United States Department of Transportation
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Abstract:Damage to concrete due to freeze-thaw (F-T) action is a serious concern for agencies in cold regions of the United
States. The most effective method to protect concrete from F-T damage is through the addition of an air entraining
agent as an admixture that creates a well distributed, closely spaced, small sized air void system in the concrete. Air
content measurement in fresh concrete is a routine quality control and acceptance test procedure that helps to ensure
good durability characteristics in the concrete structure. Most agencies, including the Colorado DOT (CDOT), in
accordance with industry recommendations, perform the air content measurement at the point of placement in a
concrete structure. Between 1993 and 2008, CDOT changed its specifications due to safety considerations of the field
personnel. Air content measurements were performed at the point of delivery during this period. While structures built
during this period did meet specifications, it is not clear whether the pumping and the placement operations altered the
air void distribution. For planning future maintenance activities, CDOT was interested in evaluating the relative
condition of bridge decks built under the point of delivery specification and verifying if they indeed possessed good FT
durability.
After a throrough review of 600 bridges in CDOT’s database, six bridges in the Denver metro area in districts 1and 4
were selected for study. NBI ratings for the selected bridges ranged from 5 to 7. Three of the bridge decks were
constructed before 1993 (using the point of placement specification), and three were constructed between 1993 and
2008 (using the point of delivery specification). Three cores from each structure were extracted, and two cores were
tested as part of the forensic evaluation plan set up for this study. The tests included petrographic analysis as per
ASTM C 856, air void analyses and total air content determination (including specific surface and spacing factor) as
per ASTM C 457, and chloride profile measurement at three different depths as per ASTM 1152. The test results
indicate that the bridge decks built under both specifications have an acceptable air void system and that the bridges
built under the point of delivery specification have better F-T resistance than the structures built under the point of
placement specification. This implies that good materials and construction practices were used and there is no
alarming difference in the quality of bridge decks built under the two specifications. This does not suggest, however,
that the point of delivery specification should be adopted. The sample size selected for this study is very small, and the
results need to be interpreted with care.
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