Examining Concrete Cores by Non-destructive Techniques

Computed Tomography (CT) scanning has served as an important non-destructive research tool by providing twoand three-dimensional images particularly in medicine, biology and materials science. An example of its application in the area of materials is reported. Experimental study was carried out to investigate the effect of location within column element on the strength and porosity characteristics of concrete by using both destructive (strength test) and non-destructive techniques (CT scanning, ultrasonic pulse velocity and rebound hammer). Tests were conducted on the cores (100mm diameter and about 200mm long) obtained from 8.0m high with 0.5x0.5m cross-section of un-reinforced Fly Ash concrete column. The results indicate that the cores from the upper part of the column exhibited more porous structure and lower rebound hammer and compressive strength values compared to those of cored specimens obtained from the lower part of the column. INTRODUCTION Concrete is a heterogeneous material both at the micro and macro level due to the nature of its constituent materials, which are difficult to mix uniformly because of differences in the specific gravity of water, unhydrated cement particles and the aggregate. Furthermore, during the initial hardening process, water tends to rise inside concrete, known as bleeding. After the hardening process, the amount of accumulated water differs between top, middle and bottom of concrete element and the effect would be more pronounced in a vertical element, e.g. column and wall elements. Particularly at the very top, the amount of trapped water is anticipated to be relatively large (Hoshino, 1989). The air content of concrete may vary between 0.2 to 10.5% depending on the variability of mix design, i.e. w/c ratio, fine and coarse aggregate types and content as well as the type and dosage of admixtures where they are used (Penttala, 2006). This implies that the constituent materials can play significant role in packing and hence the air content. Pores within concrete can also result from either hydration process or due to inadequate compaction. Engineering and durability properties of concrete are governed by internal structure, in particular by the pore system and porosity of concrete. The effect of pores smaller than 20 nm in diameter was found to be negligible (Sutan et al., 2002; Khatib and Mangat, 2003). Research by Sersale et al. (1991) showed that increasing the volume of the pores larger than 20 nm in diameter for mortar reduced the compressive strength from 60 MPa to 20 MPa. However, there is limited work on quantifying the influence of location within a concrete element on porosity and pore size distribution. Therefore, to examine the pore structure of concrete including their shapes, volumes, locations and distributions would be significant in analysing concrete. This would help to deal with structural problems associated with deteriorated concrete present in existing buildings and highway structure (Karihaloo and Jefferson, 2001). Computed tomography (CT) allows non-destructive imaging, measuring and analyzing the pore structure and flow characteristics of engineering materials, e.g. rock and concrete. Scanning provides twoor three-dimensional (2-D or 3-D) images of the internal structures of samples, reflecting relative X-ray attenuation (function of the X-ray energy and the density and atomic number of the material being scanned). The high attenuation contrast between air and other materials in samples allows direct imaging of individual pores and networks, in particular in a sufficiently high spatial resolution (Siddiqui 2001; Ketcham and Iturrino, 2005). CT scanning has been used as a non-destructive tool in a wide range of engineering including geological engineering (Siddiqui et al., 2006), mechanical engineering (Dastarac, 1999) materials science (Iovea et al., 1999) automotive and motorcycle industries (Flisch, 1999) and civil engineering (Karihaloo and Jefferson, 2001; Chotard et al., 2003). Specific fields of interest are porosity and flaw detection, fluid flow behaviour, analysis of failure, dimensional measurements of not accessible geometrical features, inspection of assemblies or statistical investigations of material properties as density distribution. In this paper, an example application of CT scanning technique in engineering materials is demonstrated. In addition to the CT scanning, compressive strength as well as ultrasonic pulse velocity (UPV) and rebound hammer readings were carried out. EXPERIMENTAL DETAILS Materials and Mix Proportions Un-reinforced column element of 8.0m high with cross section of 0.5x0.5m was constructed in open air using Fly Ash (FA) concrete. Concrete was discharged into the column element via a concrete pump. During the placing of fresh concrete into the column, vibration was applied using poker vibrator to achieve optimum compaction. The power and the duration of the vibration were kept constant throughout the filling of the column element. Properties of fresh concrete before the concrete pour by slump test (110 mm) and 28 days of compressive strength (32.5 MPa) were determined. Concrete contained 30% FA by mass of total cement content. Concrete mix proportions and the chemical properties of Portland cement and FA used in concretes are given in Tables 1 and 2, respectively. Table 1 Mix proportions (kg) for 1m of concrete used for the test.