National Academies Press: OpenBook

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

Chapter: Chapter 4 - Conclusions and Suggested Research

« Previous: Chapter 3 - Findings and Applications
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Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." 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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Page 30
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Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." 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.
×
Page 31
Page 32
Suggested Citation:"Chapter 4 - Conclusions and Suggested Research." 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.
×
Page 32

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30 C H A P T E R 4 4.1 Conclusions 1. e FEM (nite element method) analysis shows that the temperature stresses are, by far, the largest stresses. Live load stresses are fairly small in comparison and tend to act in the opposite direction of the temperature stresses. is conrms previous research that temperature cracks the shear keys, that live load stress alone will not crack the shear keys, and that live load simply drives existing temperature-induced cracks. 2. e thin full-depth (Type IV), the thick full-depth (Type V) and the mid-depth shear keys are all superior to the current Type III shear key. e Type III develops tensile stresses through- out due to temperature. e mid-depth shear key develops much lower stress due to being at the point where temperature movement are lowest. e Type IV and Type V shear keys develop high tensile stress at the top, but it quickly diminishes with depth. Near the bottom, the stresses are compressive. 3. If the top 4 inches of the thin full-depth (Type IV) and the thick full-depth (Type V) are not grouted, high tensile stresses develop only at the very top of key. e great majority of the key is either in a very low state of tension or in compression. is is because the grouted area is moved out of the zone of greatest temperature gradient. e larger bonding surfaces also help to reduce stress. 4. e bond between the shear key ll material is of great importance. a. e surfaces of the girder must be roughened and prewet prior to placing the grout. At the very least, the surface must be sand or shot blasted to a CSP of 4 as dened by the International Concrete Repair Institute. Rougher surfaces, such as exposed aggregate or higher values of CSP can improve bond even more. While a roughened surface works with concrete ll, exposed aggregate surfaces may provide a better bond with concrete ll. b. e bonding characteristics of the grout are one of the most important parameters. e best performance came from the high bond grout, however the non-shrink grout and the concrete still provided acceptable performance. c. ASTM C1583 bond test provides a means of assessing the bond strength of a given ll material and a given surface preparation of a girder. Based on FEM results, it is suggested that the bond strength be at least 200 psi. d. Although not explicitly tested in this project, data from literature suggests that UHPC may be a solution, especially for shallower girders or girders where the shear key depth is limited. 5. e full-depth shear keys appeared to bind the boxes together and made them behave as a single slab in the 3-girder test. is would be benecial in live load distribution. is should be veried with a larger test involving more girders or with a eld test. 6. e temperature gradients in Article 3.12.3 of the AASHTO LRFD Bridge Design Specications were developed from data on “I” and “T” shaped girders. Limited eld testing in the project Conclusions and Suggested Research

Conclusions and Suggested Research 31 showed they are applicable to box girders and can be used to assess temperature eects on box girders and shear keys. 7. ere is no signicant eect of girder span or skew angle on the performance of the shear keys. Girder depth is only signicant to the extent it limits the depth of the shear key. e temperature gradient extends from the top of the girder to a point 16 inches below the girder surface. Deeper girders can have a shear key that extends below the area aected by the temperature gradient. 8. Lateral post-tensioning is likely not eective. Conrming eld, laboratory and previous ana- lytical studies, the post-tensioning compresses the shear keys at the point where the post- tensioning is applied, but the eect diminishes quickly away from the point of post-tensioning application. In some cases, additional tensile stresses are developed in the shear keys. 9. Reinforced joints do not help with temperature stresses but would transfer load. However, these joints are located in the area of maximum temperature stress. e joint will likely crack, and the reinforcing bar will not prevent this. e bar may only limit the crack width. is alone cannot guarantee the joint will not leak. 4.2 Suggestions for Implementation 1. Use the deepest possible shear key and do not ll the top 4 inches with ll material. e data show that the highest stresses occur in the top 4 inches of the shear key where the temperature gradient is the most severe. Deep shear keys are more resistant to cracking as there is more bond area. In addition, full-depth shear keys sometimes have compressive stress at the bottom. e current AASHTO LRFD Bridge Design Specications (Article 5.12.2.3—Precast Deck Bridges) recommend a minimum 7-inch depth of a shear key. If the top 4 inches are not lled, the recommended minimum depth becomes 4 + 7 = 11 inches. is will t in a 12-inch-deep girder, which is the shallowest girder usually used. Full-depth shear keys are recommended, but it may be possible to use a partial-depth shear key in a deep girder. In these cases, the shear key should extend beyond a point 16 inches below the top of the girder so that at least some of the shear key will be outside of the temperature gradient. 2. e top 4 inches of the shear key should be lled with some sort of sealer. is would provide additional protection against leakage. Possible details are shown in Appendix A. If a composite concrete deck is used and the top of the shear key is lled with deck concrete, a signicant bond breaker must be used to prevent cracking from propagating into the shear key. 3. Roughen the girder surface to a concrete surface prole of at least CSP-4. Exposed aggregate surfaces can be used and are recommended if the ll material is concrete. 4. Prewet the girder surfaces prior to placing the ll material. Follow manufacturer’s recom- mendations if possible. e closer the surface is to saturated, the better the bond. 5. e ASTM C1583 test should be used as a qualifying test. It should be performed using the shear key ll material and a substrate made of girder concrete with the surface preparation used on the actual girder. It is suggested that this test be performed to qualify a combination of grout and surface preparation. It is recommended that the combination of ll material and surface preparation be retested periodically to verify that nothing has changed, but that is at the discretion of the owner. It is recommended that the test be performed at 7 days. is is due to the time it takes to perform the test. e ll material must be allowed to set suciently to be cored. Coring is usually done wet to prevent damage and to control dust. However, wet coring saturates the cores which must be allowed to dry, or the epoxy used in the test will not adhere. Finally, the epoxy must properly cure and that usually required 24 hours. Fast cure epoxies were tried but none were found suitable.

32 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems 6. Use a high bond strength, non-shrink material. High bond grout is recommended. If concrete is used, a non-shrink additive should be used. UHPC can also be used. 4.3 Suggestions for Future Research e suggestions from this project are based on literature, analytical modeling, and testing of two full-scale specimens. A total of 48 analytical models were examined. e full-scale speci- mens used full-depth shear keys and were tested under laboratory conditions. Each test took approximately 30 days. While the research team believes this work is a signicant contribu- tion to improving shear key performance, there were some aspects that were not tested. Future research could be performed to examine these aspects. 1. e full-scale tests were conducted on specimens 3 girders wide in a laboratory. Field tests of complete bridges are suggested as further verication of the results. 2. e laboratory tests took about 30 days. is is not sucient to examine the eects of long- term shrinkage or creep deformations. Longer term tests should be done to ascertain the eects of time dependent deformations. 3. Full-depth shear keys were tested. ey are recommended, especially for shallow girders. How- ever, it is possible that deeper girders could use partial-depth shear keys as long as the shear keys extend beyond the zone of temperature gradient. is should be tested.

Next: Appendix A - Suggested Details for Shear Keys »
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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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