Thermal effect on the physical properties of carbonate rocks

The effect of thermal damage on the physical properties of five carbonate rocks has been investigated. The tests were conducted on two marbles and three limestones, mainly composed of calcite but with different grain sizes, porosities, structural and textural characteristics. Cubic samples prepared from these rocks were gradually heated to a specific temperature level of 100, 200, 300, 400 and 500 °C, and gradually cooled down to room temperature without causing thermal shock in order to investigate the effect of heating temperature on physical properties such as microstructure, bulk density, effective porosity and P-wave velocity. Microscopic investigations from thin sections showed that damage in rocks at elevated temperatures was induced in different severity depending on grain size, porosity, structural and textural characteristics. Colour changes were also observed in porous limestones (Lymra and Travertine) due to organic material. In accordance with the degree of calcite dilation depending on heating temperature and in turn new microcrack occurrence, separation along intragrain and/or intergrain boundaries and widening of existing cracks, P-wave velocity decreased to various levels of the initial value, whereas porosity increased. Microscopic analyses and P-wave velocity measurements indicate that compaction of rock structure up to 150 °C occurred and induced calcite dilation had no significant damage effect on the rock material. Compaction of rock structure led to an increase in P-wave velocity and slight decrease in porosity. Most of the damage occurred within 24 h of heating time and further heating treatments brought relatively minor changes in physical properties. Damage intensity was well explained with P-wave velocity and effective porosity values depending on temperature increase.     

Thermal decomposition study of crystalline limestone using P-wave velocity

A high-calcium limestone was calcined in order to study its thermal decomposition using P-wave velocity. The onset of calcination is at approximately 750 °C, while its completion according to the size of the cubic specimens is noted between 1000 and 1150 °C. We found that P-wave velocities are a very good index for the study and estimation of calcination. P-wave velocity decreases due to a temperature rise from 650 to 1150 °C for the cubic specimens of 4, 6 and 8 cm mean edge, while for 1 and 2 cm cubic specimens mixed behavior is observed, with a considerable increase in velocities at calcination temperature higher than 1050 °C.     

Post-crack (or post-peak) flexural response and toughness of fiber reinforced concrete after exposure to high temperature

Several studies have already reported on the various effects of high temperature on the mechanical properties of fiber reinforced concrete (FRC). Some of these effects include changes in; compressive strength, compression toughness and splitting tensile strength. None of the previous studies have investigated the changes that might occur on the post-crack flexural response and flexural toughness. Post-crack (or peak) response and toughness is considered one of FRC’s key beneficial characteristics – as the purpose of adding fibers is to increase the energy absorption and load carrying capacity after an initial crack. In this study, the flexural toughness test according to ASTM C1018 was carried out on two types of concrete: plain concrete and fiber reinforced concrete with three different types of fiber (steel, polypropylene, and polyethylene) at 0.5% and 1.0% by volume fractions. Prior to the flexural test, the specimens were put in an oven chamber and subjected to high temperatures using the ISO/TR834 standards of: 400 °C, 600 °C and 800 °C. The results showed the typical load–deflection response of FRC was a double-peak response. The first peak represented the properties of concrete matrix and the second peak represented the properties of the fibers used. Under flexural load, instead of dropping (or remaining unchanged), the post-peak load and the toughness were found to increase at lower temperatures (400 °C) and later, decreased as the temperature increased (600 °C and 800 °C). Fiber type and content also played an important role. At a temperature of 400 °C, all FRCs exhibited higher flexural strength and increased post-peak response and toughness. A significant decrease in strength, toughness and load–deflection response was observed with synthetic or plastic FRC (PFRC) when the temperature approached 800 °C. When steel FRC (SFRC) was used, those effects were relatively small. It appears, SFRC has better heat resistance than the PFRC. The density (measured by ultrasonic pulse velocity) was found to decrease more in the PFRC than in the SFRC.     

Hysteresis in the ultrasonic parameters of saturated sandstone during freezing and thawing and correlations with unfrozen water content

Determining the mechanical properties of frozen rock is highly important in cold-area engineering.These properties are essentially correlated with the content of liquid water remaining in frozen rock.Therefore, accurate determination of unfrozen water content could allow rapid evaluation of mechanical properties of frozen rock. This paper investigates the hysteresis characteristics of ultrasonic waves applied to sandstone(in terms of the parameters of P-wave velocity, amplitude, dominant frequency and quality factor Q) and their relationships with unfrozen water content during the freeze-thaw process.Their correlations are analysed in terms of their potential for use as indicators of freezing state and unfrozen water content. The results show that:(1) During a freeze-thaw cycle, the ultrasonic parameters and unfrozen water content of sandstone have significant hysteresis with changes in temperature.(2)There are three clear stages of change during freezing: supercooled stage(0℃ to -2℃), rapid freezing stage(-2℃ to -3℃), and stable freezing stage(-3℃ to -20℃). The changes in unfrozen water content and ultrasonic parameters with freezing temperature are inverse.(3) During a single freeze-thaw cycle, the ultrasonic parameters of sandstone are significantly correlated with its unfrozen water content,and this correlation is affected by the pore structure. For sandstones with mesopores greater than 50%,there are inflection points in the curves of ultrasonic parameters vs. unfrozen water content at -3℃ during freezing and at -1℃ during thawing. It was found that thermal deformation of the mineral-grain skeleton and variations in the phase composition of pore water change the propagation path of ultrasonic waves. The inflection point in the curve of dominant frequency vs. temperature clearly marks the end of the rapid freezing stage of pore water, in which more than 70% of the pore water freezes. Consequently,the dominant frequency can be used as an index to conveniently estimate the unfrozen water content of frozen rock and, hence, its mechanical properties.     

川东北地区岩石纵波速度分布特征和影响因素的试验研究

 岩石的矿物组分、微构造和其所处的温度围压状态是岩石声波速度的决定性因素。针对中石化川东北重点勘探区全层位的砂泥岩和碳酸盐岩地层,选取具有代表性的49种岩样,开展常温常压、常温围压和温度围压条件下的岩石声波速度测试,系统地分析中石化川东北重点勘探区全层位岩石波速的分布规律和在温度围压条件下的变化特征,得出川东北地区砂泥岩、碳酸盐岩纵波速度的均值和变化范围;采用多种数学模型研究川东北地区砂泥岩、碳酸盐岩和全层位岩石纵波速度与地层埋深、密度的相关关系,确定最优回归方程。探讨影响川东北地区砂泥岩、碳酸盐岩纵波速度的主要因素,建立温度、围压和层位与纵波速度的定性定量关系并进行初步的机制分析。综合分析表明：川东北地区砂泥岩呈现出正常沉积压实特征;碳酸盐岩的微构造复杂,微构造是决定碳酸盐岩岩声学响应特性的重要因素。     

岩石高温相变与物理力学性质变化

 岩石内部结构随温度升高的变化会导致其物理力学性质的改变。利用MTS伺服试验机和高温炉进行常温至800 ℃花岗岩物理力学参数随温度变化特征试验。研究结果表明：(1) 岩石物理力学性质随温度变化可划分常温～100 ℃和100 ℃～300 ℃,300 ℃～500 ℃,500 ℃～600 ℃,600 ℃～800 ℃五个阶段;前3个阶段的温度范围分别对应岩石内附着水、结合水和结构水汽化逸出的温度区间。(2) 岩石物理力学性质(抗压/抗拉强度,渗透率,波速等)在400 ℃～600 ℃的温度范围内会有显著变化;受石英由? 相变为? 相的影响,岩石体积增大,微裂隙大量增加,在573 ℃附近存在强度和波速下降的加速点。(3) 温度大于600 ℃后,岩石强度和波速会继续降低,其与固体矿物膨胀和金属键断裂引起矿物熔融破裂及相变有关。     

Correlating mode-I fracture toughness and mechanical properties of heat-treated crystalline rocks

For the effect of thermal treatment on the mode-I fracture toughness(FT), three crystalline rocks(two basalts and one tonalite) were experimentally investigated. Semi-circular bend specimens of the rocks were prepared following the method suggested by the International Society for Rock Mechanics(ISRM)and were treated at various temperatures ranging from room temperature(25 ℃) to 600 ℃. Mode-I FT was correlated with tensile strength(TS), ultrasonic velocities, and Young's modulus(YM). Additionally,petrographic and X-ray diffraction analyses were carried out to find the chemical changes resulting from the heat treatment. Further, scanning electron microscopy(SEM) was conducted to observe the micro structural changes when subjected to high temperatures. These experiments demonstrate that heat treatment has a strong negative impact on the FT and mechanical properties of the rocks. From room temperature to 600 ℃, mode-I FT values of massive basalt, giant plagioclase basalt, and tonalite were reduced by nearly 52%, 68%, and 64%, respectively. Also, at all temperature levels, FT and mechanical properties are found to be exponentially correlated. However, the exact nature of the relationship mainly depends on rock type. Besides, TS was found to be a better indicator of degradation degree than the mode-I FT. SEM images show that micro crack density and structural disintegration of the mineral grains increase with temperature. These physical changes confirm the observed reduction in the stiffness of heat-treated crystalline rocks.     

高温后石灰岩的物理力学特性研究

对焦作石灰岩在常温及经历100℃～800℃不同温度作用后的物理力学特性进行了试验研究,详细分析了加温后石灰岩的表观形态、体积、质量、密度和纵横波波速以及单轴下石灰岩的峰值应力、峰值应变和弹性模量等的变化情况,并对石灰岩高温劣化的影响因素进行了分析。研究结果表明,高温使石灰岩的表观形态发生改变：在400℃以内,温度对石灰岩的物理力学性质的影响不大;200℃以下石灰岩的体积略微减少,超过200℃后石灰岩的体积明显增大,石灰岩的密度随温度的升高而逐步减少;随温度的升高,石灰岩的纵、横波波速大都呈现下降;高温后石灰岩的波速比变化呈无规律性;高温后石灰岩的动弹性模量随温度上升而下降。经历的温度超过400℃后石灰岩的峰值应力和弹性模量均有不同幅度的降低,而800℃内石灰岩的峰值应变随温度的升高变化不明显。温度引起的热应力作用、矿物组分和微结构变化导致石灰岩物理力学性质发生改变与高温劣化。     

Influence of loading and heating processes on elastic and geomechanical properties of eclogites and granulites

Increased knowledge of the elastic and geomechnical properties of rocks is important for numerous engineering and geoscience applications(e.g. petroleum geoscience, underground waste repositories,geothermal energy, earthquake studies, and hydrocarbon exploration). To assess the effect of pressure and temperature on seismic velocities and their anisotropy, laboratory experiments were conducted on metamorphic rocks. P-(V_p) and S-wave(V_s) velocities were determined on cubic samples of granulites and eclogites with an edge length of 43 mm in a triaxial multianvil apparatus using the ultrasonic pulse emission technique in dependence of changes in pressure and temperature. At successive isotropic pressure states up to 600 MPa and temperatures up to 600 ℃, measurements were performed related to the sample coordinates given by the three principal fabric directions(x, y, z) representing the foliation(xy-plane), the normal to the foliation(z-direction), and the lineation direction(x-direction). Progressive volumetric strain was logged by the discrete piston displacements. Cumulative errors in V_p and V_s are estimated to be 1%. Microcrack closure significantly contributes to the increase in seismic velocities and decrease in anisotropies for pressures up to 200-250 MPa. Characteristic P-wave anisotropies of about 10% are obtained for eclogite and 3-4% in a strongly retrogressed eclogite as well as granulites. The wave velocities were used to calculate the geomechanical properties(e.g. density, Poisson's ratio, volumetric strain, and elastic moduli) at different pressure and temperature conditions. These results contribute to the reliable estimate of geomechanical properties of rocks.     

Effect of high temperature and cooling conditions on aerated concrete properties

In this study, effect of elevated temperatures and various cooling regimes on the properties of aerated concrete is investigated. Air cooled materials are tested at room temperature and in hot condition right after the fire. Water quenching effect is determined by testing the material in wet condition right after the quenching and in dry condition at room temperature. Unstressed strength of the material tested hot is relatively higher than air cooled unstressed residual strength up to 600 °C. On the other hand, water quenching decreases the percentage of the strength particularly when the material is wet right after the quenching; strength is lost gradually as the temperature rises. As a result, if the quenching effect is disregarded, temperature rise does not have a considerable effect on the strength of the aerated concrete approximately up to 700–800 °C. It is able to maintain its volumetric stability as well. However, more care needs to be taken in terms of its use above 800 °C for fire safety.     

Hydraulic and physicochemical properties of dense thin bentonite layers permeated with variably mixed alkaline solutions of KOH and CaCl2 at various temperatures (25–75 °C) for 3 years

Hydraulic conductivity tests were performed using mixed alkaline solutions of KOH and CaCl₂ (pH ～12) on thin Na-bentonite layers under various temperature conditions (25–75 °C) for 3 years. For dense thin Na-bentonite (dry density of 1.12 Mg/m³) in a mixed alkaline solution of 0.03 M KOH and 0.03 M CaCl₂, the hydraulic conductivities at 50 °C and 75 °C were approximately 10 times higher than that at 25 °C. The bentonite samples permeated with the mixed solution at 50 °C and 75 °C achieved almost complete cation exchange of Na ions by Ca and K ions. However, only slight cation exchange occurred in the bentonite specimens permeated at 25 °C, regardless of the type of permeant. The free swell index of the reacted bentonite permeated with a mixed solution of 0.03 M KOH and 0.03 M CaCl₂ significantly decreased at 50 °C and 75 °C compared with that at 25 °C. X-ray diffraction analysis revealed that the decreases in the relative intensities of the peaks of accessory minerals, such as opal-cristobalite/tridymite, quartz, and feldspar, were enhanced at 50 °C and 75 °C.     

Determination of the thermal conductivity from physico-mechanical properties

Samples of limestone, dolomite, dolomitic limestone, marble, travertine, sandstone, siltstone, andesite, basalt and porous basalt from different parts of Turkey were tested to obtain their UCS, porosity, water absorption, density, P-wave velocity and thermal conductivity (TC). It was observed that while TC increased with density, P-wave velocity and UCS, it decreased with increasing porosity. Equations are presented to allow an assessment of TC from these physico-mechanical properties.      

Influence of weathering on the engineering properties of Harsit granitic rocks (NE Turkey)

Weathering and hydrothermal alteration of the granitic rocks in the Eastern Black Sea Region (NE Turkey) are important phenomena affecting the engineering projects in the region. The study investigated the probable paths of rock-forming mineral transformations due to weathering in the Harsit granitoid rocks, the changes of the major oxides as a consequence of weathering and the effects of weathering on the engineering properties. To identify the changes in the major oxides, the volume concentrations were considered in relation to the dry density. Chemical leaching during weathering was shown to be significant in changing the minerals. It was concluded that the engineering properties of the weathered granitic rocks can be most reliably predicted by P-wave velocity.      

A waveform modification method for testing dynamic properties of rock under high temperature

In this study,a waveform modification method was proposed using a self-designed heating device combined with the split Hopkinson pressure bar(SHPB) technique for determination of dynamic behaviors of rock at high temperature.Firstly,the temperature gradient distribution on the incident bar was measured according to the variation of elastic modulus of the bar with temperature,and the relationship between the longitudinal wave velocity and temperature of the bar was obtained based on onedimensional stress wave theory.The incident bar with a temperature gradient was divided into a series of microelements,and then the transmission coefficient of the whole incident bar was obtained.Finally,the stress wave was modified by the transmission coefficient from 25℃ to 600℃.This method was used to study the dynamic properties of rock at high temperature,which not only preserves a classical SHPB device,but also effectively ensures the accuracy of the experimental results.A dynamic Brazilian disc experiment was carried out to explore the influences of loading rate and temperature on dynamic tensile strength of sandstone at high temperature using the proposed waveform modification method.     

Influence of high-temperature thermal cycles on the pore structure of red sandstone

Analyzing variations in rock pore structure under high-temperature thermal cycles is of great significance for the development of deep resource. In this paper, red sandstone from Linyi, Shandong Province, was heated to 200 °C, 400 °C, 500 °C, and 600 °C, followed by five cold thermal cycles. The transverse relaxation times (T₂) of red sandstone treated at various temperatures were observed using nuclear magnetic resonance (NMR), and the porosity and the characteristics of the pore structure were analyzed. The results show that the variations in the pore structure of red sandstone occur primarily owing to the escape of pore water, thermal expansion, and the decomposition of some minerals. In the range of 200 to 500 °C, the escape of free water, bound water, and structural water in the sandstone resulted in a significant increase in the proportion of pores with a volume of r?<?0.1 μm. At 573 °C, the phase transition of quartz and the high-temperature decomposition of calcite led to a marked increase in the proportion of mesopores (with a volume of 0.1 μm?<?r?<?1 μm), and the connectivity between pores increased. At the same temperature, the number of mesopores and macropores increases with increases in the number of cycles. At 600 °C, the number of mesopores in sandstone increased significantly after five thermal cycles.

     

Experimental study of the influence of water and temperature on the mechanical behavior of mudstone and sandstone

Understanding the influence of water and temperature on the mechanical behaviors of coal measure rocks is important for deep coal resource exploitation. Using an electro-hydraulic servo-controlled testing system (MTS816) with a self-designed thermostatic water tank, a series of water absorption experiments and uniaxial compression experiments were performed on mudstone and sandstone samples that were immersed in water under different temperature conditions (from 25 to 95 °C). The water absorption characteristics at different temperatures and the effect of water and temperature on the mechanical strength, deformation and failure mode of the samples under uniaxial compression were systematically analyzed. In addition, computerized tomography (CT) scanning was used to examine the microstructural changes in the mudstone and sandstone before and after water saturation at different water temperatures. The results from the water absorption tests show that the water content of the mudstone and sandstone samples kept increasing with immersion time until a saturated state was reached, with the trend generally following an exponential law. The higher water temperature allowed additional water absorption in the saturated mudstone, but less water absorption in the saturated sandstone. The mechanical tests suggest that the presence of water can significantly reduce the mechanical properties of the coal measure rocks. Decreases in the uniaxial compressive strength (UCS) of 76.0 and 38.9 % and the elastic modulus of 68.1 and 48.5 % were observed in the mudstone and sandstone, respectively, because of water saturation at room temperature. Moreover, the water-weakening effect was sensitive to water temperature, and as the water temperature increased from 25 to 95 °C, the UCS and elastic modulus decreased linearly in the saturated mudstone by 53.8 and 70.4 %, respectively, and increased linearly in the saturated sandstone by 21.3 and 20.2 %, respectively. The increasing water temperature also promoted a transition in the saturated mudstone from brittle to ductile behavior, but it had a negligible effect on the failure mode of the saturated sandstone. The CT scanning tests demonstrated that new fractures are produced inside the mudstone after water saturation and that the increasing temperature can exacerbate such water-induced damage. However, no obvious fractures were observed in the CT images of the sandstone at room temperature or at high water temperatures, and the water-induced damage in the sandstone appeared as the micro-fractures at a scale below the CT resolution.     

La recherche de granulats en region centre

This study was made on the reaction between aggregate and cement paste in autoclaved concrete and its influence on compressive strength. The specimens mixed with cement and various crushed minerals (149–74 μm) were autoclaved at 181°C for 1–168 hours. Quartz and feldspars (microcline, albite and anorthite) reacted to cement paste, whereas hornblende, augite and olivine did not react. The reaction rate of minerals were, in order, anorthite > quartz > microcline=albite. 11A tobermorite was formed by the reaction between cement paste and quartz, and feldspars. Hydrogarnet was also formed by the reaction between cement paste and anorthite. The compressive strength of the specimens containing quartz and feldspars increased as the curing time was prolonged.     

Estimation of strength and deformation properties of Quaternary caliche deposits

The aim of this study was to develop and evaluate statistical models for predicting the uniaxial compressive strength (UCS) and average Young’s modulus (E _av) for caliches, using some index and physical properties. The caliche samples, from Adana, southern Turkey, were of low strength and difficult to sample. X-ray diffraction and microscopy were undertaken and the following physical parameters established: unit weight, apparent porosity, Schmidt rebound number, Shore hardness, P-wave velocity, slake durability, point load, uniaxial compressive strength and average Young’s modulus. Simple and linear regression variable selection analyses were performed. The best relationships were obtained for UCS with P-wave velocity and unit weight and for average Young’s modulus with P-wave velocity, porosity and slake durability. Empirical equations are proposed, although it is emphasised that these may only be applicable for caliche of a similar geological character.      

A correlation between P-wave velocity,impact strength index,slake durability index and uniaxial compressive strength

Impact strength index, slake durability index and uniaxial compressive strength (UCS) are important properties of a rock mass which are used widely in geological and geotechnical engineering. In this study, the mechanical properties of one igneous, three sedimentary and three metamorphic rock types were determined in the laboratory and correlated with P-wave velocity. Empirical equations have been developed to predict the impact strength index, slake durability index and UCS from P-wave velocity, which may avoid the necessity for time-consuming and tedious laboratory testing. To check the sensitivity of the empirical relations, a t test was performed which confirmed the validity of the proposed correlations.      

Strength characterization of cohesionless soil treated with cement and polyvinyl alcohol

Lime, cement, and bitumen are well-known traditional binders for improving the bearing capacity of soils. However, the production of these binders results in a massive impact on the environment due to the emission of greenhouse gases, such as carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). In this study, a novel cement–polyvinyl alcohol (PVA) mixture is proposed to fabricate strong composite geomaterials. The advantage of the proposed materials is that they can increase the unconfined compressive strength (UCS) and, combined with cement hydration, producing PVA glue, can be used to fill up the soil pores. Laboratory tests indicate a threefold increase in UCS with the cement–PVA-combined mixture compared to a cement-stabilized one. The results of scanning electron microscope (SEM) observations suggest that the cement–PVA composite can ameliorate the pore structure that is more solid than the cement-stabilized one. Moreover, by curing at 80 °C, the strength of the cement–PVA stabilized soil decreases by threefold, plateauing at the same strength as the non-PVA stabilized soil. In addition, the results of cyclic thermal exposure tests suggest that, with the increase in the number of heating/cooling cycles, the UCS gradually decreases compared to the initial one. However, the loss of UCS is less than 25 % under the three cycles of exposure. Thus, these composites have the potential to promote urban renewal projects in an ecofriendly manner.