Materials selection for concrete overlays : the final report Page: 8
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aggregate for BCO if existing concrete has siliceous river gravel as coarse aggregate, because of
limestone's lower CTE, but the opposite arrangement will make up for a BCO prone to
delamination [11]. If the existing pavement used limestone aggregate as CA, only limestone
aggregate with equal or lower thermal coefficient should be used for new BCOs.
Also, the modulus of elasticity (MOE) for BCOs should be lower than for the existing
pavement [11]. For the same strain, concrete with higher MOE will have higher stress.
Fine Aggregate (FA)
FA must be sound and nonreactive. It is necessary that FA be sufficiently resistant to tire
wear (polishing) to prevent loss of skid resistance. The polish resistance may be improved by
using durable and angular fine aggregates [4, 6, 10]. Calcium carbonate fines are known to
polish excessively. TxDOT recommends a minimum acid insoluble residue of 60 % [63].
Gradation
Using uniformly and densely graded aggregates is recommended to reduce shrinkage
because it reduces required paste. This is helpful in thin concrete overlays, because the risk of
debonding due to shrinkage and curling potential is decreased [10]. Both the top size and
gradation of the aggregate will also affect aggregate interlock at the joint, which is another
important consideration, because thin concrete overlay joints are typically not dowelled [8].
2.3.3 Fly Ash
Cement may be partially replaced with fly ash, which can lead to higher ultimate concrete
strengths and lower permeability [7]. Moreover, replacing cement with fly ash can reduce cost,
increase workability, and increased protection against deleterious environment. Due to the lower
specific gravity of fly ash, as compared to cement, replacement of cement with fly ash increases
the volume of cementitious paste in the mixture. This increased volume of paste provides an
improved coating of fibers and aggregate in the mixture, leading to improved workability and
fiber distribution [4]. However, higher the fly ash replacement lowers the early strengths. This
may lead to delay in construction and opening to traffic.
2.3.4 Slag
For concrete overlays, granulated, ground, blast furnace slab (GGBFS), blast furnace slag
(BFS), or, simply, slag is typically used in replacement proportions of 25 to 35 %. It is normally
substituted for cement by mass. The proportion of slag cement is usually dictated by
requirements for strength, durability, time of set, and the resistance of the concrete to ASR.
Mixtures should be optimized for strength and durability using trial batches and the appropriate
test methods. It is not uncommon to find that total cementitious material can be reduced by using
appropriate levels of slag cement to replace cement when strength is used as the evaluating
criteria.
2.3.5 Silica Fume
Generally, addition of silica fume will increase the compressive strength. However, an
unbalanced addition will attract agglomerated silica fume particles to provide fast crack
propagation path within the matrix [13]. And higher compressive strengths usually mean higher
modulus for concrete, and that may not be desirable in thin overlays on very low-modulus
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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/m1/28/: accessed July 17, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu.; crediting UNT Libraries Government Documents Department.