An Evaluation of the Performance of Concretes Containing Fly Ash and Ground Slag in Bridge Decks

About the report:

Cores from 36 bridge decks were evaluated to assess the condition and quality of the concrete by petrographic methods and direct and indirect measures of the transport properties. Transport properties were measured by a rate of absorption test (ASTM C 1585) and by electrical conductance using the rapid chloride permeability apparatus (ASTM C 1202). The decks were distributed across Virginia to reflect the varied geographic and climatic regions. Two bridge age groups, each constructed under different specifications, were represented: (1) from 1968 through 1971, where portland cement concrete with a maximum specified water-cementitious material ratio (W/C) of 0.47 was used with uncoated reinforcing steel; and (2) from 1984 through 1991, where the specification required a maximum W/CM of 0.45, required epoxy-coated reinforcement, and allowed the use of fly ash or ground slag as supplementary cementitious materials. The older group included 10 decks, and the younger included 26. In the younger group, 8 were identified as containing fly ash and 7 were identified as containing slag. Five of the concretes exhibited excessively high spacing factors, suggesting susceptibility to freezing and thawing damage, although only two showed signs of such damage. Four of the decks exhibited excessively small spacing factors that could significantly affect strength. Signs of poor paste quality attributable to excessive water were noted in approximately one-third of the concretes. Cracking was of significance in 12 decks but was limited to paste cracking in 6; of these, 5 contained either fly ash or slag. Four showed signs of damage related to alkali-aggregate reactions, including 3 with carbonate rocks, 1 of which contained slag. A general assessment based on petrographic observations showed a fairly even distribution of good, fair, and poor ratings. Fly ash and slag concretes tended to have initial rates of absorption in the lower third, often despite their petrographic rating, suggesting they are providing beneficial reductions in transport properties in field concretes. The secondary (longer term) rate of absorption related better with the petrographic ratings, and the fly ash and slag concretes again tended to have lower rates. Of the fly ash and slag concretes exhibiting paste cracking, only one consistently had high absorption rates. In contrast to the rate of absorption results, the electrical conductivity results suggested little differences between the concretes, raising questions about its usefulness in evaluating mature field concretes. This study demonstrates the beneficial contributions that fly ash and ground slag as supplementary cementitious materials provide to concrete durability, and they should continue to be used as an integral part of efforts to increase the service life of concrete structures. ASTM C 1585 provides a direct measure of the transport properties of concrete and should be incorporated into both the concrete materials acceptance and asset evaluation and management programs.

Authors