Improving the Durability of Coastal Bridges with CFRP Prestressed Cored Slabs

Griffith Shapack, Zachary Van Brunt, Rudolf Seracino, Gregory Lucier, Sami Rizkalla, Mohammad Pour-Ghaz

Synopsis: Steel prestressed cored slab superstructures are a common structural system for multi-span bridges in coastal North Carolina. However, due to the aggressive marine environment several such bridges are in need of major repairs or replacement after being in service for little more than 40 years. To address this issue two research projects were undertaken in parallel. The first project involved a critical assessment of non-destructive evaluation techniques in an attempt to predict the extent of corrosion deterioration and hence, the residual strength of cored slabs from existing bridges. Twelve cored slabs taken from two in-service bridges scheduled for superstructure replacement were tested to failure in the laboratory to validate residual strength predictions. The second project involved the design, manufacture and testing of a full-scale CFRP prestressed cored slab reinforced with GFRP stirrups, and a typical steel prestressed cored slab control specimen. The results of the destructive laboratory testing enabled validation of the prediction of the flexural performance and strength of CFRP prestressed cored slabs relative to existing design recommendations. Direct comparison to the new steel prestressed control cored slab and similar existing cored slabs with varying degrees of deterioration from the first the research project was also undertaken. Keywords: Bridges; FRP; Deterioration; Prestressed Concrete; Corrosion; Cored Slabs

ACI student member Griffith Shapack is an MS candidate specializing in structural engineering in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, USA. Zachary Van Brunt is an MS candidate specializing in structural engineering in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, USA. Prior to pursuing his Masters he worked for 2 years as an assistant bridge inspector in Texas. ACI member Rudolf Seracino is a Professor and Associate Head of the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, USA. He earned his PhD from the University of Adelaide, Australia in 2000. He is a member of ACI Committee 440, FRP Reinforcement, and is a Fellow of the International Institute for FRP in Construction (IIFC). His research interests include the application of advanced FRP materials in civil engineering. Gregory Lucier is a Research Assistant Professor in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, and is the manager of the Constructed Facilities Laboratory. Dr. Lucier earned his PhD from North Carolina State University in 2012. He studies large-scale structural systems including reinforced and prestressed concrete. ACI member Mohammad Pour-Ghaz, is an Assistant Professor in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University. He earned his PhD from Purdue University in 2011. He studies the durability of reinforced concrete materials and structures as well as nondestructive test methods for damage tomography in concrete structures. He is a member of ACI Committee 444, Structural Health Monitoring and Instrumentation as well as ACI Committee, Material Science of Concrete. ACI Fellow Sami Rizkalla is a Distinguished Professor in the Civil and Construction Engineering and the Director of the Constructed Facilities Laboratory at North Carolina State University. He is a Fellow of ACI, ASCE, IIFC, and PCI. He is also the Director of the NSF Industry/University Cooperative Research Center on Integration of Composites in Infrastructure (CICI).

INTRODUCTION

The North Carolina Department of Transportation (NCDOT) has used prestressed cored slabs in bridge superstructures for over 40 years. The typical design, shown in Figure 1, consists of two tubular cores in a rectangular concrete section 3.0 ft (0.9 m) wide with depth ranging from 1.5 ft (0.5 m) to 2.0 ft (0.6 m). These sections are economical for spans ranging from 40 to 60 ft (12.2 to 18.3 m). In the bridge superstructure the cored slabs are post-tensioned transversely at the third points of spans and an asphaltic wearing surface is placed directly on the slabs with no cast-in-place concrete deck.

Figure 1 — Typical cored slab section

Cored slab bridges built in the 1970s along the North Carolina coast, similar to that shown in Figure 2, are showing signs of significant deterioration. The soffit of the cored slabs, particularly at the end spans, is continuously exposed to salt-water splash leading to spalling of the cover concrete from corrosion of the bottom of the stirrups and the bottom-most layer of prestressing.

Figure 2 — Bridge #35 on US Hwy 70, North Carolina

Several of these bridge superstructures have been replaced, or are scheduled for replacement, and in 2013

the NCDOT initiated two research projects to: (1) examine the ability of non-destructive evaluation and assessment

techniques to predict the degree of deterioration before it is visible to the naked eye; and (2) assess the feasibility of replacing existing steel prestressed cored slabs with an all FRP reinforced cored slab alternative.

The scope of work for the first project included two bridges along US Hwy 70 in coastal North Carolina scheduled for complete superstructure replacement. These bridges, both constructed in 1974, are approximately 6 ft (1.8 m) above sea level and showed significant deterioration, particularly in the end spans where the concrete retaining walls, shown in Figure 3, tended to catch waves and splash seawater onto the cored slab soffits. An illustration of some of the most severe cases of deterioration is shown in Figure 4 where multiple repair patches may also be observed. The bridges are located within 60 miles (96 km) of one another, and are herein referred to as Bridge #35 with 9 – 40 ft (12.2 m) spans, and Bridge #39 with 8 – 45 ft (13.7 m) spans. The cored slabs from these two bridges were evaluated in the field (prior to replacement) and in the laboratory using visual inspection, sounding with a small hammer, and concrete resistivity (a non-destructive electrical technique). Results from these three methods of inspection were examined to evaluate the ability to predict the actual extent of corrosion of the prestressing. At the conclusion of field testing, 12 cored slabs (6 from each bridge) representative of the range of deteriorated conditions were delivered to North Carolina State University’s (NCSU) Constructed Facilities Laboratory (CFL) for flexural testing to failure. The results from the destructive testing were compared to strength predictions considering the predicted extent of corrosion from the non-destructive evaluation techniques.

Figure 3 — Retaining wall at end spans

Figure 4 — Typical spalling and patching of severely corroded regions

The second research project was a comprehensive study of the application of prestressed CFRP in cored

slabs to address the aforementioned corrosion problem. Experiments were conducted on cored slabs in flexure and shear. Material properties were investigated with beam-end specimens tested to establish the bond characteristics of the CFRP strand in addition to tensile tests of both the CFRP strands and the GFRP bars. The scope of this paper includes the behavior of the flexural tests only. This involved the design, manufacture, and testing of new cored slabs with all-FRP reinforcement. For convenience, the new cored slabs have the same concrete section geometry and strand layout as the standard NCDOT design (see Figure 1). This allows the CFRP cored slab members to be cast in existing casting beds at precast plants that regularly produce cored slabs. A steel-reinforced control cored slab and an FRP-reinforced cored slab were cast at a precast concrete plant. The FRP-reinforced cored slabs contained CFRP prestressing strands and GFRP stirrups designed according to the recommendations given in ACI440.4R (2004) and ACI440.1R (2006). The 45 ft (13.7 m) long cored slabs were delivered to the CFL and were tested monotonically to failure in four-point bending. Load, midspan deflection, strand slip, and midspan curvature were measured during testing. The observed experimental behavior was compared to theoretical predictions and the results from the first research project. Of the CFRP prestressed slabs designed and tested in the second research project, the focus of this paper is on one of the 45 ft (13.7 m) long specimens, to enable direct comparison of the flexural behavior to the deteriorated in-service slabs tested in the first research project. This paper focuses on three components of the research projects: field evaluation and non-destructive evaluation of in-service cored slabs; casting and construction of CFRP prestressed cored slabs; and laboratory flexural testing. Results and conclusions are discussed and predicted strengths are compared to measured flexural capacities.

FIELD EVALUATION OF EXISTING BRIDGES

Cored Slab Condition Overview

Through a review of routine bridge inspection reports every cored slab in both bridges was characterized according to the presence of previous patches, visible signs of deterioration, and the identified need for priority maintenance. Based on this information, three spans from each bridge (6 spans total) were chosen for detailed inspection. Each simply supported span consisted of 16 cored slabs transversely post-tensioned. As stated previously, the end spans were most severely deteriorated and of the four end spans for the two bridges, 37 of 64 cored slabs were identified as needing priority maintenance. Therefore all of the end spans were chosen for detailed inspection. The intermediate spans were in significantly better condition, with only 9 of 208 cored slabs identified as needing priority maintenance. Therefore, only two intermediate spans (Span 4 from Bridge #35, and Span 2 from Bridge #39) were chosen to capture a representative range of corrosion deterioration observed throughout both bridges. The condition summary for each cored slab for both bridges is presented graphically in Figure 5 to enable a simple visual comparison.

Figure 5 — Cored slab condition summary according to NCDOT inspection reports

For each of the cored slabs in the six spans selected, three inspection techniques were undertaken in order

to capture the condition of the cored slab in 1.0 ft (0.3 m) intervals along the length of each cored slab. These inspection techniques included visual inspection (photographic record), sounding (tapping with hammer and noting audible delamination), and concrete resistivity (electrical method considered indicative of porosity and corrosion potential in concrete).

Visual Inspection

A key focus was to calibrate common inspection methods used by DOTs to actual residual flexural capacities derived from estimates of strand deterioration. The most common method of inspecting bridges remains visual inspection, with trained bridge inspectors recording observations about the apparent deterioration of a bridge under a limited timeframe, frequently without close physical access to the bridge element (AASHTO 2008). In order to obtain a visual record of the existing condition, digital photographs were uniformly taken perpendicular to the soffit using a custom bipod. This allowed for key cored slabs to later be reexamined and their in-service condition to be shown. An illustrative composite image created from photographs of a sample cored slab from Bridge #35 is shown in Figure 6 where the red lines indicate the 1.0 ft (0.3 m) intervals. Rust stains indicate heavy corrosion of stirrups and prestressing strands, but the precise extent of corrosion is not clearly visible due to patching repairs and remaining cover concrete.

Figure 6 — Bridge #35: Span 9, Beam 7, from 10 to 33 ft (3 to 10 m)

Sounding

Another technique that is both low-tech and frequently undertaken in bridge inspection is sounding. This is the process of impacting a concrete element with a hammer or other solid object and listening to the sound produced. Areas of delaminated concrete tend to sound “hollow” compared to sound concrete, and therefore sounding can be used to identify areas where corrosion is likely to be occurring behind the concrete cover and is not visible. Cored slabs were tapped with a hammer at 1.0 ft (0.3 m) intervals with a minimum of three strikes distributed across the width of the cored slab soffit. Every point along the soffit that sounded delaminated was identified as such. Attempts to minimize subjectivity were undertaken by having at least two people present during sounding, with agreement needed between these people for each location examined. Figure 7 shows the sounding technique applied to an end span of Bridge #35 at low tide.

Figure 7 — Example of sounding using claw hammer

Concrete Resistivity

Unlike the previous two methods presented, concrete resistivity measurement is not currently in widespread use for routine bridge inspections, though it has been explored as a method to characterize in-service structures (Presuel-Moreno et al. 2010). Concrete resistivity was selected for field implementation due to the commercial availability of compact, waterproof Wenner array (4-probe) resistivity meters. Testing of concrete cylinders has shown that resistivity can be used as a predictor of chloride ion penetrability, but is affected by moisture conditions and mix design (Rupnow and Icenogle 2011). Resistivity is also affected by the presence and location of steel in concrete (Presuel-Moreno et al. 2010). In the current testing program, resistivity was also measured at 1.0 ft (0.3 m) intervals along the length of the cored slabs, with the probe placed at the center of the width. The probe is shown in use in Figure 8.

Figure 8 — Concrete resistivity measured in field using Wenner probe array

Selection of In-Service Cored Slabs

Twelve cored slabs were selected from Bridge #35 and #39 for laboratory testing. To ease transportation from the bridge during superstructure demolition, cored slabs were selected in adjacent pairs. These cored slabs included two in apparently undamaged condition (serving as controls, one from each bridge), and the remaining ten showing various types and extents of deterioration, including one with very shallow widespread spalling, one with extensive audible delamination (from sounding) but no visible spalling, and others showing varying states of patching and corrosion.

DESIGN AND CONSTRUCTION OF CFRP PRESTRESSED CORED SLABS

Cored Slab Design

The CFRP prestressed cored slabs were designed to be a direct replacement for their existing steel prestressed counterparts. This allows the cored slabs to be cast at precast plants that have existing forms and bulkheads in place for standard NCDOT cored slab sizes. Therefore, the design was kept as similar as possible to the current standard NCDOT geometric details. Key parameters controlled by the standard design details include the geometry of the concrete section and the location of the prestressing strands.

The NCDOT standard steel prestressed cored slabs, shown in Figure 1, have thirteen 0.6 in. (15.2 mm) ASTM A416 270 ksi (1860 MPa) steel strands, and No. 4 (13M) ASTM A615 60 ksi (420 MPa) steel stirrups, spaced at 9 in. (230 mm) in the end zones, and 12 in. (300 mm) in the midspan region of the cored slab. Each steel strand is prestressed to 43,950 lb (195 kN), 75% of its rupture strength. Cored slabs are produced in a range of depths. These 21 in. (530 mm) deep sections are used for spans from 25 ft (7.6 m) to 55 ft (16.8 m).

The final CFRP prestressed section is shown in Figure 9 where it can be seen that the 13 steel strands were

replaced by 15 CFRP strands of the same size. The CFRP strands were prestressed to 39,450 lb (175 kN) due to the ACI 440.4R (2004) jacking force limit of 65% of the guaranteed strength of the strand. Therefore, two additional CFRP strands were required to have a similar total prestress force so that the same flexural capacity could be achieved. Figure 10 shows a typical steel prestressing strand alongside the CFRP strand used in this experimental program. The material properties of the CFRP strand obtained from laboratory testing are compared with typical steel strand in Table 1.

Figure 9 — Final design of CFRP prestressed section

Figure 10 — CFRP (left) and steel prestressing strands

Table 1 — CFRP and steel prestressing strand mechanical properties

Tensile Modulus, ksi (MPa) Rupture Stress, ksi (MPa) Rupture Strain, % CFRP Strand 21,900 (150,000) 460 (3200) 2.1 Steel Strand 29,000 (200,000) 270 (1860) 6 to 7

The steel stirrups were replaced with GFRP stirrups in an attempt to limit the increase in cost of the CFRP prestressed cored slab. The material properties of the GFRP bars obtained from laboratory testing are compared with that of typical Grade 60 reinforcing steel in Table 2. The strain limit of 0.004 for GFRP stirrups from ACI 440.1R (2006), imposed to limit crack width, required an increase in the size of the stirrups and a reduction in stirrup spacing to maintain sufficient shear capacity. The final design included No. 5 (16M) GFRP stirrups spaced at 7 in. (180 mm) throughout the length of the cored slab.

Table 2 — GFRP and steel rebar mechanical properties

Tensile Modulus, ksi (MPa) Rupture Stress, ksi (MPa) Rupture Strain, % GFRP Rebar 6,700 (46,000) 105 (720) 1.6 Steel Rebar 29,000 (200,000) 90 (620) > 10

Cored Slab Casting

One 45 ft (13.7 m) steel prestressed control specimen and one 45 ft (13.7 m) CFRP prestressed specimen were cast at a precast plant that regularly produces cored slabs. Figure 11 shows the final arrangement of reinforcement and cores before casting. The steel strands were stressed using standard procedures. In order to avoid local crushing failure of the CFRP prestressing strands, due to transverse stresses at end anchorages, standard steel prestressing chucks cannot be used. Hence, special anchorages specific to the CFRP prestressing strands must be used, as provided by the manufacturer. Figure 12 summarizes the installation of the anchorage and coupler system for the CFRP prestressing strands. The CFRP strands were run through the end plates of the cored slab, but not the bulkheads. They were cut shorter than the casting bed and the ends were attached via the couplers to short segments of steel strand that ran from the couplers through the bulkheads at either end of the casting bed. After the couplers were attached, the ends of the steel strands were passed through the bulkheads and tensioning could take place in the same manner as that of standard steel strands. The GFRP rebar was tied using plastic zip ties, so there would be no steel in the cored slab. The rest of the casting process was the same as the process for casting steel prestressed cored slabs.

Figure 11 — Overall reinforcing layout for CFRP prestressed cored slab

(a) Helical mesh is wrapped around the end of the CFRP (b) A metal jacket is taped over the mesh wrap

(c) Wedges are placed around the strand and fed into a chuck (d) A hand pump presses the wedges into the chuck

(e) Standard steel chuck is placed onto the steel strand (f) The two halves of the coupler are screwed together

(g) Couplers are staggered so they do not touch during tensioning

Figure 12 — CFRP strand coupler installation

The CFRP and steel prestressed cored slabs were both designed with 6500 psi (40 MPa) concrete target strength and Type III cement in order to reach release strength quickly. The casting process is shown in Figure 13, and the 28 day strength of the concrete exceeded 9000 psi (62 MPa).

Figure 13 — Cored slab casting

EXPERIMENTAL PROGRAM AND RESULTS

Laboratory Setup

The cored slabs from both projects were tested in the same test frame. All of the cored slabs were tested simply supported and loaded monotonically to failure in four-point bending. A 220 kip (980 kN) hydraulic actuator hanging from a reaction frame was placed at the midspan of the beam. The reaction frame was post-tensioned to the laboratory strong floor. A 15 ft (4.6 m) long spreader beam was bolted to the bottom of the actuator. Pin and roller supports attached to plates on the bottom of the spreader beam transferred load at third points through 8 in. (200 mm) wide steel plates resting on the top of the cored slab specimen. A schematic of the test setup for the 45 ft (13.7 m) long cored slabs is shown in Figure 14.

Figure 14 — Test setup schematic

String potentiometers were used to measure deflection at the load points and at midspan. Linear

potentiometers were used to measure strand slip at one end of each cored slab. Pi gages at midspan on the top and bottom of each cored slab measured average concrete strain. All specimens were loaded at a rate of 0.15 in/min (3.8 mm/min) up to the cracking load. Post-cracking, the load rate was increased to 0.5 in/min (13 mm/min) until failure. Overall views of the test setup during testing are shown in Figure 15.

(a) Side View (b) End view

Figure 15 — Cored slab during testing

Flexural Test Results

A comparison of the overall flexural performance of select cored slabs is shown in Figure 16. This includes the deteriorated cored slabs taken from Bridge #39, the new steel prestressed control cored slab, and one CFRP prestressed cored slab.

Figure 16 — Moment - deflection response for multiple flexural tests

Typical flexural failure zones at the conclusion of testing are shown in Figure 17 to Figure 19. Figure 17

shows the severely deteriorated Span 8, Beam 12 cored slab. Bottom prestressing strands are visibly corroded and hang below the section due to loss of all cover concrete and patching material at failure. The failure mode for all significantly deteriorated cored slabs was rupture of deteriorated strands followed by increasing deflection and reduced load carrying capacity prior to crushing of compression concrete. Figure 18 and Figure 19 show the failure zones for the new steel and FRP prestressed beams, respectively. These beams and the two beams in good condition from Bridge #39 failed by crushing of compression concrete after significant elongation of the prestressing strands.

Figure 17 — Bridge #39: Span 8, Beam 12 after removal of crushed concrete

Figure 18 — New steel prestressed control cored slab after failure

Figure 19 — CFRP prestressed cored slab after failure

DISCUSSION

Cored Slab Flexural Behavior Prior to testing, the flexural capacity and behavior for all the cored slabs were predicted using standard sectional analysis. A discussion of the results in relation to those predictions follows, along with comparisons between the cored slabs tested. The behavior of the new steel prestressed cored slab provided a control comparison for both the CFRP prestressed and the in-service deteriorated cored slabs. Performance of the control specimen confirmed the prediction from the analysis. To characterize the stress-strain relationship of the high-strength concrete in the cored slab, the Popovics constitutive model was used to represent the concrete in compression. Mill certifications for the steel strand allowed a calibrated modified Ramberg-Osgood model to represent the tensile prestressing strand behavior. The midspan deflection reached 14 in. (360 mm) and the applied moment reached a maximum value of 808 kip-ft (1100 kN-m) at failure.

The CFRP prestressed cored slab exhibited nearly perfect bilinear behavior. Because of the elastic nature of the CFRP strands, the moment increased almost linearly post-cracking. Therefore, while the Popovics model was still used to approximate the constitutive behavior of concrete in compression, a simple linear-elastic relationship was used to model the tensile behavior of the CFRP strands. The slab failed due to crushing of the concrete in the compression zone. This failure mode was more violent than that of the steel prestressed cored slabs, and occurred at a midspan deflection of almost 12 in. (300 mm) and an applied moment of 882 kip-ft (1200 kN-m). Compared to the steel control specimen, the FRP-reinforced beam exhibited higher moment capacity, but somewhat lower displacement capacity at failure. This result suggests that the CFRP prestressed cored slab is a possible replacement for the current standard design.

The residual flexural capacity of the cored slabs taken from Bridge #39 varied significantly. Two cored slabs, Span 1 Beam 13 and Span 8 Beam 12, each showing extensive longitudinal rust stains and heavy delamination of previously patched areas, had less than 50% of the capacity of the strongest cored slabs. Cored slabs from Span 2 (the intermediate span) performed very well and showed almost identical response. Span 2 Beam 5 was intended as the control and appeared to be in excellent condition. Span 2 Beam 4 had widespread distinct, shallow spalls near stirrups, but no signs of rusting of the strands. The remaining two cored slabs (Span 8 Beam 13 and Span 1 Beam

14) had moderate spalling with isolated rust stains indicating corrosion of the prestressing steel. This corresponded to losses of flexural capacity of 20% and 35%, respectively.

Discussion of Inspection Techniques

Generally visual inspection worked well to predict the loss of strand sectional area, reinforcing the understanding that extensive corrosion of internal steel reinforcement is accompanied by spalling of the cover concrete. In cored slabs where extensive patching existed at a depth greater than the stirrup cover, it appears to be reasonable and conservative to assume total loss of the lowermost layer of strands in a deeply spalled region, but only partial loss in adjacent strands. Removal of delaminated concrete, where possible, is essential to accurately assess the condition of the steel. Generally, layers of strands above the bottom layer were observed to be in good condition for the cored slabs examined. Extensive deterioration of one layer apparently does not necessary equate to corrosion of layers higher in the section.

For example, Figure 20 shows the condition of the soffit of Span 1 Beam 14 at 21 ft (6.4 m), which

appeared to be the location of deepest spalling and corrosion. Spalling was deepest near the central 5 strands where longitudinal rust stains were visible. Concrete cover to the 3 left strands was visibly delaminated but concrete was not spalled off. Cover concrete to the 3 right strands appeared to be comparatively sound. Based on this, it was assumed that the central 5 strands of the bottom layer suffered total loss of sectional area with only partial loss equivalent to the sectional area of 2 - 3 of the remaining bottom 6 strands.

Sectional analysis using the reduced area of prestressing strand in the bottom layer gives predicted

maximum applied moment in the range of 441 - 483 kip-ft (598 – 655 kN-m). This prediction is only 11% - 3% less than the experimentally observed flexural capacity of 496 kip-ft (672 kN-m) for this cored slab. Figure 21 is a photo taken after flexural testing showing this same region, which was located at the point of failure. Severe corrosion of the left 3 strands and 4 of the middle 5 strands is visible, while the right 3 strands appear sound. Predictions based on visual observations were made for all 6 beams in Bridge #39 in this manner, using best estimates of strand loss from visual inspection alone. Predictions were all within 17% of the experimental capacity, with only one over-prediction of 3%. Visual observation can clearly produce acceptably accurate estimates of the cored slabs’ existing condition, but variability between inspectors remains a concern. Guidelines are currently being developed to minimize subjectivity and better ensure consistency in the visual evaluation of cored slabs.

Figure 20 — Bridge #39 field condition of Span 1 Beam 14 at 21 ft (6.4 m)

Figure 21 — Bridge #39: Span 1, Beam 14 at 21 ft (6.4 m) at failure

Sounding also appears to be effective in identifying areas of concern, but care must be taken in interpreting

the results. Several cored slabs had isolated delaminated sections limited to only 1 - 2 ft (0.3 - 0.6 m) long regions. This likely indicates corrosion of the stirrups, but in the cored slabs studied in this investigation, it did not lead to significant loss of flexural capacity since the condition of the steel prestressing strands remained in relatively good condition.

Evaluation of concrete resistivity is still ongoing, but preliminary evidence suggests substantial challenges to effective implementation in the field, with consistency across differing operators and environmental conditions potentially limiting its usefulness for decision-making. Resistivity was repeated for each of the twelve cored slabs that were brought to the CFL after each had been in the climate-controlled laboratory for a minimum of 24 hours. These resistivity measurements will be used to examine the variability and repeatability of field resistivity measurement. Preliminary comparisons of resistivity values from the field and in the laboratory on the same cored slabs suggest that broad trends in resistivity remain similar across different environmental conditions, but specific values of resistivity cannot be reliably reproduced across different moisture and temperature conditions. Environmental conditions should not vary substantially when measurements are taken, which limits the general applicability of concrete resistivity measurements in the field.

CONCLUSIONS Select results of a two phase research project have been reported in this paper. The first phase involved the inspection and testing of 12 cored slabs from in-service bridges in coastal North Carolina with varying degrees of deterioration. Interpretation from visual inspection was able to give a good indication of the existing condition, and guidelines for accurate, consistent analysis based on visual inspection and sounding techniques are being developed. Resistivity measurements are being critically examined to see whether they can add value to traditional inspection techniques, but preliminary experience suggests the concrete resistivity presents challenges for obtaining consistent field data. The second phase of the project included the design, manufacture, and testing of CFRP prestressed cored slabs. The CFRP reinforced cored slab performed well in flexure relative to current in-service cored slabs and a typical steel prestressed control specimen, and proved to be a potential alternative to current designs which are prone to corrosion deterioration.

ACKNOWLEDGEMENTS

This research was supported by the North Carolina Department of Transportation under FY2014 research projects RP2014-09 and RP2014-35. Thanks are also extended to CFL laboratory technicians Jerry Atkinson and Johnathan McEntire for their assistance with the experimental testing. The effort of Steven Thornton, undergraduate research assistant in the Department of Civil, Construction, and Environmental Engineering at NCSU, is also acknowledged for his contribution to the field testing of the existing cored slabs from the in-service bridges.

REFERENCES AASHTO (2008) The Manual for Bridge Evaluation, 1st Edition, American Association of State Highway and Transportation Officials, Washington, DC, USA. ACI 440.1R (2006) Guide for the Design and Construction Structural Concrete Reinforced with FRP Bars, American Concrete Institute (ACI), Farmington Hills, Michigan, USA. ACI 440.4R (2004) Prestressing Concrete Structures with FRP Tendons, American Concrete Institute (ACI), Farmington Hills, Michigan, USA. Presuel-Moreno, F., Suares, A., and Liu, Y. (2010) Characterization of New and Old Concrete Structures Using Surface Resistivity Measurements, Florida Atlantic University – Sea Tech Campus, Dania Beach, Florida, USA Rupnow, T. and Icenogle, P. (2011) Evaluation of Surface Resistivity Measurements as an Alternative to the Rapid Chloride Permeability Test for Quality Assurance and Acceptance, Louisiana Transportation Research Center, Baton Rouge, Louisiana, USA.