The Effects of Higher Strength and Associated Concrete Properties ...

important to PCC tensile capacity than compressive strength even in the normal strength
range (Forster, 1997; Vervuurt, 1997; and Mohamed and Hansen, 1999).
Model predictions for the relationship between the tensile strength and the compressive
strength indicate the influence of mix design parameters. This is also reflected by the
relationship between the compressive strength and the flexure strength (ACI Committee
363, 1984; and Carrasquillo et al., 1990). Furthermore, the flexure strength is not a pure
material property, but rather a structural property (Neville, 1983).
1.5.2 Compressive Strength
Although compressive failure rarely occurs in the pavement structure, the compressive
strength of concrete is perhaps the most comprehensive measure of concrete quality. The
compressive strength is directly related to the microstructure of the hardened concrete
(Mehta and Aitcin, 1990). Higher strength concrete behaves increasingly as a
homogeneous material in pre-peak loading, where the propagation of microcracks does
not initiate until just prior to failure. At the same time, the fracture behavior becomes
more brittle (Mehta and Monteiro, 1993; and Marzouk and Chen, 1995).
The compressive strength is mainly related to the w/c ratio. As the w/c ratio decreases,
the compressive strength increases (Mindess and Young, 1981; Neville, 1983; and Mehta
and Monteiro, 1986). Furthermore, strength of the coarse aggregate, mineralogical
origin, and quality can become controlling factors at high compressive strengths. In
addition, aggregate type influences the characteristics of the aggregate-paste bond at the
interfacial transition zone (Mehta et al., 1990; Zia et al., 1991; and Zhou et al., 1995).
Increasing aggregate size generally decreases the water requirements for the same
consistency, and should thereby also benefit the strength of the concrete. At the same
time, larger aggregate also increases the possibility of defects within the aggregates. For
these reasons, aggregate properties become more crucial for higher strength concrete than
for normal strength concrete (Mindess and Young, 1981; and Mehta et al., 1990).
Aggregate gradation is important to workability, strength, and durability. In general, the
sand content is from 40 to 60 percent of the total aggregate content depending on the
aggregate type and maximum aggregate size. For higher strength concrete, with higher
cement content, the sand should not be too fine because it will increase water
requirements (Cramer et al., 1995).
Certain chemical admixtures improve the workability (Aitcin and Lessard, 1994).
Entrained air also enhances workability, but decreases the compressive strength (Neville,
1983). Use of entrained air is important from a freeze-thaw durability point of view.
Mineral additives such as silica fume and fly ash are commonly used in concrete
structures. The mineral additives affect the strength of the concrete. Fly ash is not as
effective as cement in increasing early strength, and its effectiveness is dependent on the
classification of the fly ash. However, the slow pozzolanic reaction can lead to improved
long-term strength. It is recommended that fly ash not exceed approximately 25 to 35
percent of the cementitious material according to American Concrete Institute (ACI)
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