31 May 93
allowed to freeze. Provisions for windbreaks, shad-
(3) RCC cold joints require special treatment
ing, fog spraying, pounding, or other methods may
in order to assure adequate bonding with the succes-
be needed to prevent excessive moisture loss or
sive lift. Typical cold joint treatments include high
excessive heat generation in hot weather.
pressure washing or wet sandblasting to remove
(2) Specifications should require sufficient plant
a high volume, low pressure washing and vacu-
capacity to meet the high production rates common
uming to remove all excess water and debris. The
for RCC placements and to assure that horizontal
surface is then maintained in a damp condition and
lift joint surfaces are left exposed for a minimum
covered with a thin layer of mortar or concrete
amount of time. Laboratory studies indicate that
bedding mix immediately before placing the next
the loss of bonding capacity at the joints begins
lift of RCC.
almost immediately after placing and that the bond
f. Contraction joints.
strength of lift joints is therefore improved when
joints remain exposed for a minimum amount of
(1) Vertical joints may be formed in mass
RCC sections to control cracking by inserting steel
e. Cold joints and treatment.
or plastic sheets, or any other bond breaker mater-
ial, into the full thickness of the uncompacted RCC
(1) An RCC cold joint may occur at any hori-
lift. The plates or sheets are placed adjacent to
zontal lift surface that is allowed to dry, set, or
each other end to end from the upstream to down-
become contaminated prior to receiving the next
stream face to form a bond breaker that serves as a
lift. These surfaces can result in reduced bond
contraction joint. The sheets are usually 1/8-in.
strength with the successive lift, even under moist
thick by 3-ft long and are installed using a vibrating
cure conditions, unless special measures are taken
plate installed on a backhoe. The width of the
to assure bonding of the two lifts. Cold joints can
plates should be somewhat less than the lift thick-
develop at a lift surface if the RCC surface remains
ness (1 to 2 in. less) to make sure the edge of the
uncovered by the next lift long enough for the
plate will not protrude above the lift surface where
surface to attain its initial set, or approximately 1 to
it could be damaged by placing and compacting
3 hr after placing. Environmental conditions such
equipment. The contraction joint plates should be
as heat, humidity, and wind, which contribute to
installed in every lift of RCC so that 100 percent of
drying of the RCC surface, are also significant
the vertical section at the contraction joint has the
factors in reducing the strength of the RCC joints.
bond breaker plates. The practice of only installing
If the RCC surface is allowed to dry, the strength
plates in every other lift can result in the formation
development of the mortar and paste at the surface
of cracks in undesired locations, which could com-
stops, due to lack of moisture for cement hydration.
promise strength and stability.
Although the hydration process continues if addi-
tional moisture is added, the bond strength capacity
(2) The number and placement of the contrac-
at the joint is significantly reduced. Specifications
tion joints should be determined by the structural
should establish the criteria used to determine if a
engineer based on thermal studies and by examina-
lift surface is a cold joint. This is usually done by
tion of the foundation profile parallel to the axis of
the structure. Abrupt changes in the foundation
tions that require that RCC surfaces be treated as a
profile may cause stress concentrations and require
cold joint. Generally, joints that are 2000 Fahren-
additional contraction joints. Normally, contraction
heit degree-hours or more old, or that have been
joint spacing will range from 60 to 80 ft. Fig-
allowed to dry, should be treated as cold joints.
ure 1-7 shows a contraction joint being installed.
Steel or plastic sheeting is placed in the RCC to
(2) Vertical cold joints occur when placement of
form contraction joints. A vibrating blade mounted
the RCC is stopped or interrupted before an entire
on a backhoe may be used to install the sheeting.
lift is complete. The leading edge of the RCC at a
Contraction joints placed in this manner help to
vertical cold joint should be tapered to form an
control random cracking due to volume change
interlocking feathered edge with the underlying lift.