Casting box culverts: how to save time and money

Casting box culverts:
how to save time and money
Tips from a contractor who built long boxes in Colorado
hy did the City of Boulder choose cast-inplace concrete for the box culverts carrying
Colorado’s Goose Creek under the Foothills
Parkway? These 7-barrel and 5-barrel culverts, each more than 300 feet long, were built with 5x8foot barrels. Six months later the 49th Street Culvert, 253
feet long, was also cast in place with two barrels 6x11
feet and a middle one 6x14 feet. There were several reasons for all of this work to be cast in place:
1. In the Denver/Boulder area it takes 6 to 8 weeks after
shop drawing approval to obtain delivery of precast segments. By using the cast-in-place culve rt s, we could begin work immediately, and within the 6 to 8 weeks had
completed half the concrete work.
2. There are few big precasters in the De n ve r / Bo u l d e r
area who do many precast culve rt s. With this constraint,
using cast-in-place culverts resulted in a savings to the
owner of 15 percent.
3. The Goose Creek culverts go under the highway, cross
a farmer’s irrigation ditch, and intersect with numerous
utilities. Such a job does not lend itself to the straight
continuous runs that are ideal for the precast method.
Consequently, the contractor himself would have had to
do much of the forming anyway.
Anticipating problems at the prebid stage through
careful scrutiny of the contract documents can save hassles later and enable the contractor to make a more precise bid as well. For example, the State of Colorado did
not include any allowance for the weight of steel needed
for splices in its design value of pounds of reinforcement
per lineal foot of box. We figured typical lap splices of 24
bar diameters, and this gave us about a 10 percent increase in weight of rebar with splices at 30 feet as required by the owner’s engineer. This additional steel
needed was a factor in the bid price that might have
been left out without some careful examination of drawings and specifications.
The contractor should also check the amount allowed
for excavation because it may differ from the actual excavation needed on the jobsite. Colorado allows payment for 1 foot of excavation on each side of the box culvert. Howe ve r, we found it would save time to cut wider
on each side of the box. This allowed 4-foot equipment
to operate along the sides, rather than using hand compaction of backfill. Fu rt h e rm o re, in order to meet OSHA
safety standards, additional excavation is required beyond state-allowed quantities. Although extra dirt was
moved, the per-cubic-yard cost of using equipment was
far less than the unit-cost of completing smaller quantities by hand.
Water control
What to do about water flow at the site should also be
considered at the prebid stage. Typically box culve rt s
are built in a flood plain and below the water table, resulting in a water problem during construction. Depending on the box location, several methods can be
used to deal with this problem.
Drainage pipes around and under the box with pumps
carrying the water above ground into a storm drain area
can be effective when gravity flow is not attainable. This
is the approach we used at Goose Creek, draining into a
large collection sump before pumping up to a surface
drainage facility.
Daylighting the end of the drain pipe to allow water
to flow underneath and away from construction is by
essential in saving time and money. By the end of the
Goose Creek project, each person knew what his job was
when he heard, “Today we’re doing a slab pour.” Or,
“Let’s get the deck reinforcing steel distributed on the
next section.” At the end of this four-month project, our
efficiency had increased by 30 percent, as measured in
man hours needed to complete a given section of the
c u l ve rt .
It was important to recognize beginning skills of the
crew and develop a forming system and technique
around their abilities. For example, our crew was not accustomed to using wire reels to tie rebar, but preferred
using looped-end wire ties. Spending a few extra dollars
to purchase the precut ties saved labor worth three times
the extra cost of the ties.
Precast retaining walls rented
Figure 1. Rented precast retaining wall sections helped
keep traffic flowing while box culvert forming operations
proceeded within 3 feet of the ongoing traffic.
Since this project involved a heavily-used street crossing, scheduling of the forming and placing were coordinated so that a mid-street completion point was
achieved. This restricted forming process was carried
out by the use of innovative right-angle precast retaining
wall sections. These sections could be placed up against
the existing street, allowing us to form within 3 feet of the
ongoing traffic (Figure 1).
far the pre f e r red method. This worked on the 49th
St reet box. We put in a drainage system beneath the
box but were able to drain to a backhoed trench without pumping.
Slab-on-grade construction
A first attempt was made to place wall form panels at
the edges of the base slab so that they could form both
Organizing and scheduling the project are key factors
in making cast-in-place culverts economical. We strategically located the jobsite construction trailer, tools and
equipment between the two box culverts at Goose Creek
to save steps on the project. Reinforcing steel was laid
out in piles by size and length to enable workmen to
quickly locate the needed bars.
By carefully scheduling our carpenters, laborers and
finishers, along with concrete pours, we kept the construction process flowing without the common time and
dollar leaks of workmen standing on the job between
forming operations, laborers waiting for concrete trucks
to arrive, or finishers having too much or too little time
for concrete to set up.
A back-and-forth sequence was also very effective and
much was accomplished in one day. For example, instead of excavating both boxes at the same time, the
north box was excavated, and then forms were erected
while the south box was excavated. Then while carpenters finished forms on the south box, concrete was
placed in the north box. Often a slab was cast, allowing
the finishers to complete the surface, while the laborers
proceeded to fill wall forms and then return to another
slab or deck pour to complete the day for the finishers.
A well-trained and well-organized labor force is also
Figure 2. Base slab for box culvert was made 3 inches wider
on each side to provide a base for the heavy crane-handled
form panels. Labor saved in form alignment more than offset
cost of extra concrete.
the slab edge and the exterior wall of the culvert box.
This procedure caused problems in lining up the interior form panel with the first (outside) form. Because of
the uneven ground on the exterior, the alignment took
The 6-foot forms were used for all interior walls. These
shorter forms allowed workmen access to the top of the
wall for finishing and for placement of blockouts.
A rough-terrain crane picked and set each panel. The
basic gang form construction (Figure 3) was strong
enough to permit use of heavy reusable ties at relatively
wide spacing. Twelve taper ties per panel turned out to
be the most economical system for assembling and
stripping panels at Goose Creek. Ties were spaced 3 feet
apart in two rows, one 8 inches below the top of the wall
and the other 8 inches above its base.
Rather than forming tight-fitting bulkheads, we saved
time by slotting the edge of the plywood bulkhead so
that the bulkhead form could be quickly slipped over the
rebar and nailed to the gang forms (Figure 4). The slots,
about 4 inches wide and 2 inches high, seemed fairly
large, but as the concrete was poured the stones plugged
the slots and there was not a great loss of concrete.
The same slotted plywood bulkhead was used where
there were two layers of reinforcing steel on the interior
walls. The bulkhead form was installed through the two
mats of rebar, leaving a fairly large opening in the bulkhead between the mats. A 2x4 was then centered on the
inside of the bulkhead form not only to plug the slots,
but also to provide the required keyway.
Suspended concrete deck
Figure 3. Heavy duty crane-handled form panels
accommodated taper ties at wide spacing. Support
members are all 4x4s.
twice as much time as it did for the interior walls.
T h e re f o re standard metal curb forms were used to
complete the slab, with one important difference. The
base slab was made 3 inches wider on each side to allow
the exterior forms to rest on the base slab itself (Figure
2). This permitted the exterior wall to be lined up as efficiently as the interior walls. The cost of this extra 3 inches of concrete on each side of the box was offset twofold
by the saving of labor and time in lining up the exterior
wall forms.
Gang forms
Crane-handled forms were constructed of 3⁄4-inch
overlaid (plastic faced) plywood backed by hori zo n t a l
4x4s (Figure 3) about 16 inches on center. Double 4x4s
about 31⁄2 feet apart served as vertical strongbacks. The
walls at Goose Creek were 10 inches thick.
The form panels were made 18 feet long and either 6
or 8 feet high. The 8-foot gang forms were used on the
outside walls of the exterior boxes. This allowed these
forms to act as not only the wall form, but also the edge
form for the suspended deck slab.
The top of the culvert box was built as a suspended
slab, with deck forms supported on rented hori zo n t a l
shoring made of telescoping bar joists. These proprietary supports come in a range of adjustable lengths
with corresponding weights. The height of the culvert
box and the weight of the bar joist required must be
carefully evaluated when determining the bid on a specific box culvert. We selected a length and weight which
could be handled by two workmen. The 6-foot-high box
expedited horizontal shoring removal, whereas a shorter box would have required additional time and effort
due to less head room.
The bar joist shoring was set on 2-foot centers on a
brick ledge formed on each side of the wall by placing a
continuous 1x2 at the top of the gang forms. The formed
ledge allowed flexibility in locating the joists but did require 1x2 blockouts between them to control concrete
Once the bar joists were in place, 3/4-inch sheets of
forming grade plywood were laid on top of them from
box wall to box wall. These forms were not nailed to any
surface but merely laid in place. The plywood was ripped
to the necessary widths to provide this continuous form
( Fi g u re 5). Typical deck forms were plastic-coated plywood with dog-eared corners. To repair these points
sheet metal was cut and laid over these areas to complete the deck surface.
Where control joints were required, 3/4-inch chamfer
strips were nailed to the deck and edge-wall forms. Form
Figure 4. Edge of the plywood bulkhead was slotted to fit easily over the
reinforcing steel. On interior walls with two layers of rebar, a longer slot was
used. A 2x4 centered on the inside of the form plugged the slots and formed
the required keyway.
oil was sprayed over the deck surface and re i n f o rc i n g
steel was placed on elevated chairs. Just prior to pouring,
the bearing ends of the bar joists exposed on the brick
ledge were greased to facilitate easy removal during the
stripping process.
Figure 5. Rented horizontal shoring made of telescoping bar
joists supported plywood form panels for the top slab of the
culvert box. The bar joist ends rested on ledges cast in the
walls by placing a continuous 1x2 at the top of the gang
The headwalls were formed at the end of the box on
temporary shoring. This system allowed for the support
of the headwall exterior and provided a place for the
concrete finishers to stand. This short wall was formed
with the interior wall form floating from support of the
exterior wall form.
quired 1005 cubic yards of concrete and 48,400 pounds
of rebar. Forming costs amounted to $0.70 per square
foot of suspended deck form and $1.55 per square foot of
wall gang forms. These forming costs included material,
both owned and rented, labor, and equipment for setting the forms and stripping the forms.
The 5-barrel and 7-barrel boxes combined re q u i re d
4600 cubic yards of concrete and a half million pounds
of rebar. The total overall cost for reinforced concrete in
place was $104 per cubic yard. The 3-barrel culvert re-
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