Research Papers – MIT-DOWEL-SCAN for Tie Bar / Dowel Bar Alignment & Positioning

Improper placement of dowel bars can lead to reduced load transfer between slabs, which results in pavement distresses such as faulting and spalling at joints.

To determine the accuracy of dowel placement by Dowel Bar Inserter (DBI), the Nebraska Department of Roads has used an MIT Scan-2 device to scan the joints in projects where a DBI was used. This device uses a nondestructive magnetic imaging technique to capture the position of dowel bars inside the pavement. The aim of the this project is to analyze the MIT Scan-2 data of the joints constructed using a DBI, and to compare them with the corresponding field performance data. This will allow us to judge if DBI is a reliable alternative for dowel placement, and to improve Nebraska’s current specifications for dowel placement tolerances.

To meet the objectives, the MIT Scan-2 data of scanned joints were initially compared with dowel placement specifications suggested by national agencies. It was observed that the longitudinal translation and rotation of dowels in a portion of scanned joints fell outside recommended tolerances. The longitudinal and vertical translation of the dowels were respectively higher and lower than the average values reported by a similar study (Khazanovich et al. 2009). MIT Scan-2 data and field performance data were then compared to find any linkage between pavement distresses and dowel misalignment levels, enabling us to potentially improve Nebraska’s current specifications as well as conclude if any of the distresses were caused by low placement accuracy of the DBI. No linkage was found between pavement performance and dowel misalignment levels for over 220 joints that were investigated in this study. No transverse cracking was observed during field investigation, and the spalling at joints was likely to be the result of joint saw-cut operations. However, measured distress from joints with missing or completely shifted dowels show that high severity dowel misalignment has an adverse effect on joint performance.

An adequate longitudinal joint tie bar system is essential in the overall performance of concrete pavement. If designed and installed properly, tie bars prevent the joints from opening and consequently improve load transfer efficiency between slabs and between slabs and shoulders, resulting in increased load carrying capacity.

This study evaluated the longitudinal joint tie bar system currently used by CDOT, examining the criteria for proper use of tie bars and determining the maximum number of lanes that can be tied together without negatively impacting the concrete pavement structure. An improved mechanistic-empirical tie bar design method was developed.

Field studies were conducted to investigate longitudinal joint performance and further evaluate the impact of factors related to design and construction practices. The experimental plan for this round of testing included the evaluation of tie bar alignment, measurement of joint load transfer, and measurement of relative slab movement at the joints. In addition, CDOT’s current specifications and practices related to longitudinal joint construction and tie bar design and placement were compared with those of other state agencies.

Field testing results revealed that the measured joint openings at some tied longitudinal joints were in the typical range of non-tied slabs, implying that some tied joints performed as poorly as non-tied slabs. The results indicate the possibility of tie bar failure due to loss of concrete-steel bonding or yielding of tie bar steel. Another key finding was the possible impact of tie bar misalignment or misplacement on poor longitudinal joint performance. Testing indicated that the measured joint openings were wider when the tie bars did not connect to the other side of the joint, or when the embedment lengths were inadequate. On the other hand, tie bars with adequate embedment length on both sides of the joint, even when misaligned, appear to hold the joint tight.