National Academies Press: OpenBook

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

Chapter: Chapter 1 - Statement of the Problem

« Previous: Summary
Page 3
Suggested Citation:"Chapter 1 - Statement of the Problem." 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 3
Page 4
Suggested Citation:"Chapter 1 - Statement of the Problem." 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 4
Page 5
Suggested Citation:"Chapter 1 - Statement of the Problem." 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 5
Page 6
Suggested Citation:"Chapter 1 - Statement of the Problem." 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 6

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

3   C H A P T E R 1 Adjacent box girder bridges have been in service since the 1950s. Originally, these bridges were adjacent channel sections. Load was transferred between the sections using a shear key. Since there was only a top ange, the shear key was located near the top of the section. ere is no record of any design methodology for the original shear key shape. Most states that use adjacent box girder bridges still use some variation of this original shear key shape and it is a “standard detail” with no associated design calculations (Russell 2009). Eventually, a bottom ange was added, making the section into a box to provide more locations for prestressing strand. is also increased torsional stiness to prevent twisting of the section. Early box girders used cardboard forms for the void. ese forms tended to collapse or shi during the casting operation. Despite eorts to provide drainage, water was trapped in the voids due to collapsed and decaying cardboard forms (Harries 2006 and 2009). Some states (such as Ohio) used 1.5 inches of cover to the lowermost strand layer and placed the stirrups on this layer to ease fabrication. Over time, these practices were discontinued due to corrosion concerns. Solid polystyrene voids replaced cardboard, cover to the reinforcing bars was increased, and stir- rups were placed below the lower strands for connement. However, the shear key conguration has not changed signicantly over the years although alternate keyway congurations have been investigated (Russell 2009). e original practice for boxes was to omit the voids at certain points along their length creating diaphragms. readed rods were passed through holes in the diaphragms, typically below the level of the shear key but above the neutral axis of the section. Tightening the nuts on these rods pulled the girders together but did not provide lateral prestressing since the nuts were tightened by hand. e shear keyways were cast, usually with a non-shrink, prepackaged grout. e girder forms were usually made of steel and the result was an extremely smooth surface on the box. In the shear key area, this surface was oen sandblasted to remove laitance and provide some roughening, but the roughening was minimal. As a result, the bond between the grout and the concrete box was oen weak. Finally, an overlay was added. In some cases, the overlay was a composite concrete deck, but equally oen the overlay consisted of a layer of waterproong membrane and then an asphalt wearing surface. e latter method of construction is still very common for county bridges due to its low initial cost and rapid construction. e problem with these bridges was that the shear keys cracked causing leakage, and in some cases, loss of load transfer. Of the two problems, leakage is the more predominant and the one that has been most dicult to solve. e result of this leakage is that chloride-laden water seeps through the joints, permeates into the girders, and corrodes the strands (Figure 1). While cases of bridge collapse are rare, the corrosion causes these bridges to have much shorter life spans. Oen, corrosion of the strands is not detected until the concrete cover spalls o. At this point, Statement of the Problem

4 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems the strand is so badly damaged it is no longer eective. e damage to the strand and concrete is not easily repaired. A review of the literature on adjacent box girder bridges provided the following information: 1. Cracking in the shear key joints is likely caused by temperature and shrinkage. Huckelbridge et al. (1995, 1997) studied shear keys being replaced in an existing bridge. e authors found the keys cracked almost immediately but were not sure why. Miller et al. (1999) and Hlavacs et al. (1997) found that the most likely reason was temperature-induced movements. It was noted that the cracks formed within one week and that the cracks opened and closed with daily temperature cycles. Sharpe (2007) conducted an analytical study using Texas Depart- ment of Transportation girders. is study conrmed that temperature and shrinkage initiate cracking. Ulku et al. (2010) and Hussein et al. (2017b) further conrmed this behavior. Miller et al. and Sharpe found that cracks tend to start near the ends of the girders. However, a recent study by Graybeal (2017a) did not nd signicant cracking when a temperature gradient of 50oF was applied. It appears Graybeal applied the temperature gradient aer grouting the shear keys. Hussein et al. (2017a, 2017b) indicated that, in the eld, solar heating caused tempera- ture gradients in the boxes before the shear keys are cast. When the shear keys are cast, the boxes are already deformed by temperature, and this contributes to cracking. 2. Stresses from live loads do not appear to cause the cracks, but simply propagate temperature- induced cracks. is eect was conrmed by Miller et al. (1999), Sharpe (2007), Grace et al. (2012), and Hussein et al. (2017b). Graybeal (2017a) noted that even severe, cyclic loading did not crack intact shear keys. 3. e shape and position of the shear key can reduce the stresses in the key, mitigating cracking. a. Several authors (El-Remaily et al. 1996, Lall et al. 1998, Dong 2002, Kim et al. 2008, Sang 2010, Hanna et al. 2011, Patnaik and Habouh 2018) recommend full-depth shear keys. Evidence from Dong (2002) indicates that this conguration is favorable because it changes the direction of principal stress from lateral to longitudinal. b. Another possibility is using a mid-depth shear key. A mid-depth shear key was used on a high performance concrete bridge in Ohio when experimental evidence suggested it was less susceptible to temperature-induced cracking (Gruel et al. 2000). is shear key was grouted only at the mid-depth and the area above the key was lled with compacted sand. Figure 1. Evidence of and damage resulting from leaking joints in adjacent box girder structures (Steinberg et al. 2011b).

Statement of the Problem 5 A 2017 inspection of this bridge by the Ohio Department of Transportation (ODOT) showed very little leakage after almost 20 years and the leakage that did exist was a small spot at the construction joint that had been there since the bridge was built in 1998. This crack/leakage was the result of half-width construction and was caused by allowing traffic on one-half of the bridge while the other half was being constructed. No additional leakage was found in the 2017 inspection. c. Not all shapes are beneficial. Dong et al. (2007) showed that some shapes actually cause increased stress. 4. Part of the solution is to improve the bonding characteristic of the shear key grout. This solution could include using a better bonding grout, such as ultra-high performance concrete (UHPC) or intentionally roughening the surface of the shear key. Gulyas et al. (1995), Huckelbridge et al. (1997), and Issa et al. (2003) used magnesium ammonium phosphate grout and found it performed well. This material was used in Ohio in a construction joint in a box girder bridge with a mid-depth shear key (Baseheart et al. 2001) and it has performed well for over 20 years. Miller et al. (1999) found epoxy joints did not crack under thermal loads but did not recom- mend epoxy due to an incompatibility in coefficient of thermal expansion when compared to concrete and possible environmental issues. Issa et al. (2003) reported that polymer modified mortars showed good bonding characteristics in shear and direct tension tests. Semendary et al. (2017a, 2017b) found good bond performance when UHPC was used with an intentionally roughened surface. Graybeal (2017b) tested the bonding characteristics for non-shrink grout, UHPC and epoxy-modified grout. He concluded: a. 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)] should be used to assess the bond between the shear key material and the precast element. b. The grout chosen should have demonstrated bonding ability and should be dimensionally stable. c. The precast surface should have an exposed aggregate finish to promote bond. d. The precast surface should be free of contaminants. e. The precast surface should be prewet. (Note that prewetting the surface is suggested by several other authors). 5. It is believed that if done properly, laterally post-tensioning the structure after the shear keys are cast compresses the joints and helps to prevent cracking. However, determining the correct amount of post-tensioning to apply and how to apply it is difficult. There is no clear indication from literature as to the amount and placement of lateral post-tensioning needed to prevent cracking. There is some indication from literature that there is a practical limit to the number of beams that can be post-tensioned together before shear lag makes the post- tensioning ineffective at interior joints. Dong (2002) found, analytically, that the more girders that are post-tensioned together the less effective the post-tensioning is on the joints near the center. Miller et al. (2009) found that the first joint (nearest the application of the post-tensioning load) had the highest com- pression and that the interior joints had uniform compression but about half the level of the outside joint. However, this project only tensioned a group of six beams at a time. Sharpe (2007) conducted analytical studies and noted that the compressive stresses due to post- tensioning are high near the points of post-tensioning but drop off rapidly away from point of application. He stated that “One of the problems discussed with post-tensioning is that a close spacing and high tensile forces must be used if a compressive stress is needed for the entire shear key along the entire length of the bridge.” A similar conclusion was reached by Ulku et al. (2010). Graybeal (2017a) noted that the common methods used currently only confine a small area near the point of application of the post-tensioning force. Ulku et al. (2010) and Fu et al. (2011) suggested using a two-stage approach. Initially, the girders are post-tensioned

6 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems to some percentage of the final stress to pull them together before the shear key is cast. After casting the key, the girders are then post-tensioned to the final stress. Hansen et al. (2012) suggested a post-tensioning system consisting of one set of post- tensioning rods in the top flange of the girder and another set in the bottom flange. They found no significant cracking under applied load. However, they did not consider tempera- ture or shrinkage effects. In addition, this was a laboratory test where only four short girders were tied together with a single tie. It is not clear how this system would perform in the field. 6. Use of a composite concrete deck is effective for load transfer (Sharpe 2007). However, by itself the composite deck may not be enough to prevent cracking and leakage of the shear keys. There are numerous reports in the literature of reflective cracking occurring in the slab. The exact cause of this cracking is not clear, but it could be temperature- and shrinkage- induced (Attanayake and Aktan 2008). Several studies have shown that the keys can tolerate cracking and still transfer load (Huckelbridge and El-Esnawi 1997, Miller et al. 1999). Hawkins and Fuentes (2003) and Steinberg et al. (2011a, 2011b) showed that even cracked shear keys with untensioned tie rods will transfer loads. El-Remaily et al. (1996) and Hussein (2017b) indicated that transverse rods alone can be used for load transfer and shear keys can be eliminated. Alternatively, recent details have been developed that use reinforced keys and eliminate lateral tie rods. Hanna et al. (2011) used a wide, full-depth shear key, reinforced at the top and bottom of the shear key. Liu and Phares (2019) suggested using a reinforced shear key with shrinkage compensating (Type K) cement. A bridge in Ohio used UHPC and reinforced shear key joints for load transfer (Semendary 2017a, 2017b) based on an FHWA design.

Next: Chapter 2 - Research Approach »
Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Get This Book
×
 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems
Buy Paperback | $88.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!