Materials selection for concrete overlays : the final report Page: 7
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Where local sulfate contamination of the roadway is an issue, Type II or V cements are
desirable because they are resistant to sulfate attack and have lower heat of hydration than other
cements. Strength gain and set time may be regulated with admixtures and mixture
proportioning [7, 8].
To prevent alkali-silica reaction (ASR), low alkali cement (total alkalis Na20
equivalent<0.6%) should be used for any type of cement coming in contact with ASR-prone
aggregates [60]. ASR can occur when siliceous aggregates are used, and alkalis from the cement
react to form expansive gel causing deleterious effects. Cement should have low alkali content
and supplementary cementing materials (SCM) substitutions to prevent ASR from occurring.
2.3.2 Aggregates
To construct an efficient concrete overlay, the aggregate should be strong and physically
chemically stable. The aggregates make up between 65 and 75% of the total concrete volume;
therefore, their properties have a definite influence on those of the concrete.
Available aggregates should be evaluated carefully to determine whether an adequate
strength will be achieved. Performance requirements may justify purchase of more expensive
(high-strength, crushed) aggregates, or careful aggregate blending [10]. Aggregates that conform
to Item 421 of TxDOT Standard Specifications should be used.
To prevent ASR, non-reactive aggregates should be selected. Many durability problems
result from the reaction between the silica in the aggregates (e.g., siliceous river gravel) and
alkalis contained in the cement [11]. If reactive aggregates are used, proper mitigation
procedures must be used as required by TxDOT specification Item 421.
Unsaturated absorptive aggregates have a higher moisture demand and can contribute to
debonding during curing. These aggregates will absorb available moisture, hindering the curing
procedure and affecting shrinkage [4, 10, 12].
Coarse Aggregate (CA)
The maximum CA size is a function of the overlay thickness. It is recommended that the
largest practical maximum CA size be used in order to minimize paste requirements, reduce
shrinkage, minimize costs, and improve mechanical interlock properties at joints and cracks [4,
9]. Maximum CA sizes of 0.75 to 1 in. have been commonly used, but a reduction in size may
be necessary for thinner overlays. For non-reinforced pavement structures, a maximum
aggregate size of one-third of the slab thickness or less is recommended [5, 4, 11]. The lowest
allowable maximum aggregate size specified should be 0.5 in.
For BCOs only, the compatibility of materials between the old concrete and the new
concrete is fundamental for the success of the bond. The coefficient of thermal expansion (CTE)
of concrete overlay should be less or at least similar to that of existing pavement [10, 18, 42].
This is because higher slab stresses and wider joint openings can occur when aggregates with
higher CTE are used [8]. Since the CTE of the overlay is governed by the coarse aggregate
properties, the CTE of the coarse aggregates used in the overlay should be less or equal to that of
the existing pavement. Significant difference should be avoided in order to reduce the
differential movement between overlay and substrate. In other words, it is recommended that the
coarse aggregate in BCOs should have a thermal coefficient no higher than that of the coarse
aggregate in the existing pavement. For this reason, it is advisable to utilize a limestone
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Kim, Dong H.; Fowler, David W.; Ferron, Raissa P.; Trevino, Manuel M. & Whitney, David P. Materials selection for concrete overlays : the final report, report, July 2012; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth303706/m1/27/?rotate=0: accessed July 18, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.