PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 212

■

Compressive Strength
Compressive strength of mortar is measured by preparing 50-mm (2 in.)
cubes and subjecting them to compression according to ASTM C109. The
mortar is prepared with cement, water, and standard sand (ASTM C778).
Minimum compressive strength values are specified by ASTM C150 for
different cement types at different ages. The compressive strength of mortar
cubes is proportional to the compressive strength of concrete cylinders.
However, the compressive strength of the concrete cannot be predicted accu-
rately from mortar cube strength, since the concrete strength is also affected by
the aggregate characteristics, the concrete mixing, and the construction
procedures.

6.7.3

6.8 Water-Cementitious Materials Ratio

In 1918, Abrams found that the ratio of the weight of water to the weight of
cement, water-cement ratio, influences all the desirable qualities of concrete.
For fully compacted concrete made with sound and clean aggregates, strength
and other desirable properties are improved by reducing the weight of water
used per unit weight of cement. This concept is frequently identified as
Abrams's law.
Supplementary cementitious materials, such as fly ash, slag, silica fume, and
natural pozzolans, have been used as admixtures in recent years to alter some of
the properties of portland cement concrete. Therefore, the term water-cement
ratio has been expanded to water-cementitious materials ratio to include these
cementitious materials.
Hydration requires approximately 0.22 kg to 0.25 kg of water per 1 kg
of cement. Concrete mixes generally require excess moisture, beyond the hy-
dration needs, for workability. Excess water causes the development of capil-
lary voids in the concrete. These voids increase the porosity and permeability
of the concrete and reduce strength. Figure 6.7 shows a typical relationship
between the water-cementitious materials ratio and compressive strength.
It is easy to see that increasing the water-cementitious materials ratio de-
creases the compressive strength of the concrete for various curing times. A
low water-cementitious materials ratio also increases resistance to weath-
ering, provides a good bond between successive concrete layers, provides
a good bond between concrete and steel reinforcement, and limits volume
change due to wetting and drying. Air-entrained concrete includes an air
entraining agent, an admixture, which is used to increase the concrete's re-
sistance to freezing and thawing, as will be discussed later in this chap-
ter. Curing maintains satisfactory moisture content and temperature in
the hardened concrete for a definite period of time to allow for hydration
(see Chapter 7).