Materials selection for concrete overlays : the final report Page: 16
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THICK INTERLAYER (+1/2 inch)
Figure 2.6. Purpose of interlayer
2.4 Recommended Mixture Design/Proportioning
Once, the potential materials are selected, proper design/proportioning of those materials
is very important to ensure desired concrete overlay performance. In this section, brief
discussions for each design/proportioning criteria and recommendations are provided.
2.4.1 Cementitious Materials Content
A concrete overlay must have enough cementitious paste to coat the aggregates, and the
interface layer [6, 7]. Insufficient cementitious material content can lead to low early strength.
However, if more than enough cementitious paste is used; it will increase the chance of
durability issues such as shrinkage and alkali-silica reaction (ASR) and issues caused by high
heat of hydration. Based on findings from the literature, depending on surface treatments, CA
shape and texture, lower cement content is recommended [10, 23, 29].
2.4.2 Water-to-Cementitious Materials Ratio (w/c)
Lower w/c ratios are often used for concrete overlays to minimize drying shrinkage.
However, lack of water can lead to less than ideal amounts of paste, hindering the complete
coating of aggregates that may lead to reduced workability [6, 7]. The low w/c of the El Paso
BCO concrete, coupled with a very dry surface, is blamed for causing overlay debonding.
However, too much water increases shrinkage and evaporation rate [12, 1]. Higher water
contents provide greater potential for shrinkage as the water evaporates [4, 18]. For normal
placement, 0.40 to 0.45 w/c is recommended and maximum of 0.35 w/c is recommended for
expedited placement.
2.4.3 Fly Ash Content
A study [15] showed that addition of Class C fly-ash resulted in increased cracking when
using cement replacement range of 0 to 15%. However, beyond this replacement rate, one can
expect beneficial effects of fly ash addition. In other words, there appears to be a threshold
cement replacement rate of around 20% at or higher which beneficial effects of fly-ash addition16
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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/m1/36/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.