Discussion of " Performance of an AASHTO Beam Bridge Prestressed with CFRP Tendons" by Nabil Grace, Elin Jensen, Vasant Matsagar, and Prasadu Penjendra

The discussed paper presents experimental investigation on the flexural behavior of both an AASHTO-type beam and a bridge model reinforced and prestressed with carbon fiber-reinforced polymers (CFRP) to demonstrate progressive failure mode by arranging the CFRP tendons and rods in different layers along the member’s depth. The authors should be congratulated for producing an excellent paper with interesting information. This is acknowleged by the discusser who would like to make some comments for their consideration, mainly about the scale bridge model, the prestressing tendons, and the test setup. 1. The authors used a scale factor of one-third for the AASHTOtype control beam (12.5 m long and 502 mm deep) and for the 2.5-m width bridge model made up of five such beams. Regarding the cross section of the control beam, and based on the 502 mm depth and the 203 mm width of the bottom flange, it seems that the reduced scale factor was taken in relation to the AASHTO type IV beam, which is often used for span lengths up to 34 m (Gerges and Gergess 2012), or the AASHTO type V beam, which is used for span lengths from 30 to 38 m (Gerges and Gergess 2012), if the related span length (12:5 3 35 37:5m) also is considered. For an unconsidered scale factor of one-half, the cross section of the control beam seems to be related to either the AASHTO type II beam or the AASHTO type III beam if the related span length (12:5 3 25 25m) is considered. In addition, the 95-mm width of the beam web seems a broad width to be related to all the aforementioned AASHTO-type beams, except for type III (953 2 5 190mm is nearly 175 mm). What were the criteria used to choose the scale factor and to design the beam control cross section? It isworth noting that the same scale factor has different effects on length, cross section, and second area moment. 2. Beam spacing in a bridge depends on several factors, including the following: (1) span length, (2) concrete compressive strength, (3) dead and live loads, and (4) allowable concrete service load stresses in accordance with environmental conditions. Why was a beam spacing of 502 mm considered in the bridge model? Regarding the average 28-day concrete compressive strength in the AASHTO beams of 48 MPa, 502-mm beam spacing is approximately 0.6–0.7 [obtained by ð5023 3Þ=2,550 and ð5023 3Þ=2,100] times the maximum beam spacing for a scale factor of one-third (37.5-m span length) and 0.45–0.55 [obtained by ð5023 2Þ=2,200 and ð5023 2Þ=1,800] for a scale factor of one-half (25-m span length), when taking the study of Gerges and Gergess (2012) as a reference. 3. According to Gerges and Gergess (2012), the maximum number of 7-wire steel strands (15.2mm in diameter) is of the order of 22–26 for a 25-m span length with an AASHTO type III beam and 40–48 for a 37.5-m span length with an AASHTO typeV beam, resulting in a range of prestressing reinforcement ratios between 0.85 and 1.04%. The authors used a ratio of only 0.35% as an optimum. On what was this optimum based? It seems that this ratio is smaller to introduce, by pretensioning, a concrete stress level nearly 0.6 times the concrete compressive strength. 4. Apart from the diameter and number of prestressing CFRP tendons being small, they were stressed to 50% of their ultimate tensile strength instead of the commonly used 75% prestress level when prestressing steel strands. This can explain the fewer prestress losses with prestressing CFRP tendons compared to conventional steel prestressed members (Caro et al. 2013). A decompression test was conducted to determine the effective prestressing force level. Whywas this test carried out on the bridge model and not on the control beam? A possible stiffness contribution of the transverse diaphragms was avoided in the control beam. 5. In the case of pretensioned prestressed concrete members, the prestressing force is transferred from the prestressing tendons to concrete by bond. Lower tendon prestress level results in a shorter transfer length, defined in ACI 318-11 [American Concrete Institute (ACI) 2011] as the distance over which a strand should be bonded to concrete to develop the effective stress. Besides, it is noted that the prestressing CFRP tendons used were coupled with their developed anchorage system. Do the authors have any information on this anchorage system and its influence on transfer length? Transfer length can be measured (Marti-Vargas et al. 2006b, 2013a) and/or predicted from different equations based on the compatibility of relative displacements (slips) tendon-to-concrete and on equilibrium of forces (Marti-Vargas et al. 2007a, 2012b). As transfer length provisions differ for distinct codes and researchers (Marti-Vargas et al. 2007b; Marti-Vargas and Hale 2013), and no consensus has been reached as to the main parameters to be considered (Marti-Vargas et al. 2012a; Caro et al. 2012), it would be interesting to detail the transfer length provisions used by the authors to design the prestressed beams. Furthermore, the effective stress is transferred completely to concrete when assuming that concrete stresses have a linear distribution, which occurs beyond dispersion length [European Committee for Standardization (CEN) 2004]. Dispersion length depends on beam depth and tendon position, among other factors, and it is longer than transfer length. How was dispersion length considered from the different vertical location of the prestressing CFRP tendons along the beamdepth in layers? According to the scale factor, what were both the dispersion and transfer lengths in relation to the beam length? 6. Transfer length remains within the development length of prestressing tendons at loading (ACI 2011; Marti-Vargas et al. 2006a, 2013b). It is worth noting that AASHTO (2012) includes a factor to avoid bond failures caused by inadequately developed lengths in beams whose depth is greater than 610 mm because of shear effects. The beams used were 502 mm in depth (less than 610 mm) when using a scale factor, but the loading conditions for tendon