ETL 1110-2-365
31 Aug 94
for the following parameters must be selected by the
hydration. The placing temperature for the initial
design team prior to the initial analysis.
analysis should be established by the materials engi-
neer as described in paragraph A-3b(1)(a). As with
a. Lift heights. Since the heat escape from a
lift heights, if structural behavior is acceptable then
mass is inversely proportional to the square of its
consideration may be given to increasing the placing
least dimension (ACI 207 (1992a)) and since the
temperature. Increasing the placing temperature can
height of a lift will usually be the smallest dimension,
lead to cost savings due to decreased cooling require-
the height of a lift can become a factor in the behav-
ments. Changing the placing temperature is a simple
ior of a mass concrete structure. Lift heights to be
change to the initial conditions in the heat transfer
used in initial analyses will typically be selected by
analysis.
the design team based on previous experience and
practical limits. If the initial analyses indicate that
d. Start time. The time of year at which an
the behavior of the structure is satisfactory, then
analysis is started can have a significant effect on the
analyses may be performed with increased lift heights
analytical results. Analyses in the past have been
as a measure for reducing costs as was done for the
started in a time frame which has the largest mass
auxiliary lock at Melvin Price Locks and Dam
placed at the hottest time of the year. It was felt that
(Truman, Petruska, Ferhi 1992). Likewise, if results
this would create the highest temperatures within the
indicate unacceptable behavior, a decrease in lift
structure and likewise create the highest stresses.
heights may be considered to alleviate problems in
Winter starts can also be critical due to the tempera-
the structure. Changing lift heights can be accom-
ture gradients that will occur within the structure at
plished in the numerical model as described in
these times. As stated in paragraph A-2d, a mini-
paragraph A-5c(3).
mum of two NISA's should be performed using dif-
ferent start times. Each project should be evaluated
b. Placement intervals. The time allowed
by the design team to determine if there are addi-
between the placement of lifts can have an effect on
tional start times which could result in a critical con-
the performance of the structure due to the insulating
dition based on past experience and engineering
effect a new lift has on the previous lift(s). In addi-
judgment. Changing the start time in the analysis
tion, the structural performance can be affected by the
will require the ambient temperature curve to be
difference in modulus of the two lifts and the fact
adjusted so that it corresponds to the given time of
that creep and shrinkage in the older lift are occurring
year that the analysis is being started.
at a lower rate than for a newly placed lift. A 5-day
interval between lift placements is typically assumed.
e. Insulation. Insulation of the concrete during
The longer the interval between placement of lifts,
cold weather may be necessary during construction
the longer each lift will have to dissipate the heat that
and if used must be accounted for in the analysis.
has built up within the lift. When considering the
The time that insulation is in place and the amount of
aging characteristic of concrete, however, longer
insulation (the R value) to be used will depend on the
placement intervals may not be desirable, since the
project location and should be selected by the design
previous lift will be much stiffer than the new lift
team for the initial analysis. Both of these parame-
providing more restraint to the new lift. Lift
ters may be varied during subsequent analyses to
placement interval can have an effect on the
achieve cost savings or to improve performance. Any
construction cost if the change increases the length of
changes made in the insulation requirements should
the contract. If stresses at the early times are high
be coordinated among the design team members.
using the typical 5-day interval, then the design team
Changes in insulation are accounted for in the
may choose to consider analyses which use longer
analysis by changing the film coefficients which
placement intervals. In the analysis, this requires
model the convection across the boundary. Instruc-
changing the time-history designations to the appro-
tions for calculating these coefficients are given in
priate values and ensuring that the values for the
paragraph A-5d(3).
reference time in the material input are properly set.
f. Vertical construction joints. There may be
c. Placing temperatures. For many mass con-
some projects where vertical construction joints
crete structures, the temperature of the concrete at the
become necessary due to excessively large concrete
time of placement is limited to reduce the tempera-
placements. If this is the case, lift sequences creating
ture level within the mass due to the heat of
vertical joints should be accounted for in the
A-15
Previous Page
