Peak Compressive Strength Research Articles

A Comparative Study on Damage Variable and Geo-Mechanical Properties of the Pre-Damaged Rock Specimen

The initial damage of a rock mass plays an essential role in parameter evaluation of that rock mass. How to determine the initial damage of a rock mass is therefore a problem that must be solved. To study the mechanical properties of a rock mass, pre-damaged specimens with different degrees of damage are prepared using uniaxial compression. The degree of damage of the pre-damaged specimens is determined by using their physical and mechanical parameters; that is, four methods are used to calculate the damage variables. Comparing the calculated damage variables using the four methods, we find the following: (1) the damage variables of different calculation methods are quite different. (2) When the degree of damage is slight, in other words, the micro-cracks are in the initial state of growth, and the calculated damage variables have little difference when using various methods. (3) When the degree of damage is severe, that is, the micro-cracks coalesce to form macro-cracks, the calculated damage variables significantly deviate when using various methods. Only the calculated damage variables using the BTS are larger than those calculated using other methods, mainly because there is almost no friction effect of cracks in BTS testing. Finally, to verify the accuracy and feasibility of the damage variables of various calculation methods, the uniaxial compressive peak strength of the pre-damaged specimens is determined by using the effective strain hypothesis and the damage variable. The 1 : 1 slope line is used to verify the deviation between the measured values and calculated values.

The Effect of Pore Structure on Impact Behavior of Concrete Hollow Brick, Autoclaved Aerated Concrete and Foamed Concrete

Porous concrete is an energy absorption material, which has been widely used in civil engineering, traffic engineering and disaster reduction engineering. However, the effect of pore structure on the impact behavior of the porous concrete is lacked. In this study, a series of drop-weight impact tests were carried out on three typical types of porous concrete, i.e., concrete hollow brick (CHB), autoclaved aerated concrete (AAC) and foamed concrete (FC), to investigate the effect of pore structures on their impact behavior. For comparison, static load tests were also conducted as references. According to the damage to the samples, the developments of impact force, strain, contact stress–strain relationship and absorbed energy during drop-weight during the impact test were measured and analyzed. The results show that the ratio between the peak impact stress and compressive strength of CHB was 0.44, while that of AAC and FC increased to about 0.6, indicating that the small and uniform pore structure in AAC and FC had a higher resistance against impact load than the hollow cavity of CHB. In addition, the elastic recovery strain in AAC increased by about 0.2% and its strain at peak contact stress increased by about 160% for a comparison of CHB, implying that a small open pore structure could enhance ductility. Besides, the peak contact stress of FC was close to that of AAC during impact loading, while the strain at peak contact stress of FC increased by about 36% compared with AAC, revealing that the closed-pore structure could further enhance the deformation potential. Correspondingly, the energy absorption rates of CHB, AAC and FC were 85.9 kJ/s, 54.4 kJ/s and 49.7 kJ/s, respectively, where AAC decreased by about 58% compared with CHB, and FC decreased by about 10% compared with AAC.

Research on confining pressure effect of pore structure of coal-rich in coalbed methane under cyclic impact

Coal seam penetration modification is the primary technical problem facing the efficient development of coalbed methane and coal mine methane control. To study the effect of cyclic impact load on the permeability of coal rock under the confining pressure, the dynamic mechanical properties and the pore evolution of coal were conducted by using the separated Hopkinson pressure bar (SHPB) and the low-field nuclear magnetic resonance (NMR) technique. The results showed that the dynamic compressive strength existed as a turning point at a critical confining pressure of 6 MPa and the spectrum area growth rate of adsorption pores was negatively correlated with that of seepage pores. As the confining pressure increased, the pattern of the effective porosity growth rate of the coal was positively correlated with the peak compressive strength, meanwhile, the permeability growth rate of the coal showed an exponential growth trend. The evolution pattern of the permeability under the coupled action of the confining pressure and impact pressure was obtained. In addition, based on the U-shaped distribution of permeability growth rate and effective porosity growth rate with the impact pressure, an exponential relationship between them was derived. The related research results could provide important theoretical support for coalbed methane exploitation.

Influence of Water Saturation Time on Microstructure and Mechanical Properties of Jointed Limestone

In order to study the influence of water saturation time on the mechanical properties of jointed limestone, the joint limestone samples with a crack length of 20 mm and a crack angle of 30° are saturated with water for 0, 1, 3, 5 and 7 days and tested with the RMT-150C mechanical test machine. The results show that the water saturation time has a significant effect on the stress-strain curve, peak strength, elastic modulus, crack development and failure characteristics of the jointed limestone. There is a step rise or fall phenomenon near the peak stress. The peak strength and elastic modulus of the joint limestone are significantly negatively correlated with the saturated time. Compared with that of the saturated 0-day sample, the peak compressive strength of the saturated 1-day, 3-day, 5-day and 7-day samples was decreased by 16.5%, 24.7%, 43.6%, and 47.7%, respectively, and the elastic modulus decreased by 11.0%, 32.0%, 45.0%, and 67.9%, respectively. Saturated water cracks the skeleton particles of the limestone samples, and the length of the microscopic pores and fissures increased gradually. The phenomenon that the number and length of cracks increase with the increase of the water saturation time will weaken the mechanical properties of the jointed limestone. The physical and chemical reactions, lubrication, and pore water pressure produced by water-saturated water-rock interactions make it easier for the Mohr stress circle to reach the limit equilibrium state. Saturated water reduces the degree of fragmentation of the sample, and the integrity of the fracture surface is higher after failure. The findings in this paper are helpful to further reveal the mechanism of the crushing of supports and the frequent occurrence of water gushing accidents at the working face in the Xinhua region of northwestern Guizhou.

Fracture evolution of artificial composite rocks containing interface flaws under uniaxial compression

Composite rock with interfaces is widely distributed in nature. Its mechanical properties are significantly different from those of a single layer rock, which has become one of the highlighted issues in engineering design and operation. In this study, uniaxial compression tests are performed on rock-like specimens with two dissimilar layers. The evolution process of the apparent strain field of the specimen is analysed by digital image correlation (DIC). Acoustic emission (AE) technology is used to analyse the damage evolution process of specimens based on energy dissipation theory and the damage evolution model. The research results show that with the increase in the interface angle, the peak compressive strength of the specimen decreases slowly first and then decreases rapidly, increasing when reduced to the minimum. When the inclination angle of the specimen interface is 60°, its strength is the lowest. The higher the overall strength of the specimen is, the more obvious the above phenomenon. DIC technology captures the apparent strain field characteristics of each stage of specimen failure, which provides a basis for the analysis of the specimen failure mode. The failure modes of the specimens can be divided into five categories. When the inclination angles of the interface are 0°, 60°, 75° and 90°, the failure modes of the specimens are mainly affected by the inclination angle. When the inclination angles are 15°, 30° and 45°, the failure modes are affected by the inclination angle and the rock matrix strength. The damage evolution process of most specimens is divided into four stages. Some specimens only show three stages, such as those specimens with an interface inclination angle of 60°.

Geomechanical Behaviour of Clay Stabilised with Fly-Ash-Based Geopolymer for Deep Mixing

Geopolymer has recently become an attractive alternative to traditional binders (e.g., cement and lime) used for chemical soil improvement, with several environmental benefits including lower toxic emissions and energy consumption. This paper presents an evaluation of the geomechanical behaviour of soft clay treated with fly-ash-based geopolymer incorporating slag for deep soil mixing (DSM) applications. The geomechanical properties of stabilised clay were evaluated using unconfined compressive strength (UCS) tests and durability against wetting–drying. Thermal conductivity and pH tests along with microstructural analysis using scanning electron microscopy (SEM) were also performed to provide insights into the effect of geopolymer on treated clay. The results indicate that the inclusion of geopolymer with the increase in curing time and activator content considerably improves the geomechanical performance of geopolymer-treated soft clay in terms of stress–strain response and attainable peak compressive strength. Although it was found that a small percentage of geopolymer can enhance the initial compressive response, a larger dosage of geopolymer up to 30% was necessary to maintain stable durability performance over successive wetting–drying cycles. Such improved durability performance is related to the enhanced soil structure due to the cementation development and overall reduction in thermal conductivity. The reduction in thermal conductivity of treated clay was found to be activator-dependent and was suppressed steadily with the increase in activator concentration. Overall, geopolymer-treated clay showed promising potential for DSM applications due to its enhanced strength and durability responses.

Compressive experimental method and properties of C/C composites under ultra-high temperature environment

Carbon/Carbon (C/C) composites are expected to serve as structural materials over 2800 ℃. Experiments under ultra-high temperatures (UHT) are critical and demanding. In this paper, we established the UHT compressive experiment technique using simultaneous Joule heating and compressive loading fixtures. The specimen was designed and validated to achieve uniform temperature and strain at the gauge section. Compressive strengths and failure behaviors of three-directional (3D) needled, 3D woven, and four-directional (4D) woven C/C composites under UHT up to 3100 ℃ were investigated. The failure modes and mechanism of strength differences were illustrated through mesoscopic surface morphologies. Results showed that the dog-bone-shaped specimen avoided crushing at loading ends and exhibited failure at gauge sections. Temperatures with peak compressive strengths for 3D and 4D woven C/C composites were determined. Differences between the C/C composites were related to heat treatment temperatures. The sublimation phenomenon was observed for 4D woven C/C composites over 3000 ℃, degrading the compressive strengths by over 50%.

Strength and Deformation Characteristics of Dune Sand Earth Blocks Reinforced with Natural and Polymeric Fibers

In this paper, we present a strength and deformation evaluation of earth blocks reinforced with different types of fiber. A natural palm tree fiber and two other types of synthetic polyolefin fibers were used to improve earth blocks’ strength and stiffness. The soil used to cast the earth blocks was composed of dune sand, which is extensively available in the United Arab Emirates and most Gulf Cooperation Council (GCC) deserts, and clayey silt soil acquired from Al-Ain city, UAE. Sixteen different mixes were prepared for this study. Two identical earth blocks were cast in a wooden mold from each mix to form a total of 32 blocks. After compaction, the blocks were consolidated under pre-specified pressure, air dried, and tested to failure in the compression machine. The main parameters investigated in this study were clay content, fiber types and percentages, and cement content. The results indicate that the addition of natural or polymeric fiber to cast earth blocks significantly improved their compressive strength. The maximum compressive strength of improved blocks showed an increase of more than 30% above the maximum compressive strength recorded for identical soil blocks without fiber. In addition, the post-peak and pre-peak strength behaviors of the earth blocks are different with different fiber contents. Finally, the addition of 0.5% cement to the fiber-reinforced blocks showed a negligible effect on the peak compressive strength. However, cement content improved the early stage (small strain) stiffness of earth blocks.

An investigative approach on the influence of mineral admixtures on internally cured concrete

Concrete curing is vital to get the anticipated properties of the concrete structure. Among all the different available curing methods, the internal curing of concrete is the most recent and is gaining prominence over a few years. This work is engrossed on developing the internal curing concrete of M40 grade employing Polyethylene glycol-400 as an internal curing supplement. The impact of mineral admixtures, on mechanical properties, like silica fume and fly ash were examined at varying dosages. Polyethylene glycol-400 was used as a replacement to cement at 2.5% by weight of cement. By maintaining the constant dosage of Polyethylene glycol-400, the silica fume was used as 5% and 10% replacement to cement and fly ash at 15% and 25% by weights of cement, and the mechanical properties were assessed in contrast to traditionally cured concrete. The peak compressive and split tensile strengths were obtained at 5% replacement of silica fume, and at 10% replacement, the maximum flexural strength was obtained. As the percentage increase of fly ash content beyond 15%, decreased the strength. Fly ash and silica fume showed a positive impact on the mechanical properties of internally cured concrete when compared with the concrete cured conventionally.

The microstructure and compressive properties of a sintered Fe–Mn–Al porous steel produced by blended elemental powder mixture

In this study, powder compacts produced by mixed elemental Fe, Mn, Al and C powders without ball milling were sintered at 640∘C and 1200∘C, respectively. The results revealed that intermetallic phases belonging to Fe2Al5 and Al8Mn5 appeared after pre-sintered at 640∘C. When the temperature increased to 1200∘C, only α-Fe phase existed. The total porosities of the sintered samples were ∼60 vol.% at 640∘C and ∼70 vol.% at 1200∘C, and the peak compressive strength of the 1200∘C-sintered samples was ∼1.6 MPa at the strain of ∼11%.

Compressive strength prediction of basalt fiber reinforced concrete via random forest algorithm

Basalt fiber is a green non-polluting material with strong tensile mechanical properties. In this paper, the strength prediction model of basalt fiber concrete is constructed by random forest method based on experiments to study the strengthening effect of basalt fiber in concrete composites and explore the impact of different fiber sizes and fiber content on the strength of basalt fiber concrete. First, the compressive strength tests of BFRC under different stress states were performed at fiber volume fractions of 0.2%, 0.4%, 0.6% and fiber lengths of 6 mm, 12 mm and 18 mm to obtain stress–strain curves and peak compressive strength. Then, 70% of the original test data is used to establish a random forest training sample set, and the remaining 30% is used as a test set. According to the error outside the bag, the appropriate number of decision trees and leaf nodes are selected, and the influencing factors are ranked by importance. Then use the Pearson correlation diagram to analyze the correlation of each influencing factor, establish a random forest training model, and output the fitting prediction results of the model training set and the prediction set. Finally, the prediction results of random forest with BP neural network and support vector regression are measured by MSE, RMSE, and R2 evaluation metrics for performance. The results show that the random forest prediction model has a good model fitting ability. Compared with other algorithms, the accuracy of the RF model in the MSE index is increased by 8% and 18.8%, which further verifies the accuracy and reliability of the model.

Dynamical mechanical properties and microstructure characteristics of cemented tailings backfill considering coupled strain rates and confining pressures effects

This paper discusses the relationship between the average strain rate (ASR), confining pressure, dynamic peak compressive strength (DPCS), and cemented tailings backfill (CTB) crack volume through the SHPB test and microscopic computed tomography (CT). The CTB samples with a water-cement ratio of 1:4 and a slurry concentration of 75% were prepared to carry out SHPB tests with different ASR (25.6 ∼ 104.73 s−1), different ASR (12.16 ∼ 152.05 s−1) at different confining pressures (0.5 ∼ 2 MPa). The results show that with the increase of ASR when there is no confining pressure, the DPCS shows a trend of first increasing and then decreasing. However, when the confining pressure is 0.5 and 1 MPa, the DPCS increases linearly. When the confining pressure is 1.5 and 2 MPa, the DPCS increases exponentially and logarithmically, respectively. The micro-CT analysis shows that the total crack volume generated after the SHPB rises first and then falls with the increase of the ASR under confining pressure of 1 MPa. When the impact amplitude is 150 mV, the crack volume also decreases with the improvement of the confining pressure (0.5–2 MPa). The internal cracks of the CTB change from shear cracks to tensile cracks, and the initial pores of the samples first decrease and then increase. The failure pattern of all CTB samples by SHPB was mainly shear, tensile, and hybrid failure (shear and tensile). The research results can provide a valuable reference for the dynamic performance of CTB under confining pressure and the design of mining backfill.

Constitutive relations and finite element modelling of Infra Lightweight Concrete (ILC)

ILC stands for “Infra” Lightweight Concrete, a high-performance Lightweight Aggregate Concrete (LWAC) with a dry density of no more than 800 kg/m3. It possesses a unique combination between a sufficient bearing capacity and good insulation properties. Therefore, monolithic cross sections of ILC can meet the energy consumption requirements without adding additional insulation layers. ILC, hence, allows for a simple and a sustainable construction as the complex, highly insulation, multi-layered exterior walls of buildings can be replaced by one material only. During recent years, ILC has been thoroughly investigated and several ILC houses were successfully built. However, predictive formulas and constitutive models for the structural analysis were not fully developed. This paper deals with the derivation of constitutive mechanical relationships and the flexural behavior of ILC. Relationships for estimating the elastic modulus, peak compressive and tensile strength have been proposed. A stress-strain behavior in compression and a simplified tension-stiffening model are presented. The flexural behavior of different grades of reinforced ILC beams has been experimentally and numerically carried out. The comparison between the numerical and the experimental results in terms of moment-displacement curves showed significant agreement, thereby validating the proposed relationships. Furthermore, an analytical approach for estimating the flexural strength of ILC is also introduced.

Experimental investigation of the compressive behaviour of GFRP wrapped spruce-pine-fir square timber columns

An experimental program investigating the behaviour of square timber columns wrapped with fibre-reinforced polymer (FRP) composites under compression loading was undertaken. A total of six unwrapped control specimens and thirty glass FRP (GFRP) specimens were tested with emphasis on characterizing the altered stress–strain behaviour and examining the failure modes. The effects of varying GFRP composite configurations (fibre direction, thickness) were investigated, and each specimen was dissected after testing to study how the wood failed. Test results showed that with proper thickness, FRP wrapping can prevent undesirable splitting and shearing modes of failure in the wood and negate the influence of naturally occurring wood defects to induce crushing in the wood fibres. The crushing mode of failure promoted by FRP wrapping contributed to significantly improved post-peak behaviour with FRP wrapped specimens sustaining up to 61% of their peak compressive strength at a strain of 0.04 mm/mm compared to 41% in the control specimens. The reinforced specimens were on average capable of dissipating between 1.35 and 1.68 times more energy compared to the control group. It is clear from the test results that FRP wrapping can help the overall behaviour of wood members become more reliable and uniform. In particular, the consistency in the post-peak stress–strain behaviour and wood failure is conducive to the development of an analytical model. The improved post-peak behaviour could also benefit wood members in flexure and wood structures subjected to extreme loads such as earthquakes and blast loads.

Behavior of steel clamp confined brick aggregate concrete circular columns subjected to axial compression

Re-use of recycled aggregates in concrete has become a very sustainable solution owing to its environment-friendly and cost-effective nature. Clay bricks are most widely used in construction all over the world. Subsequently, waste generated from the demolition of clay-brick structures is most troublesome. Inherent porosity of bricks compromises the compressive strength of concrete with clay brick aggregates (C-CBA) that has rendered its use in structural concrete for quite some time. With some external clamping pressures, this deficiency can be countered. This study presents an experimental investigation into the use of steel clamps as a strengthening material for C-CBA. Steel clamps are cost-effective, readily available, and very easy to use. C-CBA were tested in two groups depending upon compressive strength: 15 & 35 MPa. Further, 3, 5, and 11 steel clamps were used in each group. Results suggested that steel clamps can effectively be used in enhancing the compressive strength of C-CBA to a level that is acceptable to be used in structural concrete. Increase in peak compressive strength of C-CBA was found to be proportional with the number of steel clamps employed. Three analytical concrete models were used to predict the compressive strength of C-CBA after strengthening. It was concluded that all three analytical models predicted well C-CBA compressive strength for 3-clamp specimens. However, their pre-diction accuracy was highly compromised as the number of steel clamps were increased.

Exploitation of Crushed Granite Fine as Replacement to River Sand in Concrete

The suitability of Crushed granite fine (CGF) to replace river sand in concrete production for use in rigid pavement was examined. Slump, compressive and indirect tensile strength tests were performed on fresh and hardened concrete.28 days Peak compressive and indirect tensile strength values of 40.70N/mm2 and 2.30N/mm2 respectively was obtained, with the partial replacement of river sand with 20% CGF, as against values of 35.00N/mm2 and 1.75N/mm2, obtained with the use of river sand as fine aggregate. Based on economic analysis and results of tests, river sand replaced with 20% CGF is recommended for use in the production of concrete for use in rigid pavement. Conservation of river sand in addition to better ways of disposing wastes from the quarry sites are some of the merits of using CGF.

Optimization of bentonite modified cement mortar parameters at elevated temperatures using RSM

The inaccessibility of industrial wastes necessitated the search for natural pozzolanic materials. This paper deals the optimization of bentonite utilization in cement mortar at elevated temperatures using response surface methodology. The substitution of calcined bentonite (0%, 10%, 20% and 30%) and temperature exposure (24°C, 200°C, 400°C and 600°C) were taken as variables. The compression strength and strength activity tests for all mixes were carried out for bentonite. Design Expert 11.0 version was utilized for modelling using RSM. The peak compressive strength was displayed by 20% replaced bentonite calcined at 200°C cement mortar after 28 days curing. Strength activity was improved upon increasing the percentage of bentonite calcined at 200°C. The generated models from RSM are significance in all the factors considered. Optimum solutions were proposed with more than 0.90 desirability.

The effect of geometric characteristics on mechanical properties of asperities

The deformation and failure behavior of fractured rock mass is closely related to the unevenness of the fracture surface. The deformation behavior of asperities on rough fracture surfaces has not been studied comprehensively. This paper aims to study the effect of geometric parameters on the mechanical properties of asperities under normal load. In this work, the uniaxial compression experiment using the discrete element software PFC3D was used to investigate the strength and stiffness law of asperities. The results show that compressive strength and stiffness are less affected by the height-width ratio of spherical asperities. However, the curvature of the asperities tip has a significant impact on the peak compressive strength and stiffness. As the curvature of asperities decreases, the peak compressive strength and stiffness rise. In addition, the strength and stiffness of the unevenness of the surface are also affected by the size effect. The peak compressive strength and stiffness increase as the size of the unevenness of the surface grows. The results show that the mechanical properties of asperities are significantly affected by the curvature and the size effect.

Effect of Freeze-thaw Cycles on Microstructure and Mechanical Properties of Limestone

In studying the effect of freeze-thaw cycles on the microstructure and mechanical properties of limestone, scanning electron microscope, low-field nuclear magnetic resonance, and uniaxial compression tests are carried out on the limestone samples dried and subject to 5, 10, 15, and 20 freeze-thaw cycles. Through analysis and discussion, it was found that the number of freeze-thaw cycles has a significant effect on the microstructure of the limestone samples. Compared to the dry samples, the number of microcracks of the limestone samples subjected to freeze-thaw cycles is increased. Moreover, the crack width is positively correlated to the number of freeze-thaw cycles. With an increase in freeze-thaw cycles, the signal intensity of the T2 spectrum is considerably increased, and the connectivity of the micropores and fissures is increased. In addition, the peak compressive strength and elastic modulus showed a significant negative correlation with the number of freeze-thaw cycles. Again, compared to the dry samples, the peak compressive strength of the samples subject to 5, 10, 15, and 20 freeze-thaw cycles decreased by 14.7%, 22.0%, 28.7%, and 37.5%, respectively. The elastic modulus also decreased by 8.0%, 11.4%, 11.7%, and 21.5%, respectively. Therefore, these findings provide basic parameters for the stability evaluation of rock slopes in Northwest Guizhou.

Survey of Mechanical Properties of Geopolymer Concrete: A Comprehensive Review and Data Analysis.

Mechanical properties and data analysis for the prediction of different mechanical properties of geopolymer concrete (GPC) were investigated. A relatively large amount of test data from 126 past works was collected, analyzed, and correlation between different mechanical properties and compressive strength was investigated. Equations were proposed for the properties of splitting tensile strength, flexural strength, modulus of elasticity, Poisson’s ratio, and strain corresponding to peak compressive strength. The proposed equations were found accurate and can be used to prepare a state-of-art report on GPC. Based on data analysis, it was found that there is a chance to apply some past proposed equations for predicting different mechanical properties. CEB-FIP equations for the prediction of splitting tensile strength and strain corresponding to peak compressive stress were found to be accurate, while ACI 318 equations for splitting tensile and elastic modulus overestimates test data for GPC of low compressive strength.