Casting box culverts: how to save time and money Tips from a contractor who built long boxes in Colorado BY BRYAN G. PENNER PRESIDENT MARANATHA CONSTRUCTION COMPANY ENGLEWOOD, 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: W 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. PREBID PLANNING 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). FORMING OPERATIONS 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 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. Bulkheads 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 spillage. 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 forms. Headwalls 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. COST DATA 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- PUBLICATION#C860867 Copyright © 1986, The Aberdeen Group All rights reserved