Differential Thermal Analysis (DTA) – Lab Test – Assessment of qualitative and quantitative composition of sample of concrete
X-ray Diffraction (XRD) – Lab Test – To determine the extern of deterioration in concrete subjected to fire
Differential Thermal Analysis (DTA)
DTA is used to study the physical and chemical changes that occur in a material when it is heated so as to characterize the material. It is concerned with the rate of change of temperature of a sample as it is heated at a constant rate of heat flow.
The principle on which DTA is based is that when a material is slowly heated, its temperature rises but when the material undergoes any endothermic reaction, i.e. losing water, losing CO2, changes in crystalline structure or decomposition, its temperature remains constant.
The results of DTA are represented in the form of DTA curves. The sample and an inert material are heated in separate crucibles and the difference of temperature between the two is recorded by means of thermocouples which generates an electrical signal whenever there is a temperature difference between the reference and the sample. When there is no endothermic reaction in the sample, there would not be any difference in the temperature between the reference and the sample and hence no electrical signal would occur. Any endothermic change would create a thermal differential and hence an electric pulse. The occurrence of the pulse appears as a peak in the DTA curve which is a plot of temperature versus electrical signal generated. Advantage of DTA over TGA is that the changes not involving weight loss can also be detected.
By comparing the DTA curve of a sample with that for known compounds, qualitative composition of the sample can be judged. Quantitative composition of the sample can also be judged by measuring the sixe of the peak in the DTA curve. The peak size is directly linked with amount of heat involved in a transition.
If a concrete sample is analysed, endothermic peak due to calcium hydroxide is found at 500 degC. The peak due to conversion of silica occurs at 570 deg.C. When various fire damaged samples of concrete are subjected to DTA, the presence of peak at 500 deg.C indicates that calcium hydroxide is present and that the sample has not been subjected to temperature more than 500 deg.C. If peak of 570 deg C is only present, it means that silica conversion had not occurred in the sample and it was not subjected to temperature more than 570 deg.C. If no peak is there, it means that sample was subjected to temperature exceeding 500 deg.C.
X – ray Diffraction (XRD):
X – ray diffraction is based on the principle that a crystal of a substance has a unique diffraction pattern. When monochromatic X –ray beam falls on a crystal it gets reflected by the various crystalline planes. Interference occurs among the various reflected beams resulting in a diffraction pattern consisting of dark and bright fringes, depending upon the phase difference among the interfering beams.
A crystal whose composition is unknown can be identified by obtaining its diffraction pattern and comparing it with diffraction pattern of already identified crystal. The diffraction pattern of a single crystal consists of a number of single spots but when powder sample is used, the diffraction on pattern consists of a series of diffraction lines.
By this technique the size of crystal planes the molecular structure of the sample can be determined. It can also be found whether the sample is a single compound or consists of more than one compound. In case of polymers, degree of crystallinity can be found because the non-crystalline portion will scatter the X-ray beam to give a continuous background while the crystalline portion will give discontinuous diffraction pattern.
As regards to the assessment of the fire damage, X-ray diffraction can be used to determine the extent of deterioration in concrete that has been subjected to fire. The temperature to which the damaged concrete has been subjected to can be determined by this method.
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