MITIGATING WEB CRACKING IN POST-TENSION APPLICATIONS

Transcription

1 MITIGATING WEB CRACKING IN POST-TENSION APPLICATIONS August 8, 2014 Erin Segal, Lehigh University REU Site: University of Nevada, Reno Primary Investigator: Dr. David Sanders Mentor: Brett Allen

2 Abstract The Nevada Department of Transportation (NDOT) has observed during the construction process of post-tensioned bridge beams that web cracks were forming. Cracking leads to a greater chance of corrosion and high repair costs. The purpose of this research project was to determine the factors that affect and cause the web cracking. Six large-scale beams were produced to NDOT standards, each varied in at least one of the following factors: curvature of duct path, spacing between ducts, and duct tie reinforcement. Each beam was stressed and air pressure tested while monitored with strain gauges in various locations. To determine if duct material was a factor, small-scale concrete duct specimens were also constructed. These were tested for compression strength and air pressure capacity. The large-scale beams illustrated that a medium amount of curvature and greater spacing between ducts helped reduce cracking. The tests showed that cracks were forming during the air pressure testing, which put strain on the ducts and surrounding concrete. The addition of duct tie reinforcement helped counteract the tension forces between the ducts which reduced the amount of visible cracking. The duct material testing illustrated a higher deformation in plastic ducts during compression loading. Further testing would need to be completed to reach more conclusive results regarding duct material. i

3 Table of Contents 1. Introduction Project Scope Literature Review Methods Large-Scale Beam Configurations Large-Scale Beam Set Up and Initial Testing Large-Scale Beam Stressing and Air Pressure Testing Large-Scale Beam Post-Processing Duct Material Configurations Duct Material Testing Duct Material Analysis Results Large-Scale Beams Duct Material Conclusions Contact Information Acknowledgements References Appendix A Duct Material Test Data Appendix B Large-Scale Beam Testing ii

4 List of Figures Figure 1: Strain Gauge Labeling and Setup on Configuration 4; Brett Allen, Figure 2: Configuration 6; location and layout of ducts; the blue and red ducts were used for posttensioning, Brett Allen, Figure 3: Setup depiction with dead load in place, the top picture is birds eye view, the bottom picture is a side view, the colored lines represent the duct paths... 7 Figure 4: North end of Configuration 4, the jack is being moved into position to post-tension the lower duct... 8 Figure 5: The Concrete blocks with embedded ducts, the left is a PPE specimen and the right is a steel specimen Figure 6: Configuration 4 Top Left Steel Gauge; Jack Load vs. Microstrain Figure 7: Configuration 5 Duct Tie 8.75 inches Left, Microstrain vs Time Figure 8: Configuration 5 Duct Tie 8.75 inches Left, air pressure vs. microstrain Figure 9: Configuration psi after 0.75 fpu bottom duct on left, Configuration psi after 0.75 fpu top duct on right, west faces Figure 10: Configuration 4, 5, and 6 cut through the center after testing, cracks formed between the ducts Figure 11: Thin compression test steel specimen Figure 12: Total Compression Plastic and Steel no concrete specimens, load vs displacement Figure 13: Configuration 4 full view before stressing of bottom duct Figure 14: Configuration 4, surface gauge setup Figure 15: Thin Compression, steel specimen setup Figure 16: Total compression, steel duct no concrete setup Figure 17: Air pressure test set up, duct material specimen Figure 18: Total Compression test results for both plastic and steel Figure 19: Thin Compression test results for both plastic and steel List of Tables Table 1: Test Beam Configurations... 4 Table 2: Duct Material Specimen Testing Designations Table 3: Duct material compression test data Table 4: Air pressure results on material specimens Table 5: Large-scale beam configurations with each radius iii

5 1. Introduction Prestressed concrete is widely used in the U.S. and around the world. It was first introduced after World War II by Eugene Freyssinet (Abeles and Bardhan-Roy, 1981). The main principle behind prestressed concrete is the application of stresses on a beam or structure before the applied service loads are added. This set of initial stresses place the concrete beam into compression. Once the tensile loads are added the member will have compressive stresses to balance them out. Since concrete has higher strength in compression than in tension, this method makes the concrete more resistant against loads and gives it a high strength capacity. Prestressing concrete is either done by pre-tensioning or post-tensioning. The focus of this research project is post-tensioning. Post-tensioning allows the concrete to be cast before any stresses are put on the beam. This is done by setting ducts or pipes through the concrete while the concrete is being poured. The tendons, usually made up of steel cables, are then passed through the ducts. These tendons are placed in tension by anchorages and a jack, which pull the tendons taught. The ducts are then filled with grout to make it a bonded posttensioned system. The tension in the cables places the concrete in compression. In order for the concrete to fail in tension, the compression stress must first be overcome. Pre-tensioning is similar but the tendons are pulled in tension before the concrete is poured. The Nevada Department of Transportation (NDOT) has been using post-tensioned bridge beams in multiple construction projects throughout the state. NDOT construction crews have observed that through the process of post-tensioning, cracks appear along the paths of the posttensioning ducts. After the concrete has been poured the ducts undergo air pressure testing to ensure that the structure can be grouted for a bonded post-tensioned system. Web cracks form in the bridge beams during tendon stressing, air pressure testing, and the grouting phases of construction. If the cracks are severe enough this can cause delamination of the concrete in the web, causing the beam to lose its strength. The purpose of this project is to determine the cause of these web cracks and to find ways to better construct post-tensioned beams to avoid such issues. The parameters being considered for the possible causes are the spacing distance between ducts, the radius of curvature of the duct paths, the reinforcement between ducts, and the material type of the duct. At the conclusion of the project the results will be used to determine if the codes for construction of post-tensioned systems need revising. 1.1 Project Scope The project included six large-scale beams modeled after NDOT s specification for bridge beams. Three beams were for testing the radius of curvature of the duct paths while the other three beams were used to observe what design changes based on the medium radius best minimized cracking. The second set of beams considered the effect of duct spacing and extra reinforcement between ducts. The larger beams were compared using data collected from strain gauges and air pressure testing. Twenty seven miniaturized specimens of concrete and duct samples were tested to determine the effect of the duct material type on the strength of the specimen. The data collected were compiled and using Microsoft Excel and MATLAB, the large-scale and miniature beams were compared and analyzed to reach conclusions regarding 1

6 the factors that contribute to the observed cracking. The research experience for undergraduate (REU) student was responsible for analyzing and comparing all data and constructing figures and testing the miniature duct type specimens. 2. Literature Review Previous research has been done on post-tensioned systems explaining the advantages of prestressing (Abeles and Bardhan-Roy, 1981; Huang, 1973; Luthi et al., 2008; Muttoni et al., 2006; Ahuja, 1991; Stone and Breen, 1981). Prestressed concrete is more durable than reinforced concrete and also holds up well in corrosive environments such as a bay area that is exposed to sea spray (Abeles and Bardhan-Roy, 1981). Less steel is required for prestressed concrete than reinforced concrete, allowing members to be made longer, shallower, and more slender (Abeles and Bardhan-Roy, 1981). Prestresssing concrete also allows structures to have more flexibility which helps them to withstand seismic loads (Abeles and Bardhan-Roy, 1981). Although there are several advantages to prestressed concrete, prestressed losses need to be considered (Huang, 1973). In Huang s (1973) research on the prestress losses in pre-tensioning found that estimating the prestress losses separately was impractical because they were interconnected with multiple characteristics of the beam. Huang (1973) found that the losses depended on the concrete and steel characteristics, initial tensioning, and geometric design parameters, and the growth of long term loss can be approximated using a semi-logarithmic relationship with time. According to Abeles and Bardhan-Roy (1981) the main prestress losses for post-tensioned members are relaxation, elastic shortening, friction, and creep. Relaxation is the ability of the steel cables to lengthen if the stresses on the steel are too high. Elastic shortening is a loss of stretch in the wire tendons due to creep that occurs over time because of the prestress forces on the concrete causing the concrete to deform. Friction happens between the duct and the tendons, which limits the amount of tension the cables can take. These losses all take away from the overall effectiveness of prestressing concrete (Abeles and Bardhan-Roy, 1981). The general concepts that explain pre-tensioning losses should also apply to post-tensioning systems (Huang, 1973). Prestressed concrete can be utilized better if the losses are minimized. One way researchers have thought of doing this is by determining the effects of duct material type on the prestressed structure (Luthi et al., 2008). Different duct types have been researched and experimented on in the past to determine if a specific material is better for the prestressing process (Luthi et al., 2008). Luthi et al. (2008) researched the effects of duct type for post-tensioned concrete on bond and friction losses. In this project, live end displacement behavior of the specimens was studied in most detail. Galvanized steel ducts continually had the highest peak loads compared to high density polyethylene, HDPE, ducts and smooth steel pipes. Corrugated ducts reduced the sliding effect and created a better connection with the concrete. Luthi et al. s (2008) study also concluded that the friction coefficient or friction loss was 40% lower for the HDPE plastic ducts than the galvanized steel ducts. The smooth steel pipes had a 30% higher friction coefficient compared to the galvanized steel ducts. This positive effect of the HDPE ducts was offset by their effect on 2

7 bond stresses. The corrugated galvanized steel ducts had 20% to 40% higher bond stresses than the HDPE ducts. Bond stresses and friction losses are important to determining duct type, but it is also relevant to consider the effect of design and construction practices on shear strength. The Network for Earthquake Engineering Simulation (NEES) project at the University of Nevada, Reno (UNR) tested large-scale members that were experiencing cracking in the webs. In an early study done at the University of Texas, Austin by Muttoni et al. (2006) the effects of duct type on shear strength were examined. The NDOT construction codes for post-tensioning details the factors and restraints for corrugated steel pipes, but gives few specifics about plastic ducts. Muttoni et al. s (2006) research showed that the presence of ducts in post-tensioned systems decreased the strength of webs in girders. A decrease in strength reduces load capacity allowing cracks to form earlier than they would if there was a high strength. The loss of strength is high if there is a greater ratio of total duct section to overall web width. The total duct section can mean larger diameter ducts or more ducts in the member. It was also concluded from Muttoni et al. s (2006) study that steel ducts with grout had the highest strength while HDPE ducts had approximately 20% lower compressive strengths in thin webbed members. The duct type was not the only component to have an effect on member strength. The radius of curvature and duct placement also affected strength. The positioning of ducts and the radii of curvatures were investigated in previous research (Muttoni et al., 2006; Luthi et al., 2008; Stone and Breen, 1981). Comparisons were done to determine if one larger duct or two smaller ones would yield different results. It was observed that two ducts set side by side produce less strength reduction than one duct of twice the diameter (Muttoni et al., 2006). The study found that the strength loss of placing ducts one diameter apart was practically the same as having only one duct (Muttoni et al., 2006). It was also seen that close duct spacing and sharp radii of curvature of duct paths led to cracking and breakouts on curved post-tensioned box girders (Ahuja, 1991). In contrast, Luthi et al. (2008) found that the radius of curvature had negligible effect on friction losses. It seems that in past research the factors that affected one aspect of the prestressed members did not affect all the others (Muttoni et al., 2006). In Muttoni et al. s (2006) study the total load losses were higher for smaller radii. In the study conducted by Luthi et al. (2008), it was determined that a bridge girder with a radius of nine meters had smaller load losses than a specimen with a three meter radius. When post-tensioned girder anchorage zones were investigated at the University of Texas, Austin by Stone and Breen (1981), it was found that significant curvature sections with multiple strands in ducts created large lateral splitting forces at the point of minimum curvature. To remedy this, the researchers put auxiliary reinforcement which helped with the cracking along the tendon path and anchorage regions. In previous research conducted by Abeles and Bardhan-Roy (1981), Huang (1973), Luthi et al. (2008), Muttoni et al. (2006), Ahuja (1991), Stone and Breen (1981) it is noted that reinforcement, curvature radius, duct spacing, and duct type all have effects on the properties and capabilities of prestressed concrete members. The different studies conducted to test specific elements of prestressed post-tensioned concrete include the same properties as in the NEES project at UNR of reinforcement, curvature radius, duct spacing, and duct type. In the project outlined for this NEES research, these 3

8 elements will be combined in an attempt to create a single cohesive analysis of why web cracks form and how to alter construction processes to eliminate this problem. The factors tested in the NEES project at UNR will be considered and brought together to detail final construction and design specifications for post-tensioning concrete bridge beams. 3. Methods 3.1 Large-Scale Beam Configurations The large-scale beams in the NEES@UNR project were at 0.7 scale of existing Nevada Department of Transportation s (NDOT) bridge beams. All beams were tested at 28 days after construction or later. Each beam s configuration is listed in Table 1. The first three configurations each had 0.7 inches of spacing between ducts and no duct tie reinforcements (NDOT protocol has 1 inch spacing between ducts but the tested beams were 0.7 scale.) The variable factor between the first three configurations was the duct path curvature. Configuration 1 had the largest average radius and configuration 3 had the smallest radius of curvature with configuration 2 being in between. Beams 4 through 6 all kept the same curvature as configuration 2 (medium curvature). Configurations 4 through 6 also had 1.05 inches of spacing between the ducts instead of the original 0.7 inches. Configuration 4, had no duct tie reinforcement. This allowed a more accurate comparison to be drawn for the increase in spacing between ducts. Since configuration 4 had medium curvature it was compared with configuration 2. Configuration 5 had duct tie reinforcement spaced at 17.5 inch increments and configuration 6 had duct tie reinforcement at 7 inch increments. The variations of the three factors allows for accurate analysis of the effects of each on the beam performance and appearance of cracks. Table 1: Test Beam Configurations Average Curvature Spacing Duct Tie Radius (ft) Between Ducts Reinforcement Configuration inches None Configuration inches None Configuration inches None Configuration inches None Configuration inches spaced 17.5 inches Configuration inches spaced 7.0 inches As the testing progressed, the number of strain gauges used to collect data increased. The gauges were located on the concrete surface, the steel reinforcement, the duct tie reinforcements, and embedded in the concrete. In later configurations, gauges were placed on both the east and west sides of the web and also to the left and right of the centerline of each side. The gauges were also positioned at varying heights at all locations. The gauges were placed as shown in Figure 1 which depicts the length of the concrete beam. A few extra gauges on the surface manufactured by novotechniks monitored web bulging displacement along the height of the web. Configuration 1, which had the fewest gauges had about twenty in the 4

9 various locations. The final beam, configuration 6, had about sixty gauges. The data collected from the testing was processed afterwards to relate displacement and strain data to time, jack load, and air pressure. KEY Red Lines: Top Duct Blue Lines: Middle Duct Green Lines: Bottom Duct, not stressed Figure 1: Strain Gauge Labeling and Setup on Configuration 4; Source: Brett Allen 5

10 3.2 Large-Scale Beam Set Up and Initial Testing The six large-scale bridge beams were made to Nevada s Department of Transportation specifications. Before the concrete was poured, the empty ducts were placed in the beam molds, which is done for any post-tensioning application. There were three ducts in each beam but one remained empty throughout the entire experiment. The extra duct allowed two areas of interest between ducts instead of one. The duct placement is shown in Figure 2. Figure 2: Configuration 6; location and layout of ducts; the blue and red ducts were used for post-tensioning, Source: Brett Allen During testing, the large beams were placed on concrete reaction blocks which supported the center third of the beam (Figure 3). This raised the beams off the laboratory floor and allowed the portion near either end to be suspended in air. It also simulated the column the beam would normally be supported by. This setup can also be seen in Appendix B. Once the beam was setup with all the gauges and connected to the computer to record data, the dead load was applied by pressure jacks. The magnitude of load was calculated to be the average load the beam would experience if it were used in a bridge. This was kips per pressure jack. 6

11 Pressure Jacks Pressure Jacks Figure 3: Setup depiction with dead load in place, the top picture is bird s eye view, the bottom picture is a side view. The colored lines represent the duct paths To verify that there were no cracks or leaks before the beam was post-tensioned, both ducts that were used for post-tensioning underwent initial air pressure testing. This initial air pressure was 50 psi which is standard for NDOT. Once initial air pressure testing was completed, steel strands used as the tendons were pulled through the ducts. NDOT normally uses nine tendons in each duct, but for this testing twelve were used. The high number of tendons was to ensure the cables would not fail in the prestressing process. The testing focused on how well the beams performed at higher than normal stresses, so extra tendons were used to ensure they would not be the cause of failure. The ducts at one end of the beam were secured and caped while all stressing was done at the other end. Figure 4 shows the jack in place and the setup for stressing. 7

12 NDOT Loading Protocol Figure 4: North end of Configuration 4, the jack is being moved into position to post-tension the lower duct 3.3 Large-Scale Beam Stressing and Air Pressure Testing The beams were post-tensioned using a jack at the open end. The tendons were marked before and after each stressing to determine the elongation caused by the prestressing. The process for prestressing and air pressure testing were done using NDOT s standard protocols. Each strand had a 0.6 inch diameter. The ultimate strength of a post-tension strand is 270 ksi. The stresses used in the testing were measured as a percentage of the ultimate strength (f pu ). The order of testing was as follows: 1. Apply dead load on beam (22.5 kip on each end) 2. Air pressure test lower duct (Locked off at 50 psi, no more than 25 psi lost in 1 min) 3. Air pressure test top duct (Locked off at 50 psi, no more than 25 psi lost in 1 min) 4. Stress lower duct to 0.15 f pu to take slack out of strands 5. Stress top duct to 0.15 f pu to take slack out of strands 6. Stress lower duct to 0.45 f pu 7. Stress top duct to 0.45 f pu 8. Stress lower duct to 0.75 f pu (100% target stress) 9. Stress top duct to 0.75 f pu (100% target stress) 10. Air pressure test lower duct to 50 psi 11. Air pressure test top duct to 50 psi 12. Air pressure test lower duct to 75 psi 13. Air pressure test top duct to 75 psi 14. Air pressure test lower duct to 100 psi 15. Air pressure test top duct to 100 psi 16. Air pressure test lower duct to 125 psi 8

13 17. Air pressure test top duct to 125 psi 18. Stress lower duct to 0.85 f pu (113% target stress) 19. Stress top duct to 0.85 f pu (113% target stress) 20. Air pressure test lower duct to 50 psi 21. Air pressure test top duct to 50 psi 22. Air pressure test lower duct to 75 psi 23. Air pressure test top duct to 75 psi 24. Air pressure test lower duct to 100 psi 25. Air pressure test top duct to 100 psi 26. Air pressure test lower duct to 125 psi 27. Air pressure test top duct to 125 psi Up to and including step 11 is typical NDOT loading protocol for post-tensioning beams in bridges. The NEES@UNR project tested beyond 0.75 f pu stress and 50 psi air pressure to determine if the beams would fail when subjected to the higher stresses. During the air pressure testing if more than half the pressure decreased within a minute of valve closure it was considered a failure. This is the same test NDOT uses on bridges under construction to confirm ducts are capable of being grouted for a bonded connection. If the duct was unable to hold a specified air pressure, then it was tested at that air pressure with a constant supply. During every stage of the testing process visual inspection and markings of cracking along the webs were noted and tagged. Once all the testing was completed the beams were dismantled and cut in half for further inspection. A camera was also run through the ducts after the strands were removed to observe any duct deformation. Split cylinder and compression testing were conducted on testing days to get the elastic modulus and tension strain of each beam. Three cylinders were used for each test and the averaged result was used in the evaluations. The procedures for each test were followed using ASTM C39 and C496. For the compression tests the loading rate was 800 to 1200 pounds per second on the 6x12 standard cylinders. The split cylinder loading rate was 120 to 240 pounds per second for the 6x12 cylinders. These tests were also conducted similarly on the small duct material specimen test days. 3.4 Large-Scale Beam Post-Processing During the casting process, strain gauges were embedded in the concrete. Once the beams were set in place to be tested, strain gauges were added to the concrete surface on both the west and east sides. Gauges were also embedded in the steel. For the final two beams tested, strain gauges were put on the duct tie reinforcements. These gauges were all connected to the computer through cables that ran between the beam and computer. The data was collected during each step of the testing process. This included each stress and air pressure test. Using MATLAB, each step of the testing process was compiled into a final list or matrix in the order in which it was completed. Each column of data signified a different gauge and each row was at a different time. On configurations 4 through 6, which the REU student post processed, readings were taken every 1/32 of a second. Before the data was graphed it was filtered using the Butterworth method. Graphs of each gauge vs time were first composed. The gauges were 9

14 also plotted against the jack loading. Each gauge is compared to the air pressure readings taken in both of the two ducts that were post-tensioned. In cases where a gauge had broken during testing, the readings were set to zero. 3.5 Duct Material Configurations Seven groups of duct specimens were compared in this portion of the project. The main two were the steel ducts and the PPE ducts embedded in concrete. The sizes of the duct specimens were 12 x8.4 x4.45 and 12 x8.4 x4.875 respectively and are pictured in Figure Figure 5: The Concrete blocks with embedded ducts, the left is a PPE specimen and the right is a steel specimen The difference in height for the two types of duct specimens is due to the variation in duct diameter. Both the top and bottom of the ducts are covered with 0.7 inches of concrete, which is equivalent to the duct spacing in the large-scale beams discussed in Section 3.1. The PPE ducts were selected based on their similar diameter to the steel ducts. This allows a direct comparison from the air pressure testing to be utilized in the analysis. The similar size also allowed the PPE ducts to be rated for the same number of tendons as the steel ducts. The other specimens tested are concrete slabs containing ducts with seam rupture steel, concrete slabs with no ducts, and ducts of both types that are not embedded in concrete. There are twenty seven specimens in concrete and six not embedded in concrete. Table 2 shows the number of ducts that will be used in each type of test. 10

15 Table 2: Duct Material Specimen Testing Designations Number of Specimens Tested from each Type Air Pressure Test Thin Compression Test Total Compression Test Steel Duct PPE Duct Seam Ruptured Steel Duct Steel No Duct PPE No Duct Steel No Concrete PPE No Concrete Each test specimen was monitored during the experiment through novotechniks, which are position transducers that measure displacement. The strengths of the specimens were also recorded using the compression testing machine (SATEC Systems, Inc) and the ducts air pressure capabilities were documented as well. After completion of all testing, the data was used to determine if there was strength loss for specimens with ducts and between duct types. The final results of this portion of the project will be used to determine if more testing and additional research should be done to consider the practicality of PPE ducts as a suitable substitute for standard steel ducts in NDOT construction. 3.6 Duct Material Testing Three tests were completed on duct material specimens. Standard air pressure tests, thin compression tests, and total compression tests. There are a total of twenty seven specimens with six extra that are not in concrete; refer to Table 2. Each test was run on three specimens of each type to get more accurate data. The steel and PPE ducts in the concrete were subjected to all three tests. The ducts with seam rupture were only used in the air pressure tests. The concrete specimens with no ducts and the ducts with no concrete were subjected to the total compression test only. The air pressure tests reached pressures of 50 psi and 75 psi in some cases. The thin compression tests compressed the specimens only over the area of the ducts. This is an approximate width of three inches along the twelve inch length specimens. This test used a narrow steel plate of dimensions 3 x12 x1 on the top of the specimens and a larger steel plate of 8.4 x12 x1 on the bottom. The total compression tests compressed the total width of each specimen. The larger steel plates (8.4 x12 x1 ) were used in the total compression testing. The only exception to this were the ducts not embedded in concrete. They utilized the split-cylinder testing head on the compression machine to perform the total compression tests. The novotechnik data, which monitored the compression machine displacement, and the applied force were recorded and used in the analysis. Air Pressure Test Procedure: 1. Mark specimens at ½ and 1/3 of total length on three sides 2. Place specimen in testing frame which places novotechniks at the marked lines on each side 3. Use silicone to secure gasket to both ends of duct opening 4. Fasten steel plates onto each end, one side has the air pressure tube connected 11

16 5. Zero novotechniks displacement and record data 6. Raise pressure to 50 psi and then to 75 psi if specimen is capable Total and Thin Compression Test Procedure: 1. Place specimen in testing machine with steel plates above and below 2. Attach novotechniks to machine to monitor displacement 3. Zero novotechniks displacement and record data 4. Compress the specimens until cracking using the compression machine Through the entire testing process pictures and comments regarding testing process were taken and recorded. 3.7 Duct Material Analysis Using Excel and MATLAB, graphs were created to show the resulting data from the project. These figures include close examinations of which material, steel or PPE held a higher strength during both compression tests. This was then interpreted using the material properties, the testing machines deflections, and force exerted. The results collected from the air pressure tests were summarized in air pressure vs. deflection graphs. The determination of which duct material is better suited for air pressure testing was also considered. Through the analysis it is hoped that full conclusions will be reached to better guide NDOT in whether or not to pursue further testing for replacement of steel ducts with PPE ducts. 4. Results 4.1 Large-Scale Beams Graphs of each strain gauge, measuring microstrain were compared with time, jack load, and air pressure. Figure 6 shows the relationship of jack load vs microstrain on a steel gauge in configuration 4. Each spike in the data is a different occasion of stressing. They are grouped in twos because two ducts were stressed at every loading stage. The levels are.15 f pu,.45f pu,.75f pu and the last two are.85 f pu. The graph illustrates a large gap between the third and fourth set of stressing. This is when the air pressure testing took place before the final stressing level. It is noted that there is very little change in microstrain between the stressing levels. The gap can be interpreted to mean that most of the strain recorded on the gauges took place during the air pressure testing. This signifies that during construction of a post-tensioned beam the majority of the strain and damage occurs during the air pressure stage. Figure 6: Configuration 4 Top Left Steel Gauge; Jack Load vs. Microstrain 12

17 For the last two configurations there were duct tie reinforcement gauges put in place to monitor their strains. In Figure 7 strain from a duct tie reinforcement gauge is graphed against time. Figure 7: Configuration 5 Duct Tie 8.75 inches Left, Microstrain vs Time The loading stages are written on the graphs. It shows that the strain on the duct ties increases greatly when the air pressure testing starts. Again this illustrates that the majority of strain and cause of problems in the beam configurations happens when the air pressure is applied. Figure 8 shows the air pressure effect on the amount of strain. The higher the air pressure the larger the strain. The configurations that showed the least amount of strain were configurations 5 and 6. The configuration with the most amount of strain was configuration 1. This means that the ducts and concrete around the ducts were pushed outward and expand during the air pressure testing. Configuration 5 and 6 had duct tie reinforcement which took most of the tension force produced by the air pressure. After the first three configurations were tested, the middle curvature was selected for the subsequent three tests because it had sufficient curvature to get the desired effect. Therefore configurations 4 through 6 all had the same curvature as configuration 2. The farther apart the ducts were spaced, the better they performed. This is because increasing the spacing allowed for a larger concrete tension capacity. This was seen in the later test by the decrease in cracking and strain recorded compared to the first three configurations. 13

18 Figure 8: Configuration 5 Duct Tie 8.75 inches Left, air pressure vs. microstrain Through observation during testing, configuration 6 had the least amount of cracks appear on the web surface. Configurations 5 and 6 passed all air pressure tests by holding more than half of the original pressure within a one minute period. In configuration 5 web surface cracking occurred on the side the ducts were tied to during 0.75 f pu 100 psi bottom duct pressure testing (protocol step 14). Configuration 6 showed a hairline crack at 0.75 f pu 100 psi top (protocol step 15). Configuration 4 first showed multiple cracks along the length of the beam after 0.75 f pu 100 psi bottom (protocol step 14). This is seen in Figure 9. During testing of configuration 4, the air pressure did not pass all the stages and was unable to be tested for 0.85 f pu 125 psi bottom duct (protocol step 26) due to too high percentage of leaking air. Figure 10 shows the damage done to configuration 4, 5, and 6 through the center cross-section. Configuration 4 had extensive damage in the center of the beam between the ducts while configuration 6 had virtually none. Configuration 5 showed small cracks between the ducts. Figure 9: Configuration psi after 0.75 f pu bottom duct on left, Configuration psi after 0.75 f pu top duct on right, west faces 14

19 Figure 10: Configuration 4, 5, and 6 cut through the center after testing, cracks formed between the ducts The embedded concrete gauges in configurations 4 through 6 did not present a consistent beam with the lowest strains for the stages before 0.75 f pu 100 psi bottom. This could indicate that before major air pressure is applied, the three configurations acted similarly due to such small stress fields. After 0.75 f pu stressing and 100 psi of air pressure was applied to the bottom duct a clearer pattern emerged from the data. Configuration 6 consistently performed better with the lowest maximum microstrain in the majority of embedded concrete gauges. Configuration 6 had the best strains in both tension and compression. Also in configuration 4 seven strain gauges broke during the testing due to high strains while configurations 5 and 6 each only had one gauge break due to testing. In cases where configuration 4 seemed to perform better than configuration 6, it may have been due to cracks forming where gauges were not placed. The surface concrete gauges were placed on the beams to measure the strain vertically. This was done because once cracks were to form through the center cross-section of the beam the beam would essentially act as two separate pieces held in place with an air pressure pushing them outward in either direction. This outward force would cause the beam to be put into strain vertically due to the stressing pushing the beams to bow outward. The concrete surface gauges showed very inconsistent data. In order for the gauges to show relevant information the cracks would have to form over the area of the gauge. Since it was difficult to predict where these cracks would form most of the gauges did not read relevant strains. 4.2 Duct Material For each air pressure test conducted on the duct material specimens, graphs of air pressure vs displacement were created. The compression tests yielded graphs of load vs average displacement of the compression machine. The air pressure test was not conducted on one of the ruptured steel specimens due to too much damage from construction and removal of the mold. Due to an inadequate isolation of the specimens during air pressure testing it was deemed unsafe to test any specimens with pressures higher than 75 psi for fear of unexpected 15

20 ruptures. A steel specimen incurred cracking along the bolt path in the concrete while the bolt was straightened to fit the air pressure cap. Other specimens were also susceptible to cracks while the nuts were being tightened onto the bolts. These discrepancies would reduce the capacities of the specimens to hold the designated air pressures and therefore give less accurate results. Two of the three plastic specimens tested for air pressure performed well by holding 75 psi and showing no cracking. The last of the three reached 75 psi but was then unable to hold the pressure. The steel specimens were rather inconsistent. One steel specimen was unable to hold 50 psi and a second was unable to reach 75 psi. At 65 psi the concrete failed by splitting apart due to too much pressure. The final steel specimen reached 75 psi and performed well. The two ruptured steel specimens were unable to make it past 50 psi and failed due to too much cracking in the concrete and an inability to hold pressure. The plastic overall was best able to maintain the 75 psi pressure and had the lowest average deflection at the high air pressure. To reach more conclusive results, further testing is required with significant changes to the testing procedure. The solid blocks with no ducts were unable to be compression tested because of limitations of the compression machine. One plastic solid block was tested prior to determination of the inability of the machine. Its results are limited due to the lack of available comparison. The thin compression tests for both the plastic and steel types caused cracks at angles around the ducts or vertically which would then follow the curvature of the duct (Figure 11). At the completion of these tests it was possible to completely remove the ducts from the concrete; the bond had been destroyed. Figure 11: Thin compression test steel specimen The plastic specimens had on average a higher deformation for a set load than the steel specimens for both the thin and total compression tests performed on specimens with concrete and ducts. The trend also showed that steel could reach a higher peak load for both the thin and total compression tests. The results from these compression tests can be found in Appendix A. When the plastic and steel ducts were tested without concrete, it was observed that the steel deformed more and the plastic had a higher peak load. Figure 12 shows that the plastic-no-concrete specimens would gradually deform while the steel specimens would take as much load as they could at the beginning and then start to deform at a quicker rate than their 16

21 plastic counterpart. This illustrates that when the steel ducts are not embedded in the member, they resist a higher compression load before deformation while the plastic ducts begin to deform at the onset of the loading. Figure 12: Total Compression Plastic and Steel no concrete specimens, load vs displacement 5. Conclusions The large-scale beams varied in duct path curvature, duct spacing, and duct tie reinforcement. The majority of the strain experienced by each beam occurred during the air pressure testing. The pressure pushed the ducts outward causing cracks to form in the concrete along the webs. Configuration 6 with the most duct tie reinforcement, the middle curvature, and the larger spacing between ducts performed best in the later stages of testing and in visual inspection. Comparison of the strain gauge values shows that for the stressing stages and beginning air pressure stages the configurations 4 through 6 would all perform relatively well. Through visual inspection of the beams during testing and after cutting, configuration 6 had the fewest visible damages on the exterior and center cross-section. Further analysis would need to be done on the data to make a more certain and applicable recommendation to NDOT. The duct material specimens illustrated that PPE ducts deform more during high loads than steel ducts. The steel ducts were able to maintain their form better at lower loads than the plastic ducts. For post-tension beams it is more valuable for ducts to deform less. Therefore the 17

22 steel ducts tested better in this research project. In order to reach full conclusions specifically on the air pressure capacities of duct materials, it is recommended that further testing be considered. 6. Contact Information For further information please contact the NEES REU student, Erin Segal at or Brett Allen at or Dr. David Sanders at 7. Acknowledgements This project was funded by the Nevada Department of Transportation and partially supported by the National Science Foundation through EEC and CMMI Special thanks to Dr. David Sanders, Brett Allen, and Mark Lattin for their support and mentorship. I would also like to thank Kelly Doyle, Rebecca Kloster, Alicia Lyman-Holt, Thalia Anagnos, and Sean Brophy for their guidance. 8. References Abeles, P. W., Bardhan-Roy, B. K. (1981). Prestressed Concrete Designer's Handbook. Third ed. Slough: Cement and Concrete Association. Ahuja, D. (1991). Effect of Duct Spacing on Breakout of Post-Tensioning Tendons in Horizontally Curved Concrete Box Girders, Master of Science thesis, Civil Engineering Dept., The University of Texas at Austin, Austin, TX. ASTM Standard C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. ASTM International, West Conshohocken PA, DO: /C0039_C0039M ASTM Standard C496, Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens. ASTM International, West Conshohocken PA, DO: /C Huang, T. (1973). Prestress Losses in Pretensioned Concrete Structural Members, Report No , Civil Engineering Dept., Lehigh University. Luthi, T., Diephuis, J., Icaza A., J., Breen, J., and Kreger, M. (2008). Effects of Duct Types and Emulsifiable Oils on Bond and Friction Losses in Posttensioned Concrete. J. Bridge Eng., 13(1), Muttoni, A., Burdet, O., and Hars, E. ( 2006). Effect of Duct Type on Shear Strength of Thin Webs, ACI Structural Journal, 103-S75, "Prestressed Concrete." (2014). PCA America's Concrete Manufacturers. Portland Cement Association, Web. 06 June Ragab, N., and Elbadry, M. (2010). Effects of Post-Tensioning Ducts on Shear Strength of Thin Webs of Bridge Girders, TRB 2011 Annual Meeting,

23 Stone, W.C., and Breen, E.J. (1981). Design of Post-Tensioned Girder Anchorage Zones, Report No. FHWA/TX-8l/l F, The University of Texas at Austin, Austin, TX. 19

24 9. Appendix A Duct Material Test Data Table 3: Duct material compression test data 20

25 Table 4: Air pressure results on material specimens 21

26 Figure 13: Thin Compression, steel specimen setup Figure 14: Total compression, steel duct no concrete setup 22

27 Figure 15: Air pressure test set up, duct material specimen 23

28 Figure 16: Total Compression test results for both plastic and steel Figure 17: Thin Compression test results for both plastic and steel 24

29 10. Appendix B Large-Scale Beam Testing Figure 18: Configuration 4 full view before stressing of bottom duct 25

30 Figure 19: Configuration 4, surface gauge setup Table 5: Large-scale beam configurations with each radius Curvature Radius (ft) Spacing Duct Tie Top Duct Middle Duct Bottom Duct Between Ducts Reinforcement Configuration inches None Configuration inches None Configuration inches None Configuration inches None Configuration inches spaced 17.5 inches Configuration inches spaced 7.0 inches 26