Page 79 - Handbook of Thermal Analysis of Construction Materials

Section 11.0 - Concrete Properties 61

11.2 Setting

The setting of concrete is determined by using the mortar contained
in it. A penetrometer is used for determining the initial and final setting
times of mortar. A needle of appropriate size has to be used. The force
required to penetrate one inch in depth is noted. The force divided by the
area of the bearing surface of the needle yields the penetration resistance.
The initial setting time is the elapsed time after the initial contact of cement
and water required for the mortar sieved from the concrete to reach a
penetration resistance of 500 psi (3.5 MPa). The corresponding resistance
for the final setting time is 4000 psi (27.6 MPa).

11.3 Bleeding and Segregation

In a freshly placed concrete which is still plastic, settlement of
solids is followed by the formation of a layer of water on the surface. This
is known as bleeding. In lean mixtures, localized channels develop and the
seepage of water transports some particles to the surface. Bleeding may,
thus, give rise to laitance, a layer of weak, nondurable material containing
diluted cement paste and fines from the aggregate. If bleeding occurs by
uniform seepage of water, no undesirable effects result and such a bleeding
is known as normal bleeding. Bleeding is not necessarily harmful. If
undisturbed, the water evaporates so that the effective water:cement ratio
is lowered with a resultant increase in strength.
The amount of bleeding can be reduced by using proper amounts
of fines, increasing cement content and admixtures such as pozzolans, or
air-entraining admixtures. Bleeding characteristics are measured by bleed-
ing rate or bleeding capacity applying the ASTM C232 standard.
During the handling of concrete mix, there may be some separation
of coarse aggregates from the mixture resulting in a non-uniform concrete
mass. This is known as segregation. Segregation may lead to flaws in the
final product, and honeycombing may occur in some instances. By proper
grading of the constituents and handling, this problem can be controlled.

11.4 Mechanical Properties

The hardened concrete has to conform to certain requirements
for mechanical properties. They include compressive strength, splitting-