What are Dowel Bars in Concrete Pavement Joints?

Definition

A dowel bar is a plain steel bar that crosses a joint in a concrete pavement and carries load from one panel to the next. The FAA uses the word panel for a slab. AC 150/5320-6G says dowels provide load transfer across the joint and prevent relative vertical displacement of adjacent panel ends (3.16.10.2).

The bar is plain, not deformed. A plain bar can slip in the concrete, so the joint can open and close. Item P-501 asks for plain steel bars to ASTM A615, free of burrs that restrict slip. Each bar has an epoxy coating to ASTM A1078 Type 1, thicker than 10 mils, and a bond-breaker from the manufacturer. Dowel sleeves and inserts are not permitted (501-2.7). The engineer may ask for chromium bars (ASTM A1035) or stainless steel bars (ASTM A955), after coordination with the FAA.

How a dowel bar works

When a wheel loads one panel, the dowel passes part of the load to the next panel. FHWA says dowel bars give positive load transfer across a joint. That greatly reduces the deflections and stresses at the joint, and so the potential for pumping, faulting and slab cracking (FHWA-HIF-07-021, p. 1).

Research shows that the main load transfer mechanism of a dowel is shear. Bending of the bar matters little, especially where the joint opens less than 1/4 in (Snyder, p. 2). Snyder reports that the critical dowel in a highway joint generally carries less than 3,000 lb. So shear and bending do not control the diameter of a steel bar. The diameter matters for a different reason. A stiffer dowel lowers the bearing stress between the bar and the concrete, and that slows the growth of faulting (Snyder, p. 2 to 3).

The bar must also let the joint move. Temperature and drying shrinkage open and close the joints. For 15 ft (4.6 m) panels, FHWA gives a typical joint movement of 2 to 3 mm (0.08 to 0.12 in). This holds even in colder areas (FHWA-HIF-07-021, p. 3). The bond-breaker on the bar lets it slide in the concrete during this movement. Snyder advises a bond-breaker on at least half the length of each bar. A semi-solid coating must be thin, because a thick layer can look like a loss of load transfer (Snyder, p. 11).

Steel dowels are old technology. The first reported US installation was in the winter of 1917 to 1918, and by 1930 nearly half of the states required them (Snyder, p. 1).

Dowel bars and tie bars

A tie bar is a different part. It is a deformed bar that holds a longitudinal joint closed. AC 150/5320-6G says tie bars do not act as load transfer devices. They keep the panel faces in close contact, so aggregate interlock can transfer the load (3.16.10.1). The two parts differ as follows:

• A dowel is plain steel, coated with epoxy and a bond-breaker. A tie bar is deformed steel to ASTM A615 (501-2.7).
• A dowel lies parallel to the pavement surface and slips in the concrete (FHWA-HIF-07-021, p. 2). A tie bar is never painted, greased or put in a sleeve, and it is held in the middle of the slab depth (501-4.10).
• A dowel passes load across a contraction or construction joint. A tie bar holds a longitudinal joint closed. AC 150/5320-6G ties longitudinal contraction joints within 20 ft (6 m) of a free edge when the panel is 9 in (225 mm) or thinner. A panel of 6 in (150 mm) or less gets No. 4 bars, 20 in (510 mm) long, at 36 in (900 mm) centers. A thicker panel gets No. 5 bars, 30 in (762 mm) long, at 30 in centers (3.16.10.1).

Where airports use dowels

• The last three transverse contraction joints from a free edge (3.16.10.2.1). The AC says a stabilized base gives panel support, so more dowels bring little benefit.
• All construction joints, longitudinal or transverse. A thickened or reinforced edge can replace the dowels (3.16.10.2.2). For aircraft of 30,000 lb (13,610 kg) or more, the AC uses a doweled Type E joint. A butt joint (Type F) has no dowels. It may serve pavements for aircraft under 30,000 lb that are on a stabilized base or are a concrete overlay of a flexible pavement (3.16.9.5).
• Longitudinal contraction joints 20 ft (6 m) or less from a free edge, in panels thicker than 9 in (Table 3-5, joint type C).

Where a longitudinal and a transverse joint meet, the AC removes a dowel or tie bar from the longitudinal joint if needed. The remaining bar stays 12 in (300 mm) from the end of the transverse dowels (Figure 3-5, note 1).

Dowels in isolation and expansion joints

Isolation joints do not get dowels. P-501 says an isolation joint is mainly used to separate structures with different foundations and pavements with different joint patterns. It does not provide for expansion by the material compressing. It lets the joint slip, so there should rarely be an occasion to dowel it. A thickened edge is the preferred load transfer method there (501-4.10).

Highway practice differs. An Iowa State guide advises expansion caps on dowel ends in new construction only at expansion joints. The cap goes on the unwelded end of each dowel in the basket. It says caps are not necessary at contraction and construction joints (Snyder, p. 11). The FHWA retrofit checklist also asks for plastic end caps on each end of a retrofit dowel, to account for pavement expansion.

Size and spacing

AC 150/5320-6G Table 3-6 gives the size, length and spacing of steel dowels by panel thickness.

The AC says to size dowels for the shear and bending stresses from the load. It also says to use a length and spacing that prevent failure of the concrete from the bearing stress (3.16.10.2.3). A drafting note in P-501 adds a limit near a joint line. A dowel or tie bar is not placed closer than 0.6 times its length to the planned joint line. If the last regular bar is closer, it moves away to that distance, but not closer than 6 in (150 mm) to its neighbor (501-4.10).

Highway practice is similar but not the same. The rule of thumb since the 1960s sets the dowel diameter, in eighths of an inch, equal to the slab thickness in inches. The bars are 18 in long at 12 in centers. ACI Committee 325 recommended the 18 in length and 12 in spacing in 1956. Agencies now differ. In Snyder’s 2009 survey, the diameters that states use for a 10 in slab lie between 1 1/4 in and 1 1/2 in (Snyder, p. 1 to 3).

Placement

Alignment and elevation decide how a doweled joint performs (3.16.10.2.4). The AC holds the dowels in a wire cage or basket that is firmly anchored to the base. It also allows a paving machine with an automated dowel bar inserter.

Item P-501 sets these limits (501-4.10):

• Horizontal spacing within 3/4 in (19 mm).
• Vertical position within 1/2 in (12 mm).
• Horizontal and vertical alignment not more than 1/4 in per foot (6 mm per 0.3 m).

The moving part of each dowel gets a thin film of lubricating oil or light grease before the paver places the concrete. The contractor may insert dowels into plastic concrete. The RPR approves that method from a control strip, where cores or non-destructive rebar location devices show the dowels are within tolerance. P-501 lists the MIT scanner, the pachometer and the R-meter as examples. In hardened concrete, dowels go into drilled holes and epoxy resin bonds them. The holes are 1/8 in (3 mm) larger than the dowel. The concrete must be 7 days old or reach the strength in the specification before drilling starts.

Misalignment

FHWA names four types of misalignment (FHWA-HIF-07-021, p. 2):

• Horizontal skew: the bar turns in the horizontal plane, so it is not parallel to the longitudinal joint.
• Vertical tilt: the bar tilts in the vertical plane, so it is not parallel to the pavement surface.
• Longitudinal translation: the bar sits off center, so one side has less embedment.
• Vertical translation: the bar sits above or below mid-depth, so there is less cover on one face.

In general, skew and tilt affect the free movement of the joint. Translation affects how well each bar transfers load. The critical level of skew or tilt is the level at which the joint may lock or the concrete around the bar may spall. The usual US limit for skew or tilt is 6 to 10 mm (0.25 to 0.375 in) on an 18 in bar. The usual limit for translation is 25 mm (1 in), because the margin for translation is much greater (FHWA-HIF-07-021, p. 2). P-501 limits alignment to 1/4 in per foot, which is 3/8 in on an 18 in bar and so falls inside this range.

The effect of misalignment is real but smaller than many specifications imply. FHWA says many in-service pavements have a few badly misaligned bars and no visible distress. In the lab, spalling and cracking appeared only at displacements several times larger than the joint movement in the field. One locked joint does not harm a pavement, but many consecutive locked joints are not desirable, because restraint stress builds up (FHWA-HIF-07-021, p. 3). Severe misalignment can concentrate stress, which leads to socketing (funneling) around the bar and a loss of load transfer.

A 2020 FHWA study used MIT scanning on Long-Term Pavement Performance test sections. It found that a majority of dowels were well aligned. It found that misalignment had its biggest effect on load transfer at the transverse joints, and that the effect on faulting depends on other factors. It found no definitive relationship between misalignment and cracking or spalling, except for some effect in three states. FHWA also says this does not mean that severely misaligned dowels have no effect, especially on localized distress (FHWA-HRT-20-070). These are highway findings. The FAA documents set their own tolerances for airfields.

Corrosion and looseness

Carbon steel dowels corrode readily, especially where deicing chemicals reach the joint. Snyder describes two harms. When corrosion starts at the joint and moves into the slabs, the gap between dowel and concrete grows. Deflections and stresses increase, load transfer drops, and pumping and faulting follow. The corrosion products also expand around the bar, which can cause severe joint spalling or mid-panel cracks (Snyder, p. 6).

Epoxy coating is the most widely used protection. It acts as a barrier. Its long-term performance has varied. Where deicing salts are used, a small defect from manufacture, transport or handling can start corrosion (Snyder, p. 6 to 7). Stainless steel dowels are an alternative. P-501 allows the engineer to ask for chromium or stainless bars after coordination with the FAA (501-2.7).

Load transfer efficiency

Load transfer efficiency (LTE) shows how much load a joint passes to the next panel. The AC defines deflection LTE as the deflection of the unloaded panel divided by the deflection of the loaded panel, times 100 percent (Equation C-3). FWD and HWD tests at joints measure it (C.8.3). The amount of load transfer depends on gear configuration, tire contact area, pavement temperature, dowels and a stabilized base (C.15.3).

FAA design assumes that load transfer cuts the free edge stress in a panel by 25 percent. The relation between deflection LTE and stress LTE is not linear, so the AC asks for extra analysis (C.15.5). The AC also says that joint deterioration and falling LTE raise the deflections at panel corners, which can create voids under the panel (C.5.3).

Highway agencies often act on a limit. Snyder reports typical action thresholds of 50 to 70 percent LTE for load transfer restoration, undersealing or joint replacement. He also warns that LTE alone does not tell the whole story. A well-supported pavement can show 40 percent LTE with very small deflections. A weak one can show 70 percent with large deflections and a large difference across the joint. Test for LTE when the slab is at 70 degrees F (21 degrees C) or cooler. A warm slab closes the joint and gives high LTE values (Snyder, p. 20).

Dowels in an inspection

Dowels are inside the slab, so an inspector sees symptoms. AC 150/5380-6C lists these (3.2.2):

• Joint spalling, which also results when dowels become misaligned.
• Corner breaks, where loss of support can come from pumping or from loss of load transfer at the joint.
• Settlement or faulting, a difference in elevation at a joint or crack.
• Pumping, the ejection of water and underlying material through joints or cracks. Pumping near joints indicates poor joint load transfer, a poor joint seal, or ground water.

The PAVER manual rates faulting by the difference in elevation across the joint. On runways and taxiways, a difference of less than 6 mm (1/4 in) is low severity. A difference of 6 to 13 mm (1/4 to 1/2 in) is medium, and more than 13 mm (1/2 in) is high. On aprons, low is 3 to 13 mm (1/8 to 1/2 in) and medium is 13 to 25 mm (1/2 to 1 in). More than 25 mm (1 in) is high. A straightedge or level measures the difference.

To see the dowels themselves, an inspector needs a tool that looks into the slab:

• Cores show the bar, its coating and the gap around it, but they damage the slab.
• Non-destructive rebar locators such as the MIT scanner, the pachometer and the R-meter. P-501 uses them to check the installation (501-4.10). FHWA says MIT scanning measures dowel alignment without damage. In 2007 it reported that a crew of two could test 400 or more joints in an 8 hour day with MIT Scan-2 (FHWA-HIF-07-021, p. 1).
• Ground penetrating radar (GPR) can locate rebar in concrete, and high frequencies give the best resolution at shallow depth (AC 150/5320-6G, E.1).

Dowel bar retrofit

AC 150/5320-6G says a dowel bar retrofit or reconstruction may be required if extensive areas pump or rock (4.10.2). In a retrofit, crews saw parallel slots across the joint and set coated dowels in the slots. They then fill the slots with patching material and re-form the joint. The FHWA checklist gives these points for inspection:

• The slots are parallel to each other and to the centerline, typically within 1/4 in per 12 in of bar length.
• There are typically three or four slots per wheel path.
• Plastic end caps sit on each end of the dowel, and a bond release compound keeps the patch from bonding to the bar.
• Chairs hold the dowel in the slot, typically with 1/2 in clearance under it, so the patch material surrounds the bar.

A full-depth repair of a corner break is a different job. AC 150/5380-6C gives two options (Appendix A). Crews can save exposed dowels by sawing just beyond their ends. They can also cut the joint and install new dowels of the same type and size in drilled and epoxied holes.