Boris Milmann

COMPLEMENTARY APPLICATION OF RADAR, IMPACT-ECHO, AND ULTRASONICS FOR TESTING CONCRETE STRUCTURES AND METALLIC TENDON DUCTS

Nondestructive testing of concrete structures plays an increasing role in civil engineering, although until now the full potential of such techniques has not been tapped. For posttensioned structures, the investigation of tendon ducts is one of the most essential testing problems. The location of tendon ducts, the determination of concrete cover and, especially, the detection and quantification of ungrouted areas inside the ducts are the relevant questions. Recent developments and opportunities of radar, impact-echo, and ultrasonics for the investigation of tendon ducts are presented. Although the obtained results on positioning and concrete cover determination are sufficient, the location of ungrouted areas is still a matter of research. Thus, new approaches for this testing problem have to be considered. Additionally, the combined use of complementary techniques offers a high potential to increase the reliability of results. Data will be displayed on the combined application of acoustic and electromagnetic impulse-echo methods and of data fusion related to the investigation of tendon ducts.

Nondestructive Detection of Delamination in Concrete Slabs: Multiple-Method Investigation

The delamination of concrete slabs is the separation along a plane roughly parallel to, and generally near, the surface. Corrosion-induced delamination is a common problem in old concrete bridge decks. If undetected, delaminations could expand, reach the surface, and result in spalling. Early detection of delamination is necessary for planning timely repairs that prevent costly deck replacement projects. Most bridge owners rely on routine visual and traditional surveys of bridge deck conditions. These surveys are highly subjective and can locate only large shallow delaminated zones. Several nondestructive testing (NDT) techniques have recently been employed for bridge deck evaluation to obtain more objective and comprehensive assessment. The reliability of applicable methods needs to be established before a greater role for NDT in routine inspections can be encouraged. This paper presents a validation study aimed at evaluating the effectiveness of three NDT techniques, namely impact echo, ultrasound (US) echo, and US linear array, in detection of delamination. This study is unique because the subject test specimens were deteriorated bridge deck segments preserved from the demolition of a prestressed box girder bridge. The results of the tests conducted on one of the specimens are presented and discussed here: impact echo provided satisfactory overall assessment, but the individual results were often difficult to interpret; US echo detected deep delaminations but not shallow ones; and US linear array located the extent of deep delaminations and provided indications of shallow ones.

IMAGING OF CRACKS AND HONEYCOMBING IN CONCRETE ELEMENTS

During more than ten years of ultrasonic inspection of concrete members the inspection methods achieved an enormous progress. This is due to the development of broadband transducers in the low frequency range (centre frequency 80 kHz to 250 kHz) and to the application of synthetic aperture measuring and evaluation techniques [ 1-3]. Since 1994 the Fraunhofer IZFP and BAM co-operate in the area of ultrasonic imaging. BAM develops measuring techniques applying monostatic impulse echo measurement, twodimensional scanning of concrete elements using transducer arrays, multistatic scanning, and a scanning laser-vibrometer as ultrasonic receiver. IZFP develops and applies the data evaluation by means of reconstruction calculation (3D-SAFT; Synthetic Aperture Focusing Technique). The results obtained up to now concerning localising and investigation of tendon ducts in concrete elements, multi layer systems (for example ballast free railway tracks) and the investigation of concrete slabs [4-6] are very encouraging.

Progress in ultrasonic tendon duct imaging

ABSTRACT Ultrasonic imaging of tendon ducts enables the precise localisation of grouting faults in many cases. Additionally to well known criteria for detecting grouting faults in tendon ducts recently new possibilities were developed. Two main effects influence the complex behaviour of wave propagation and reflection: a) different arrangement and number of strands or internal rebars; b) superposition of reflecting signals and different wave modes propagating into the interface region around the tendon duct. In order to understand the experimental results, EFIT modelling is performed (EFIT: Elastodynamic Finite Integration Technique).

Non-destructive detection of surface-bond defects in carbon composite-strengthened concrete structures

For almost 15 years, carbon fibre-reinforced polymer (CFRP) plates and sheets have been used for strengthening and repair of civil engineering structures. CFRP plates were applied to upgrade structures for change in use or increasing traffic load. CFRP laminates have also been used to repair deteriorated structures such as multi-storey parking or bridges. The execution quality and condition of the strengthening measure in terms of bond integrity were neither monitored nor tested systematically. Inspectors eventually applied simple hammer knocking during regular main inspection for subjective noise distinction as a measure for eventual insufficient bond. This paper presents the feasibility and the limitations of low-frequency transversal ultrasonic waves with dry-coupling point-contact sensors for inspection and defect detection between CFRP laminate and concrete surfaces. The German Centre of Competence in Civil Engineering (DIBt) has initiated a national research project [Helmerich et al. (2012). Condition analysis of bonded CFRP-applications on concrete structures by means of non-destructive testing [in German: Zustandsuntersuchung von CFK-Klebeverstärkungen an Betonbauteilen mittels zerstörungsfreier Prüfung (ZfP)]. Final, internal project report], carried out at the Federal Institute for Materials Research and Testing (BAM), Germany. The aim of the project was to present a potential non-destructive testing method for bond defect detection as a measure for the durability of the strengthening system more than 10 years after application.

New Possibilities for Ultrasonic Imaging of Concrete Elements

In recent years the combination of measuring large concrete areas with dry ultrasonic transducers and reconstruction calculation imaging the location of construction elements and defects has been further developed and presents itself now as a precise tool to survey the inner condition of concrete elements. Examples of large area applications at test specimens and post-tensioned concrete bridges are presented.