SIMULATED GPR INVESTIGATION OF DETERIORATION IN REINFORCED CONCRETE BRIDGE DECKS

Transcription

1 SIMULATED GPR INVESTIGATION OF DETERIORATION IN REINFORCED CONCRETE BRIDGE DECKS Kimberly Belli, Ralf Birken, Reid Vilbig, and Sara Wadia Fascetti Northeastern University, Boston, MA This work was performed under the support of the U.S. Department of Commerce, National Institute of Standards and Technology, Technology Innovation Program, Cooperative Agreement Number 70NANB9H9012 March 19 th,

2 Outline Bridge Deck Inspection Why GPR? Objective Background EM properties of concrete EM modeling program Cracking Models Chloride graded Concrete Models Conclusion Outlook March 19 th,

3 Bridge Deck Maintenance Problem 600,000 bridges are in dire need of maintenance or repair (FHWA, 2008) 1 in 3 urban bridges are either structurally deficient or functionally obsolete (ASCE, 2009) 5 year cost of nearly $1 Trillion (ASCE, 2009) sheet/bridges March 19 th,

4 Bridge Deck Inspection Problem Traffic Congestion and Delays Congestion due to road repairs costs an estimated $78 billion annually in lost productivity and wasted fuel 2007 University Transportation Center for Mobility Statistic There is a critical need to make: * The right repairs * In the right place * At the right time March 19 th,

5 Motivation Why GPR? What if we can inspect bridge decks without road or lane closures? GPR has potential to accomplish this Could substitute for Half Cell Potential (previous talk) a slow point measurement method requiring lane closures Need Fast GPR array to be deployed in traffic flow Needs complementary accurate positioning information Needs threshold for interpretation (previous talk) March 19 th,

6 Objective Use advanced modeling capabilities to represent different cracking schemes around reinforcement steel and chloride graded concrete to determine potential relationships between measured GPR signals and attenuation, and how they relate to potential deterioration states of bridge decks. March 19 th,

7 MODELING BACKGROUND March 19 th,

8 EM Properties of Concrete March 19 th,

9 Modeling Program Finite Difference Time Domain (FDTD) modeling tool (Belli et al., 2011) Maxwell s Equations, which govern wave propagation, are discretized according to Yee s formulation (Yee, 1966). March 19 th,

10 Computational Modeling Overview 2.6 GHz center frequency ground coupled Transmitter/receiver separation of 50 mm The bridge deck is 200 mm thick, and a single #4 rebar (roughly 13 mm diameter) is buried under 60 mm of concrete cover When B scans are presented, data were simulated horizontally along the deck every 12.5 mm. Simulations use a discretized spatial step of 0.5 mm, and a temporal step of 1ps. March 19 th,

11 Two Deterioration Models 1. Simulations for a rebar with significant cracking 2. Simulations for concrete with graded electromagnetic properties 1. Change associated with varying Chloride content in concrete layer above rebar 2. How to derive valid material properties for simulation March 19 th,

12 CRACKING MODELS March 19 th,

13 Crack Pattern around Rebar (Zhao et al. 2012) (Chen and Mahadevan, 2008) (Arndt et al., 2010) (Hong et al., 2012) March 19 th,

14 Modeling Deterioration as Cracking Concrete: r =5.00, =2e 3S/m 2mm thick rust layer around the rebar Cracking extending from the rebar (app. 1mm thick), filled with: Air ( r =1, =0 S/m) Water ( r =81, =4.8 S/m) Rust ( r =8.42, = S/m). March 19 th,

15 Crack Geometry Results Case 1 Case 2 The water filled cracks are crazy, and not like we see in the field. The air and rust filled cracks are more likely candidates, but those do not yield a lot of signal attenuation. March 19 th,

16 Crack Geometry Results Spider crack geometry time histories with antenna centered over rebar. March 19 th,

17 CHLORIDE GRADED CONCRETE MODELS March 19 th,

18 Chloride Graded Concrete No publications were found that discussed EM properties varying with depth in a concrete slab for GPR frequencies If you know of any please let us know? Found final technical report listing measured chloride content versus depth for four concrete in service bridge decks in 1996 and 2006 (Pincheira et al., 2008) Measured moisture content values of in service bridge deck cores versus depth (Carrier et al., 1975) These limited published chloride content and moisture values for various depths are used to estimate the electromagnetic properties for the models with gradual changes March 19 th,

19 Estimation of EM Properties Varying with Depth CRIM model (Halabe et al, 1993) March 19 th,

20 March 19 th,

21 Calculating Complex Permittivity from Measured Data Measured Chloride Ion Content (lb/cy) Measured Water Content Assumptions: water content = degree of saturation Conversion to Salinity (ppt), where ppt = g/l Compute Relative Complex Permittivity of Water (Halabe Thesis) Assumptions: Temperature = 20 degree C NaCl solution for normality of saline water computations (A = 1) εsw = 4.9 εsw Compute Complex Relative Dielectric Permittivity using Concrete Mixture (Eq 5, Halabe, 1993) Assumptions: Porosity of concrete = 0.1 Dielectric constant of concrete solids = 5 Compute Conductivity if desired March 19 th,

22 Modeling Deterioration with Varying Electromagnetic Profiles Input Moisture Content Constant moisture through 1 inch cores (Carrier et al., 1975) Chloride Content Profiles from field data from bridge deck samples (Pincheira et al., 2008) Calculations Relative permittivity and conductivity CRIM (Complex refractive index method) March 19 th,

23 Modeling Deterioration with Varying Electromagnetic Profiles Output Discretized with a 1 mm resolution March 19 th,

24 Graded Geometry Results App. 56% reduction in amplitude in Model 3 compared to the healthy model. March 19 th,

25 Conclusions It can be seen from the results of the two sets of simulations that the attenuation of the signal due to severe cracking does not govern over the variation of moistures and chlorides (or permittivity and conductivity) throughout the bridge deck. The variation of electromagnetic properties with depth results in attenuation of the signal scattered from the rebar similar to observations in the field March 19 th,

26 Outlook Simulate models combining both presented models Use real experimental Chloride Content and Moisture Profiles from same core Use Complex permittivity determined in laboratory from same core as another comparison March 19 th,

27 This work was performed under the support of the U.S. Department of Commerce, National Institute of Standards and Technology (NIST), Technology Innovation Program (TIP), Cooperative Agreement Number 70NANB9H9012 Thank you! Supporting Organizations March 19 th,