RESEARCH NEEDS for Structures TAG

RESEARCH NEEDS FOR Structures TAG

EFFECTIVE/UPDATED 8/15/2017

ISSUE: Crack Initiation and Propagation through Concrete Bridge Deck Overlays

SUMMARY OF PROBLEM: Bridge decks are routinely overlaid to upgrade decks which have sustained surface deterioration from the effects of surface spalling, cracking, exposure of rebar and eventual punch-out. The surface and subsurface deterioration of concrete is due to several causes, often acting in concert. These causes include: (a) the combined action of truck and passenger car axle loadings; (b) freeze-thaw action on concrete; (c) application of deicing salts and their penetration due to the permeability of concrete, leading to rebar corrosion; and (d) restraint-induced cracking due to thermal contraction and expansion effects of the bridge itself.

Although the surface degradation is always apparent and the degraded concrete can be swept away by hydro-demolition, the underside of the deck can still be cracked, whereby such cracks can then re-propagate through the new overlay. The extent of deck underside cracking, along with the other factors previously noted, influences the life of the new overlay.

A recent project using consulting funds to overlay a bridge in McHenry County was completed. This project compared one lane of a two-lane bridge using Portland cement concrete (Pcc) reinforced with welded wire fabric, whereas the opposite lane was overlaid with Pcc containing polypropylene fibers. Unfortunately, when the underside of the bridge was surveyed, one side of the bridge contained a greater number of cracks than the opposite side. This disparity could seriously skew results as to crack propagation when comparing the fabric reinforcement vs. the concrete overly with polypropylene fibers.

To obtain a more quantitative evaluation of crack initiation and propagation for different overlays, actual 8 in thick x 36 in wide slabs, either simply supported or subject to end fixity, need to be loaded in flexure at various stress levels and cycles to failure to simulate wheel loadings experienced by actual bridge decks. The 8 in thickness will permit tracking of crack formation and propagation as the number of loading cycles progresses to failure.

Bridge deck overlays to be tested include: (1) Pcc with a layer of welded wire fabric (WWF); (2) Pcc with polypropylene fibers; (3) Pcc with a layer of WWF set in an epoxy or polyester polymer, in-between two pours of Pcc; (4) one layer of WWF in a matrix of Pcc with polypropylene fibers; (5) overlays with steel fibers.

DESIRED RESULTS/OUTCOME: Using different concrete compositions and reinforcement that simulate the overlays will help determine which composition is best at: (1) delaying crack initiation; (2) which has the slowest rate of crack propagation; (3) what are the threshold stress levels where crack initiation first appears; and (4) if there is a relationship between rate of crack propagation and increasing stress levels. Overlays with better resistance to crack formation and propagation and maintaining structural integrity could then be considered as better choices for future overlays.