Civil Engineering Reference
In-Depth Information
Excessive bleeding increases the
water-cement ratio near the top surface;
a weak top layer with poor durability
may result, particularly if finishing oper-
ations take place while bleed water is
present. A water pocket or void can
develop under a prematurely finished
surface.
After evaporation of all bleed water,
the hardened surface will be slightly
lower than the freshly placed surface.
This decrease in height from time of
placement to initial set is called settle-
ment shrinkage.
The bleeding rate and bleeding
capacity (total settlement per unit of
original concrete height) increases with
initial water content, concrete height,
and pressure. Use of properly graded aggregate, certain
chemical admixtures, air entrainment, supplementary ce-
mentitious materials, and finer cements, reduces bleeding.
Concrete used to fill voids, provide support, or provide
watertightness with a good bond should have low bleed-
ing properties to avoid formation of water pockets. See
Powers (1939)
,
Steinour (1945)
, and
Kosmatka (1994)
.
Fig. 1-8. Good consolidation (left) is needed to achieve a dense and durable
concrete. Poor consolidation (right) can result in early corrosion of reinforcing
steel and low compressive strength. (70016, 68806)
with paste and the less aggregate surface area there is to
coat with paste; thus less water and cement are needed.
Concrete with an optimally graded aggregate will be easier
to consolidate and place (Fig. 1-8 left). Consolidation of
coarser as well as stiffer mixtures results in improved qual-
ity and economy. On the other hand, poor consolidation can
result in porous, weak concrete (Fig. 1-9) with poor durabil-
ity (Fig. 1-8 right).
Mechanical vibra-
tion has many advan-
tages. Vibrators make it
possible to economi-
cally place mixtures
that are impractical to
consolidate by hand
under many conditions.
As an example, Fig.
1-10 shows concrete of a
stiff consistency (low
slump). This concrete
was mechanically vi-
brated in forms contain-
ing closely spaced rein-
forcement. With hand
rodding, a much wetter
consistency would have
been necessary.
Consolidation
Vibration sets into motion the particles in freshly mixed
concrete, reducing friction between them, and giving the
mixture the mobile qualities of a thick fluid. The vibratory
action permits use of a stiffer mixture containing a larger
proportion of coarse and a smaller proportion of fine aggre-
gate. The larger the maximum size aggregate in concrete
with a well-graded aggregate, the less volume there is to fill
0
20
Flexural strength
Modulus of elasticity
Fig. 1-10. Concrete of a stiff
consistency (low slump). (44485)
40
Hydration, Setting Time, and Hardening
The binding quality of portland cement paste is due to the
chemical reaction between the cement and water, called
hydration.
Portland cement is not a simple chemical compound, it
is a mixture of many compounds. Four of these make up
90% or more of the weight of portland cement: tricalcium
silicate, dicalcium silicate, tricalcium aluminate, and tetra-
calcium aluminoferrite. In addition to these major com-
pounds, several others play important roles in the hydration
process. Each type of portland cement contains the same
four major compounds, but in different proportions.
60
80
Compressive strength
100
0
5
10
15
20
25
30
Voids, %
Fig. 1-9. Effect of voids in concrete due to a lack of consoli-
dation on modulus of elasticity, compressive strength, and
flexural strength.
