Detection of delamination in concrete bridge decks

A major problem with the reinforced concrete bridge decks located in coastal areas, or areas where de-icing salts are used on roadways during winter, is the premature deterioration of concrete. The intrusion of chloride from these salts into the concrete causes the embedded reinforcement to corrode, which eventually causes concrete to crack or delaminate. Marine structures are also subjected to this type of deterioration.

The simplest method to detect delamination in concrete involves sounding the concrete with a hammer or heavy chains, which produces a characteristic hollow sound when delamination is present. Although the method is effective, it is adversely affected by the presence of traffic noise. Since sounding is a contact method, its use requires closure of traffic lanes, which is often costly and undesirable. Because of the need for an alternative to the sounding method, the use of non-contact and non-destructive methods such as infrared thermography and radar have been studied.

13.7.3.1. Principle of radar operation on concrete bridge decks

When a beam of microwave energy is directed at a reinforced concrete slab (Fig. 13.6), a portion of the energy is reflected from the surface of the concrete and the remaining energy penetrates this interface.

FIG. 13.6. Radar echoes from the cross-section of a reinforced concrete deck.

The surface reflection has a negative polarity since the dielectric constant of concrete, which has been reported to range from 6 when dry to about 12 when saturated, is considerably higher than that of air, which is 1. It must be noted that the actual in situ relative dielectric constant of concrete and most materials will vary because it is affected to varying degrees by not only its water content but also by its conductivity, mineral composition, etc. As the remaining microwave energy propagates into the concrete, a portion of the beam will be completely reflected and scattered as it strikes the top mat of reinforcement. This reflection will also have a negative polarity, since the dielectric constant of metal is infinite compared with that of the surrounding concrete. The remaining energy will continue deeper into the concrete slab until a portion of it strikes the second mat of reinforcement and the same reflection and scattering processes occur. Eventually some portion of the original beam of microwave energy will reach the bottom of the concrete slab and some of it will be reflected at the concrete air/interface to give a positive reflection signal. The remainder will penetrate through this interface and be lost from the receiving antenna. When the concrete slab is delaminated, usually at the level of the top mat of reinforcement, there is an additional reflection from the deteriorated section. This additional reflection, usually of negative polarity, serves as an indicator of the presence of a delamination in the concrete slab.

The presence of a delamination causes additional reflection of the incident energy.

13.7.3.2. Test principles

Two types of antenna are typically used for bridge deck evaluation:

(1) Air coupled (0.2-0.5 m above the ground) horn antenna designed to operate at 1GHz or 2.5GHz. The higher the frequency the better the resolution, but the lower the depth penetration. Thus a 2.5GHz antenna may give the required resolution of say 3 cm but be restricted to a penetration of 35 cm. That may mean that the bottom of the deck may not be identified.

(2) Bow tie antenna is normally close coupled to (i.e in contact with) the ground. Very little energy would be transmitted into the bridge deck if a bow tie antenna is held at more than λ/10 above the ground.

Research and practice in mid to late 1990’s in Europe and the USA indicated that good results could be obtained with the 1.5 GHz bow tie antenna close coupled to the road surface of the bridge deck.

One main reason for choosing air coupled horn antennas was to enable high speed radar scans to be made of highways and bridge decks at, say 50, km/h. By contrast, ground coupled bow tie antenna surveys would be undertaken at 10-15 km/h, requiring lane closures at considerable expense and disruption to traffic. However, the radar systems for high speed measurement using horn antennas typically cost double than that of a normal radar system.

Also, the resolution may not be as high as the ground coupled 1.5 GHz bowtie antenna.

In the inspection of bridge decks, in which a relatively large concrete area has to be inspected, the antenna is mounted on the front or the rear of an inspection vehicle, which is also instrumented with a horizontal distance-measuring device. If a single antenna radar system is used the vehicle has to make several passes over each traffic lane from one end of a deck to the other at a selected speed, usually in the range from 8 to 16 km/h. During each pass, the antenna scans a different area in the lane. The stream of radar signals is recorded continuously with an instrumentation tape recorder. With a two-antenna or a multiple antenna radar system, a single pass may be made over each lane. It must be emphasized that when a lane is scanned with a two antenna radar system in a single pass only, a significant portion of the lane will be missed. This procedure creates continuous recordings of the reflections from the entire depth of the deck along the paths of the antenna. These recordings are played back at a later time for signal interpretation.

Full recording of a lane (2.5 m wide) in high traffic (65 mph) was the goal of a radar system developed at Lawrence Livermoore National Laboratory and the Federal Highway Administration (FHWA) in the USA. The system, named High Speed Electromagnetic Roadway Mapping and Evaluation System (HERMES), Fig. 13.7 collects data simultaneously from 64 wide band air coupled horn antennas mounted on a trailer that is towed by a truck.

The design specification includes a resolution of 3 cm and a penetration depth of 35 cm. The system is presently under test.

A scaled down version of the FHWA HERMES is the Precision Electromagnetic Roadway Evaluation System (PERES), Fig. 13.8, which has only one pair of horn antennas that can be moved along a line that is 2 m long. By moving the system large areas can be tested. The software allows one to plot depth slices of the tested area, giving a visual representation of the internal features as indicated in Fig. 13.9.

FIG. 13.7.HERMES system for bridge deck inspection.

FIG. 13.8. PERES scanning radar system for bridge deck inspection.

FIG. 13.9. Synthetic aperture reconstruction of horizontal plane through bridge deck using PERES system.

14. RADIOISOTOPE GAUGES