Tie Bar Alignment – Concrete

This Appendix contains information related to:

• Dowel placement in concrete pavements to identify:
  • problems with alignment,
  • extent of misalignment,
  • methods for determining misalignment, and
  • effect of misalignment on performance;
• Factors contributing to dowel bar misalignment;
• Available approaches for estimating the effects of misalignment on pavement performance.

Developed by Magnetic Imaging Tools GmbH (Dresden, Germany), the MIT Scan‐2 is a nondestructive testing device that uses magnetic imaging technology to measure the position of metal dowel bars embedded in concrete. It is very quick and easy to use. Less than five minutes are needed to scan each joint and reposition the device for the next joint.

The hand held computer that controls the device allows the user to see the positions of the dowel bars in the joints immediately after scanning. It also has a very easy to use analysis program called MagnoProof that uses the specified dowel locations, along with allowable tolerances, to automatically identify misaligned dowels.

Dowel alignment can be measured efficiently and accurately using MIT Scan-2, a state-of-the-art nondestructive testing device for measuring and recording the position and alignment of dowel bars (FHWA 2005; Yu and Khazanovich 2005). The device is easy to use, the dowel alignment can be checked within a few hours of concrete placement, and the results can be printed using the onboard printer immediately after scanning. Up to 400 or more joints can be tested in an 8-hr workday using MIT Scan-2.

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.

The purpose of this study was to demonstrate and evaluate the use of magnetic tomography technology through the use of Magnetic Imaging Tools’ (MIT) MIT Scan-2. The main objective was to measure the alignment of dowel bars in a few jointed plain concrete pavements in Virginia and demonstrate the applicability of the technology. The MIT Scan-2 was obtained on loan from the Concrete Pavement Technology Program. This program is managed by the Federal Highway Administration (FHWA) through a partnership with state highway agencies, industry, and academia.

Dowel alignment measurements were successfully performed on both mechanically inserted dowel bars and bars on dowel baskets. Although the verification with field coring showed reasonably accurate measurements, signal interference from uncut dowel baskets, the presence of foreign metal in nearby locations, and bars deeper than 8 in can result in unreliable quantitative results. The repeatability of the measurements for bars on dowel baskets showed general agreement with the data reported from FHWA. The device was found to be user-friendly, and its field operation was simple. Thus, there is a potential benefit from using the MIT Scan-2 as a quality assurance tool for detecting dowel bar misalignment in jointed plain concrete pavement in Virginia.