Dowel Bars vs Tie Bars in Concrete Pavement

Dowel bars and tie bars both cross joints in rigid (concrete) pavement, but they do opposite jobs. A dowel bar is a smooth bar across a transverse joint that carries wheel loads from one slab to the next while letting the joint open and close. A tie bar is a deformed bar across a longitudinal joint that bonds to both slabs and holds the joint tight so lanes do not drift apart. Swap them and the pavement fails: a tie bar in a moving joint locks it, and a dowel in a lane joint lets it separate.

This post compares the two for contractors, owners and estimators working on concrete parking lots, drive aisles, loading aprons and roads. For the broader question of how dowels differ from ordinary rebar, see our post on dowel bars vs rebar.

Dowel bars and tie bars at a glance

What dowel bars do

Concrete pavement shrinks as it cures and moves with temperature, so it is cut into panels by transverse contraction joints. Those joints crack through below the saw cut, as intended. The problem is load: when a truck wheel crosses a joint, the slab it is leaving deflects and the slab ahead does not. Repeated thousands of times, that leads to pumping of the base, faulting (a step at the joint) and broken corners.

A dowel bar fixes that by letting each slab bear on the other through the bar. The US Federal Highway Administration's 1990 technical advisory on concrete pavement joints (since replaced by newer guidance) put it simply: the purpose of dowels is to transfer load across a joint without restricting the joint movement caused by thermal contraction and expansion. It recommended dowels on any route carrying more than a low volume of heavy trucks, with aggregate interlock alone reserved for low-truck streets.

To slide, a dowel must be:

• Smooth and round. Ribs would key into the concrete.
• Debonded on one end with a coating, bond breaker or sleeve. At expansion joints an end cap leaves room for the bar to travel.
• Aligned. A dowel that is tilted or skewed acts as a lock even if it is smooth. That is why dowels are set in wire baskets or inserted by the paver, and checked afterwards.
• Corrosion resistant. Joints collect water and de-icing salt. A rusted dowel seizes and the joint locks up. Epoxy-coated and stainless dowels are common answers; our comparison of epoxy-coated vs galvanized dowel bars covers the options.

Dowel diameter, length and spacing come from the pavement design. Diameter generally grows with slab thickness and traffic, so do not size dowels by habit. Our post on smooth dowels, sleeves and baskets covers the hardware.

What tie bars do

Longitudinal joints run along the pavement, usually at lane lines or between paving passes. They relieve warping stresses in wide pavements so the slab does not crack lengthwise on its own. Load across a longitudinal joint is carried mainly by aggregate interlock: the rough faces of the crack under the saw cut bear on each other.

Aggregate interlock only works while the crack stays tight. If the lanes creep apart, the faces lose contact, load transfer drops, and water and debris get in. Tie bars prevent that. They are deformed bars that bond to both slabs, so the joint can hinge slightly with warping but cannot open up. The same FHWA advisory described their job as preventing lane separation and faulting at longitudinal joints.

Tie bar design points that come up on drawings and specifications:

• Deformed bar, not smooth. The bar has to develop bond on each side, so length is tied to bar size and grade.
• Mid-depth placement, either inserted into the plastic concrete by the paver or placed on supports before the pour.
• Kept clear of transverse joints. Tie bars right at a transverse joint would restrain the corner where the joint needs to move, so specifications keep them back a set distance.
• Corrosion protection where salt exposure is expected, since corrosion reduces the bar's capacity.
• Bending is discouraged. Where a tie bar would have to be bent out of the way and straightened later (at a longitudinal construction joint, for example), two-piece threaded tie bars with couplers are a common alternative.

Spacing usually depends on slab thickness and how far the joint is from the nearest free edge. Those values are set by the pavement designer or the owner's standard, not by rule of thumb.

Mixing them up: what goes wrong

Most problems with pavement joint steel come from treating dowels and ties as the same item.

• Rebar across a transverse contraction joint. The most common mistake on private lots and residential flatwork. The deformed bar locks the joint, the panel cannot shrink freely, and a crack forms beside the joint instead.
• No ties at a longitudinal joint. Lanes or pours drift apart, the joint opens, and the edge starts to fault and ravel.
• Smooth dowels at a longitudinal joint where tie bars were specified. The joint is free to open, which defeats its purpose.
• Ties too close to a transverse joint, which restrains the slab corner and can cause cracking there.
• Uncoated bar in a salted joint where the specification called for coated or corrosion-resistant bar.
• Misaligned dowels after a basket was knocked over or not staked down. A crooked dowel restrains the joint almost as much as a tie bar.

If a drawing just says "dowels" or "joint bars" without a detail, ask the designer which joints move and which are tied before ordering.

Rigid pavement types and where each bar shows up

Concrete pavement comes in a few forms, and the joint steel follows the type:

• Jointed plain concrete pavement has no distributed reinforcement. Steel is only at the joints: dowels at transverse joints where load transfer is needed and tie bars at longitudinal joints. This is the common type for parking lots and many roads.
• Jointed reinforced concrete pavement uses longer panels with distributed reinforcement to hold intermediate cracks tight, plus dowels and ties at the joints.
• Continuously reinforced concrete pavement has no regular transverse contraction joints. Heavy longitudinal steel controls the cracking, and tie bars are still used at longitudinal joints.

Parking garages and suspended decks are a different structure entirely; they are designed as structural slabs, not pavement. Our post on rebar for parking lots and concrete pavement covers when a lot gets distributed rebar and how industrial floors differ.

Ordering dowel bars and tie bars

Because they are different products, list them separately on your order.

For tie bars, give:

1. Bar size and grade, for example 15M Grade 400W to CSA G30.18 (or whatever the specification calls for).
2. Length, and whether bars are straight, bent or two-piece with couplers.
3. Coating, if specified.
4. Quantity by joint, worked out from the joint length and the specified spacing.

For dowel bars, give:

1. Diameter and length from the pavement design.
2. Coating type and the standard it must meet.
3. Loose bars or assembled baskets, with basket height and spacing.
4. Debonding treatment, sleeves or end caps.

Estimating quantities is straightforward once the joint layout is known. Our post on how to calculate dowel bar quantity and weight works through it.

Getting tie bars and joint steel supplied

Tie bars are deformed rebar, so they come off the same cut-and-bend line as the rest of a bar list. We cut and tag tie bars and other rebar from 10M to 55M, along with any distributed slab steel, and deliver across the GTA and Ontario. If your job also calls for smooth or coated dowels, include the specification when you ask for pricing so we can tell you what we can supply. Send us your drawings or see our rebar products.