9.2.1 Visual investigation

The primary means of visual evaluation was by up-close observations using aerial lift access. Concrete distress conditions were documented by way of field notes and crack mapping. Predominantly, the two straddle bent beams exhibited map cracking on all surfaces, with visible white staining exuding from cracks, as shown in Figs 9.3 and 9.4. Crack widths ranged from hairline to 3 mm on the surface of the bent beams. Severe concrete deterioration, in the form of wide cracking and concrete spalling, was observed at the ends of the bent beams (Fig. 9.5). At these locations, concrete crack widths up to 12 mm were recorded. Less notable map cracking was also observed near the top of the reinforced concrete columns supporting the substructure bent beams.

9.2.2 Non-destructive evaluation

Non-destructive evaluation techniques were deployed to study the concrete deterioration in the two straddle bents. The substructure elements were scanned using ground (surface) penetrating radar (GPR) to identify the presence of reinforcing steel and to verify as-built conditions. Where possible, the bent beams were scanned in a single horizontal pass for side-by-side comparison with the as-built construction drawings. As shown for a particular bent beam end in Fig. 9.6, the horizontal GPR scan of the T-beam revealed conformance of reinforcing steel stirrups and longitudinal column steel placement with the as-built drawings. This finding was typical for the placement of reinforcing steel at both straddle bent beam end locations. The reinforcing steel placement was a key factor in determining the type and extents of the CFRP repair solution discussed later in this chapter.

Acoustic sounding techniques were deployed to obtain measurements of crack depth. Impact-echo testing was conducted at cracked concrete elements at selected locations. Impact-echo testing showed typical crack depths of 50–150 mm at locations of map cracking. Most notably, these crack depths exceeded the concrete clear cover at the reinforcing steel. Further testing of the bent beam ends showed full depth cracking, through a bent beam width of approximately 1.1 m. Crack depth measurements were further corroborated with crack length measurements along cores excised through cracks.

9.2.3 Concrete materials testing

Although there were some visual cues to possible mechanisms causing premature concrete deterioration, laboratory testing of excised concrete specimens was utilized to identify the specific deterioration mechanism. Exploratory coring was conducted to evaluate the concrete material properties and deteriorated condition of the concrete. Core samples were excised to obtain representative samples of deteriorated (cracked) and non-deteriorated concrete areas for subsequent laboratory testing and comparative study.

Concrete petrography was conducted on excised specimens at a testing laboratory in accordance with ASTM C 856 (standard practice for petrographic examination of hardened concrete) to study the cause of the concrete deterioration. Concrete samples were cut and examined under a stereomicroscope at 45 times magnification, and then as thin sections under polarized light up to 400 times magnification. The petrographic analysis indicated that the general quality of the original concrete was good, based largely on paste properties, water–cement ratio, and consolidation. The excised concrete cores, however, showed signs of randomly-oriented microcracks under microscopic evaluation, with evidence of ASR gel formation in voids and around quartzite aggregate particles. The ASR gel reaction product in the presence of moisture was believed to be the primary contributor to the expansion within the concrete and subsequent premature concrete deterioration. If left unchecked, further expansion of the ASR gel would lead to continued and more severe concrete deterioration of the bridge substructure.

Concrete deterioration due to ASR gel expansion can be influenced by concrete confinement, degree of restraint, and placement of reinforcing steel to control concrete cracking. In focusing on the substructure beam ends experiencing the most severe cracking, a review of the as-built construction drawings revealed the placement of limited reinforcing steel stirrups at the substructure beam ends. While additional reinforcing steel was not required for structural design purposes, the lack of confining steel stirrups provided ripe opportunity for ASR gel expansion in the presence of moisture. In addition, column longitudinal reinforcing steel extending up into the substructure bent beams for continuity was curtailed just below where the most severe cracking developed at the beam ends (Fig. 9.6).

9.4.1 Design basis

In the repair methodology, CFRP composites were designed in a manner to provide external confinement of the bent beams. Research and experimentation in the confinement of concrete elements using CFRP has produced varied results. CFRP composites are more effective in confining circular concrete elements (i.e. columns). Confinement of non-circular concrete sections using CFRP can be achieved to a lesser extent using an equivalent diameter and shape factors (ACI, 2008). An important distinction to make in this case study is that CFRP was provided to confine further concrete expansion rather than to provide additional concrete compressive strength or joint confinement, as is the typical application for CFRP confinement in axially loaded members. In addition, some confinement is already provided by way of the reinforcing stirrups in the bent beams at locations of map cracking. Ideally, confinement and arrestment of future cracking would be achieved by a combination of existing reinforcing stirrups and external CFRP application. One drawback of this CFRP repair methodology is that the concrete element wrapped in CFRP must first expand in order to engage the CFRP and initiate restraint through confinement. This is somewhat counterintuitive, in that the CFRP composites are being provided to prevent further expansion due to ASR. However, applications of CFRP for confinement can be justified considering that some residual expansion is to be expected given the latent moisture conditions contributing to ASR within the bent beams.

CFRP composites were also designed to control expansion at the beam ends where there were minimal steel reinforcing stirrups. Calculation of the amount of CFRP composite strengthening was achieved by determining the design strength of the reinforcing stirrups at the typical spacing within the bent beam, had they been provided at the bent ends. Computation of the CFRP material requirements was similar to that for shear strengthening.

9.4.2 CFRP design for substructure beam

CFRP composites were designed for placement in locations to control concrete expansion along the straddle bent beams. A single ply of CFRP was designed for application along the beam where there was extensive map cracking. Given the angular geometry of the inverted T-beam, a single wrap was not possible, due to re-entrant corners. CFRP wrapping was therefore designed in two pieces. A single ply CFRP was designed to be applied as a three-sided U-wrap to the top leg of the inverted T-beam and a three-sided U-wrap on the flange of the inverted T-beam (Fig. 9.7 – Section ‘B’). Since the confinement procedure was designed in two pieces, CFRP end anchorage was provided to create pseudo-continuity between top and bottom of the beam in CFRP applications. End anchorage was provided by saw cutting a slot in the beam and embedding saturated plies in resin in the slot (Fig. 9.7). A single U-wrap application to the bottom flange of the beam was provided where access to the top leg of the inverted T-beam was not possible due to interference with the precast bridge beams and bearing seats (Fig. 9.8).

9.4.3 CFRP design for substructure beam ends

CFRP composites were designed for placement in locations to control concrete expansion at the straddle bent beam ends. In the case of the beam ends, a horizontal CFRP application was designed to control lateral expansion of the outside face of the bent beam end. Additionally, calculations showed that three vertical plies of CFRP would be required to supplement the existing minimal reinforcing steel stirrups, over a distance of approximately 2 m. These three plies were designed to be applied as a three-sided U-wrap to the top leg of the inverted T-beam. The CFRP installation drawings provided work phasing such that the horizontal CFRP application at the outside face of the beam was applied first (to the top leg and then bottom flange of the inverted T-beam) followed by the vertical U-wrap application (see Fig. 9.7 – Section ‘A’).

9.4.4 CFRP design specifications

Drawings and technical specifications were developed prior to the application of CFRP composites. Documentation for concrete surface preparation was provided to include the removal of existing paint finishes on the concrete surfaces as well as surface roughness criteria using contained abrasive blasting. Although most CFRP confinement designs are generally not bond critical, surface roughness was specified to enhance bond to control concrete surface expansion. Procedures for crack repair were also provided in this construction documentation. Cracks with widths less than 6 mm were filled by injection of epoxy. Cracks with widths exceeding 6 mm, at the bent beam ends for example, required additional concrete removal and patching with concrete repair materials (Figs 9.9 and 9.10).

Several approved CFRP systems were specified for possible use on the project. Each system provided a layered application approach consisting of a primer, epoxy putty for surface irregularities, saturant, CFRP fabric, and protective top coat. The CFRP composite could be applied using either a wet or dry layup, depending on the contractor’s repair approach. A protective UV stable coating was specified for application over the finished CFRP composite. The coating was color-matched to other coated concrete surfaces in order to minimize the visibility of the CFRP application.

9.6.1 CFRP bond testing

In this case study, CFRP test patches were installed for each application day and batch of resin used. Patches were tested for adhesion after curing to ensure conformance with ASTM D7522/D7522M–09 (standard test method for pull-off strength for FRP bonded to concrete substrate). A minimum pull-off tensile strength of 1.4 MPa was specified for bond.

9.6.2 Acoustic sounding

Acoustic sounding techniques were utilized to rapidly detect CFRP bond defects that were not visible during initial inspection. Delaminations detected by acoustic sounds, typically light tapping, were noted to have a dull, hollow sound upon testing. Although the testing can be performed quickly, there can be a high degree of inaccuracy, often leaving voids under the CFRP systems undetected. Those defects that are detected can be marked and subsequently repaired after acoustic sounding.

9.6.3 Infrared thermography

Infrared (IR) thermography is the science of acquisition and analysis of thermal information from non-contact thermal imaging devices. IR thermography detects emitted radiation in the infrared range of the electromagnetic spectrum. This corresponds to wavelengths longer than the visible light portion of the spectrum. Thermal imaging can therefore be utilized to detect defects in the CFRP installation that may not be visible. Voids or delaminations in the CFRP installation are shown with a unique thermal signature due to entrapped air between the CFRP composite and the concrete substrate. As shown in Fig. 9.16, CFRP delaminations exhibit a dark thermal signature in comparison to well bonded CFRP applications. As such, defects were marked and subsequently repaired after thermal inspection.

9.6.4 Impulse response testing

Impulse response is a non-destructive testing technique used to evaluate the integrity of concrete elements. In this test, a low-strain impact is applied to excite the structure and the response of the structure is measured. The concrete element is struck by using a small impactor, which has a built-in load cell that measures the impulse imparted. The response (vibration) of the concrete element is monitored by a velocity transducer (geophone) placed adjacent to the impact location. The geophone is connected to a data acquisition and processing system, which calculates the dynamic mobility as a function of the frequency of the excitation. The dynamic mobility was analyzed as a secondary means to characterize the condition of the CFRP composite and concrete substrate conditions, and the probability of delaminations in the CFRP application that may not be visible at the surface. Defects identified by impulse response testing were marked and subsequently repaired after inspection (Fig. 9.17).

9.6.5 Laboratory testing of CFRP materials

Laboratory testing is typically conducted to verify the mechanical properties of the CFRP composite. In this case study, witness panels were constructed by the repair contractor for each application day and batch of resin used in order to ensure quality control. Three-ply CFRP witness panels were created to match the installation condition on the straddle bents (Fig. 9.18). A portion of these fully cured witness panels were sent to a qualified testing laboratory with experienced personnel to validate the CFRP material properties with those used in the design. Witness panel testing was conducted to determine the CFRP composite thickness, tensile strength, modulus of elasticity, and percent elongation. The test results were then compared to the published mechanical properties from the CFRP material manufacturer.