The bowing potential of granitic rocks: rock fabrics, thermal properties and residual strain

The bowing of natural stone panels is especially known for marble slabs. The bowing of granite is mainly known from tombstones in subtropical humid climate. Field inspections in combination with laboratory investigations with respect to the thermal expansion and the bowing potential was performed on two different granitoids (Cezlak granodiorite and Flossenbürg granite) which differ in the composition and rock fabrics. In addition, to describe and explain the effect of bowing of granitoid facade panels, neutron time-of-flight diffraction was applied to determine residual macro- and microstrain. The measurements were combined with investigations of the crystallographic preferred orientation of quartz and biotite. Both samples show a significant bowing as a function of panel thickness and destination temperature. In comparison to marbles the effect of bowing is more pronounced in granitoids at temperatures of 120°C. The bowing as well as the thermal expansion of the Cezlak sample is also anisotropic with respect to the rock fabrics. A quantitative estimate was performed based on the observed textures. The effect of the locked-in stresses may also have a control on the bowing together with the thermal stresses related to the different volume expansion of the rock-forming minerals.     

Bowing of dimensional granitic stones

Bowing is a well-known phenomenon seen in marbles used as building veneers. This form of rock weathering occurs as a result of external factors such as temperature, humidity, the system for anchoring the marble slabs or the panel dimensions. Under the same external conditions, many factors will determine the degree of deformation including petrography, thermal properties and residual locked stresses. The usual way to solve the problem of bowed marble slabs is to replace them with other materials, such as granites, in which the deformation still exists but is less common. In this study, eight ornamental granites with different mineralogy, grain size, grain shape, porosity and fabric were tested in a laboratory to assess their susceptibility to bowing. Three slabs of granite, each cut with a different orientation, were studied under different conditions of temperature (90 and 120°C) and water saturation (dry and wet) to investigate the influence of these factors together with that of anisotropy. At 90°C, only the granite with the coarsest grain size and low porosity exhibited deformation under wet conditions. At 120°C and wet conditions, three of the granites showed evident signs of bowing. Again, the granite with the coarsest grain size was the most deformed. It was concluded that the wide grain size distribution influences microcracking more than other expected factors, such as the quartz content of the rock. Also, mineral shape-preferred orientation and porosity play an important role in the bowing of the studied granites.     

Moisture expansion as a deterioration factor for sandstone used in buildings

The expansion processes that develop in building stones upon changes of moisture content may be an important contributing factor for their deteriorations. Until recently, few data could be found in the literature concerning this parameter and weathering processes. Moreover, the processes that may be responsible for the moisture related expansion of natural building stones are not yet completely understood. To further elucidate this process, extensive mineralogical, petrophysical and fabric investigations were performed on eight German sandstones in order to obtain more information regarding the weathering process and its dependence on the rock fabric. The analysed sandstones show a wide range of pore size distributions and porosities. A positive correlation with the fabric and the pore space can be found for all studied petrophysical parameters. The intensity of the expansion and related swelling pressure cannot be attributed only to the swelling of clay minerals. The investigations suggest that the micropores and the resulting disjoining pressure during wet/dry cycles also play an important role. The results obtained suggest that the mechanism is related to the presence of liquid water within the porous material.     

Weathering of volcanic tuff rocks caused by moisture expansion

Moisture expansion in natural building stones is considered one of the most important factors affecting their weathering and deterioration. The processes that may be responsible for the expansion under determinate relative humidity (hygric dilatation) and water-saturated conditions (hydric dilatation) are generally attributed to the presence of swellable clay minerals. In contrast to this assumption, our investigations show that moisture expansion also takes place in volcanic tuff building stones almost free from clay minerals. To provide a deeper understanding of the processes, swelling and deterioration were performed on 14 volcanic tuffs used as important building stones of different ages, compositions and weathering stages from Mexico, Germany and Hungary. The investigations undertaken include extensive chemical, petrophysical and fabric analyses. The samples show a wide range of effective porosity, microporosity, capillary water absorption, moisture expansion, and CEC values. High moisture expansion does not seem to depend on clay mineral content alone. We also observed that there is no significant effect on dilatation if clay minerals are present but only form a thin coat on the outer shell of bigger pores. Moreover, we identified a correlation between microporosity, average pore radius and moisture expansion. The investigations highlight the fact that moisture expansion cannot only be attributed to swellable clay minerals, and suggest that the presence and accumulation of micropores and their average radius and distribution play an important role for non-clay associated swelling intensity, which can most probably be attributed to the disjoining pressure.     

The combined effect of moisture and temperature on the anomalous expansion behaviour of marble

Three calcitic marble types often used as cladding material and different in lattice preferred orientation (texture), grain shape preferred orientation, grain size distribution and grain interlocking are investigated to study the combined effect of thermal cycles (day-night) and moisture on the decay of marble, particularly on the bowing phenomenon. Repetitive heating-cooling under dry conditions leads to considerable inelastic residual strain only in the first thermal cycle. The residual strain continuously increases again if water is present, whereby the moisture content after a thermal cycle has a certain impact on the decay rate. The water-enhanced thermal dilatation strongly correlates with the deterioration rate obtained from the laboratory bow test. All applied approaches reveal that the texture in combination with the grain shape preferred orientation control the intensity and anisotropy of marble deterioration, thus, the cutting direction of facade panels has to be considered in terms of durability. On-site analysis from building facades confirm the laboratory results such as the bowing tendency of different marbles, directional dependence, relevance of moisture content during cyclical heating and the loss of strength due to environmental impact.Special Issue: Stone decay hazards     

Prediction of weathering by thermal degradation of a coarse-grained marble using ultrasonic pulse velocity

Engineering properties of building stones can vary because of degradation by weathering agents. Thermal fluctuation is one of the most important agents on deteriorations such as sugaring, bowing, cracking and spalling of marble. As a result, physical and mechanical properties of marble used in the construction of old and/or modern structures are adversely affected by time. On the other hand, some properties of building stones are always required for decision of rehabilitation works. Several kinds of conventional tests have been suggested for characterization of stones, to measure their properties or to evaluate conservation or repair works. However, in most cases, sampling from historical buildings is not possible. Therefore, nondestructive testing methods are often suggested for the prediction of weathering grade of building stones. One of the most practical methods for similar purposes is ultrasonic pulse velocity measurement, which can be performed easily. The main goal of this study is attainment of sound empirical correlations between the ultrasonic pulse velocity and engineering properties of previously deteriorated marble. Experimental works were conducted on seven different specimen categories of a coarse-grained marble having different micro-crack frequencies induced by both cyclical heating–cooling and freeze–thaw actions. The experimental results indicated that physical and mechanical properties of Mu?la marble can be reliably estimated for different environmental cases by ultrasonic pulse velocities. P-wave velocities in dry and saturated cases are two sound indicators of both the apparent porosity and the coefficient of capillary absorption, and whereby the sugaring type of deterioration for coarse-grained marbles.     

中高温三轴应力下鲁灰花岗岩热破裂声发射特征的试验研究

在中高温三轴应力作用下岩石内部物理化学性质和结构特性将发生变化,随着岩石内部结构的变化会引发一系列的声发射现象。通过试验研究了大试件花岗岩在三轴压力状态下声发射随温度的变化规律。试验研究表明：①随着温度升高,岩石的声发射现象是间断发生的;②花岗岩存在一个开始发生热破裂的门槛值温度,其值为120 ℃左右;③试验温度范围内花岗岩热破裂的声发射现象可分为5个阶段,即岩石原生裂隙整合阶段、热破裂前声发射静默阶段、热破裂声发射阶段、大规模热破裂后声发射静默阶段、二次热破裂开始阶段。     

花岗岩低温热力效应参数及强度规律研究

长期处于变低温环境中的岩石热力学效应主要表现为材料的力学特性参数、热力系数和饱和冻结状态孔隙冰胀系数等随温度改变而发生变化,这种变化对岩体的变形及强度特性均有显著影响。根据变物性参数的非线性热弹性理论,建立了考虑热力系数和冰胀力系数的本构方程,给出了单轴压缩条件下热力效应系数随温度变化的分析格式和确定方法;借助于花岗岩不同低温干燥和饱和冻结状态的单轴压缩试验资料,获得了花岗岩低温热力效应与温度的关系,并探讨了其对花岗岩抗压强度的影响特性。分析表明,在变低温环境下花岗岩热力系数和冰胀力系数均随相对温差的增大而逐渐减小,热力系数降低速率小于冰胀系数降低速率,在同一温差下热力系数大于冰胀系数;热应力和冰胀应力与温度改变量呈非线性增长关系;花岗岩抗压强度在两种状态下均呈增大趋势,但主要以热应力为主,热力系数引起的试样轴向应力增量总是大于冰胀力引起的轴向应力增量。所给方法及研究成果可用于长期处于变低温状态下材料的力学性能研究及其工程应用。     

Marble decay induced by thermal strains: simulations and experiments

Thermoelastic behavior of different marble types was analyzed using computational modeling and experimental measurements. Eight marble samples with different composition, grain size, grain boundary geometry, and texture were investigated. Calcitic and dolomitic marbles were considered. The average grain size varies from 75 μm to 1.75 mm; grain boundary geometry differs from nearly equigranular straight grain boundaries to inequigranular-interlobate grain boundaries. Four typical marble texture types were observed by EBSD measurements: weak texture; strong texture; girdle texture and high-temperature texture. These crystallographic orientations were used in conjunction with microstructure-based finite element analysis to compute the thermoelastic responses of marble upon heating. Microstructural response maps highlight regions and conditions in the marble fabric that are susceptible to degradation phenomena. This behavior was compared to the measured thermal expansion behavior, which shows increasing residual strains upon repetitive heating–cooling cycles. The thermal expansion behavior as a function of temperature changes can be classified into four categories: (a) isotropic thermal expansion with small or no residual strain; (b) anisotropic thermal expansion with small or no residual strain; (c) isotropic thermal expansion with a residual strain; and (d) anisotropic thermal expansion with residual strain. Thermal expansion coefficients were calculated for both simulated and experimental data and also modeled from the texture using the MTEX software. Fabric parameters control the amount and directional dependence of the thermal expansion. Marbles with strong texture show higher directional dependence of the thermal expansion coefficients and have smaller microstructural values of the maximum principal stress and strain energy density, the main precursors of microcracking throughout the marble fabric. In contrast, marbles with weak texture show isotropic thermal expansion behavior, have a higher propensity to microcracking, and exhibit higher values of maximum principal stress and strain energy density. Good agreement between the experimental and computational results is observed, demonstrating that microstructure-based finite-element simulations are an excellent tool for elucidating influences of rock fabric on thermoelastic behavior.     

Environmental impact on the Roman monuments of Tarragona,Spain

The extant remains of the Roman monuments of Tarragona, Spain are made of different types of Miocenic rocks from the quarries surrounding the city, which vary from calcarenite to bioclastic limestones, showing different degrees of dolomitization, depending on their diagenetic evolution. The decay of these monuments is highly dependent on the mineralogy and the fabric of the stone as well as on the environmental conditions to which the monument subjected. As a consequence, different forms of decay are observed on these monuments, namely, granular disintegration, differential erosion between sparitic and micritic areas of the rock, and development of black crust and orange patinas, some of them attributed to a sulfation process. A number of processes have been established as being responsible for the decay forms observed: sulfation on sheltered areas of the building in the urban environment; differential dilatation because of the NaCl of the marine spray that crystallizes inside the porosity; hydric and thermal expansion of the stone, both related to the amount and crystallinity of the clay minerals forming the rock matrix; and biocolonization on the stone surface. An empirical model is proposed to explain the decay forms studied in relation to these factors (rock and environment).     

Natural building stones of Mexico–Tenochtitlán: their use, weathering and rock properties at the Templo Mayor, Palace Heras Soto and the Metropolitan Cathedral

In the Aztec period and in colonial times different natural stones originating in the Valley of Mexico were used for building construction. Stone weathering was investigated onsite at various historical buildings within the old quarter of Mexico City. In this study, different aspects of weathering and deterioration at three significant historical buildings will be presented, the Aztec excavation site Templo Mayor, the Metropolitan Cathedral, and the colonial palace of the dukes of Heras Soto. Petrophysical properties of the main building stones of these structures were investigated like density, porosity, pore radii distribution, water uptake rate and coefficient, thermal and hygric expansion, and the mechanical properties of uniaxial compressive strength. A relationship between single critical property values, according to anisotropy fabric characteristics, and specific weathering forms could be deduced.     

Marble bowing: comparative studies of three different public building facades

The veneer cladding of the Oeconomicum (OEC, Göttingen), the State Theatre of Darmstadt (STD, Darmstadt) and of the State and University Library (SUB, Göttingen) is characterised by pronounced bowing after a short time of exposure. Direct comparison of bowing data related to measurements from 2000 to 2003 at the SUB clearly show that the amplitude in bowing had significantly increased. The bowing is different in intensity and orientation (concave, convex). The cladding material (Peccia marble, Rosa Estremoz marble and Carrara marble) are different in lattice preferred orientation, grain size distribution and grain interlocking. Depending on the bowing, panels may show cracks mostly initiated at the dowels. The percentage of visible cracks and breakouts increases with the amplitude of bowing except for the STD. Repetitive heating–cooling under dry conditions leads to considerable inelastic residual strain only after the first or second thermal cycle. The residual strain continuously increases again if water is present, whereby the moisture content after a thermal cycle has a certain impact on the decay rate. The water-enhanced thermal dilatation strongly correlates with the deterioration rate obtained from the laboratory bow test. Detailed petrophysical investigations provide evidence that with increasing bowing a decrease of mechanical properties (flexural strength or breaking load at dowel hole) occur. Marble degradation is also connected with the increase in porosity and a general shift of the maximum pore radii to larger pore sizes. On-site damage analyses were combined with laboratory tests of the bowing potential to constrain factors that may influence the risk failure. The experimental bowing data clearly demonstrate that after 40 heating cycles combined with the effect of moisture a certain impact on the decay rate is observed. In the case of demounted panels the bowing tests show that already strongly deformed panels from the building exhibit a lower bowing potential than those with lower amplitudes of bowing. This is not the general case for all marble types. Finally, the artificial bowing causes a significant reduction of the flexural strength and the breaking load at the dowel hole. The strength loss of the experimentally aged claddings combined with on-site damage analyses led to conclusions concerning risk assessment and the predicted lifetime of the investigated marble claddings.     

高温下花岗岩的细观结构与声发射特性研究

利用LEICA DM4500P偏光显微镜对实时温度作用下山东临沂花岗岩的细观形态进行了观测,结合其在高温下单轴压缩与声发射检测试验结果,对不同温度下花岗岩的强度和声发射与细观结构形态关系进行了初步的探讨。研究表明,高温下花岗岩细观结构形态的变化主要体现在不同温度下裂纹萌生及扩展速度的不同;随温度的升高,花岗岩内部形成的裂纹越多,内部损伤越严重,单轴压缩下其声发射活动越频繁;花岗岩的力学特性及声发射特征与岩样内部裂纹网络的形成具有对应的关系,裂纹扩展缓慢则其峰值应力曲线和振铃累计数曲线走势平稳,而裂纹网络急剧扩展则峰值应力曲线和振铃累计数曲线出现拐点导致突变。通过观测岩石在热作用下内部结构形态的变化,以期推断其在热破裂过程中物理力学特征参量发生变化的原因。     

Physical weathering of marbles caused by anisotropic thermal expansion

 Marbles as building stones as well as in their natural environments show complex weathering phenomena. The most important damage scenario is based on the highly anisotropic thermal expansion coefficient α of calcite, i.e. extreme expansion parallel and contraction normal to the crystallographic c-axis. Therefore, the rock fabric and especially the lattice-preferred orientation (texture) of calcite and/or dolomite as the predominant mineral phases in marbles have a significant influence on the mechanical weathering. The textures of marbles from five different locations vary from a more or less perfect prolate to moderate oblate shape of the [006] pole figure tensor. Accordingly, the texture-derived bulk thermal dilatation anisotropy covers a broad range from –0.048 to 0.680. The modelled thermal dilatations correlate with those obtained from experimental measurements. The difference in magnitude is basically explained by the microcrack fabrics which was not considered in the computations. All samples show a deterioration due to thermal treatment regardless of the strength of texture. The directional dependence of (a) the total magnitude of the thermal dilatation coefficient and (b) of the residual strain is highest in marbles with a strong texture, whereas the Carrara marble with a weak texture exhibits a uniform crack formation. The progressive loss of cohesion along grain boundaries due to dilatancy may serve as an example for the initial stage of physical weathering. Received: 10 February 1999 / Accepted: 16 October 1999     

Statistical approach of the influence of petrography in mechanical properties and durability of granitic stones

The influence of petrographic features on the strength of granitic stones is a wide studied topic which finds different correlations depending on the research and the granite type. The aim of this article was to provide an accurate statistical analysis in which the amount of analysed data did not imply any doubt about the representativity of the samples and the accuracy of the results. The focused principal component analysis was used because it allows to explain a determinate property in relation to several variables. In addition, the expression of the results was done as a simple and graphical representation that allowed to interpret the results in a global way. Data of texture, mineralogy and strength of 12 granites were obtained in this study and were completed by those of more than 100 granites obtained from the literature. The durability of the twelve granite characterized was also assessed. A thermal fatigue test was carried out in 5?×?5?×?5 cm cubes revealing that the thermal expansion experimented by the different minerals was enough to produce variations in the crack network even if temperature was lower than the microfissuration threshold.     

The effect of weathering on pore geometry and compressive strength of selected rock types from Turkey

Weathering processes cause important changes in rock porosity. Besides porosity, distribution of pore sizes is significant for the identification of changes due to rock weathering and its effects on fabric. The formation of secondary porosity in different types of rock taken from different parts of Turkey was examined and the results are presented in this paper. The aim of this study was to observe changes in porosity due to weathering. Effective porosity, mercury intrusion porosimetry, and optical and scanning electron microscopy were used to evaluate the changes in pore geometry of the rocks. Additionally, the dry density, water absorption and uniaxial compressive strength of the rocks at different weathering stages were determined. Analysis of experimental data showed that microstructure of the rocks in relation to weathering is the main feature, which controls their physical and mechanical properties. The study revealed that fabric characteristics, particularly the pore and fracture geometry are very important characteristics for assessment of the behaviour of weathered rock.     

磁化率各向异性与剪切带

岩石组构记录了地壳形成与演化的关键信息,提取这些信息对分析和恢复地球动力学过程具有重要意义.磁化率各向异性(AMS)是一种重要的岩石组构方法,可以有效地揭示岩石的应变特征,分析其地球动力学过程,是研究构造变形性质以及应力作用方式的有效手段.本文在梳理AMS的研究历史、主要成果和最新进展的基础上,系统阐述了AMS的基本原...     

Field meeting report: the geology of some West London Churches, 22nd September 2012

This excursion report describes briefly the ecclesiastical geology of seven localities in West London. It reveals that building stones related to local geology were particularly important in the construction of early churches. For this reason wall building fabric analyses of churches are also able to indicate an enormous amount of local geological information. Where London Basin rocks were unsuitable for structural aspects of these churches it was sometimes necessary to use rock types from more distant sources.     

华南燕山期两种不同矿化花岗岩类中锆石标型特征的研究

华南地区燕山期两种不同矿化花岗岩类中锆石的标型特征有较明显区别。第一类花岗岩锆石的比重、硬度、红外光谱频率均低于第二类花岗岩锆石,而晶胞常数正相反,第一类花岗岩锆石较富含W、sn、Be、Nb、U、Th REE,第二类则富含Cu、Pb;第一类花岗岩锆石表面微结构特征有熔蚀坑、裂纹呈网状、附生物形态不定,第二类则表面无熔蚀坑,裂纹和附生物均较规则等。本中探讨了锆石标型特征控制因素和形成规律。为锆石及母岩的成因物质来源、成岩成矿提供矿物学标志和信息。     

Weathering behaviour and construction suitability of dimension stones from the Drei Gleichen area (Thuringia, Germany)

The construction suitability of a dimension stone depends on its weathering properties along with the petrology and the petrophysical properties. The aim of this study was to evaluate the suitability of the dimension stones from the “Drei Gleichen” area for construction and replacement purposes. In total, six sandstones (Ingersleben, Wachsenburg, Hindfelden, Seeberg, Röhnberg, Gleichenberg; Upper Triassic) as well as two carbonates (Wachsenburg sinter; Quaternary, Wandersleben dolomite; Middle Triassic) were analysed. The results from our laboratory and on-site studies of the dimension stones show that rocks from the same stratigraphic layer, like the sandstones from the upper Triassic, can show major differences in their petrophysical and weathering properties. These differences are attributed to their different diagenesis, resulting, e.g. in varying pore space, water balance and strength properties. The pore size distribution can be divided into four different groups based on their occurring maxima and micropore content. The determined water balance properties as well as moisture expansion and salt attack depend on these groups. Next to this, the mineralogical composition significantly influences the weathering resistance. Sandstones with a high content of altered lithoclasts show a high amount of moisture expansion, low strength and, in consequence, a low weathering resistance against salt attack. Based on the results of the present study, an evaluation of construction suitability could be accomplished. From the analysed sandstones, only the Seebergen sandstone is suitable for construction purposes due to its good availability, good strength properties (high compressive and tensile strength, low softening degree) as well as a low porosity. Furthermore, the Wachsenburg sandstone also shows good petrophysical and petrological properties, but exploitable deposits are too sparse to be of commercial interest. From the carbonates, the Wachsenburg sinter shows very suitable rock parameters, but only sparse outcrops occur, which are not appropriate for mining.