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Joint Mechanics

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Dowel bars pre-placed for paving to provide load transfer

There are many mechanisms by which joints are evaluated. Some of the most common metrics that make up transverse joint performance include load transfer efficiency, differential deflection, total deflection and joint opening. A number of these also apply to longitudinal joints. Joint opening is important to longitudinal joints because the paved lanes need to remain close for proper performance.

Load Transfer

Load transfer from one slab to the next in concrete pavements is essential for long term performance. Proper load transfer minimizes joint related distresses including faulting and pumping. The primary factors that contribute to load transfer are aggregate interlock, the use of embedded steel in the form of dowels, and subbase support. All of these methods help the pavement system share the applied loading between the slabs such that a loaded slab does not carry the full loading by itself. Load transfer efficiency will also impact the differential deflection between the two joints where the load is being applied and the total deflection of the joint. This can create a smoother ride and reduce noise created by joint slap.

Aggregate interlock is the result of sawed contraction joints in jointed plain concrete pavement (JPCP). The saw cut controls where the crack develops and since the cut is typically only 1/3 to 1/4 of the pavement thickness, the bottom portion has aggregates that bridge the propagated crack to help transfer load from one slab to the next. Aggregate interlock is also a very important load transfer mechanism at cracks in continuously reinforced (CRCP) and jointed reinforced concrete pavements (JRCP). Embedded steel reinforcement helps to keep the cracks tight so that aggregate interlock can continually provide load transfer throughout the service life of these pavements. The reinforcing steel itself does not provide any load transfer. Similarly, tie bars are used to hold adjacent lanes and shoulders tightly together and not to provide load transfer.

Dowels are one of the most important tools used for load transfer. JPCP typically uses dowels to transfer loads across transverse contraction and construction joints and even occasionally isolation joints. Dowels are typically cylindrical objects made of metal placed into the pavement for load transfer. Dowels can be pre-placed before paving using dowel baskets or inserted during paving using a dowel bar inserter (DBI). Dowels can also be placed via dowel bar retrofit (DBR) to restore or improve load transfer over cracks or joints that were not initially dowelled.

Dowel Design

Dowel design for pavements has been largely empirical, with engineers either following past specifications and designs or trying new designs based on what has not performed well. The typical dowel design now calls for twelve dowels spaced at 12 in. on-center. Typically 18-in. dowels are specified and dowel diameters are 1.25 in. for pavements 8-10 in. thick and 1.5 in. for pavements greater than 10 in. Through the use of DBR since the 1980's, the typical design has been found to be overly conservative. DBR is now typically only performed in the wheelpaths using between three and five dowels. Additionally, the dowels being used are often shorter than 18 inches due to placement and alignment accuracy with this technique. These ultimately result in cost savings that many agencies are trying to leverage in new construction. By only utilizing dowels in the wheelpaths, significant savings can be achieved without sacrificing performance. Some agencies are going down to as few as 4 bars per wheelpath to optimize their designs.

Concrete Design Thickness, in. Dowel Bar Size, in.
< 8 in. and cracking is predicted cause of failure Dowels not required
< 8 in. and faulting is predicted cause of failure 1.00 in.
>= 8 in. and < 10 in. 1.25 in.
>= 10 in. 1.50 in.

Standardization of dowel design can help to reduce the cost of dowel baskets. For instance, by allowing dowels to be placed in the middle third of the slab, the same dowels and baskets could be used for 8 and 9 inch thick pavements. Dowel coating thickness is another thing that can be generalized to decrease costs since many states require different coating thicknesses. A trend towards generalization and standardization can lead to cheaper designs.

Little research has been performed on the spacings of the dowels to determine if 12 inches on-center is actually the optimum spacing when dowels are only placed in wheelpath. An added benefit of eliminating dowels in the wheel path is the opportunity to move the edge dowels away from the longitudinal joint. This can be beneficial as it can ease the construction of the joints. It can also help eliminate any interference with the placement and performance of adjacent tie bars in the longitudinal joints. There are tools available to help quantify the change in stresses associated with alternate dowel configurations. One such tool is DowelCAD which is a free ACPA software that can predict pavement behaviors based on the type of dowel bar, dowels spacings, and dowel configurations. EverFE is a similar software that can analyze slab stresses and joint deflections.

Dowel Technologies

In the past dowels have primarily been cylindrical steel bars of approximately 18 in. in length. However, now there are numerous emerging dowel technologies including various geometries, materials, coatings, and even baskets. While the most common dowels are round, there are also elliptical dowels and flat plate dowels. Some of these dowel geometries are more common in industrial pavements due to the very heavy loads where bearing stresses can become a concern. Most of the new dowel materials are being utilized to combat corrosion and improve joint performance. Some of the new materials include stainless steel, fiber-reinforced composite, zinc jacketed, fiberglass and stainless filled tubes, MMFX, and steel pipe. Typical steel dowels are primarily epoxy coated but other coatings include red oxide paint, tectyl, and RC 250 among others. Variations in baskets include the bottom, top, and leg wire gauges, whether the baskets are square or skewed, on-center spacing of dowels, and the number of dowels per frame. Even the style of the dowel basket leg can change.

Tie Bar Design

Dowel and tie bars

Tie bars design for pavements, like dowel bar design, is typically never optimized for given applications. Requirements must change based on support (subgrade and subbase) and climate. Current default designs are based on subgrade drag theory. There are a number of issues with this, the first being that it is based on drag. This simplistic method of modeling slab/base friction does not account for temperature drop from set and has a large safety factor. Additionally, free edge condition does not apply after two or more lanes. Finally this method does not account for displacement of the subbase.

Mechanistic empirical tie bar research has been undertaken to account for considerations such as subbase type, number of lanes/shoulders, concrete material properties, climate (location and placement time), thickness and steel yield. This optimization is important because less steel may be required to keep joints closed. This research was developed into the Mechanistic-Empirical Tie Bar Designer.

One mistake often made with tie bars is how closely they are placed to the transverse joint. Tie bars should never be placed within 6 in. of dowel bars. This will allow the dowels to transfer load as desired without locking up the joint. When possible, steel in longitudinal joints should be placed 12 in. away from dowels in transverse joints. This should be considered a higher priority than tie bar spacing.

State Tiebar Spec. Steel Grade Typical Diameter Typical Length Typical Spacing Coatings Allowed Bending of Coated Tiebars Allowed Degree of Bend Allowed Date Updated
Arizona AASHTO M31; ASTM A615 40, 60 5/8 in. 24 in. 30 in. None No N/A
Arkansas AASHTO M31 40 5/8 in. 30 in. 30 in. Epoxy Yes 90 deg
British Columbia -- -- 25 mm 3,000 mm 750 mm None Yes --
Calgary -- -- 15 mm 800 mm 750 mm Epoxy -- --
Delaware AASHTO M31 40, 60 5/8 in. 30 in. 30 in. None No N/A
Edmonton ASTM D3963 40 10 mm 400 mm 750 mm None No N/A
Florida ASTM A615 40 1/2-5/8 in. 25-30 in. -- None Yes --
Hawaii AASHTO M42; ASTM A615 40, 60 5/8 in. 30 in. 30 in. None Yes --
Idaho AASHTO M31; AASHTO M284 40, 60 5/8 in. 30 in. 30 in. Epoxy No N/A
Illinois AASHTO M31, M42, M53 40, 60 -- -- -- None No N/A
Indiana ASTM A615 40 -- 36 in. 36 in. Epoxy Yes 60 deg
Iowa ASTM A615, A616, A617 40, 60 1/2-5/8 in. 30-36 in. 30 in. Epoxy Yes 90 deg
Kansas AASHTO M284 40, 50, 60 5/8 in. 30 in. 24 in. Epoxy No N/A
Manitoba AASHTO M31, ASTM A615 40 20 mm 915 mm 1,000 mm Epoxy No N/A
Maryland -- -- 5/8 in. -- -- Epoxy Yes 90 deg
Michigan ASTM A615, A616, A617 40, 60 5/8 in. 30 in. Varies Epoxy Yes 90 deg
Missouri AASHTO M31, M42, M55 40, 60 5/8-3/4 in. 30-40 in. 30 in. Epoxy Yes 90 deg
Montana ASTM A615 60 1 in. 24 in. 30 in. Epoxy No N/A
Montreal ASTM D3963 -- 15-19 mm 760 mm 300 mm Epoxy Yes --
Nevada AASHTO M31 40, 60 1/2-5/8 in. 24-30 in. 30 in. Epoxy Yes 90 deg
North Carolina AASHTO M31 40, 60 5/8 in. 30 in. 30 in. None -- --
Ohio ASTM A615, A775 -- 5/8 in. 24 in. 20 in. Epoxy No N/A
Oklahoma AASHTO M31, M42 60 5/8 in. 30 in. 36 in. Epoxy No N/A
Pennsylvania 2016
Quebec ASTM A615 40 15 mm 750 mm Varies None No N/A
South Carolina AASHTO M31, ASTM A615 40, 60 -- -- -- None -- --
South Dakota AASHTO M31 40 5/8 in. 30 in. 48 in. Epoxy Yes 90 deg
Tennessee ASTM A615 40 5/8 in. 30 in. 18 in. None -- --
Utah AASHTO M31 60 5/8 in. 30 in. 30 in. -- No N/A
Virginia ASTM A615 40 5/8 in. 30 in. 30 in. None -- --
Washington AASHTO M31, M284 60 -- 32 in. 36 in. Epoxy Yes 90 deg
West Virginia AASHTO M31, M42 -- 5/8 in. 30 in. 30 in. Epoxy -- --
Wisconsin ASTM A615 40, 60 1/2 in. 24 in. 24 in. Epoxy Yes --
Wyoming ASTM A615 40, 60 1/2 in. 24 in. 24 in. Epoxy -- --

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From the National Concrete Pavement Technology Center: