Materials selection for concrete overlays : the final report Page: 17
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on shrinkage cracking occurs. A 30% replacement of cement with fly ash improved workability,
reduced heat of hydration, noticeably increased long-term strength, and enhanced resistance to
environmental attack. Too much fly ash will reduce short term strength gain. A replacement of
at least 25% of fly ash is recommended.
2.4.4 Aggregate Ratio
Since cement paste shrinks and expands more than aggregates, using as much aggregate
as possible while using as little cement possible is beneficial (while meeting other requirements.
An increase in the aggregate/cement (a/c) ratio was highly effective in reducing shrinkage
cracking [15].
2.4.5 Fiber Content
Fiber addition impacts water demand and workability. Changes to water content and
water-reducing admixture dosage will be required for adequate workability. The extent of these
changes will be dependent on fiber type and fiber dosage rate [19]. A main factor that will
determine the content or even the usage of fibers at all will be the economic feasibility. A cost-
benefit analysis will need to be performed to determine if or which fibers can reduce long-term
cost (obviously, initial cost is going to be much higher).
Each type of fiber has a recommended dosage from the manufacturer. Typical amounts
of synthetic fibers used are at least 0.1% by volume or around 3 lb/yd3. Normal steel fiber
contents range widely from 0.25% to 2% by volume or 33 to 265 lb/yd3.
Numerous studies have shown indefinite trends of compressive and/or tensile strength
change due to the addition of fibers [19, 21, 23, 33]. There were instances where the strengths
increased, decreased, or stayed the same. For each project, the amount of fiber desired should be
calculated in terms of cost and required strength. TxDOT has developed a minimum average
residual strength (ARS) requirement based on ASTM C1399.
2.4.6 Air Entrainment
Enough air must be entrained for durability as required for environmental reasons and
chloride exposure. Typically, in the northern-most parts of Texas 4 to 7% of air is entrained
depending on project specifications [63]. The interaction between air entrainment and other
admixtures should be considered.
2.4.7 Admixture Dosage
Admixture dosage can be different for each batch. The dosage should be adjusted
through trial batches, and the interaction between admixtures should be considered. Too much
HRWR can cause the mixture to get sticky and make finishing more difficult.
2.5 Recommendations for Construction
Construction is a crucial phase that can determine the level of performance. Many
failures in concrete overlays are caused by poor construction procedures [34]. Good selection
and mixture/proportioning of the materials need to be supplemented with good construction
practices to produce desired performance in concrete overlays.17
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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/m1/37/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.