What is a Load Transfer Device in Concrete Pavement?

A load transfer device is a joint feature that carries wheel load from one concrete pavement panel to the next.

Concrete airfield pavement is built in panels with joints between them. When an aircraft wheel crosses a joint, the loaded panel deflects. A load transfer device makes the next panel share that load. FAA AC 150/5320-6G says pavement life extends when more load goes to the unloaded panel, because the bending stress in the loaded panel falls.

Types

• Dowel bars. Dowels carry load across the joint and prevent relative vertical movement of the panel ends. The FAA specifies plain steel bars with an epoxy coating and a bond-breaker coating, so the bar can slide in the concrete.
• Aggregate interlock. The irregular faces of a joint or crack lock against each other. The FAA keeps joints closed to preserve this, for example with tie bars.
• Thickened edge. A Type A joint has a thicker panel edge. It carries the load that dowels or aggregate interlock would otherwise carry.

Tie bars are not load transfer devices. They hold the joint closed so that aggregate interlock keeps working.

FAA rules for dowels

• Dowels are required in the last three transverse contraction joints from a free edge.
• Dowels are required in all construction joints, unless the edge is thickened or reinforced.
• Panels thicker than 9 inches (225 mm) need doweled intermediate longitudinal contraction joints next to a free edge.
• Table 3-6 sets size and spacing by panel thickness. A 5 to 7 inch (125 to 178 mm) panel uses 3/4 inch (20 mm) dowels at 12 inch (305 mm) spacing. A 7.5 to 12 inch (191 to 305 mm) panel uses 1 inch (25 mm) dowels at the same spacing.
• Item P-501 limits dowel misalignment to 1/4 inch per foot (6 mm per 0.3 m).

Load transfer efficiency

Load transfer efficiency (LTE) is the deflection of the unloaded panel divided by the deflection of the loaded panel, as a percentage. An FWD or HWD test at the joints measures it. The result depends on the gear configuration, tire contact area, pavement temperature, dowels and the type of base. A reflective crack in an asphalt overlay can give a wrong value.

The FAA design method assumes that load transfer cuts the free edge stress in a panel by 25 percent. The deflection LTE does not convert directly to this stress value.

Why it matters for inspection

Falling LTE raises the deflection at panel corners. This can create voids under the panel. Water then collects at the joints and speeds up damage to the concrete.

On the surface, the related distress is pumping, corner breaks, joint spalling and faulting. Misaligned dowels can cause joint spalling. Faulting is a difference of elevation at a joint. The PAVER manual rates its severity by the height of the step. A visual or drone survey records these defects, but a deflection test gives the LTE value.