Slipform Paving
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Set-up
Prior to paving the concrete surfacing, a 4" HMA base was paved. Surveyors then determined the HMA elevation at 50' intervals so that the guide wire could be installed (Figure 1). The thin metal wire spans both sides of the road path and is positioned to ensure that the paver runs on the desired path and at the exact height; these two variables are critical for a smooth ride. If bumped or knocked, the wire must be re-set; unfortunately, it is the number-one tripping hazard for 1st year engineers. Once the wire is set, paving can commence as soon as the paver is set to the correct width. For the observed day of paving, the slipform paver was adjusted from 36' to a width of 38'. This process required 3 mechanics and approximately 40 hours of labor. Because of the inefficiency in adjusting the width, add/drop lanes and other small areas are usually constructed by hand. An important part of set-up is equipment and material layout. The critical items include the slipform paver, curing machine, dowel bar bundles and curing compound (Figure 2).
Water trucks, as shown in Figure 3, are also stationed nearby and make trips to the site to wet the base-course surface prior to dumping the concrete. This serves two purposes: 1) to prevent all of the moisture from the mix from being sucked into the asphalt base, and 2) to cool the base from heat of the day (when needed).
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Discussion
Since the design of the new roadway consists of a different alignment from the original SR 240, there is adequate space for the slipform paver. Slipform paving through urban areas can create challenges due to limited lateral spacing. A typical slipform paver requires an additional 2-3' on both sides of the machine to account for the tracks. More space should be considered to allow access for laborers. These issues were not a problem for this project.
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Paving
Beginning the paving process is typically the most complicated and time consuming part of the work-day. Because the initial batch of concrete is usually wetter than the later batches to facilitate the movement of the mix through the machine, it is harder to form straight edges behind the paver; therefore, more laborers are needed to correct the slumping edges. When the concrete is dumped in front of the paver, the process begins. By the time 2 loads are dumped (side-by-side), theslipform paver approaches the mix which then passes under the screed and eventually into the paver. Another problem with the initial loads is that a "head" has not formed in front of the profile pan (sort of a second screed) and voids are more likely to result (Figure 4). The resulting voids must be filled in and finished by hand (discussed in detail in the Finishing & Curingsection). This process requires additional laborers, slowing productivity. Many times, a front end loader is used to push the concrete mix in front of the paver and towards the screed, shown below in Figure 5.
During the dumping process the mix is not of a uniform height in front of the paver and must be leveled out. To level the material there is a bucket, attached to a track, which travels the width of the paver. The bucket is controlled by the paver operator and can be rotated up or down depending on height of the mix (Figure 6). This allows the operator to redistribute the mix to a ensure a uniform amount and height is directly in front of the paver. Many pavers utilize an auger to accomplish this leveling task.
There are a few steps to examine as the mix passes into the paver. First, the screed serves as a strike off plate to level the PCC at the correct elevation. After this the concrete is consolidated by a series of vibrators (Figure 7). The vibrators are close enough to ensure their influence zones overlap across the width of the paver. The vibrators are adjustable and set according to mix specifications, which in turn reduces segregation.
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Dowel and Tie Bars
Meanwhile, as the mix passes through the paver there are many other jobs being performed to support the paving process. On one side of the paver a team works on feeding dowel bars into the paver. These dowel bars are necessary to transfer the load from one slab to another. A crane attached to the paver lifts the pre-placed bundle of dowels (as previously shown in Figure 2) and loads them onto the side of the paver. Before insertion, one worker thoroughly sprays the bars with form-oil to ensure that the bars do not stick to the setting concrete. Two workers manually load the bars into a dolly which transports them across the width of the paver into predetermined, individual slots. They are then pushed into the slab at the appropriate time using forked rods. These rods vibrate when inserting the dowels into the concrete in an effort to fill the voids that are created in the mix. This process is shown below in Figures 8,9, and 10. Dowel bar insertion is one of the determining factors of the paver speed. If the workers can not load the dowels fast enough or the paver is moving too fast, the inserter can not place the dowels satisfactorily. In either case the paver speed would need to be reduced. If the dowel bars are not loaded onto the dolly and placed into the slots across the width of the paver, the entire paver will actually stop to allow the dowel bars to be inserted.
To ensure that the dowel bars are inserted in the right locations, a "bicycle" wheel (Figure 11) is attached to paver to record the distance traveled by the paver. The wheel is calibrated and connected to the onboard computer; this information is used to inform the operator of the speed of the paver and, more importantly, to insert the dowel bars in the concrete at the required locations.
When the dowel bars are inserted into the concrete, a paint dot is sprayed onto the base at the midpoint of the dowel bars (Figure 12). This paint dot allows the contractor to locate the dowel bars after the paving is complete. Once paving is complete, the contractors will use a string line and match paint dots on both sides of the slab allowing them to saw acontraction joint at the midpoint of all the dowel bars. One possible problem with the paint dot is due to the wet asphalt base. Since the asphalt base is wetted prior to placement of concrete, the paint dot will not always "stick". As a result, the contractors for this project were measuring a distance of 15' (typical slab length) and placing a small nail into the side of the slab to inform the sawers of the location of the dowel bars.
Another aspect of the paving process is the insertion of Tie barswhich were installed on one side of the slab. Tie bars are 30", #5 steel bars which hold abutting slabs together. They are placed every 3' along the outside of the slab and are usually done during the paving operation automatically. One laborer loads tie bars into the inserter individually and they are inserted into the wet mix (Figure 13). The back tracks on the concrete paver are designed to allow the tracks to pass by on the outside of tie bars (Figures 14 & 15). The front tracks, however, are not wide enough to pass on the outside of the tie bars, and, as a result, the tie bars at the starting point of the day were not installed. This was done to allow the concrete paver to set back and begin paving in the opposite direction and not have the front tracks run over the tie bars. These remaining tie bars will be installed later by hand, by first drilling into the concrete slab and epoxying the tie bars into place. The side of the slab that received tie bars was located in a taper section that will be paved with concrete. As stated above, tie bars are used to hold abutting concrete slabs together. The other side of the slab will have an adjacent asphalt shoulder. As shown in the WSDOT standard plans, tie bars are not required in a slab when an asphalt shoulder is placed adjacently.
Two tie bars were also inserted within the width of the slipform paver. These mid-slab tie bars were inserted between the 12 foot lanes. A longitudinal contraction joint will be sawn over these dowel bars. The bars are loaded by hand into a machine that places them automatically into the concrete mix as shown in Figure 16. One laborer remained on the paver throughout the paving process to load both machines with tie bars.
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Discussion
Due to the paving startup location, a transverse construction joint was needed (Figure 17). The contractor's forms, however, were not tall enough to match the 0.92' concrete pavement depth. Therefore, shims were placed underneath to elevate the forms to the correct height. Utilizing this method to elevate forms can create problems because the end of the shims are placed into the concrete pavement and this can create a stress point. This stress point can cause the pavement to crack within a matter of days, as shown in Figure 18. It is unknown whether the shims used for this project caused cracking.
After the initial startup period, the slipform paver stopped only three observed times because the dowel bars were not in the correct locations for installation. Minimizing the number of these stops enhances the smoothness of the pavement. The paver operator said that he tries to maintain the paver speed at about 4 feet per minute. The measured speed throughout the case study ranged between 3.5 and 4 feet per minute. The paver operator also said that this was the best mix that he had seen in the past few years. His two main concerns during paving are, 1) ensuring the concrete dumped in front of the paver is uniform and not too far in front of the paver and 2) ensuring the surface behind the paver is not tearing or ripping. If the surface is tearing, it is usually due to a dry mix and the operator can subsequently increase the vibration rate. This was not needed during the time of the case study.
For this project, changes were made to WSDOT standard plans regarding the dowel bar placement. As shown in the standard plans, a typical slab has dowel bars spaced 12" apart across the entire width. Due to the low percentage of trucks using this part of SR 240 and in an attempt to decrease project costs, this spacing was altered. The designers of the project labeled the lanes as either "slow" or "fast". Slow lanes were either the far right (outside) lane in the 6-lane section (3 lanes in each direction) or the two outside lanes in the 8-lane section (4 lanes in each direction) of the project. The fast lanes were the other lanes of the roadway. Dowel bar placement in the slow lanes is as shown in the standard plans. For the fast lanes, dowel bars are placed only in the wheel paths. That is, 4 dowel bars are placed at the edge of the lane, there is then a 4' gap and then 4 more dowel bars are placed at the other edge of the lane for a total of 8 dowel bars. This alteration saves the project approximately $42,000 per project mile (assuming dowel bars are $10 each). A detail in the contract plans showed this alteration. This detail also showed that the placement of the dowel bars in the "fast" lanes would be 1' on the inside of the longitudinal lane joint rather than 0.5' as shown in the Standard Plans. The paving contractor's slipform paver was not able to efficiently place dowels 1' on the inside and they were allowed to place the dowel bars at the typical 0.5'. The contractor had 2 different options for the type of dowels to use on this project: 1) Stainless Steel Tube Dowel Bars, and 2) MMFX 2 Dowel Bars. The contractor chose to use the MMFX 2 Dowel Bars.
