Nondestructive Testing of the Lawrence Street Bridge

Abstract

The University of Colorado at Denver (UCD) has been involved in cooperative research efforts with the City and County of Denver (CCD) since 1997. The CCD public works department owns, inspects, and maintains 531 bridges in its inventory of which 264 are considered major structures spanning over 6.1 m (20 ft) in length. In this paper, a comprehensive nondestructive analysis of a major structure, the Lawrence Street Bridge, will be presented. The Lawrence Street Bridge is a two-span (each span 23.2 m (76 ft) in length), prestressed concrete girder bridge that spans over Cherry Creek and an adjacent bike path, in downtown Denver. The total bridge width is 19.1 m (62.5) ft. The current structure was built in 1958 and replaced an existing bridge at the site. The original abutments remain, however, and consist of masonry abutment walls founded on concrete footings with timber piles. The pier was comprised of cast-in-place concrete cap and columns. There were five 710-mm (28-in) diameter columns spaced at approximately 5.8 m (19 ft) on center. The pier cap was 27.6 m (90.5 ft) long, 1.07 m (3.5 ft) in height, and varied from 0.76 m (2.5 ft) at the bottom, to 1.07 m (3.5 ft) wide at the top The primary focus of the condition assessment was the existing pier cap and columns. Each span of the two-span superstructure was simply supported with an expansion joint over the pier. Over the years, water had leaked through this expansion joint and directly onto the supporting pier and foundation system. Water penetration and cyclical climatic changes of freezing and thawing had resulted in the deterioration of the supporting substructure system below. The cap suffered from spalling, cracking and exposed steel reinforcement that was corroding. The supporting columns exhibited cracking, from the top downward. In the spring of 2003, a team of UCD researchers began an in-depth nondestructive inspection analysis of the bridge substructure. The objective of the pier analysis was twofold: the first was to provide an assessment of the existing structural condition in order to determine rehabilitative design options. The second was to provide a case study comparing the data from different types of nondestructive and destructive testing. The testing methods performed were as follows: (1) Nondestructive Testing is (a) Visual Inspection, (b) Hammer Sounding, (c) Surface Hardness, (d) Ultrasonic Pulse Velocity, (UPV) (e) Acoustic Tomography, and (f) Ground Penetrating Radar (GPR) Scan; (2) Destructive Testing is (a) Coring and (b) Partial Concrete Removal for Rehabilitation. This particular project was unique in that it provided an opportunity to apply a succession of nondestructive testing methods that ultimately allowed the researchers to identify areas to be further examined using more intensive nondestructive testing as well as destructive testing. Because portions of the concrete cap and columns were removed during rehabilitation, the data from the nondestructive analysis was compared with actual visual inspection of what was found beneath the surface. This paper describes how the nondestructive testing methods were implemented at the bridge site, provides data analysis of the nondestructive test results, and lastly, compares the structural assessments made from nondestructive testing data with observations made during destructive testing and partial concrete removal of the substructure.