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Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems (2023)

Chapter: Chapter 3 - Findings and Applications

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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
×
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Suggested Citation:"Chapter 3 - Findings and Applications." National Academies of Sciences, Engineering, and Medicine. 2023. Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems. Washington, DC: The National Academies Press. doi: 10.17226/27029.
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19   Findings and Applications 3.1 Introduction e research project consisted of a literature search, an analytical investigation of the param- eters that aected shear key performance, and physical testing. e physical testing consisted of testing the bond between various shear key materials and various conditions of the preparation of the girder sides, and testing two, full-scale specimens. e full-scale specimens used two dif- ferent shear key congurations and were tested under temperature and live loads. Details of the literature search are found in Chapter 1. A summary of the methods used for the analytical work and physical testing are found in Chapter 2. A detailed summary of the analytical work and physical testing are found in Appendices B and C, respectively. 3.2 Findings of the Analytical Work As discussed in Chapter 2, the analytical model consisted of a bridge with a cross section consisting of seven 48-inch-wide box girders. Each model had six shear keys. e depth, span, and skew of the girders varied. Models were subjected to temperature-induced deformations, live loads, and lateral post-tensioning loads. A total of four shear keys were examined: the currently used partial-depth, top shear key (Type III); a thin, full-depth shear key (Type IV); a thick, full-depth shear key (Type V) and a mid-depth shear key that was not grouted to the top of the girder. Complete details of the analytical model and more in-depth ndings are found in Appendix B. 3.2.1 Temperature Analysis Figure 10 shows typical results for transverse stresses in the currently used, partial-depth, Type III shear key. e gure shows that the stresses near the top of the shear key, in the “throat” area, are nearly 1 ksi in tension. is is well beyond any reasonable bond strength for the shear key material. Also, the transverse stress remains tensile throughout the depth of the shear key at the ends. Moving away from the ends toward midspan shows the transverse stress becoming compressive. Information found in both the literature and in eld observations indicates that cracking starts near the end of the girders, and this is consistent with the analytical results. It is believed that cracking at the ends of girders is caused by the restraint of the bearing pads in transverse direction. ere was no clear inuence of span, girder depth, or skew on the stresses. Figure 11 shows typical transverse shear key stresses for the thin full-depth (Type IV) shear key. As shown, the stresses in the throat of the key are in excess of 700 psi. However, the stresses decreased with depth of the key and compressive stresses existed at the bottom of the key near the end. is implied that cracking was still likely to occur near the top but may not fully C H A P T E R 3

20 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Figure 10. Typical results for transverse stresses in the currently used, partial-depth Type III shear key. Figure 11. Typical results for transverse stresses in the full-depth Type IV shear key. penetrate through the full depth of the key. At the bottom of the throat, tensile stresses were less than 300 psi. ere was no clear inuence of span or skew on the stresses. Figure 12 provides typical results for the transverse shear key stresses of the thick full-depth (Type V) shear key. Like the thin full-depth key, the stresses were high near the top and exceed 800 psi. However, the transverse stresses at the bottom of the key were compressive even at the very end of the key. is suggested that the full-depth shear key may be successful because

Findings and Applications 21 cracking will not penetrate the entire depth of the shear key, and this may prevent leakage. ere was no clear inuence of span or skew on the stresses. Figure 13 shows typical results for transverse stresses in a mid-depth shear key. In this case, the Type III shear key was simply moved to the mid-depth of the girder and the area above the shear key was not grouted. e stresses were high at the top of the key (about 900 psi) and dropped quickly with depth. By the midpoint of the key, the tensile stresses were nearly zero and small compressive stresses developed at the bottom of the mid-depth shear key. e results suggest these shear keys may crack near the top, but the cracking will be minimal, and cracks may not propagate the entire depth of the shear key. e results shown in Figure 13 suggest that not grouting the very top of the shear keys may lessen the stress. As seen in Figure 11 and Figure 12, the stresses in full-depth shear keys drop o very quickly. is is not unexpected. e temperature gradient used in the model was the Zone 1 gradient from Article 3.12.3 in the AASHTO LRFD Bridge Design Specications. e gradient over the top 4 inches is 40o F while the gradient between the points 4 inches and 16 inches from the top is only 14oF. us, removing the material from the top 4 inches of the shear key may lessen the stresses. Figure 14 shows typical results for the Type IV shear keys when the top 4 inches of the shear key are not grouted. e transverse stresses were highly tensile near the top but dropped sig- nicantly and quickly with depth, eventually turning to compression. is was consistent with previous research by Huckelbridge et al. (1995, 1997) that cracking starts in the throat and that removing the throat grout (as is done in the mid-depth shear key) may prevent cracking. In the conguration, the throat could be lled with a ller or sealer material which may help prevent leakage. It is expected that similar results would be obtained for the Type V shear key, although that was not explicitly modeled. Figure 12. Typical results for transverse stresses in the full-depth Type V shear key.

22 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Figure 13. Typical results for transverse stresses in the mid-depth shear key. Figure 14. Typical transverse stresses in the Type IV shear key when the top 4 inches are not grouted.

Findings and Applications 23 e results suggest that using a full-depth shear key and not grouting the top 4 inches would be eective in preventing or lessening cracking. Note that Figure 10 shows that removing the grout from the throat of the currently used Type III shear key may not be eective as that entire shear key is seen to be in tension. 3.2.2 Live Load Analysis Live load stresses were evaluated by placing an HL-93 truck loading on the bridge. Models were run with live load only and with a combination of live load and temperature. It was found that the transverse tensile stresses due to live load only were very low, always less than 150 psi and oen less than 70 psi. Another important point is the location of these tensile stresses. While the maximum transverse tensile due to temperature is near the top of the shear key, the maxi- mum tensile stress due to live load is typically at the bottom of the shear key. In the currently used Type III shear key, these stresses add to the tensile stress at the bottom of the shear key. is conrms the ndings of earlier studies that live loads do not cause cracking but may simply drive existing cracks. In the full-depth shear keys, these stresses occur in areas of small tensile or areas where the transverse stress is actually compressive. us, the full-depth shear key may be more eective since live load stress may not drive existing cracks. Full details in the live load analysis are found in Appendix B. 3.2.3 Effect of Lateral Post-Tensioning Lateral post-tensioning is oen used in adjacent box girder bridges. e post-tensioning can be benecial in pulling the girders together and in load transfer. However, post-tensioning is oen used to compress the shear keys in the belief that this will prevent cracking. Several models were run with lateral post-tensioning. e post-tensioning was always applied at the ends of the girders, but dierent models applied the post-tensioning at the midspan only, at the third points only or at the midspan and quarter points. Post-tensioning was applied at the mid-depth of the girders. e post-tensioning force at each point was chosen such that the average force would be 11 k/; that is, the girder length was multiplied by 11 k and the total force was divided between the post-tensioning points. See Chapter 2 and Appendix B for complete details on the models. Figure 15 shows a typical result for a model where only the lateral post-tensioning is applied. e gure shows that the post-tensioning is eective at compressing the areas around the point Figure 15. Stresses in a laterally post-tensioned model; post-tensioning forces only. Post-tensioning applied at the ends, midspan, and quarter points of the bridge.

24 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems of post-tensioning force application, but it falls o quickly away from the points of load applica- tion and actually causes tensile stresses in some areas. is is consistent with other analytical models from the literature, lab experiments, and eld observations (see Chapter 1). Additional models were run combining post-tensioning and temperature eects. e results showed that the post-tensioning reduced the stresses in the shear keys near the points of the application of the post-tensioning force but had little eect on areas away from the point of force application. In some cases, the transverse tensile stresses slightly increased in the areas away from the points of post-tensioning load application. It was concluded that applying lateral post-tensioning forces was ineective in reducing transverse tensile stresses in the shear keys. Complete details of the analysis and results for all models are in Appendix B. 3.2.4 Reinforced Shear Keys Several models were run with reinforcing bar in the shear keys. e conclusion was that the presence of reinforcing made no dierence in transverse stresses that were developed. Rein- forcing will not prevent cracking but may limit crack widths and propagation. Details are in Appendix B. 3.3 Physical Test Results 3.3.1 Results of Bond Testing A series of pull-o tests were performed to evaluate the tensile strength of various shear key materials and their bonding characteristics with the girder surface. A total of four shear key materials—one high bonding grout (Masterow 4316), two non-shrink grouts (Sika 212 and Masterow 928) and one small aggregate concrete mix—were tested. Eects of surface prepa- rations such as surface roughness, prewetting the surface, and covering the surface with wet burlap were studied. Four dierent surfaces were used—steel formed (smooth), exposed 1-inch rounded aggregate, exposed 1-inch angular aggregate, and sandblasted to a CSP-4. e ASTM C1583 test was used as the standard testing procedure. A summary of the testing program and procedure are found in Chapter 2 and a detailed description of the tests along with the data for individual tests is found in Appendix C. e overall ndings of the pull-o testing are as follows: 1. All the materials and surface conditions had a pull-o failure strength of at least 200 psi, and most were above 300 psi. De la Varga et al. (2016) suggested that a strength of 150 psi was needed, and all the grouts and all the conditions met this minimum. Lopez de Murphy et al. (2010) suggested 300 psi for bond and most of the specimens met this requirement as well. 2. e higher bonding grout performed the best and is recommended, however, other grouts also showed satisfactory performance and could be used. e concrete also showed satisfac- tory performance. 3. e surface roughness aected the bond performance. Both the exposed aggregate surfaces and the CSP-4 surface performed better than the steel formed surface. In many cases the fail- ure modes were shied from bond failure on steel formed surface to either substrate or grout failure on roughened surface. Any surface roughened to CSP-4, or more is recommended. 4. Prewetting the surface increased the pull-o strength in 10 out of 12 cases. In the remain- ing two cases, the strength remained almost identical in one case while in the other it was reduced. e reduction in strength was for the steel formed surface. It was noticed that prewetting caused water to pool on the steel formed surface and this might have reduced the

Findings and Applications 25 bond strength. erefore, for roughened surfaces, prewetting of the girder surface is highly recommended. 3.3.2 Findings of the Full-Scale Tests 3.3.2.1 Box Girders e box girders were 36 feet long, 48 inches wide, and 21 inches deep. A standard ODOT cross section was used, and ODOT design data sheets were used to choose the main and shear reinforcement. e girders were instrumented with a combination of vibrating wire strain gages and thermistors. Appendix C contains complete details about fabrication and instrumentation of the girders. 3.3.2.2 Field Measurements of Thermal Gradients For the analytical and full-scale tests, the temperature gradient from Article 3.12.3 of the AASHTO Bridge Design Specications was used. is gradient was developed from studies on “I” and “T” shaped girders. Aer fabrication, the box girders used in this study were stored in the fabricator’s yard until needed. Since this was during the summer, the instruments in one girder were monitored for temperature to determine what the actual gradient was. e girder had one side exposed to direct sunlight which would represent the exposed side of a fascia girder. e other side was shaded and would represent the interior side of a fascia girder or either side of an interior girder. Figure 16 shows the gradient on the shaded side of the girder. e maximum and minimum AASHTO gradients (Zones 1 and 4) are superimposed. Although the data is limited to 2 days of monitoring, the data suggests that the AASHTO gradients are representative of the actual gradient in the box girders. A complete description of the test and additional data are found in Appendix C. 3.3.2.3 Results of the Full-Scale Tests of the Type IV Shear Key e full-scale test of the narrow, full-depth (Type IV) shear key was conducted as described in Chapter 2 with full details found in Appendix C. e test consisted of three girders connected by two shear keys. One shear key used a high bond grout and the other used a standard non- shrink grout. As stated in Chapter 2, the girders were preheated prior to placing the grout, and the sides of the girders were prewet when the grout was cast. e top 4 inches of the shear keys were not Figure 16. Differential temperature distribution over the depth of the girder on the shaded side of the girder.

26 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems grouted. The high bond grout shear key was cast without any problems. There was a problem with the non-shrink grout. Initially, the grout was mixed to the manufacturer’s specification for a flowable condition, as this is the condition usually specified by state departments of trans- portation (DOTs). The first batch was found to be too stiff to flow and consolidate properly so additional water was added to future batches. A far larger problem occurred when about half the shear key with the non-shrink grout had been cast. The grout on bottom of pallet was found to be compromised. The unmixed material was hard and lumpy, as though it was either old or perhaps had been exposed to moisture. The research team had purchased the grout a few weeks earlier and had no reason to believe that some of the grout had been compromised. Since the shear key with high bond grout had been cast, as had about half of the non-shrink grout shear key, the research team used the compro- mised grout to complete the shear key. A total of 30 cycles of temperature gradient were applied. At various points, the joints were flooded with fabric dye to check for leaks. The shear key with the high bond grout did not leak. The shear key with the non-shrink grout leaked in the area of the compromised grout. The areas of good non-shrink grout did not leak. Complete details of the temperature testing and signifi- cant data are found in Appendix C. After application of the heat cycles, 100,000 cycles of live load were applied. No additional leakage was detected. The high bond grout and the areas of good non-shrink grout did not leak. The load testing further showed that the girders behaved as single unit with the load equally distributed to all three girders. Complete details of the live load testing and significant data are found in Appendix C. When the testing was complete, the girders were cut apart. The cut surfaces of the shear keys were checked for delamination by sounding with both a hammer and a gear-tooth device. No delamination was found in the high bond grout. Delamination was found in the area of the compromised non-shrink grout. There was some delamination near the end of the girder in the area of the good non-shrink grout, but this was where the first batch of grout that did not flow well was placed. Figure 17 shows marked areas of delamination. After locating areas of delamination, grout in both sound and delaminated areas was removed by chipping hammer. The high bond grout could not be easily removed and had bonded so well that attempts to remove it resulted in damage to girder concrete. The non- shrink grout was easier to remove. Areas of leakage were clearly shown by the dye penetration (Figure 18). 3.3.2.4 Results of the Full-Scale Tests of the Type V Shear Key The full-scale test of the wide, full-depth (Type V) shear key was conducted as described in Chapter 2 with full details found in Appendix C. The test consisted of three girders connected by two shear keys. Both shear keys were cast using a small aggregate concrete. The mix design is found in Appendix C. As stated in Chapter 2, the girders were preheated prior to placing the grout and the sides of the girders were prewet when the concrete was cast. The top 4 inches of the shear keys were not filled with concrete. The first shear key was cast with no problems. At the start of casting the second shear key, the concrete became very stiff as the effects of the high-range water reducer (HRWR) had worn off. The concrete was re-dosed with the HRWR but a small portion near the end of one shear key had been cast with this stiff concrete. The concrete may not have consolidated properly. A total of 30 cycles of temperature gradient were applied. At various points, the joints were flooded with fabric dye to check for leaks. No leaks were found. After the thermal cycles were

Findings and Applications 27 Figure 17. Areas of delamination in the non-shrink grout shear key. Figure 18. Removal of the non-shrink grout. Figure b shows the dye penetration in the area of leakage. Figures c and d show no dye penetration in sound areas.

28 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems applied, 100,000 cycles of live load were applied. Again, no leaks were found. Girder end reac- tions showed the three girders behaved as a single unit. Aer testing, the joints were cut and examined for delamination and leakage. Figure 19 shows some delamination and dye penetration at the end of one joint. is was the area where the concrete had lost slump and did not consolidate properly. Figure 20 shows delamination at the ends of other joints. However, there is no dye penetration. is is believed to be damage caused by cutting the girders apart. In sound areas, it was very dicult to remove the concrete with the chipping hammer. e total testing time was about one month. e researchers do not believe there was su- cient time to see if shrinkage cracking would occur as this cracking usually takes more than one month to appear. As a precaution, the researchers suggest that the concrete used for shear keys in eld applications contain a non-shrink additive. 3.3.3 Applications e results of the testing show that shear keys that do not leak and transfer load are achievable. is is done by using a deep shear key, not grouting the top area of the shear key that experiences the highest temperature movements, properly roughening the sides of the girders, and using a material with a high bond strength. is research provides a methodology for engineers to specify a shear key shape that will be more resistant to cracking. It also provides a means to evaluate the eectiveness of the surface preparation of the sides of the girder and the bond strength between a shear key ll material and the side of a girder. While this testing was done specically for box girders, the results can be applied to any adja- cent, concrete structure such as deck bulb Ts, concrete slabs, or precast concrete deck panels. Figure 19. Delamination (a) and dye penetration (b) at the end of one joint. This area had poor consolidation of the concrete (c and d).

Findings and Applications 29 Figure 20. Delamination at the end of joints, but no dye penetration is found. This is thought to be damage due to cutting the joints.

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Bridges constructed with adjacent precast prestressed concrete box beams have been in service for many years and provide an economical solution for short and medium span bridges. A recurring problem is cracking in the longitudinal grouted joints between adjacent beams, resulting in reflective cracks forming in the asphalt wearing surface or concrete deck.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 1026: Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems presents guidelines developed for the design and construction of various adjacent precast box beam bridge systems to enhance the performance of connections and bridge service life and to propose design and construction specifications.

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