Materials selection for concrete overlays : the final report Page: 13
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Figure 2.4. Deformed steel fiber
Blended fiber systems combine macrofibers with microfibers or steel fibers. The
microfibers in these systems provide resistance to plastic shrinkage and settlement cracking,
while the macrofibers or steel fibers provide long-term secondary reinforcement. Blended
systems provide higher levels of fatigue resistance, greater flexural toughness, and improved
durability. Additional benefits include improved impact, abrasion, and shatter resistance [62].
2.3.8 Bonding Agents [11]
Bonding agents, e.g., portland cement grouts, latex-modified portland cement grout, and
epoxy resins, are sometimes used to improve bond. However, bonding agents cannot compensate
for bad substrate surface preparation and may act as a bond breaker when used inappropriately,
thus it is not recommended to use bonding agents unless under special circumstances. The use of
bonding agents leads to two interfaces and thus to the creation of two possible planes of
weakness instead of one. Besides, a grout often has a high water-cement ratio leading to a low
strength and the risk of a cohesive failure within the bonding agent itself.
Under normal placement conditions, the performance of the BCOs and whitetoppings is
better if no bonding agent is utilized [25], as long as the surface has adequate texture and has
been cleaned and free of dust, white water, other debris, and a saturated surface dry (SSD)
condition is achieved.
The shear strength at the bond interface should be at least 1.4 MPa (200 psi) [11, 26].
Bond strength can be improved by increased surface roughness, which exposes aggregates to
lock the layers together [1].
Grout
If the surface happens to be wet, a concrete grout will assure better bond strength. If a
grout is used, the overlay should be placed before the grout dries; otherwise, the bond strength of
the overly may be significantly reduced, because dried grout increases the probability of
delamination by acting as a bond breaker [11]. Past experiences have shown that grouting is not
needed, especially when the existing surface has been milled and cleaned well. A cleaned and
properly moistened surface is enough to ensure proper BCO bonding.
Figure 2.5 shows that immediately before paving, a grout can be uniformly broomed over
the full width of the prepared surface. The prepared surface must be in SSD condition.
Typically water-to-cement ratio of the grout is around 0.62 to 0.70 by weight, or approximately
seven gallons of water per sack of cement [27].13
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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/m1/33/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.