replacing some of its fine aggregate with granulated rubber chips from old rubber tires. These granulated chips are achieved through a process called continuous shredding, which is necessary to create chips small enough to replace an aggregate as fine as sand. This would produce a type of concrete that is lightweight and durable, which could be used in applications where great strength is not necessary but resistance to cycles of expansion and contraction is needed. One such example of an application like this is a building faŤade. Building facades are attacked daily by many different factors. These include weather aspects such as rain, snow, wind, and the harmful rays of the sun.
This research program was designed to test the effects of ultraviolet (UV) radiation on the compressive strength and elasticity of rubberized concrete. Although UV radiation does not have a great impact on concrete itself, due to its inability to penetrate more than about a millimeter into the surface of the concrete, it is possible that the UV radiation will be able to damage the rubber particles added to rubberized concrete, therefore decreasing its compressive strength and/or its modulus of elasticity. This program also studied the effects of different water/cement ratios and the amount of rubber added on the compressive strength and elasticity of the rubberized concrete.
Purpose
The purpose of this experiment is to determine the effects of UV radiation on rubberized concrete with varying water/cement ratios, rubber content, and UV exposure periods. Various relationships can be developed among these three variables, which can be analyzed in order to determine whether or not rubberized concrete could suitably withstand the rays of the sun in any of its proposed applications. This program also allowed for the analysis of the exposure lengths used and therefore the correlation of these exposure periods to real life time periods in different cities across the United States. This will show how rubberized concrete would fare if used in an application in any of these cities for a given length of time.
Parameters
The following three parameters varied in this study:
- Water/Cement Ratio (47%, 54%, and 61%)
- Rubber Content (0%, 5%, 10%, and 15%)
- UV Exposure Period (0 min, 7000 min, 28000 min, and 56000 min)
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Testing Program
The testing program used in this study involved the creation of 144 cubes of concrete. Each cube had a side length of 2 inches. There were a total of 12 different mixtures of concrete used including variations in the water/cement ratio and the percent of rubber in the mix. Each specimen was numbered and after hardening, was subjected to the amount of UV exposure designated by its number. For each different combination of water/cement (w/c) ratio, rubber content, and exposure period, three cubes were made. This was done to reduce any accidental error that may occur. If any one cube in any triplet produced results that delineated greatly from the average, they were later ignored in the analysis. The following tables, Tables 1, 2, and 3, show the numbering, and consequently the testing program, of the 144 cubes for w/c ratios of 47%, 54%, and 61%, respectively.
Table 1. Specimen Numbering and Testing Program for w/c = 47%
Table 2. Specimen Numbering and Testing Program for w/c = 54%
Table 3. Specimen Numbering and Testing Program for w/c = 61%
As shown, one quarter of the cubes were not subjected to any length of UV exposure, and therefore were able to serve as a control group to show what the effects of the exposure truly were.
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Concrete Mixture Design
A research program that tested the compressive strength of rubberized concrete, with the varying parameters being water/cement ratio (47%, 54%, and 61%), rubber content (0%, 5%, 10%, and 15%), and the age of the specimen (1, 7, 14, 21, and 28 days), was done previously by a Union College student, Jed Cahill. In this research program, the concrete mixtures were designed by volume in accordance with tables provided by the ACI 211.1-91, "American Concrete Institute (ACI) Standard Practice for Selecting Proportions for Normal Heavyweight, and Mass Concrete." The parameters considered in this study for the concrete mixtures were the maximum and minimum slump values, the nominal maximum size of aggregate, the water volume for non-entrained concrete, the amount of entrapped air, the compressive strength (Su) at the age of 28 days, the water/cement ratio, the unit weight of coarse aggregate, the fineness modulus, the volume of coarse aggregate per unit volume of concrete, the first estimate of concrete weight, and the volume of concrete desired for the mixture. The volumes necessary for the different mixtures of concrete were determined and then converted into weights that could be used in mixing the concrete ingredients. Due to the similarity in the nature of these two research programs, the volume and weight calculations done by Jed Cahill, were re-used in this research program. They were converted into deviations of mixes made for 3 cubes and 9 cubes, rather than the mixes made for 1 cube, 8 cubes, or 9 cubes used previously.
There were three molds used to from the 2-inch concrete cubes, each of which was capable of forming 3 cubes. Therefore 9 cubes could be made at one time. Before the samples could be taken out of the molds, they needed to set for at least 12 hours. This allowed up to 18 cubes to be made per day. The different mixes were either prepared for a full nine cubes at a time, or for the remaining three cubes which would make up the total of 12 cubes for each of the 12 different concrete mixtures. The ingredients by volume and weight for each of the cubes are shown in Tables 4 through 15.
Table 4. Ingredient proportions for W/C = 47%, 0% rubber by volume of mixture.
Table 5. Ingredient proportions for W/C = 47%, 5% rubber by volume of mixture.
Table 6. Ingredient proportions for W/C = 47%, 10% rubber by volume of mixture.
Table 7. Ingredient proportions for W/C = 47%, 15% rubber by volume of mixture.
Table 8. Ingredient proportions for W/C = 54%, 0% rubber by volume of mixture.
Table 9. Ingredient proportions for W/C = 54%, 5% rubber by volume of mixture.
Table 10. Ingredient proportions for W/C = 54%, 10% rubber by volume of mixture.
Table 11. Ingredient proportions for W/C = 54%, 15% rubber by volume of mixture.
Table 12. Ingredient proportions for W/C = 61%, 0% rubber by volume of mixture.
Table 13. Ingredient proportions for W/C = 61%, 5% rubber by volume of mixture.
Table 14. Ingredient proportions for W/C = 61%, 10% rubber by volume of mixture.
Table 15. Ingredient proportions for W/C = 61%, 15% rubber by volume of mixture.
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Procedure for Making Specimens
Once all the weights were calculated the process of molding either 9 or 18 cubes a day began. The first step in the procedure was to clean and prepare the molds. Each mold held 3-2" cubes of concrete. The molds were scraped of any excess concrete with a trowel and then brushed with motor oil with a small paintbrush to make the removal of the concrete cubes easier. Once the molds were prepared the mixing of the concrete began.
The first step was to mix all the dry ingredients, including the coarse aggregate, fine aggregate, cement, and rubber (if any). Each dry ingredient was weighed on a digital scale with 0.5-gram accuracy. Photograph 1 shows all the dry ingredients before being thoroughly mixed, along with the digital scale used to weigh them.
Photograph 1. Dry Ingredients
The dry ingredients were then thoroughly mixed with a shovel to ensure the homogeneity of the mixture. The next step was to add the water. The water was measured with a 250 mL graduated cylinder. All the ingredients were then mixed completely. Photograph shows the mixing of all ingredients.
Photograph 2. Mixing of Ingredients
After the ingredients were mixed all three molds were filled with the concrete mixture using a small shovel. This is shown in Photograph 3.
Photograph 3. Filling the Molds
After the molds were filled they were placed on a vibrating table in order to pack down the concrete. While the molds were on the table, they were pressed with a trowel and concrete was added to fill in any molds that needed it. Photograph 4 shows a mold with concrete on the vibrating table.
Photograph 4. Vibrating the Molds
After the concrete was securely packed into the molds, it was left to harden for at least 12 hours. The cubes were then removed from the molds and the process began again for the next 9 samples. As the cubes were removed they were labeled with their specimen number and placed on a shelf for 24 hours and then a water bath for 27 days to reach their maximum hardness. They were then removed from this water bath 24 hours before they were to receive their predetermined degree of exposure.
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Exposing the Specimens
After the 28-day hardening period had passed the cubes were placed into the UV crosslinker (oven) for the appropriate amount of time as designated by their specimen number. The oven used was a Model CL-100 UV Crosslinker with 5 x 8 watt UV dual bipin discharge type tubes, which produced a wavelength of 254 nanometers UV. The dimensions of the interior chamber where the cubes were placed is 5"H x 12"D x 10"W. This oven is shown in Photograph 5, below.
Photograph 5. Ultraviolet CrossLinker
The oven had a maximum time limit of 1000 minutes per day and was turned on once a day, in the morning. Therefore, the cubes that received 7000 minutes of exposure time remained in the oven for one week (7 days), the cubes that received 28000 minutes remained in the oven for four weeks (28 days), and the 56000 minutes cubes remained in the oven for 8 weeks (56 days). 48 cubes could fit in the oven at any one time and were arranged in two 24-cube layers. The top layer received direct UV exposure, while the bottom layer received indirect exposure. The cubes were rearranged throughout their exposure to allow for half of their exposure to be direct, while half of it was indirect. Due to the fact that in reality, only one side of a concrete surface can be exposed to the harmful rays of the sun, only one surface of each cube in the lab was directly exposed to UV rays. The same cube face was always facing upwards towards the UV source, and it was this cube face that faced upward when compression testing was done.
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Compression Testing
All 144 cubes were tested using a computer-run loading machine. The computer program for the machine allowed for the design of a program with which samples could be tested. The program designed included a loading rate of 21 kN per minute (which complies with ASTM specifications), SI units, and the display of the maximum compressive load applied to the specimen. For each cube, the computer recorded the load vs. displacement at specified time intervals throughout the testing, until a few seconds after failure occurred. This information was saved for each cube and exported to excel so analysis could be done. This analysis included the pinpointing of the maximum load (load at failure) and its corresponding displacement in order to calculate average stresses and strains
.Before each cube was tested it was weighed using the same 0.5-gram accuracy digital scale that was used to weight the dry ingredients for the concrete mixtures. Each cube was then placed under the center of the loading plate, and the plate was manually moved downward until it was in position for testing to begin. Photograph 6 shows a cube ready to be loaded.
Photograph 6. Specimen in Loading Machine
The computer program then did the rest of the testing. It was necessary to input the side length of the cube, which was always 2 inches, and then run the program. The program would show a real-time load vs. displacement graph of the test being run along with a timer. The test would stop once failure occurred, with the exception of a few load vs. displacement points that would be plotted after the maximum was reached. These files were then saved, as mentioned earlier, for later analysis. The bottom portion of the cube, which was usually pyramid shaped, was saved for every cube.
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Results
There were many comparisons made from the results of the 144 cubes tested. Tables and graphs comparing the effects of W/C ratio, rubber content, and period of exposure on the average stress for each triplet of cubes (calculated using the maximum loads found during testing) were formed. These were split into four different categories. These relationships were:
- Average Stress Vs. W/C Ratio for different Rubber Contents
- Average Stress Vs. Rubber Content for different W/C Ratios
- Average Stress Vs. Exposure Length for different Rubber Contents
- Overall Graphs Comparing Exposure Length, Rubber Content, and Average Stress for different W/C Ratios
The load at failure and its corresponding displacement were used to calculate the maximum stress (load/surface area) and strain (displacement/height). These were then used to calculate the average stress and strain for each triplet of cubes. The individual weights, maximum loads, displacements, stresses, and strains, along with the average stresses and strains for each cube are shown in the included Appendix A. The average stress for each combination of W/C ratio, rubber content, and exposure length is shown in Table 16 below.
Table 16. All Calculated Average Stresses (in MPa).
Although further analysis is shown later on through more tables and graphs, it is possible to see, even with just this table, that there is, first of all, an obvious drop in the average stresses as the rubber content increases. It is also shown that, although not in every case, there is a general trend towards a decrease in the average stress as the W/C ratio increases. Also, in almost every case there is a significant drop in the average stress from 0 minutes of exposure to 7000 minutes, while the change from 7000 to 28000 to 56000 minutes is not as significant. This initial drop, however, leads to the belief that the average stress of the rubberized concrete does also decrease with the length of exposure time.
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Relationship of Stress and W/C Ratio
The next table and its four corresponding graphs show the first relationship mentioned, that is, the average stress versus W/C ratio for each different rubber content. The data for these graphs is shown in Table 17, and the graphs are Figures 1, 2, 3, and 4, which represent the relationship for rubber contents of 0%, 5%, 10%, and 15%, respectively.
Table 17. Data for Average Stress (MPa) Vs. W/C Ratio for different Rubber Contents.
Figure 1. Average Stress Vs. W/C Ratio for 0% Rubber.
This graph, along with those shown in Figured 2, 3, and 4 are made up of 3 points, each which represents a different water/cement ratio. The graphs are also comprised of 4 lines, each of which represents a different period of UV exposure. There is a general trend shown in these graphs that the average stress of rubberized concrete will decrease as the W/C ratio increases. There are some apparent discrepancies from this theory but these can be attributed to the fact that concrete is in fact a non-homogenous material and there is also a certain human factor which must be considered. This trend, however, is especially apparent in Figure 1 for the 7000 minute and the 28000 minute samples, and the latter half of the 0 minute and the 56000 minute graphs. This proves the theory that the lower the water/cement ratio, the stronger the concrete is.
Figure 2. Average Stress Vs. W/C Ratio for 5% Rubber.
This general downward trend is also shown in this Figure. It is especially apparent in the 0 minute section and the latter half of the 7000, 28000, and 56000 minute sections of this graph. Despite the very low values for some of the 47% W/C cubes, which can most likely be explained by some type of heterogeneity in the mix, this graph, for the most part, also shows that the increase in W/C ratio will decrease the average stress.
Figure 3. Average Stress Vs. W/C Ratio for 10% Rubber.
This overall downward trend is shown once again for the 10% rubber. This graph once again shows the same trend for the 0 minute section and the latter half of the 7000, 28000, and 56000 minutes sections of this graph. This further justifies the theory that the maximum stress will decrease as the W/C ratio rises.
Figure 4. Average Stress Vs. W/C Ratio for 15% Rubber.
This graph, the final one in this section, perhaps best shows the theorized trend. The only sections of this graph to slightly deviate from the trend is the first half of the 0 minute line, and even that is only a slight deviation and also the second half of the 56000 minute section. Therefore, it is safe to conclude, despite the outlier points shown, that for any given rubber content, as the water/cement ratio increases, the average stress at failure of a sample will decrease.
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Relationship of Stress and Rubber Content
The second section of results is categorized by the average stress versus rubber content for different W/C ratios. The expected result is that an increase in rubber content will decrease the strength of the rubberized concrete. This means that as the rubber content increases, the average stress at failure will decrease. The tabular results for this section are shown in Table 18, and also in graphical form in Figures 5, 6, and 7, with W/C ratios of 47%, 54%, and 61%, respectively.
Table 18. Data for Average Stress (MPa) Vs. Rubber Content for different W/C Ratios
Although further analysis is shown later on through more tables and graphs, it is possible to see, even with just this table, that there is, first of all, an obvious drop in the average stresses as the rubber content increases. It is also shown that, although not in every case, there is a general trend towards a decrease in the average stress as the W/C ratio increases. Also, in almost every case there is a significant drop in the average stress from 0 minutes of exposure to 7000 minutes, while the change from 7000 to 28000 to 56000 minutes is not as significant. This initial drop, however, leads to the belief that the average stress of the rubberized concrete does also decrease with the length of exposure time.
Figure 5. Average Stress Vs. Rubber Content for W/C Ratio = 47%
Figure 6. Average Stress Vs. Rubber Content for W/C Ratio = 54%
Figure 7. Average Stress Vs. Rubber Content for W/C Ratio = 61%
The previous three graphs shown in Figures 5, 6, and 7 are made up of 4 points, each of which represents a different rubber content. They are also made up of a 4 lines, each of which represents a different UV exposure period. There is once again an overall downward trend shown in these graphs. The trend in these three graphs is in fact very clear. For every exposure length and at every W/C ratio, there is an obvious downward slope to the graph. This indicates clearly that as rubber content increase from 0 to 15%, the average compressive stress at failure decreases. This shows that rubberized concrete weakens with the addition of more rubber particles in place of the fine aggregate.
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Relationship of Stress and Exposure Length
The third relationship shown is that of the average stress versus the exposure length for each different rubber percentage. The expected trend for this comparison is that the average compressive stress at failure will decrease as the length of exposure increases, due to the fact that it is believed that the UV rays will harm the rubber chips in the concrete. The data for this comparison in is shown in tabular form in Table 19, and in graphical form in Figures 8, 9, 10, and 11 for rubber contents of 0%, 5%, 10%, and 15%, respectively.
Table 19. Data for Average Stress (MPa) Vs. Exposure Length for all Rubber Contents
Figure 8. Average Stress Vs. Exposure Length for 0% Rubber.
The graph shown in Figure 8, and the upcoming graphs shown in Figures 9, 10, and 11 are made up of four points, each of which represents a different UV exposure period. They are also made up of three different lines, each of which represents a different water/cement ratio. In all of the graphs in this section there is a general downward trend showing that as he exposure length increases, there is a decrease in the average stress. There are some deviations from this theory shown in the graphs but these can, once again be attributed to the fact that concrete is not a homogenous material and the human factor. There are, in fact many discrepancies shown in Figure 8, above, however, if the overall trend of Figure 8, along with the following Figures 9, 10, and 11 are analyzed, the general downward trend is apparent.
Figure 9. Average Stress Vs. Exposure Length for 5% Rubber.
This graph, once again, shows a general trend, which shows that as the exposure length increases, the average stress will decrease. For this graph each W/C ratio shows a drop in the average stress from the unexposed samples to those that received no exposure. Once, again, after that though the numbers vary very little and show no true common trend, as some of the increase and some decrease with the increase in exposure length. This shows that the addition of exposure definitely lowers the strength slightly but perhaps the length of the exposure itself is not as large a factor as originally thought.
Figure 10. Average Stress Vs. Exposure Length for 10% Rubber.
On this graph, once again, all the W/C ratios show a drop in the average stress when comparing the 7000 minute samples to the no exposure samples, but afterwards the number vary only slightly. The 47% samples show a continuous downward trend while that of the 54% and 61% is only partially downward. Overall, though, the downward trend mentioned earlier is also apparent in this Figure.
Figure 11. Average Stress Vs. Exposure Length for 15% Rubber.
This last graph in this series shows completely downward trends for the 54% sample, while the 47% and 61% samples have a partial downward trend. Therefore, overall this graph, once again, upholds the theory that the average stress would decrease with an increase in the exposure length.
Overall, the results for this section are very interesting. There is shown a general trend, which states that as the exposure length increases, the average stress will decrease. There is also a clearer trend of this nature as the exposure length increase from 0 to 7000 minutes. This may show that despite the fact that exposing the specimens does seem to slightly reduce their compressive strength, their strength is not affected as greatly by an increase in this period of exposure. However, all results in this section help to draw the conclusion that despite any downward trend, the addition of UV exposure to rubberized concrete does not create a significant loss in its compressive strength which would hinder its use in any various applications. Therefore, any reservations about the usage of rubberized concrete due to possible damage by UV exposure should be dismissed.
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Relationship of Stress, Rubber Content, and Exposure Length
The last section of graphical results shows an overall comparison of exposure length, rubber content, and average stress for different W/C ratios. These graphs are shown in 3-D in Figures 12, 13, and 14, for W/C ratios of 47%, 54%, and 61%, respectively. These graphs give a general overview of the findings of this research.
Figure 12. 3-D Graph for w/c = 47%
Figure 13. 3-D Graph for w/c = 54%
Figure 14. 3-D Graph for w/c = 61%
These three graphs can be analyzed along each axis to show the decrease in the average stress at failure with the increase in rubber content, and also the decrease in the average stress at failure with the increase in UV exposure. Also, if the relationship along the diagonal axis, towards the front-right of each of the graphs the overall downward trend in compressive strength as both rubber percentage and exposure length increase together. Also, when compared to each other, these graphs can also be used to show the relationship of the decrease in the average stress at failure of the rubberized concrete with the increase in the water/cement ratio.
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Modulus of Elasticity
The second section of results calculated focused on the modulus of elasticity of the rubberized concrete. This was determined by analyzing the stress versus strain graphs plotted for each cube. The straight-line section of the curve, which represents the elastic portion of the overall curve, was used to determine the elastic stress and strain, which were then used to determine the modulus of elasticity. The modulus of elasticity is a measure of the stiffness of a material, or in this case it is a measure of the deformation of the rubberized concrete. This means that the lower the modulus of elasticity a sample has, the lower the amount of deformation it could withstand before breaking.
The results for this section were found in the following way. First, a straight line was fit to the straight-line portion of the load vs. displacement graph for each cube. Then, using this line as a guide, the x and y coordinates were recorded for the beginning and end of the linear portion of the curve. If, for some reason, the straight line fit to match the linear portion of the curve did not go through the origin, then a line parallel to it was drawn through the origin. In this case, the corresponding points for the beginning and end of the linear portion of the curve on the new parallel line were recorded. The difference in the x coordinates could be divided by the original height of the cube in order to find the elastic strain, and then the difference in the y coordinates could be divided by the original surface area of the cube in order to find the elastic stress. It was then the ratio of this elastic stress to this elastic strain that was equivalent to the modulus of elasticity.
Overall, the results from this section were again separated into four different categories. These categories were:
- Average Modulus of Elasticity Vs. W/C Ratio for different Rubber Contents
- Average Modulus of Elasticity Vs. Rubber Content for different W/C Ratios
- Average Modulus of Elasticity Vs. Exposure Length for different Rubber Contents
- Overall Graphs Comparing Exposure Length, Rubber Content, and Average Modulus of Elasticity for different W/C Ratios
Similar, to the procedure for the stress results, the elastic stress, elastic strain, and modulus of elasticity were determined for every individual cube. The averages for each triplet of cubes were then determined. All of these values can be viewed in Appendix A. The average moduli of elasticity for the triplets are shown in Table 20 below.
Table 20. Average Modulus of Elasticity for all Samples
Due to the seemingly erratic nature of the results above, it is very hard to distinguish trends from the data alone. Therefore, the next four sections will display, in graphical form, the results above.
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Relationship of Modulus of Elasticity and W/C Ratio
The fifth relationship shown is that of the average modulus of elasticity for each triplet of cubes versus the water/cement ratio for each of the different rubber contents. The expected trend for this relationship is that the modulus of elasticity will decrease as the water/cement ratio increases. This is due to the fact that, as shown earlier, as the water/cement ratio decreases, the rubberized concrete becomes weaker, and therefore will be able to withstand less deformation, or in other terms, have a lower modulus of elasticity. The data for this relationship is shown in Table 21, and in graphical form in Figures 15, 16, and 17 for 47%, 54%, and 61% water/cement ratios, respectively.
Table 21. Data for Average Modulus of Elasticity (MPa) Vs. W/C Ratio for different Rubber Contents
Figure 15. Average Modulus of Elasticity Vs. W/C Ratio for 0% Rubber
The four graphs in this sections are all made up of three points, each of which represents a different water/cement ratio, and four lines, each of which represents a different period of UV exposure. The expected trend for this relationship is only portrayed in half of the graph above, that is, in the change from 54 to 61 percent water/cement ratio. The discrepancies in the first section in the graph can be attributed to the fact that concrete is not a homogeneous material, and unfortunately, there is also a human factor which must be taken into account. This general downward trend for the second half of the graph does confirm the theory, however the many discrepancies in the first half suggests that more testing may be required in order to obtain more accurate results
Figure 16. Average Modulus of Elasticity Vs. W/C Ratio for 5% Rubber
Figure 17. Average Modulus of Elasticity Vs. W/C Ratio for 10% Rubber
Once again, the expected trend is apparent for the latter half of Figures 16 and 17, but does not appear for the first section. Despite the fact that the trend is partially confirmed, more testing is necessary to draw solid conclusions.
Figure 18. Average Modulus of Elasticity Vs. W/C Ratio for 15% Rubber
The last graph in this section shows a general downward trend, as expected, in three out of the four curves plotted. This helps to further confirm the theory that as the water/cement ratio increase, the average modulus of elasticity for each triplet will decrease.
Therefore, the overall result for this section is that there is a general trend, which displays the expected relationship or that as the water/cement ratio increase, the modulus of elasticity decreases. However, in this section there were many discrepancies noted. Therefore, although this research does help to confirm the theory, further research should be conducted in order to substantiate the results found here.
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Relationship of Modulus of Elasticity with Rubber Content
The sixth relationship discussed was that of the average modulus of elasticity versus the rubber content for different water/cement ratios. The expected trend for this relationship is that as the rubber content increases the modulus of elasticity will decrease due to the fact there should be less deformation possible in the weaker (higher rubber content) samples. The data for this relationship is sown in Table 22, and also in graphical form in Figures 19, 20, and 21 for 47%, 54%, and 61% water/cement ratios, respectively.
Table 22. Data for Average Modulus of Elasticity (MPa) Vs. Rubber Content for different W/C Ratios
Figure 19. Average Modulus of Elasticity (MPa) Vs. Rubber Content for W/C=47%
Figure 20. Average Modulus of Elasticity Vs. Rubber Content for W/C=54%
Figure 21. Average Modulus of Elasticity (MPa) Vs. Rubber Content for W/C=61%
Each of the graphs in this section is made up of four points, each of which represents a different rubber content, and four lines, each of which represents a different exposure length. There is a general downward trend apparent in all of the figures above, therefore proving the theory that as the rubber content increases the modulus of elasticity decreases. There are also some minor discrepancies shown, all of which are apparent on graph for 47% water/cement ratio, in Figure 19. However, due to the very small number of these discrepancies, and the fact that, even with discrepancies, there is still a downward trend shown, even in Figure 19, it is possible to confirm the expected trend for this relationship.
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Relationship of Modulus of Elasticity and Exposure length
The seventh relationship shown is that of the average modulus of elasticity for each triplet of cubes versus the exposure length for the different percentages of rubber in the mix. The expected trend for this relationship is that as the exposure length increases the modulus of elasticity will decrease due to the slightly weakened nature of the exposed concrete. The data for this section is shown in Table 23 below, and also, in graphical form, in Figures 22, 23, 24, and 25 for 0%, 5%, 10%, and 15% rubber respectively.
Table 23. Data for Average Modulus of Elasticity (MPa) Vs. Exposure Length for all Rubber Contents
Figure 22. Average Modulus of Elasticity (MPa) Vs. Exposure Length for 0% Rubber
Each of the graphs in this section, similar to the one above, is made up of four points, each of which represents a different period of UV exposure, and three lines, each of which represents a different water/cement ratio. It is difficult to draw very solid conclusions from the results shown in the graph above. There is no clear trend to describe the points. There is a downward trend apparent in some sections of the graph, such as the 7000 to 28000 minute sections of the 47% and 54% samples and also the last section of the 61% curve; however, there are also some upward curves apparent. The erratic nature of these results can be attributed to the fact that concrete is a non-homogeneous material and also to the fact that there is a human factor that must be taken into account. These results also show that in order to develop a clearer trend, additional research should be done.
Figure 23. Average Modulus of Elasticity (MPa) Vs. Exposure Length for 5% Rubber
The results in this graph show a clearer trend. It is apparent here that there is in fact an overall downward trend, with a non-downward trend occurring in only one exposure section of the graph. This leads to the belief that for the most part it is true that as the length of UV exposure increases the modulus of elasticity will decrease.
Figure 24. Average Modulus of Elasticity (MPa) Vs. Exposure Length for 10% Rubber
Figure 24 shows a very similar trend to that shown in Figure 23. There is a general downward shown in all of the curves except in the 7000 to 28000 minute section for the 47 and 61% curves. Due to that fact these are minor discrepancies, this graphs does verify the theory that as the exposure length increases the modulus of elasticity decreases.
Figure 25. Average Modulus of Elasticity (MPa) Vs. Exposure Length for 15% Rubber
The results in this graph are again slightly erratic. However, a downward trend is shown in four out of the nine separate sections of the graph, and in some of the remaining sections it is only a very minimal increase that is displayed. Therefore this also leads to the belief that there is a general downward trend shown in the graph and therefore the theory that as the exposure length increase, the modulus of elasticity decreases is verified.
Despite the discrepancies and the seemingly erratic nature of the results in this section, the expected theory that with an increase in exposure period there will be a decrease in the modulus of elasticity is proven. In addition, it is possible to say that because of all of the discrepancies and out of trend results that this decrease in the modulus is not a drastic one, and therefore will not be overwhelmingly damaging to the concrete. It is also true though, that in order to further confirm these results further testing should be conducted.
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Relationship of Modulus of Elasticity, Exposure Length, and Rubber Content
The last section of results shows the relationship between the period of exposure, the percent of rubber in the mix, and the average modulus of elasticity for each triplet of cubes. These graphs are basically a summary of the results above and are shown in Figures 26, 27, and 28 for water/cements ratios of 47%, 54% and 61%, respectively.
Figure 26. 3-D Graph for w/c = 47%
Figure 27. 3-D Graph for w/c = 54%
Figure 28. 3-D Graph for w/c = 61%
The three graphs in the section can be analyzed along each axis in order to view trends in the graphs. For example, if the Rubber % axis is analyzed, it can be shown that as the rubber content increases there is generally a decrease in the average modulus of elasticity in each graph. Also if the Exposure Length axis is analyzed a slight decrease can be generally seen in the average modulus of elasticity. Also, if a diagonal axis is drawn from the back left corner of the graph to the front right, there is an overall decrease in the modulus shown as the exposure length and rubber content both increase simultaneously. These 3-D graphs help to illustrate the theories mentioned above in their respective sections.
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CORRELATION BETWEEN LAB AND FIELD EXPOSURE
The next step in this research was to correlate the UV exposure in the lab to UV exposure in reality. This was done by first determining the amount of energy that was affecting the concrete in the UV CrossLinker in the lab. The energy came from 5-8 watt bulbs and affected a surface area of 40 by 35 cm. Therefore is the product of these numbers is taken it is determined that the energy affecting the concrete was 28.57 mW/cm^2. Next the energy in reality needed to be determined. The UV bulbs in the oven produced a wavelength of 254 nm. This value could be found on a chart of solar spectral irradiance from NASA Technical Report R-351 and, in turn, correlated to an energy value of 0.306659 mW/cm^2. This value is equivalent to the amount of energy produced by UV radiation of this wavelength in reality. Therefore, a lab/reality ratio can be developed, which is equal to 93.2. This means that the energy given off in the lab is 93.2 times stronger than that which is given off in reality. This also means that one unit of time in the lab is equivalent to 93.2 units of time in reality.
This data was then used to correlate the lab time to reality time in various cities throughout the United States. This was done by first obtaining the UV exposure for different cities throughout the United States from the Energy Technology Handbook. These values were then compared to the times in the lab and converted into a reality time in years. This means that for each length of exposure, 7000, 28000, and 56000 minutes, the number of years of exposure in reality for various locations was calculated. This data is shown on the following page in Table 24. Due to the fact that the most damaging rays of the sun are those that impact a surface at a ninety degree angle, the locations that are closer to the equator, which therefore have a higher percentage of perpendicular rays, are calculated to have a longer exposure time in reality than those further from the equator. The values on the chart reflect this, such as the fact that the reality exposure time for a place such as Hawaii is higher than the reality exposure time for a place such as Alaska, which is further from the equator than Hawaii.
There is a discrepancy in the data shown though. This is due to the fact that the reality exposure times are calculated on the basis of continuous exposure. In reality, however, exposure is not continuous. This is due, first of all, to the fact the UV rays are not present during the night, and also due to the fact that the most damaging, perpendicular rays only occur for one to two hours per day, depending on the distance of the location from the equator. Therefore, the values in years on the table can be multiplied by a factor of either 12 or 24 in order to account for this non-continuity in the exposure. Due to the fact that the values for the 56000 minutes samples range from approximately 4 to 9 years, this factor of multiplication would results in values slightly over 100. This is why the value of 56000 minutes was chosen. Most structures are designed to have a life span of approximately 100 years. This means that the exposure time of 56000 minutes realistically covers the life span of most structures. Therefore the 7000 and 28000 minute samples were used to make any trends in the data visible.
Table 24. Correlation Between Lab and Field Exposure
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CONCLUSIONS
- For any given rubber content, the stress at failure decreases with the increase in water/cement ratio.
- For any given water/cement ratio, the stress at failure decreases with the increase in rubber content.
- For any given water/cement ratio or rubber content, the exposure of rubberized concrete to UV radiation decreases the stress at failure.
- In many cases, the length of the UV exposure period is indirectly proportional to the stress at failure.
- The decrease in compressive strength due to UV exposure is not significant enough to prevent the use of rubberized concrete in real-life applications.
- For any given rubber content, the modulus of elasticity decreases with an increase in the water/cement ratio.
- For any given water/cement ratio, there is a decrease in the modulus of elasticity as the rubber content increases.
- For any given water/cement ratio, there is a general decrease in the modulus of elasticity as the period of UV exposure increases.
- The decrease in modulus of elasticity due to UV radiation is not significant enough to hinder the usage of rubberized concrete.
- There is a clear correlation between the lab exposure times and reality time, in years, in various locations throughout the United States, and the times used in this study realistically cover the life span of most structures.
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REFERENCES
Ahmad, Shuaib; Fedroff, David; and Sayas, Banu Zeynep (1997). "Freeze-Thaw Durability of Concrete With Ground Waste Tire Rubber," Transportation Research Record, No. 1574.
Avcular, N. and Topcu, I.B. (1997). "Collision Behaviors of Rubberized Concrete," Cement and Concrete Research, V. 27, No. 12.
Bayomy, Fouad M and Khatib, Zaher K. (1999). "Rubberized Portland Cement Concrete," Journal of Materials in Civil Engineering, V. 11, No. 3.
Cahill, James (2001). "Strength and deformation characteristics of rubberized concrete," Internal Report, Civil engineering Department, Union College, Schenectady, NY.
Considine, Douglas M., ed (1977). Energy Technology Handbook. New York: McGraw-Hill Book Company.
Eldin, Neil N. and Senouci, Ahmed B. (1993). "Observations on Rubberized Concrete Behavior," Cement, Concrete, and Aggregates, V. 15, No. 1.
Thekaekara, Matthew P., ed. "The Solar Constant and the Solar Spectrum Measured from a Research Aircraft," NASA Technical Report R-351.
Topcu, I.B. (1997). "Assessment of the Brittleness Index of Rubberized Concretes," Cement and Concrete Research, V. 27, No. 2.
Topcu, I.B. (1995). "The Properties of Rubberized Concretes," Cement and Concrete Research, V. 25, No. 2.
Toutanji, H.A. (1996). "The Use of Rubber Tire Particles in Concrete to Replace Mineral Aggregates," Cement and Concrete Composites, V. 18, No. 2.
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LIST OF SYMBOLS
ACI American Concrete Institute ASTM American Society of Testing and Materials Cc coefficient of curvature Cu coefficient of uniformity E modulus of elasticity Sc maximum compressive or normal stress Su compressive strength (maximum compressive or normal stress at 28 days) W weight W/C water/cement ratio
