By: Malcolm Lim, Peter Foster and David Gress, Ph.D.
Many state departments of transportation set tolerances for the accuracy of dowel-bar placement in a concrete-pavement joint.
By limiting the allowable misalignment of dowel bars, these tolerances are designed to improve bar placement at the joints and thus increase the load transfer efficiency of the joint.
Highway officials often use nondestructive testing methods such as impulse radar and MIT Scan-2 to evaluate dowel alignment and its effect on load transfer between slabs in doweled concrete pavements. To do so, however, technicians work from the assumption that slabs crack and divide perpendicular to the pavement surface at saw-cut joints. All the current techniques base their determination of dowel misalignment in relation to the position of the saw cut, which is not necessarily where the crack crosses the dowel bar. Typically, when the pavement cracks, the cracking occurs where the cement paste/aggregate bond in the concrete is weak. Most cracks form around aggregate particles, not through them, which makes a vertically plumb crack highly unlikely.
Saw-cut depth depends on the technique that is used. Early-age saw cutting produces a relatively shallow, 1-in.-deep saw cut, which increases the area between the bottom of the saw cut and the base/sub-base. This distance makes it even less likely that a strictly vertical crack will form and the crack path will extend across the middle of the dowel bar. Conventional cutting methods typically require a deeper cut (1?3 D or 1?4 D), which reduces this overall effect. In either case, though, the cracking characteristics of the pavement and the depth of the saw cut will determine the point at which the crack passes across the dowel bar.
If the crack propagates toward the “longer” end of the dowel bar (Crack A), the misalignment may not increase the bearing stresses on the bar and may not affect the performance of the pavement. On the other hand, the effect is magnified when the crack propagates toward the “shorter” end of the dowel bar (Crack B). The net result of Crack B is a considerable increase in bearing stresses, which will likely result in localized premature failure of the pavement slab. Research has shown that extreme longitudinal translation will reduce the shear capacity of the pavement.
The initiator
Having encountered these issues in a variety of field situations, the authors undertook a research project to determine whether attaching a crack initiator to the middle of the dowel bar could control the crack path. The goal was to ensure that the crack would propagate through the middle of the dowel bar. The research used slabs that were cast, and then tensioned to simulate stress development. The slabs were 10 ft long by 2 ft wide by 9 in. thick. Each slab was cast with three initiators, attached at the midpoint and quarter points of the slab. Four 3?4-in.-diam. by 12-ft-long threaded rods also were cast into each slab, positioned so that when the forces were applied to the slab, a 6-in. zone of influence was established on either side of the initiator. This concentrated the forces around the initiator. A reaction frame was then affixed to the slab in order to apply a tension force to the threaded rods. The load was constantly increased until a level of stress was achieved in the slab sufficient to cause the slab to crack. Cracks formed around the initiators because the reduced surface area there created a zone of weakness.
The researchers experimented with initiators of various shapes, including V-shaped, square, circular and rectangular, as well as varying thicknesses. One goal of the research was to determine how small an initiator could be used to produce the desired effect. Approximately 300 slabs were cast, with each slab incorporating three initiators. The tests showed that a rectangular initiator produced the most efficient and consistent results. The research also investigated the relationship between the initiator’s thickness and its influence on the cracking patterns within the large aggregate distributed above it. Using several different thicknesses ranging from 1?4 to 11?2 in., the researchers found that the thickness of the initiator had no influence on whether the slab cracked or not, and so concluded that 1?4 in. would be sufficient thickness for the initiator. Examination of the cracks did not reveal any visible honeycombing in the vicinity of the initiator.
They further noted that in order to determine the propagation path of the crack, the crack initiators should be attached to the dowel basket assembly. This configuration served to facilitate the formation of the crack through the middle of the dowel bar. Research has shown that the sides of the dowel bars do not contribute to the load transfer during loading, so these serve as an ideal place to connect the initiator.
The position of the initiator also was examined to determine the effects of having the initiator at different positions in the slab. Crack initiators were attached at the bottom and in the middle of the slab. Results showed that the cracking was more predictable with the initiator in the middle of the slab. Initiators that were attached to the sub-base or base were less effective in dictating the crack path.
The study concluded that with crack initiators present, the overall cross-sectional area of the pavement is reduced to the area bounded by the bottom of the saw cut and the top of the initiator. The crack initiators provide a predetermined weakened plane in the pavement that directs the crack path and ensures that the crack propagates through the middle of the dowel bar. Minor offsets in saw cutting will not affect the overall crack propagation location, which makes pin-point locating of the saw cut less critical.
Stuck in the middle
This study has shown that having crack initiators in place within a slab creates a weakened plane that ensures the crack propagates through the desired path. The installed crack initiators’ wide and thin shape is designed to direct the crack path along one edge of the initiator. With a saw cut present at the top of the pavement slab, once the slab begins to set up and contract, the crack follows the weakest path. The crack initiator reduces the cross-sectional area of the pavement considerably and serves a similar function as saw cutting but is inherent within the pavement slab. The area between the bottom of the saw cut and the top of the initiator becomes the critical component in the pavement, controlling the propagation of the crack to ensure that it passes through the middle of the dowel bar. With a crack initiator attached to the middle of the dowel bar, it becomes relatively easy to find the center of the dowel bar nondestructively and thus determine where the crack passes the dowel bar. This allows for an accurate determination of the amount of misalignment actually present in the pavement.
Even if the saw cut is not accurately positioned, the research concluded that an offset of 10% of the overall pavement thickness would not affect the initiator’s influence on the cracking characteristics. The crack will propagate from the bottom of the saw cut, along the crack initiator and down to the sub-base. Despite minor offsets in saw cutting, the distance between the bottom of the saw cut and the top of the crack initiator remains the governing area of influence. This relieves the contractor of the need for pin-point accuracy in saw cutting and of having to make the saw cut directly over the center of the dowel bar.
In addition to the benefit of directing the cracking through the predetermined plane within the pavement slab, a crack initiator ensures that a shallow (1 in.) saw cut is sufficient. This eliminates the need for the more expensive and time-consuming 1?3 D or 1?4 D cuts required for conventional joints without initiators. As with early-age saw cuts, the 1-in. depth is sufficient to create the weakened plane in the concrete pavement.
Saw cutting is still required in order to maintain a visually pleasing joint and also minimize spalling of the pavement joint. When the pavement slab cracks, saw cutting provides the necessary control of the crack propagation to the surface of the pavement. Contractors should continue to observe current practices governing when to initiate saw-cutting operations. The cost/benefit analysis of crack initiators must include the increased costs of installing the initiators and the additional steps required to secure the baskets prior to concrete placement. This overall cost increase is minimal, however, compared with the potential benefits of utilizing initiators, as discussed above. Improved performance can provide significant cost savings by reducing maintenance and repair needs over the course of the pavement’s designed service life. The initiators also will eliminate the long-standing debate over how much misalignment is allowable, since the initiator influences the overall propagation path of the crack through the middle of the dowel bar.
The right attachment
Past studies have shown there is potential for significant problems with the nondestructive methods currently used to evaluate dowel misalignment in pavement joints. This work is very expensive, and the nondestructive evaluation tools lack technical sophistication, which can result in inconclusive predictions of the expected performance. Even at their best, these evaluations are based on the position of the saw cut and not where the crack passes over the dowel bar.
Our research has shown that the use of crack initiators has advantages over current practices. The 1?4-in.-thick rectangular initiator with rounded sides proved to be the most optimal size and shape to influence the pavement’s cracking characteristics while still allowing proper concrete consolidation. Attaching crack initiators to the dowel bars ensures that the pavement joint crack propagates through the middle of the dowel bar. It minimizes potential problems such as longitudinal translation due to markings or dowel baskets misplaced within the pavement slab. The research has shown that the crack propagation can be controlled even if the saw cut is offset, and cost savings are achieved through reduced depth of saw cutting with the use of crack initiators. A shallow 1-in. saw cut works well for both early and conventional saw cutting.
The benefit gained by adding an initiator to the dowel baskets far outweighs the small increase in overall cost. When compared with the overall economic impact of reduced pavement performance, the cost of adding initiators at the construction stage becomes insignificant.
About The Author: Lim is a consultant working out of Buffalo Grove, Ill. Foster is a research consultant working out of Chicago, Ill. Gress is a professor of civil engineering at the University of New Hampshire, Durham, N.H.
