28-12-2012, 04:39 PM
Application of Radar and Ultrasonic Pulse Echo for Testing Concrete Structures
1Application of Radar.pdf (Size: 2.65 MB / Downloads: 152)
Abstract
Nondestructive testing of concrete structures is one of the essential task for estimating or
assessing the quality. In order to validate the advanced NDT equipments like Radar and Pulse
Echo, a unique large scale reinforced concrete specimen was cast at SERC, Chennai, with
columns and beams of different sizes, cross section having different percentages of
reinforcement. The defects such as honeycombs, cracks, delamination, presence of conduits,
ducts, etc. were also incorporated while casting the large scale reinforced cement concrete
specimen. In the present study, the top and bottom slabs were investigated by dividing into
grid lines of 50 x 50 mm in both the direction using radar and pulse echo techniques. For
radar measurements, 1.6 GHz antenna was used over the grid lines. The pulse echo technique
was adopted with an antenna array of 55 KHz and the data was collected over the same grid
points. Two different data analysis programs were used for the analysis of the data. This paper
highlights the test results from the application of these two methods Radar and Pulse Echo on
the concrete slabs. The need for automatic scanning for both the methods compared to manual
measurement is highlighted. The advantages and the limitations of the test methods are also
discussed with respect to this specimen.
Introduction
Nondestructive testing in reinforced concrete structure is one of the essential task for
estimating or assessing the quality. This includes identification of defects such as
honeycombs, voids, cracks, etc., and, thickness measurement, location of reinforcements,
ducts, etc., The Ground Penetrating Radar (GPR) technique is a very effective method for
investigating the integrity of concrete, thickness measurement, reinforcement identification in
concrete structures (1-4). The Ultrasonic Pulse Echo is a one-sided technique which can be
used effectively for the thickness measurement, localization of reinforcement and ducts, and
the characteristics of surface cracks (5-6). The main aim of the paper is to demonstrate the
application of radar and ultrasonic pulse-echo testing techniques in measuring the thickness,
identification of reinforcements, defects in concrete and other embedments.
The Test Specimen
The test specimen is a reinforced concrete specimen, designed and constructed at the
Structural Engineering Research Centre (SERC), exclusively for the data generation and
validation of different NDT techniques. It consists of two slabs of sizes 4.15m × 4.15m
(bottom slab) and 3.0m × 3.0m (top slab with cantilever projection at one end) with beams
and columns. The entire block is supported on four pedestals at a height of 1.2m to have
access for the bottom slab. The top slab is made with two different thicknesses (150mm and
250mm) and bottom slab with three different thicknesses (200mm, 300mm and 400mm) for
validating the thickness measurements using NDT methods. At the edge the slab is tapered.
Top slab is provided with construction joints, different sizes and shapes of honeycombs, PVC
conduits, cracks for their identification and quantification. Bottom slab is provided with bolt
holes, PVC inserts and conduits to locate and quantify the defects. Both the slabs have various
reinforcement ratios to locate the position of the bars, spacing etc. Beams are provided with
different diameter of reinforcements with different spacing of shear reinforcements and
different cover thickness.
Radar measurements
For the radar measurements, SIR-20 model of GSSI has been used with 1.60 GHz antenna.
This frequency was chosen for a maximum penetration depth of 500 mm. For data collection
the bottom slab was divided into grids of size 50 mm x 50 mm. A portion of 2.0 m x 2.0 m
within the beams was considered for scanning. The data was collected from the top face on
the bottom slab. Fig. 4 shows the measurement on the bottom slab. The upper portion of Fig.
5 shows the raw data of the radargram for the bottom slab. The lower portion of Fig.5 shows
the dataset after correcting for the surface reflection to time zero and also the migration.
Conclusions
The application of radar and pulse echo have been demonstrated for the thickness
measurement, identification of reinforcements, steel embedment, and honeycombs. The need
for automatic scanning for data collection vs manual collection of data on a large scale
structure for both the radar and also for the pulse echo is highlighted. The B-scans and
C-scans as obtained for the radar measurements gives the reinforcement distribution. The
depth slice also provide useful information in identifying the steel embedment and the PVC
conduits. For the radar measurements it was observed that the spacing of the reinforcement
affects the penetration of the waves in to the concrete. The ultrasonic pulse echo technique
provide information on the exact thickness of the concrete member. In addition, the
embedments such as steel plate or PVC pipe can be identified. However the presence of
reinforcements cannot be identified. Additional research is required for the effect of spacing
and the size of the reinforcement on the penetration of radar waves in concrete.