Portland Cement Concrete (PCC) Design FAQ
Note: Both the Long Life Pavement FAQ and the AC Design FAQ are being developed.
1. Why do we use concrete shoulders with PCC lanes when asphalt concrete shoulders are less expensive?
Answer: Tied concrete shoulders (with tie bars) provide stress reduction along the longitudinal joint between the outside lane and the shoulder. The largest amount of stress from truck tires is along the outside edge of pavement, so the tied shoulder spreads the load more uniformly across the longitudinal joint. An asphalt concrete shoulder does not connect to the PCC and does not provide that edge support like a tied PCC shoulder. Normally, over time, there will be shoving of the asphalt between the lane and shoulder. Remember, the outside lane is normally where trucks are traveling. Truck axle loading is the main distress that pavements endure. The HDM does allow an alternative design of asphalt shoulders with a widened PCC slab. This option keeps a 3.6 m (12-ft) lane width, but the concrete is paved at 4.27 m (14-ft) wide with an AC shoulder. The widened slab provides an approximate 0.6 m (2 ft.) buffer between the edge of travelled way and edge of PCC which reduces the edge stresses on the PCC pavement and extends the pavement life.
2. Why does the Department place dowel bars in shoulders?
Answer: Experience has shown, particularly in urban areas, that shoulders do get converted to traffic lanes to improve mobility. In some instances, shoulders which were intended to be converted to future traffic lanes, were not designed with dowel bars which lead to expensive change orders. It is also difficult to predict when a shoulder will be converted to a lane in the future. Because the cost to incorporate dowel bars by change order or to retrofit them into existing shoulders is expensive when compared to placing them when the PCC is first built, it was determined that it is in the best interest of the State to include dowel bars in new PCC shoulders and except them on a case-by-case basis. Considerations for exceptions will be based on the likelihood of the shoulder being converted into a traffic lane in the future and the geometric configuration of the shoulder. The HDM includes a blanket exception for shoulders next to existing undoweled PCC lanes. Also, the shoulder portion of widened slabs (see PCC FAQ #1) will always be doweled. The Pavement Standards Team is always looking for ways to improve this guidance and would welcome input. Please forward your comments to the Office of Pavement Design.
3. Why do we have two types of sawn joints?
Answer: See Standard Plan P20. There is a single joint cut and a double saw cut joint. The joint selection is left up to the Contractor. Either type of sawn joint works well. A designer does not have to worry about what type of joint is selected. The Contractor will cut one type of joint to control cracking. If however there is a preference by the District, then one type of joint saw cut could be eliminated in the SSP.
4. Why are joints sealed when they were not in the past?
Answer: Joint sealing keeps debris and surface moisture from entering the joint. Over the course of a day, temperature changes cause slabs to contract and expand. This movement allows the joints to open more, thus being susceptible to debris embedding in the joint. If enough hard particles such as gravel build up in the joint, this may cause joint movement restriction, which would lead to spalling or joint lock up. Sealants also reduce moisture from entering the base below, reducing the possibility of pumping of fines. This will help to prevent premature erosion of the base.
In prior years, sealants were not used regularly. But with better performing sealants available, sealants are recommended for all sawn joints. Note that with an extruded sealant, there should be a foam backer rod placed in the reservoir before applying the sealant. Compression seals do not need a backer rod.
5. Why is no saw cut needed on a lane poured previously with a cold joint along its longitudinal edge?
Answer: When a lane is slip formed, and allowed to cure for a day, a cold joint is formed. This cold construction joint would have tie bars placed into it. Then an adjacent lane is poured up against this new lane. The longitudinal joint will remain tight due to the tie bars. It is anticipated that very little water will penetrate this joint. Therefore, to saw cut the longitudinal joint would be difficult at best and not very beneficial. With a tight cold joint, a sealant would not be needed. However, when the time arrives to reseal the pavement joints, the cold joints should be sealed with the rest of the joints.
6. How do I know if the alignment of the dowel bars is correct?
Answer: The dowels require coring. The contractor provides coring at each end of one dowel. By comparing the two cores, the dowel tips should provide an idea of the alignment of the dowel relative to the surface. If the dowel is misaligned, then there are special measures in the special provisions to handle this issue. Tolerances are covered in the specifications and schematics are shown on Standard Plan P10.
There are no approved non-destructive techniques for determining dowel locations. The Department is looking into promising potential research that may change the way that dowels are checked for alignment in the future.
7. Are there dowel baskets for replacing existing skewed joints if dowels are to be used?
Answer: There are a few dowel manufacturers who can make skewed dowel baskets. Skewed baskets are used infrequently. If a significant quantity is not being ordered, it would be assumed that this would be an expensive option. However, this should not prevent the Design Engineer from specifying dowels at skewed transverse joints if they are warranted. If the Engineer includes a schematic of the skewed dowel basket in his Construction Detail plan sheets, then this would be covered. The Designer needs to put information into the Resident Engineer's file to ensure that the location of these dowels is documented for future use. Future retrofitting will be a problem if there is no information on locating dowels during slab replacement or lane replacement.
8. Are early entry saws allowed for crack control?
Answer: The latest version of SSP 40-010 and 40-011 will allow early entry saws to be used. The saw cut depth for the early entry saw will be D/4. However, spalling of joint is still unacceptable during sawing for transverse joints.
9. Is there a rapid setting lean concrete base specification?
Answer: Not yet. The use of rapid setting PCC is suggested when a quick cure of base material is necessitated. In the future, a SSP for this may be developed that would utilize a lean concrete base with additional non-chloride accelerators. If the District wishes to use rapid setting lean concrete base for a particular project, they should contact Mr. Tom Pyle, Chief of the Office of Rigid Pavement Materials and Structural Concrete for assistance in developing a specification.
10. Can we use dowels in transverse joints when replacing panels?
Answer: The Slab Replacement Guidelines, which are available on the Pavement website has a good table that describes when and where you should consider the use of dowel bars. Remember, the use of dowel bars for a joint to be replaced by a major rehabilitation project in the near future is not a cost effective use of dowel bars.
11. Why don't we have a standard plan for intersection joint layouts?
Answer: It is not practical to develop a standard plan for intersections because each intersection is different. Intersections have various widths and lengths with possible turn pockets, cross walks, etc. Also, various manholes and drainage inlets may be located in the intersection that impact joint locations. With all these various elements, it requires good judgment to determine the best design for each particular intersection to layout the jointing pattern for saw cutting. The Office of Pavement Design is currently developing training for placing joints in an intersection.
Please forward any questions/comments/suggestions to Shira Rajendra(firstname.lastname@example.org). Last updated 04/29/2011.