National Academies Press: OpenBook

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

Chapter: Chapter 2 - Research Approach

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Suggested Citation:"Chapter 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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 2 - Research Approach." 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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7   Research Approach 2.1 Introduction e purpose of the research was to determine the best methodology to prevent cracking of shear keys in adjacent box girder bridges. e research portion of this project had three dis- tinct phases: literature review, analytical modeling, and full-scale testing. e literature review was used to nd the critical issues that needed to be addressed in the analytical modeling. e analytical modeling was used to determine which parameters, such as span length, girder depth, and skew, aected the performance of the shear keys. Using the results of the analytical model, two full-scale specimens were designed and tested. Additional testing on bond strength between the girder surface and the shear key material was conducted. is chapter details the methodology used. 2.2 Literature Review e results of the literature review are summarized in Chapter 1. Table 1 summarizes the signicant ndings of the literature review by considering various permutations of the shear key, including size and position, material, load transfer mechanisms, surface preparation, and decks. As described in Chapter 1, the weight of evidence from the literature review is that cracking in the shear keys is caused by temperature variations. Shear keys tend to be cast during warm weather. e concrete girders expand due to warm ambient temperatures. In addition, the tops of the girders are heated by direct sunlight causing the tops of the girders to expand. According to Article 3.12.3 in the AASHTO LRFD Bridge Design Specications, there is a gradient of 29oF to 40oF over the top 4 inches of the girder, depending on the solar radiation zone dened in the same article. us, the shear keys are oen cast when the girders are expanded due to warm temperatures. When the girders cool and contract, tensile stresses develop. Research shows that live loads do not tend to initiate cracking but will propagate temperature- induced cracking. Figure  2a shows the traditional, top, partial-depth shear key, often called a Type III. Figure 2, b and c, shows possible full-depth shear keys, called Type IV and Type V. Research suggests these are more resistant to cracking due to a higher bond area and due to most of the shear key being outside of the zone of severe temperature gradient. Another possible solution is to move the shear key to the mid-depth of the girder, out the zone of highest temperature gradient (Figure 2d). is shear key, which was tried in a bridge in Ohio, did not place grout in the “throat” of the shear key to avoid having any material in the zone of highest temperature movement. C H A P T E R 2

8 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Variable Effectiveness at preventing leakage Effectiveness at load transfer Comments Shear Key Shape Traditional, top, partial depth Ineffective Varies with material Evidence shows this configuration is subject to very high stress due to thermal movements and possible lateral (plate) bending of the system. This configuration cracks and leaks with great frequency. If the crack occurs at the interface, the shape of the key can sometimes transfer load mechanically. If the crack occurs in the box girder (as happens with epoxy grouts) or the key material, the surface is too smooth to transfer shear. Mid-depth shear key with area above not grouted May be effective Varies with material There is one full-scale experimental test and one bridge in service with this design. Evidence suggests the position of the key makes it less susceptible to thermal stresses. Ungrouted area above the key can be filled with sand or polymer to resist leakage. Load transfer characteristics are the same as traditional, top key. Full-depth shear key Likely effective Likely effective Evidence suggests that the full-depth shear key has a number of advantages. The larger bonding surface distributes the stress more effectively. The full depth has a large moment of inertia so it distributes applied stresses more effectively and may change the direction of principal stress. The top may crack due to thermal movement, but the crack may not completely penetrate the key, resisting leakage. Load transfer is enhanced by the large, bonded area. Evidence in literature suggests that the full-depth feature is important, and width and shape of the key is less important. Shear Key Material Conventional grout without surface preparation Generally ineffective Depends on shear key shape and position Conventional grout applied to unprepared surface cracks at very low load levels. This makes it ineffective at preventing leakage in many applications. However, cracking tends to occur at the interface so there is mechanical interlock which can transfer some shear. Conventional grout with surface preparation Can be effective, depends on shear key shape Depends on where cracks form Conventional grout applied to a prepared surface cracks at higher load levels. Research suggests a roughened surface can greatly improve bond. Effectiveness depends on shear key configuration and level of stress developed. If the cracking tends to occur at the interface, mechanical interlock can transfer some shear. If the bond is strong enough to force cracking outside of the interface, the resulting surfaces may be too smooth to transfer stress. Epoxy grout Effective Effectiveness depends on stress level Generally, not recommended. Epoxy grouts have very high bond strength and will generally resist the stresses generated in the keys. Tests show if they do crack, cracking occurs in the concrete box limiting load transfer. The coefficient of thermal expansion for epoxy is about 3 times greater than concrete and thermal incompatibility will likely cause distress. UHPC with or without surface preparation Effective Effectiveness depends on stress level The much higher bond strength of the UHPC will likely resist stresses in the shear keys. Surface preparation, such as a roughened surface improves the bond. The material is high strength and not likely to crack but if it does crack the cracks will occur in the concrete box and the smooth surface may limit load transfer. Table 1. Evaluation of the signicant shear key parameters.

Research Approach 9 Variable Effectiveness at preventing leakage Effectiveness at load transfer Comments Load Transfer Mechanisms Other Than Shear Keys Untensioned transverse bars Ineffective Effective Evidence suggests that lateral tie bars, tensioned or untensioned, will transfer load between adjacent box girders. Untensioned bars provide no cracking or leakage benefit. Post-tensioned transverse bars Likely ineffective Effective In theory, post-tensioned transverse tie bars compress the shear keys and prevent cracking. Both experimental and analytical evidence suggests this is not true and the compression due to lateral post-tensioning is primarily at the locations of the post-tensioning bars. Reinforcing bars in the shear key using conventional grout Likely ineffective Effective but perhaps impractical Use of reinforcing bars in the shear keys will not prevent cracking; bars simply limit the crack width. Whether or not bars can limit the cracking such that leakage will not occur has not been proven. Reinforcing bars in the shear keys can provide a load transfer mechanism. However, the reinforcing bars in the boxes cannot extend beyond the limits of the form without expensive modification to the forms, so overlapping bars may be impractical. Lap splices are possible but lap lengths are long so frequent box outs would be required. Reinforcing bars in the shear key using UHPC Likely effective Likely effective Use of reinforcing bars in the shear keys will not prevent cracking but the high bond strength of UHPC addresses this issue. Reinforcing bars in the shear keys can provide a load transfer mechanism. Lap splices are practical because lap lengths in UHPC are short. Surface Preparation None Ineffective unless a high bond material Not applicable Smooth steel forms used for box girders provide a poor surface for bonding. (UHPC or Epoxy) is used Sandblasting or shotblasting May be effective Not applicable Bond depends on resulting surface profile. Surface roughness is defined by a concrete surface profile (CSP). CSP-1 has the roughness of sandpaper while CSP-9 is a roughness of about ¼ inch. Rougher surfaces bond better. Exposed aggregate finish May be effective Not applicable Exposed aggregate finishes seem to greatly improve the bond of most materials. Prewetting Helps but not effective alone Not applicable Prewetting the surface seems to improve the bond, but the smoothness of the surface seems to have more impact. Deck Concrete composite deck Generally ineffective Effective Concrete decks provide suitable connectivity for load transfer, but reflective cracking due to shrinkage and temperature movements are frequently reported. Leakage through these cracks is not uncommon. Non-composite asphalt deck with waterproofing Can be effective but proper installation of waterproofing is critical Ineffective Recent studies indicate that the use of waterproofing over the entire bridge deck with additional waterproofing over the joints may prevent leakage, but proper installation is critical. Table 1. (Continued).

10 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems 2.3 Analytical Modeling Aer completing the literature review, the research team proposed several analytical models to assess the shear key performance. e purpose of the analytical modeling was: 1. To assess the stresses in various shear key congurations under temperature movements. 2. To assess the stresses in various shear key conguration under live load application. 3. To determine parameters that aect shear key performance. Parameters considered were: a. Span length, b. Girder depth, c. Skew, d. Deck type, e. Amount of lateral post-tensioning, and f. Keyway reinforcement. e model cross section consisted of seven, 48-inch-wide box girders (Figure 3). e depth of the girders varied. Decks, either asphalt or concrete, were taken as 6 inches thick. e material properties, unless noted otherwise, are listed in Table 2. A total of 48 separate models were analyzed (Table 3). Twenty-eight were various congura- tions of spans, girder depths, shear key types and decks without lateral post-tensioning. Sixteen Figure 2. Current shear key (a) and proposed shear keys (b–d). Figure 3. Cross section of bridge used in the analytical models (PT = post-tensioning).

Research Approach 11 models were various congurations of spans, girder depths, shear key types and decks with lateral post-tensioning. e nal four models had reinforced shear keys. e analysis was performed using the ABAQUS nite element program. To simulate the temperature eects, the nite element analysis was run using the following steps: 1. e bridge was modeled. e shear keys and concrete deck (for models with a concrete deck) were present but were “turned o ” to allow the elements to follow the deformations of the girders. 2. e temperature gradient from the AASHTO LRFD Specications, Article 3.12.3 was applied to the box girders. e worst case of Solar Radiation Zone 1 was used. is caused the top of the boxes to expand. 3. e shear key and deck material were then “turned on” to simulate the casting of the shear keys and deck. For asphalt decks, the asphalt is not modeled but the weight is added. 4. The temperature gradient was then removed from the girder to simulate cooling of the girders. 5. Stresses due to temperature in the girders and keys were assessed aer cooling. 6. Post-tensioning was assessed separately by adding lateral post-tensioned forces to the model with the deck and shear keys “turned on.” ese stresses were added to the temperature results. 7. Live load stresses were assessed by placing an HL-93 truck on the bridge. ese stresses were assessed separately and added to the temperature results. e complete results of the analyses are found in Chapter 3. Specic details of the analyses are found in Appendix B. Girder Concrete Compressive Strength 6 ksi Modulus of Elasticity 4,500 ksi Poisson Ratio 0.2 Tensile Strength 590 psi Coefficient of Thermal Expansion 5.5 x 106/oF Deck Concrete Compressive Strength 4.5 ksi Modulus of Elasticity 3,800 ksi Poisson Ratio 0.2 Tensile Strength 500 psi Coefficient of Thermal Expansion 5.5 x 106/oF Grouta Compressive Strength 3 ksi at one day 8 ksi at 28 days Modulus of Elasticity 3,000 ksi Poisson Ratio 0.2 Tensile Strength 600 psi Coefficient of Thermal Expansion 5.5 x 106/oF (assumed same as concrete) Steel Modulus of Elasticity 29,000 ksi Coefficient of Thermal Expansion 6 x 106/oF Bearing Pads 9 in. x 6 in. x 1 in. thick 9 in. dimension placed transverse to the bridge span aGrout properties were determined as typical properties of non-shrink grout materials listed on the Ohio Department of Transportation Approved Product list in flowable condition. Table 2. Material properties used in analytical models.

12 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Standard Type III Concrete None Standard Type III Concrete None Thin Full-Depth Type IV Concrete None Thin Full-Depth Type IV Concrete None Thin Full-Depth Type IV Concrete None Thin Full-Depth Type IV Concrete None Thick Full-Depth Type V Concrete None Thick Full-Depth Type V Concrete None Thick Full-Depth Type V Concrete None Thick Full-Depth Type V Concrete None Mid-Depth Concrete None Mid-Depth Concrete None Mid-Depth Concrete None Mid-Depth Concrete None Standard Type III Asphalt None Thin Full-Depth Type IV Asphalt None Thick Full-Depth Type V Asphalt None Mid-Depth Asphalt None Standard Type III Concrete None Thin Full-Depth Type IV Concrete None Thick Full-Depth Type V Concrete None Mid-Depth Concrete None Standard Type III Concrete None Thin Full-Depth Type IV Concrete None Thick Full-Depth Type V Concrete None Mid-Depth Concrete None Standard (Type III) Concrete Ends/Midspan Standard (Type III) Concrete Ends/Thirds Standard (Type III) Concrete Ends/Quarters Thin Full (Type IV) Concrete Ends/Midspan Thin Full (Type IV) Concrete Ends/Thirds Thin Full (Type IV) Concrete Ends/Quarters Thick Full (Type V) Concrete Ends/Midspan Thick Full (Type V) Concrete Ends/Thirds Thick Full (Type V) Concrete Ends/Quarters Mid-Depth Concrete Ends/Midspan Mid-Depth Concrete Ends/Thirds Mid-Depth Concrete Ends/Quarters Standard Asphalt Ends/Thirds Thin Full Asphalt Ends/Thirds Thick Full Asphalt Ends/Thirds Mid-Depth Asphalt Ends/Thirds Standard Type III Reinforced Asphalt None Standard Type III Reinforced Asphalt None Thick Full-Depth Type V Reinforced Asphalt None Thick Full-Depth Type V Reinforced Asphalt None 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30 30 30 30 30 30 30 30 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 42 42 27 27 42 42 27 27 42 42 27 27 42 42 27 27 27 27 27 27 27 27 42 42 42 42 27 27 42 27 27 42 27 27 42 27 27 42 27 27 27 27 27 42 27 42 60 80 45 60 60 80 45 60 60 80 45 60 60 80 60 60 60 60 60 60 60 60 80 80 80 80 45 60 80 45 60 80 45 60 80 45 60 80 60 60 60 60 60 60 60 60 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Shear Key Deck Lateral Post- Tensioning Locations Standard Type III Concrete None Standard Type III Concrete None Skew (°) 0 0 Girder Depth (in.) 27 27 Span (ft) 45 60 Model 1 2 Table 3. Summary of the nite element analysis models.

Research Approach 13 2.4 Full-Scale Testing of the System Full-scale testing had two components: 1. A full-scale bridge section consisting of three girders and two shear key joints was constructed in the laboratory. e girders were heated prior to casting the shear keys to duplicate the temperature gradient in the eld. Aer casting the shear keys, the girders were allowed to cool. e specimen was then subjected to 30 cycles of temperature and 100,000 cycles of live load. e shear keys were checked for cracking and leakage at various times. 2. To assess the bond strength of the shear key ll material, the ASTM C1583 pull-o test was performed on various combinations of shear key material and girder surfaces roughness. 2.4.1 Testing of the Bridge Section Based on the literature search and the analytical work, it was determined that the most impor- tant parameters for shear key performance were: 1. Overall depth of the shear key. 2. Position of the shear key within the zone of temperature gradient. 3. Bond and tensile strength of the shear key material. 4. Surface condition of the sides of the precast girder. e most important nding was that not grouting the top 4 inches of the shear key had a tremendous eect on the maximum stress in the shear key. As noted earlier, the top 4 inches of the girder experience the most severe temperature gradient. Not grouting this area removes the material from the zone of greatest gradient and reduces the overall stress. It was found that span length and skew had little eect on shear key stresses. Girder depth did not aect the maximum stresses in any shear key conguration and did not aect overall stress in partial-depth shear keys (Type III or mid-depth). Girder depth did aect the overall stresses in full-depth shear keys because deeper girders had more contact area between the girder and shear key and more of the shear key was outside of the zone of most severe temperature gradients. Two full-scale tests were conducted using full-depth shear keys: the Type IV, thin full-depth shear key and a modied form of the Type V, thick full-depth shear key (Figure 4). e Type V was modied to remove the gap at the bottom (Figure 2) to avoid having to form the bottom of the girder. e Type IV shear key was lled with non-shrink grout. e Type V shear key has a very large volume. Non-shrink grout usually comes in 50–60-pound bags, and there was Figure 4. Shear keys used for full-scale testing.

14 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems concern about being able to mix large quantities of grout quickly enough. e Type V shear key was lled with concrete. e girder depth was chosen as 21 inches. e temperature gradient in Article 3.12.3 of the AASHTO LRFD Specications shows that the gradient extends over the top 16 inches of the girder. e thinner the girder, the greater the proportion of the shear key that is within the tem- perature gradient zone so using a thin girder would be the worst case. e thinnest box (voided) section available was 17 inches, but this was not considered to be a commonly used section. e 21-inch section was a reasonable compromise between having a section thin enough to have most of the shear key in the temperature gradient zone but still having a commonly used depth. e girders were 48 inches wide and had standard 5.5-inch-thick webs and anges. Specics of the girder properties are found in Appendix C. A practical consideration in determining the girder size was that the girders had to be heated to duplicate the temperature condition in the eld. e longer the girder span, the more energy that would be needed to heat the girders or the longer it would take to heat and cool them. e analysis showed span and skew did not have a huge inuence on the shear key stresses so the span length chosen was 35 . (36 . overall girder length) as this was a reasonable, but short span for the 21-inch-deep girder. e skew was taken as zero. Four girders were constructed. One girder had a Type IV shear key on each face, and one had a Type V shear key on each face (Figure 5). e remaining two girders had a Type IV shear key on one face and a Type V on the other face. ese two girders could be used for both tests by rearranging their position in the specimen. e specimen with the Type IV shear key used a dierent grout in each shear key. One was a standard non-shrink grout, and the other was a high bond strength non-shrink grout. e Type V shear key was lled with a high strength, small aggregate concrete. Details about the girders, the grout and shear key concrete are found in Appendix C. Originally, the surfaces of the girders were to have an exposed aggregate nish, but the fabri- cator was not able to obtain this nish. Subsequently, the surfaces were sandblasted to roughen them. Surface roughness can be dened by a concrete surface prole (CSP) as dened by the International Concrete Repair Institute (ICRI). A CSP of 1 (lowest) has a surface similar to a Figure 5. Details of the two bridge section specimens used for full-scale testing. The Type IV shear key is the top diagram, and the Type V is the bottom diagram.

Research Approach 15 rough sandpaper. A CSP of 9 has a roughness of about ¼ inch. CSP is determined visually by matching a chip to the surface. Figure 6 shows the sides of the girder compared to the CSP-3 and CSP-4 chips. e research team judged the surface to be a CSP-4. An insulated box was built over the girders, similar to the one used by Shi et al. (2019) as shown in Figure 7. Heaters were used to heat the air in the box and fans circulated the heat as evenly as possible. It was not possible to duplicate the AASHTO gradient exactly, but the top 4 inches of the shear keys were not lled with grout (Figure 5) so the gradient here was of less importance. e girders were heated such that gradient between a point 4 inches from the top of the girder and a point 16 inches from the top of the girder was 14oF, representing the Zone 1 temperature gradient. Vibrating wire strain gages were used to measure strains in the girders and across the joints. ermistors were used to measure temperature. Load cells measured girder end reactions. Deflections were measured by wire potentiometers and linear variable differential trans- formers (LVDTs) were used to measure dierential vertical movement at the joints. Details are in Appendix C. Figure 6. Finished girder surface alongside CSP-3 and CSP-4 chip. Figure 7. Insulated box over girders.

16 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems The sequence of testing was: 1. The girders were heated and cooled several times prior to casting the shear keys to establish thermal properties. 2. The girders were heated to obtain the required thermal gradient prior to casting the shear keys. Since the girders would cool during the casting process, they were slightly overheated so that the gradient would be near the correct value of 14oF after casting. 3. The sides of the girders were prewet and shear keys were cast. If the girders had cooled too much, heat was applied to restore the gradient. This typically took 1 hour or less. 4. Starting the next day, heat was applied to the girders to raise the temperature gradient between a point 4 inches from the top of the girder and a point 16 inches from the top of the girder to 14oF. The girders were then allowed to cool. Since the grout/concrete was still gaining strength, only one cycle per day was used for the first 5 days. After that, it was possible to apply two cycles per day. A total of 30 cycles were applied. 5. At various points, the joints were flooded with liquid fabric dye. A head of at least 1 inch was maintained in the joint for at least 2 hours. Any leaks were detected. The first dye was black and subsequently lighter dyes were used. After testing, the joints were cut open and inspected. The lighter dye could not be seen over the darker dye, so the lighter dye showed cracks which formed later in the process. 6. After the temperature cycles, 100,000 cycles of load corresponding to the tandem axle load plus impact (AASHTO LRFD Article 3.6.1.2.3) were applied. This load controls for the 35-foot span. Since the instruments do not read fast enough to measure responses under cyclic loads, static tests were performed at various time to determine girder response. After all load testing was done, a final dye test was performed. 7. After testing, the joints were cut and inspected for cracking and soundness. Complete details of the testing program are found in Appendix C. 2.4.2 ASTM C1583 Pull-off Testing The strength of the interface between the shear key material and the side of the girder is a criti- cal factor. This is dependent on the bond strength of the shear key material, the tensile strength of the shear key material and the girder concrete, and the roughness of the side of the girder. Since these parameters can vary, the ASTM C1583 Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method) was used to evaluate these parameters. This test is a relatively simple, standard technique that the research team has used in the past and it is recommended by Graybeal (2017a). The test is suitable for both field and laboratory use. The test methodology used in this research is outlined here. 1. A substrate was prepared to match the desired surface condition. It was 5.5 inches thick, the same as the web of a box girder. Four different surface conditions were used: a. Steel formed, smooth. b. Exposed aggregate, 1-inch maximum size, rounded river gravel. c. Exposed aggregate, 1-inch maximum size, fractured/angular. d. Sandblasted to CSP-4. 2. A 1.5-inch layer of shear key material, the same thickness as the Type III or Type IV shear keys, was cast on top. Prior to casting the shear material, some of the substrate was prewet and some was left dry. Four different materials were used: a. Non-shrink grout, Sika 212. b. Non-shrink grout, high bond, Masterflow 4316.

Research Approach 17 c. Small aggregate concrete. d. Non-shrink grout Masterow 928 (not used on the CSP-4 specimen). 3. Aer the shear key material had set, nominal 2-inch diameter cores were cut into specimen. Wet coring was used to avoid damage to the specimen and to limit the dust. In accordance with ASTM C1583: a. e cores penetrated at least 0.5 inch into the substrate. b. e center-to-center distance between the cores was at least 4 inches. c. e distance from the center of a core to the edge of the region was at least 2 inches. d. e cores’ diameters were measured with a caliper in two perpendicular directions to an accuracy of 0.001 inch. e average diameter was used to nd the area. 4. A 2-inch diameter by 1-inch-thick steel disk was epoxied to the core as required by ASTM C1583 (Figure 8). A tensile load was applied using the apparatus shown in Figure 9. Figure 8. Overlay on angular aggregate substrate showing cores for ASTM C1583 pull-off test. Figure 9. Apparatus for pull-off testing.

18 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems 5. The following was recorded: a. The failure loads. b. Type of failure as described in ASTM C1583: a = substrate; b = bond; c = overlay (grout); d = epoxy. c. If the failure was in two modes, the percentage of each mode was estimated (e.g., 80% bond and 20% grout). 6. Testing was done until at least three “good” tests were obtained for each region. Good was defined as: a. Epoxy failures (mode d) were discarded. b. The failures were of the same or similar modes. c. The tensile strengths were reasonably similar and obvious outliers were eliminated. 7. 2-inch × 2-inch × 2-inch cubes of each material were tested for compressive strength. Specifics of the test procedure are found in Appendix C.

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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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