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Assessing Highway Infrastructure

Friday, October 14, 2016

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Contributed by Stephen Garrett, Wiss, Janney, Elstner Associates Inc.

The American Society of Civil Engineers (ASCE) has given America’s civil infrastructure an overall grade of a D+ based on ASCE’s 2013 Report Card, while bridges received only a slightly better grade of C+.

As these systems continue to degrade, significant resources will need to be allocated to improve their current state to ensure safe and reliable use. Regular, targeted inspections incorporating field investigations to determine appropriate repair programs for these structures are of vital importance.

Preparing for Field Investigations

Preparation for effective field investigations begins with a thorough review of available construction documents. An understanding of the as-built construction is necessary for correct interpretation of field conditions.

Depth and type of reinforcement are of particular importance in assessing durability potential of bridge decks, and field verification is critical. Uncoated, black-bar reinforcement is common in reinforced concrete construction and is prone to chloride-related degradation from de-icing salt application.

black bar uncoated epoxy coated
Photos: Wiss, Janney, Elstner Associates Inc.

Corrosion of embedded reinforcing steel (black bar at left, epoxy coated at right).

While epoxy-coated reinforcement offers an added layer of protection against chloride attack, corrosion can still occur due to defects or damage in the coating, or given sufficiently high chloride concentrations (especially at deck cracks).

Field Investigation

The first step in deck assessment is visual evaluation of cracking and distress, generally performed on the topside and, if possible, from the sides and underside of the structure. Important conditions to note are cracking, signs of moisture and corrosion staining.

Cracking in reinforced concrete bridge decks is almost guaranteed, but the type and location of cracking may offer insight into the behavior and future performance of the deck. Regularly spaced transverse cracks are common and are generally associated with early age shrinkage or thermal contraction of the deck; whereas irregular cracking may be associated with corrosion-related distress or other deterioration mechanisms.

typical reinforced concrete bridge deck crack closeup

A standard reinforced concrete bridge deck shown at left, with cracks (yellow) and delaminations (white). A closeup of a typical crack is shown at right.

As reinforcing steel corrodes, expansive stresses lead to cracking and formation of near-surface planar delaminations. A simple but effective tool for locating these delaminations is “sounding,” where the deck is impacted with a hammer or chain, and audible changes in the tone are correlated to the location and extent of possible distress.

Locations of cracking and delaminations are recorded on plan views of the structure, which are later used to compare against results from other analysis methods and in developing repair documents.

overlay bond testing overlay bond testing

Overlay bond testing (pull-off method) with portable load-frame.

Decks with overlays need special consideration because sounding methods are not able to differentiate between delaminations resulting from corrosion and delaminations at the overlay-substrate interface due to debonding. A standardized portable bond test system is available to evaluate in-situ bond strength of overlays to the deck substrate.

One widely used tool for evaluating depth of reinforcement is ground penetrating radar (GPR). GPR uses propagation of electromagnetic waves to detect anomalies in an otherwise homogenous medium (e.g., reinforcing steel in a concrete bridge deck).

GPR scanning GPR scan

GPR scanning with cart-mounted antenna (left) and collected scan showing locations of embedded reinforcing steel.

For bridge deck inspections, rapid scanning is important, and cart-mounted units are available to collect full-length scans at a walking pace. Multiple scans are usually taken to accurately capture variability in as-built conditions.

Laboratory Analysis

Core samples are collected at “representative” locations including regions of the deck with and without distress, as well as at any notable atypical conditions.

There are three main laboratory tools commonly used for evaluating performance of concrete structures: petrographic examination; core strength evaluation; and chloride concentration profiles.

Petrographic examination of concrete involves visual and microscopic evaluation to understand the composition and determine the relevance of material distress.

Compressive strength evaluation is also useful in evaluating performance of a bridge deck. While high concrete strengths are desired for structural capacity, strength and stiffness are generally proportional and stiffer concrete is more prone to cracking.

typical chloride concentration results typical results

Typical results from chloride concentration analysis.

Determination of chloride concentrations and depth of chloride ingress is useful in estimating future performance of the deck and reinforcement. Thin slices, at predetermined depths, are cut and ground into a fine powder, then chloride concentration is evaluated for each depth.

Service Life Modeling

The results of the field and laboratory investigations formulate the parameters for developing long-term performance models of the structure through service life modeling. Modeling considerations include: chloride exposure on the deck surface; diffusion and permeability characteristics of the concrete; and type of reinforcement and cover depth. Probabilistic approaches (e.g., Monte Carlo simulations) are implemented to consider variability inherent in these input conditions.

service lfie prediction

Service life prediction compared to damage at current age and repair threshold.

Predictions from the service life model are reported as a damage percentage versus age. The results are first compared to the level of damage observed during the field investigation, as a means to evaluate the validity of the predictions, then the predictions are compared to a threshold level of damage above which repair is indicated.

Development of Repair Recommendations

Field testing and laboratory methods offer additional insight beyond visual observations, and service life modeling offers invaluable insight into the future behavior of the structure. Considered in total, a synthesis of the field investigation, laboratory analysis and service life modeling provide a powerful basis for identifying appropriate repairs of bridge decks.

About the Author

Stephen Garrett is an associate with Wiss, Janney, Elstner Associates Inc. with experience in bridge infrastructure investigation, nondestructive testing and laboratory evaluation. He has been involved with assessment and repair projects for a variety of structures including bridge decks, residential high-rise buildings and stadium facilities.

For more stories like this, see WJE’s blog “Solving for Why” on our sister site Durability + Design. Since 1956, WJE’s primary goal has been to provide the best solutions for its clients’ new and existing construction-related problems. The firm’s highly qualified engineers, architects, and materials scientists possess a collective knowledge gained from solving, as well as helping clients avoid, thousands of problems.

   

Tagged categories: Asia Pacific; Bridges; concrete; EMEA (Europe, Middle East and Africa); Infrastructure; Inspection; Inspection equipment; Latin America; North America; Quality Control; Roads/Highways; Site/field testing; Steel; Transportation; Wiss, Janney, Elstner Associates

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