International Concrete Abstracts Portal

A total of 25 papers are included in this Symposium Publication on HPC. The general topics include HPC bridges, HPC structural lightweight concrete, material science of HPC, and structural safety of HPC. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP189

The fracture process of cement-based material cause by compressive loading is a complex, three-dimensional phenomenon that occurs as a result of material heterogeneity and complicated mixed-mode cracking mechanisms. However, obtaining three-dimensional information describing this process requires equipment-and time-intensive techniques. One such technique is X-ray Microtomography, which provides high resolution data, but is limited to small specimens. A more straightforward technique, Digital Image Correlation (DIC), can be used to characterize the fracture pattern of a wider range of specimens, but only in two dimensions on the surface. In order to determine the relationship between surface and internal cracking mortar specimens were tested in compression and examined using both DIC and X-ray Microtomography. Microtomography is an X-ray technique that can be used to produce three-dimensional images which reveal the full internal structure of the specimen, including cracks and pores. DIC is a Computer Vision technique that compares successive images to measure two-dimensional deformation on the surface of the specimen, providing information on the location and opening of surface cracks. Rectangular mortar specimens (25.4 mm by 12.7 mm by 12.7 mm) were loaded to certain levels past the peak stress to induce significant cracking and then unloaded. Deformations of the unloaded specimen were measured with DIC and used to determine crack shape, size and location. This surface phenomena was compared to the shape and size of internal cracks seen in tomographic reconstructions of the same specimen. It was observed that these techniques give complementary information about crack geometry and development.

Concrete may be considered a three phase material, consisting of cement matrix, aggregate and the interfacial transition zone (ITZ). The ITZ is studied usually by collecting backscattered electron (BSE) images of polished samples of concrete and by carrying out a quantitative analysis of these images. This technique makes use of the contrast between pores and cement hydrates to analyse the microstructure. Once the images are collected, grey level thresholding is used to segment regions associated with capillary porosity. The size of the ITZ is determined using this data. Different techniques have been suggested to minimise errors introduced at this stage of the analysis. The authors have carried out an investigation on 17 different concrete mixtures in an attempt to assess the role of threshold values. The results indicated that the size of the transition zone is not affected by the range of threshold values used. However, the porosity of both the ITZ and the bulk cement paste varied with different threshold values. The paper proposes a method which can be used to determine a reliable threshold value.

Portland cement concrete is a composite material, where the matrix consist of porland cement paste. Cement paste forms from the hydration reaction of portland cement with water. The microstructure of cement paste changes drastically over time period of about one week, with slower changes occurring over subsequent weeks to months. The effect of this hydration process on the changing microstructure can be represented using computer simulation techniques applied to three dimensional digital image-based models. Percolation theory can be used to understand the developing microstructure of cement paste in terms of three percolation thresholds: the set point, capillary porosity percolation, and the percolation of the C-S-H phase. The percolation of the interfacial transition zones also plays an important role in concrete microstructure, and can be displayed using computer simulation models. These percolation aspects help determine the transport properties and therefore the durability and service life of concrete.

Nitrogen adsorption is quick, inexpensive method for measuring the surface area of mesoporous materials. The use of this method for studying cement-based materials has generated much controversy in part because the values obtained are quite variable and differ from those measured using water adsorption, SAXS, SANS, or NMR techniques. No two measure the same surface area of cement paste. Although nitrogen may not measure all of the surfaces in a cement paste, a quantitative measurement of the surfaces accessible to nitrogen is till a valuable number. When variation due to sample preparations minimized, the effects of variables on the nitrogen B.E.T. surface area become clear. Evidence from the effects of w/c and age on surface area shows that there are two types of C-S-H can be manipulated, with potential effects on physical properties such as drying shrinkage and creep.