Page 81 - Masonry and Concrete

Concrete Construction Techniques
80 CHAPTER THREE

3.9 Avoiding Common Problems

There are a number of problems which can occur in concrete as a result
of improper mixing, placing, or curing. The following are common
problems that are easily avoided if proper procedures are followed.
Segregation is the tendency of the various constituents of a con-
crete mix to separate, especially the separation of the large aggregate
particles from the cement mortar. Segregation can result in rock pock-
ets or honeycombs in the hardened concrete, sand streaks, porous lay-
ers, scaling, laitance, and bond failure at construction joints. Harsh
mixes have a tendency to segregate, usually those that are too wet but
sometimes those that are too dry. A well-proportioned mix with a
slump of 3 to 4 in. resists segregation, but any mix can segregate if it is
not properly handled, transported, and placed. Once segregation has
occurred, the aggregate cannot be reintegrated and the mix must be
discarded. Segregation can be caused by overmixing or by improper
handling during placement operations.
Bleeding occurs when the cement and aggregate in newly placed
concrete begin to settle and surplus water rises to the top surface of the
concrete. Bleeding continues until the cement starts to set, until bridg-
ing develops between aggregate particles, or until maximum settlement
or consolidation occurs. Mix proportions, sand grading, sand particle
shape, the amount of aggregate fines, the fineness of the cement, water
content of the mix, admixtures, air content, temperature, and depth or
thickness of the concrete all influence the rate and total amount of
bleeding. A slab placed on a plastic vapor retarder will bleed more than
one placed directly on soil because the soil absorbs some of the surplus
water. Some bleeding is a normal part of concrete curing, but excessive
bleeding can decrease the durability of the surface, interfere with the
bond of cement paste to reinforcing bars, and increase porosity of the
hardened concrete. Air entrainment reduces bleeding, as does a well-
graded sand, an increase in cement content, or a reduction in water
content. If changes are made to some ingredient quantities, the mix
must be adjusted to maintain the proper proportions required for
strength and durability. Bleed water must be allowed to dry naturally,
as there is little way to remove it from the soft surface of the fresh con-
crete. Excessive bleeding will delay the start of finishing operations.

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