Control of Concrete Cracking in Bridges (2017) / Chapter Skim
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5 Cracks in reinforced concrete members can be classified into two main categories (Leonhardt 1977) : • Cracks caused by externally applied loads, and • Cracks that occur independent of the loading conditions.
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6 The likelihood of settlement cracking can be reduced by proper vibration of the concrete, use of the lowest possible slump, and increasing the concrete cover in conventional concrete or using viscosity-modifying admixtures in self-consolidating concrete (SCC)
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7 • Cracking in curved bridges caused by torsional forces; • Longitudinal cracks at the ends of spans, particularly where the bridge deck is integral with the abutment; and • Cracks at construction joints. Because some cracks are inevitable, their width and spacing need to be controlled through the use of reinforcement.
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8 at random in eight states and a detailed survey of 70 bridges in four states. The study concluded that transverse cracking was the dominant type of cracking.
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9 cited strategy was a reduction in the cement and paste content. The full list of strategies is provided in the answer to Question 7 in Appendix B
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10 Change of Hardened Hydraulic Cement Mortar and Concrete, but the initial curing period may be different. A low shrinkage alone does not guarantee that cracking will not occur.
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11 were either bridges with reconstructed or re-overlaid decks, or bridges that had decks placed during extreme temperature conditions. Cracking as a result of deck reconstruction was attributed to shrinkage of the deck being restrained by the aged prestressed concrete girders.
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12 0.073 ft/ft2 transversely; 0.008 ft/ft2 diagonally; and 0.042 ft/ft2 longitudinally, indicating that most cracks were in the transverse direction. The results also indicated that, in some cases, the use of HPC reduced bridge deck cracking, whereas in other cases the crack lengths were greater.
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13 consist of 8¼-in.-thick concrete decks with stay-in-place forms on four precast concrete bulb-tee girders at 8 ft 4 in. centers.
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14 was caused by the restraint of the abutment and the temperature differences between the abutment and the deck. Although not likely to induce cracking, shrinkage of the deck concrete may have exacerbated cracks that developed from thermal effects.
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15 Although implementation of the new specifications reduced deck shrinkage cracking, minor cracking was still evident. This was attributed to the differential temperature between the deck peak hydration temperature and the ambient temperature.
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16 are controlled by the reinforcement in the CIP concrete. In comparative laboratory tests, Tsui et al.
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17 Overall, both structures exhibited the largest total crack density of the 19 bridges included in the investigation (Mokarem et al.
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18 panel is usually the full width of the bridge, unless that makes the panel too long or heavy to ship or staged construction is used. The panels generally are pretensioned in the transverse direction and may be posttensioned in the longitudinal direction.
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19 cracking in adjacent Box Beam Bridges and slaB Beam Bridges Adjacent box beam and slab beam bridges consist of precast, prestressed concrete beams that are placed next to each other (Russell 2009)
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20 Ahlborn et al.
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21 (graybeal 2014)
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22 more frequently in girders with draped strands. Based on experimental research, the following equation was proposed to control the size of these cracks: = ×0.021 (2)
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23 Based on analytical and experimental research, Kannel et al.
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24 • Lifting the beam from the bed. • Hoyer effect.
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25 control methods were debonding the strands at the ends rather than using draped strands, locating the lifting loops at a distance equal to the girder depth from each end, and detensioning the strands beginning with the innermost ones. Increasing the vertical reinforcement area in the end zone alone was not recommended because it did not eliminate cracking, although it did help control crack widths.
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26 Flexural cracking in nonprestressed concrete beams is inevitable because cross sections are designed to be cracked. Crack control is provided by using a minimum amount of reinforcement and a maximum bar spacing.
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27 different concrete mixes, shorter curing period, or a different exposure factor in Equation 5.6.7-1 (formerly 5.7.3.4-1) of the AASHTO LRFD Specifications (AASHTO 2017)
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28 frequent cracking in columns or abutments, and one agency reported that cracking always occurred. The cracking was identified as being caused by flexure, shear, shrinkage, formwork settlement, or foundation settlement.
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29 • Specifying and ensuring minimum and maximum concrete temperatures at the time of placement as 55°F and 75°F, respectively. • Minimizing cement content.
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From page 30... ...
30 non-Prestressed concrete Beams Cracking in nonprestressed concrete beams is almost inevitable but is controlled by providing minimum amounts of reinforcement to control the widths of cracks caused by flexure or shear. substructures In general, cracking in substructures occurs infrequently.
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