Thaw Damage Research Articles

The Dynamic Compressive Properties and Energy Dissipation Law of Sandstone Subjected to Freeze–Thaw Damage

To investigate the dynamic compressive properties and the law of energy dissipation of freeze–thaw-damaged sandstone, static and dynamic compressive experiments were conducted. The influences of the number of freeze–thaw cycles and strain rate on strength characteristics, energy dissipation rate and the fractal dimension characteristics of sandstone were evaluated. Based on the peak energy dissipation rate, a freeze–thaw damage variable was established. The results show that peak strength increases exponentially with strain rate, and there exists a strain rate threshold. When strain rate is below this threshold, the increasing rate of the DIF slows down with the increase in the number of freeze–thaw cycles; when strain rate is higher than this threshold, the increasing rate of the DIF increases with the increase in the number of freeze–thaw cycles. In addition, the fractal dimension increases with the number of freeze–thaw cycles as well as the strain rate. Based on the freeze–thaw damage variable established, the damage degree of sandstone under freeze–thaw cycling can be characterized.

Experimental study on freeze–thaw damage characteristics of coal samples of different moisture contents in liquid nitrogen

In this study, the surface crack-propagation law and pore damage characteristics of coal samples of different water contents after they undergo leaching in liquid nitrogen are investigated using a 4 K scientific-research camera, HC-U7 non-metal ultrasonic detector, nuclear magnetic resonance testing technology, and self-made multi-functional three axial fluid–solid coupling test system. Experimental investigations are conducted on coal samples of different water contents before and after they undergo liquid-nitrogen freezing and thawing in order to determine the propagation law of surface fissures, the development law of internal micro-fissures, the development process of internal pores, the change law of the pore-size distribution, and the law of coal-sample deformation and gas seepage during the stress process. The test results show that, with the increase in water content in the liquid-nitrogen leaching process, the frost heave force on the coal surface increases, and the greater the increase ratio of the coal porosity, the faster is the development of micro-cracks and pores. Under the action of liquid nitrogen, the number of micro-pores, meso-pores, and macro-pores in the coal sample increased, and with the formation of new cracks and the connection of the original cracks, liquid-nitrogen freezing and thawing can promote the development of the pore structure in the coal body. The permeability changes of coal samples of different water contents during unloading failure exhibit obvious stage characteristics. The above results demonstrate that the moisture content of coal has a significant effect on the development of surface cracks and pore-damage characteristics of coal after liquid-nitrogen freezing and thawing, and there is a positive correlation between the surface crack expansion and internal damage of the coal samples of different moisture contents leached in liquid nitrogen.

Effect of hospital waste pyrolysis hydrocarbon (HWPHC) on fracture behavior of Warm Mix asphalt (WMA) under freeze–thaw damage (FTD)

Warm mix asphalt (WMA) is more susceptible to macroscopic cracks than other mixtures due to insufficient aggregate-bitumen adhesion, especially at low temperatures. In this study, the behavior of low-temperature cracking (LTC) and intermediate temperature cracking (ITC) of WMA mixture with and without the environmentally friendly additive called hospital waste pyrolysis hydrocarbon (HWPHC) was evaluated at −15 and +15°C under mode I loading conditions using three geometries: symmetric specimen SCB, classical-modified specimen SCB-1, and symmetric specimen ENDB. In order to determine the effect of freeze–thaw damage (FTD) on the mixtures, samples with and without the HWPHC additives were subjected to 0 and 3 FTD cycles. The results showed that adding 3 % and 6 % of HWPHC additive increased the fracture resistance of the samples, i.e., fracture energy (GF) and fracture toughness (KIC) at temperatures of ±15 °C using all three mentioned geometries under 0 and 3 FTD cycles. At temperatures of ±15 °C and for all mixtures with and without additives, the average GF and KIC for symmetric specimen ENDB (vertical crack) are lower and higher than for symmetric specimen SCB (vertical crack) and classical-modified specimen SCB-1 (angular crack), respectively. Therefore, the complete fracture potential and crack initiation for the ENDB geometry were higher and lower than the other two geometries (under 0 and 3 FTD cycles). Also, at temperatures of ±15 °C and under 0 and 3 FTD cycles, the flexibility indices showed that adding HWPHC reduced the flexibility of the WMA mixture slightly. Finally, the increase in the fracture resistance and resistance to elastic deformation of the mixtures modified with HWPHC additive was achieved using two Tensile Stiffness Index (TSI) and Tensile Strength (TS) indices. Therefore, in addition to helping to reduce the amount of environmental waste and increase cleaner production, the mentioned mixtures can be used to improve the LTC and ITC fracture performance of the WMA mixture under mode I and FTD conditions.

Study on the simulation method and mesoscopic characteristics of rock freeze-thaw damage

The damage to saturated rock caused by freeze–thaw cycles seriously affects the stability of projects in cold regions. To accurately obtain the mechanical characteristics of rock after freeze–thaw damage, this paper extracts the microscopic pore structure parameters of freeze–thaw sandstone based on CT scanning tests. Functional relationships between the number of freeze–thaw cycles and both the frost heaving coefficient and thawing coefficient of pore particles are established, and a new method to simulate the mechanical behavior of freezing-thawing rock is proposed based on PFC3D. The CT test results show that the freeze–thaw cycle causes a certain degree of damage to the mesopore structure of sandstone. The sensitivity of each pore structure parameter to characterize the damage is pore volume > effective porosity > porosity > pore radius > fractal dimension > throat length > throat radius > coordination number. Numerical simulation results show that with the increase in the number of freeze–thaw cycles, the internal damage degree of sandstone is more obvious, and the bearing capacity declines. In addition, the rock mainly produces tensile cracks during the freeze–thaw cycle, and failure occurs first outside the rock. The research results reveal the freeze–thaw damage mechanism of rock more accurately.

Freeze–Thaw damage characteristics of composite modified open graded friction course

To reasonably describe the damage characteristics of composite modified open graded friction course (OGFC) after multiple freeze-thaw cycles, based on the Able viscoelastic constitutive equation, a viscoelastic model of freeze-thaw damage was constructed and analyzed using the Weibull distribution function, damage mechanics, and fractional derivative theory. Under the conditions of composite modified OGFC mixtures with different mixing ratios (12%, 0%), (12%, 1%), and (12%, 2%), and multiple freeze-thaw cycles (0–16), low-temperature bending and creep tests of the mixtures were carried out. The stress-strain curve data obtained were fitted to analyze the physical significance of the model parameters. The results show that the model is suitable for characterizing the viscoelastic stage of composite modified OGFC under 10 freeze-thaw cycles. The freeze-thaw damage model parameters of three types of composite modified OGFC with different mixing ratios were compared and analyzed. The order of the fractional derivative of the composite modified OGFC model (12%, 1%) was 0.2223, the maximum damage threshold was 1.108, and the maximum viscosity coefficient was 371.84. This composite modified OGFC had the best low-temperature crack resistance.

Frost resistance of fiber-reinforced self-compacting recycled concrete

Abstract Freeze–thaw damage and the lack of aggregate resources have become two major problems facing the construction industry. The use of recycled aggregate to produce recycled concrete has good social and economic value. In this study, the author used recycled coarse aggregate as a substitute for stone to mix recycled concrete. In order to enhance the performance of concrete, a microwave heating modification process was introduced and 0.10% volume fraction of polypropylene fiber (PPF) was added. The effect of microwave heating modification on the frost resistance of fiber-reinforced self-compacting recycled concrete was studied. The results show that with the increase in the number of freeze–thaw cycles, the degree of denudation and deterioration of the three groups of concrete surfaces becomes more and more serious, the number of surface cracks gradually increases after the concrete is damaged, and the plastic deformation becomes more and more serious, W n gradually increases, E n gradually increases, D n gradually increases, σ 0 gradually decreases, ε 0 gradually increases, ε u gradually increases, E c gradually decreases, and μ gradually decreases. Under the same number of freeze–thaw cycles, the frost resistance of the three groups of concrete is A > C > B. Microwave heating modification improved the frost resistance of group C concrete compared to that of group B concrete. Due to the incorporation of PPF into the concrete, the load did not drop sharply when the load acting on the concrete reached the failure load. The prediction results of the established constitutive model are in good agreement with the experimental results.

A constitutive model of freeze–thaw damage to transversely isotropic rock masses and its preliminary application

A constitutive model was developed to study damage in transversely isotropic rock masses caused by freeze–thaw cycles. The model was based on the Weibull distribution function, the macroscopic bedding plane damage variable and a new freeze–thaw damage variable. The model was verified by the results of uniaxial and triaxial compression tests of Carboniferous slates at various bedding angles that corresponded to different numbers of freeze–thaw cycles. The predictions of the model and the experimental results followed similar trends. Thus, the model objectively reflected the complete evolution of the stress and strain of a transversely isotropic rock mass due to freeze–thaw cycling. Then, the constitutive model was embedded into UDEC numerical analysis software to analyze the bedding–freeze–thaw stress coupling response of a whole section of the Guanjiao Tunnel during excavation. The results of the calculation accurately reflected the failure mechanism of the rock surrounding the tunnel induced by excavation and correlated with field monitoring data.

Experimental study of chloride transport law in concrete considering the coupling effects of dry-wet ratio and freeze–thaw damage

In marine environment, dry-wet ratio (D/W) in the dry-wet cycling zone of coastal structures and freeze–thaw (FT) damage can both affect the chloride transport characteristics in concrete. In this paper, FT cycling tests of concrete specimens were conducted by the FT cycles testing machine to establish the relationship between the number of FT cycles and FT damage. Then, the chloride natural diffusion tests of concrete specimens with different FT damage under the dry-wet cycling zone were carried out to investigate the chloride transport law considering the coupling effects of D/W and FT damage on the condition of the dry-wet cycling period = 24 h. Under the influence of dry-wet cycles, there is a convection zone in the transmission of chloride in concrete specimens. Based on the Fick's second law, the convection zone peak concentration, the convection zone depth and the apparent chloride diffusion coefficient considering the coupling effects of D/W and FT damage were determined by fitting the chloride concentration distributions at various exposure time. The results show that the effect laws of D/W on unfrozen and frozen concrete specimens are basically the same. The convection zone peak concentration, the convection zone depth and the apparent chloride diffusion coefficient increase with the increase of FT damage in concrete specimens. The influences of FT damage on the convection zone peak concentration, the convection zone depth and the apparent chloride diffusion coefficient were quantified by introducing the impact coefficients of FT damage. Finally, a prediction model of chloride transport in concrete considering the coupling effects of D/W and FT damage was proposed, and the accuracy and reasonability of the prediction model were validated by comparing the predictive results of the model and the measured values of the tests.

Pure mode I fracture resistance of hot mix asphalt (HMA) containing nano-SiO2 under freeze–thaw damage (FTD)

• Pure mode I fracture resistance of Nano-SiO2-modified asphalt mixture under FTD was evaluated. • The semi-circular bending and edge notched disc bend tests were performed on samples at ± 15 °C. • Nano-SiO 2 improved the behavior of LTC and ITC asphalt mixtures under mode I loading and FTD conditions. • Nano-SiO 2 improved the fracture energy and fracture toughness of asphalt mixture under FTD. • SCB geometry with an angle of 35°was critical due to lower average fracture toughness. In cold climates, increased traffic load has increased the damage caused by low-temperature cracks (LTCs) in asphalt mixes. Due to the viscoelastic behavior of asphalt mixtures, cracks caused by intermediate temperatures (ITCs) have led researchers to think more about improving the technical properties of asphalt mixtures with additives. On the other hand, asphalt pavements are affected by freeze–thaw cycles during their service life, a process that intensifies distress and reduces crack resistance. However, despite the effect of freeze–thaw cycles on the fracture behavior of LTCs and ITCs, this process has received less attention from researchers. Hence, in this study, the effect of Nano-SiO 2 on the occurrence of low and intermediate temperature cracks under freeze–thaw damage (FTD) conditions in asphalt mixtures has been investigated experimentally. For this purpose, Semi-Circular Bending (SCB) and Edge Notched Disc Bend (ENDB) tests were used under mode I loading to investigate the effect of Nano-SiO 2 on the crack resistance of the asphalt mixture (i.e., vertical and angular cracks) at temperatures of ± 15 °C. Before the fracture test, the samples were subjected to 0 and 3 FTD cycles to investigate the effect of these conditions on the long-term fracture resistance of the samples. The results showed that adding Nano-SiO 2 increased the brittleness of the mixtures, fracture energy, and fracture toughness of asphalt mixtures containing vertical and angular cracks in both 0 and 3 FTD cycles. Also, it was concluded that Nano-SiO 2 could recover a large part of the resistance reduced by the 3 FTD cycles; therefore, using Nano-SiO 2 additive improved the long-term behavior of LTC and ITC asphalt mixtures under mode I loading and FTD conditions. Finally, by comparing all the geometries used in this research, it was concluded that from the point of view of fracture energy, the ENDB geometry, and from the point of fracture toughness, the SCB geometry with an angle of 35°was critical due to lower average fracture energy and lower average fracture toughness, respectively (temperatures of ± 15 °C under 0 and 3 FTD cycles).

Evaluation of fracture behavior of Warm mix asphalt (WMA) modified with hospital waste pyrolysis carbon black (HWPCB) under freeze–thaw damage (FTD) at low and intermediate temperatures

Asphalt pavement, as an important engineering structure under various conditions such as freeze–thaw cycles and temperature fluctuations, suffers from failures such as low-temperature cracks (LTCs) and intermediate-temperature cracks (ITCs). Therefore, studying and providing a suitable solution is a concern of researchers in this field. On the other hand, the amount of hospital waste is increasing due to the Covid-19 pandemic; hence, one of the appropriate solutions is to recycle and use them in engineering structures such as asphalt pavement to reduce environmental pollution. In this study, a hospital waste pyrolytic carbon black (HWPCB) additive from recycled hospital waste resulting from the COVID-19 pandemic was used to improve the LTCs and ITCs resistance of Warm Mix Asphalt (WMA). For this purpose, three geometries called Symmetric specimen SCB (containing vertical cracks), Classical-Modified specimen SCB-1 (containing angular cracks), and Symmetric specimen ENDB (containing vertical cracks) were subjected to mode I loading conditions. In order to adapt the actual conditions of the pavement to the laboratory conditions, the samples were subjected to 0 and 3 cycles of freeze–thaw damage (FTD). They were fractured at 15°Celsius. The results showed that adding 18 % HWPCB to the WMA mixture increased the fracture energy and fracture toughness of all three geometries under mode I in both 0 and 3 FTD cycles at ± 15 °C. Also, it was concluded that the HWPCB additive was able to compensate for the reduction in resistance created by the 3 FTD cycles and increase it to some extent. On the other hand, the results of the Tensile Stiffness Index (TSI) and Tensile Strength (TS) indices showed that adding 9 and 18 % HWPCB increased the resistance to elastic deformation at ± 15 °C (under 0 and 3 FTD cycles) in addition to improving the crack resistance of the WMA mixture. The results of the Flexibility Index (FI), Toughness Index (TI), and Cracking Resistance Index (CRI) brittleness indices for WMA mixtures containing different percentages of HWPCB additive showed that this additive slightly reduced the flexibility of the mixture under 0 and 3 FTD cycles at temperatures of ± 15 °C. Finally, it was concluded that using HWPCB additive in the WMA mixture improved the behavior of LTC and ITC under mode I and FTD conditions.

Effects of Polypropylene Fibers on the Frost Resistance of Natural Sand Concrete and Machine-Made Sand Concrete.

In order to study the effect of polypropylene fibers on the frost resistance of natural sand and machine-made sand concrete, polypropylene fibers (PPF) of different volumes and lengths were mixed into natural sand and machine-made sand concrete, respectively. The freeze–thaw cycle test was carried out on polypropylene-fiber-impregnated natural sand concrete (PFNSC) and polypropylene-fiber-impregnated manufactured sand concrete (PFMSC), respectively, and the apparent structural changes before and after freezing and thawing were observed. Its strength damage was analyzed. A freeze–thaw damage model and a response surface model (RSM) were established used to analyze the antifreeze performance of PFMSC, and the effects of the fiber content, fiber length, and freeze–thaw times on the antifreeze performance of PFMSC were studied. The results show that with the increase in the number of freeze–thaw cycles, the apparent structures of the PFMSC gradually deteriorated, the strength decreased, and the degree of freeze–thaw damage increased. According to the strength damage model, the optimum volume of PPF for the PFNSC specimens is 1.2%, and the optimum volume of PPF for the PFMSC specimens is 1.0%. According to the prediction of RSM, PFNSC can maintain good antifreeze performance within 105 freeze–thaw cycles, and when the PPF length is 11.8 mm, the antifreeze performance of PFNSC reaches the maximum, its maximum compressive strength value is 33.8 MPa, and the split tensile strength value is 3.1 MPa; PFMSC can maintain a good antifreeze performance within 96 freeze–thaw cycles. When the length of PPF is 9.1 mm, the antifreeze performance of PFMSC reaches the maximum, its maximum compressive strength value is 45.8 MPa, and its split tensile strength value is 3.2 MPa. The predicted values are in good agreement with the measured values, and the model has high reliability.

Analytical solution of mechanical response in cold region tunnels under transversely isotropic freeze–thaw circle induced by unidirectional freeze–thaw damage

During the operation stage of cold region tunnels, the isotropic surrounding rock in a freeze–thaw circle suffers long-term unidirectional freeze–thaw cycles and gradually transforms into transversely isotropic material, which induces the variation of stress and displacement distribution of cold region tunnels. Aimed at this phenomenon, an analytical solution of mechanical response in cold region tunnels under transversely isotropic freeze–thaw circles induced by unidirectional freeze–thaw damage is proposed. The analytical solution is derived under two different states of the freeze–thaw circle: 1) transversely isotropic and unfrozen state (state TU) and 2) transversely isotropic and frozen state (state TF). In addition, the stress distribution in the lining and surrounding rock with a transversely isotropic freeze–thaw circle is analyzed. The transformation of the surrounding rock in a freeze–thaw circle from isotropic material into transversely isotropic material leads to the increase of stress in the lining, especially for a significant increase under state TF. Finally, the influence of the deterioration coefficient and the degree of anisotropy on the stress distribution in the lining is analyzed. The stress in the lining increases linearly as the deterioration coefficient decreases, while it increases nonlinearly as the degree of anisotropy decreases. The smaller the degree of anisotropy is, the greater the increase rate of the stress is. Moreover, the increase of stress with deterioration coefficient and degree of anisotropy under state TF is much greater than that under state TU. Both deterioration coefficient and degree of anisotropy decrease from 1.0 with increasing unidirectional freeze–thaw cycles suffered by surrounding rock, and, thus, induce the increase of stresses in the lining. In addition, the deterioration coefficient has a greater influence than the degree of anisotropy on the stress in the lining.

A novel pneumatic structure for reducing train-induced mass and heat transfers in a permafrost tunnel

As a train passes quickly through a confined space such as a tunnel, it induces dramatic airflow movement in the tunnel. The substantial mass transfer results in a noteworthy energy transfer in the air-tunnel system, forming complex aerodynamic and thermodynamic processes (ATPs). The aerodynamic process has been studied well enough to solve many practical engineering issues. However, little attention has been given to the thermodynamic process in tunnels, especially heat transfer in permafrost tunnels. To study the train-induced mass and heat transfers in the Fenghuoshan permafrost tunnel and design a novel pneumatic structure (PS) to mitigate thaw damage to the tunnel, we built a train-induced mass-heat transfer (MHT) model for permafrost tunnels based on mass, momentum and energy conservation and simulated the ATPs in tunnels without and with the PS. By comparing the mass and heat transfers in the two tunnel structures, we found that the air temperature maximally increases by 1.7 °C in the tunnel without the PS, whereas it increases by only 0.5 °C in the tunnel with the PS. In addition, the PS also reduces the convective heat transfer coefficient (CHTC) by 38.6 % compared with that of the tunnel without the PS. Therefore, the PS can significantly decrease the temperature rise in the tunnel system and prevent excess heat energy from penetrating into the surrounding permafrost. Consequently, it is an efficient method for mitigating thaw damage to permafrost tunnels. Moreover, the study is also helpful for comprehending train-induced mass and heat interactions in permafrost tunnels.

Mechanical recovery of lime-stabilised clays subjected to freeze–thaw damage

Lime stabilisation of clay soils is a widely used technique for the improvement of existing soils in the construction of embankments and sub-bases for road pavements. The mixing of clayey soils with lime initiates chemical reactions allowing a significant increase of geotechnical properties. This study aims at investigating the ability to recover strength of a conventional lime-stabilised clay soil after being distressed by multiple Freeze–Thaw (F–T) cycles. This capability was investigated as a function of the percentage of lime additive, post-compaction curing time and the presence of a disturbing element in clay-lime interaction. The stabilisation of clay soils with lime provides capability of recovering compressive strength after distress from freeze–thaw cycles,regardless of the post-compaction curing time. The percentage of lime added assumes a very important role in the extent of this capability. The presence of disturbing elements negatively affects the success of pozzolanic reactions between clay and lime.

Microstructure and damage evolution of hydraulic concrete exposed to freeze–thaw cycles

In cold regions, hydraulic concrete structures are vulnerable to freeze–thaw (FT) damage, which is manifested by the evolution of microstructure in concrete. In order to reveal this evolution law, the rapid FT test is carried out on hydraulic concrete, and the damage evolution of hydraulic concrete under FT action is studied by using the combination of X-ray computed tomography (X-ray CT) technology and deep convolutional neural network (U-Net) image segmentation method. The results show that the U-Net image segmentation has higher accuracy than traditional threshold segmentation, and can capture microcracks and capillary pores caused by FT damage. The pores and cracks developed due to the FT cycles connect the isolated primary pores with each other, resulting in a significant increase in the connectivity and skeleton length of the internal pore structure of hydraulic concrete. In addition, the FT damage increases the number of pores and microcracks in interfacial transition zone (ITZ), and even degums aggregates from their surrounding mortar matrix, resulting in a continuous increase in the surface area of pores and microcracks in ITZ. Considering the FT damage of hydraulic concrete as a kind of fatigue effect, a FT damage evolution model based on continuous damage mechanics (CDM) is established, and the effectiveness of the model in predicting FT damage of hydraulic concrete is verified by comparing with the experimental results.

The laboratory performance of asphalt mixture with thermoplastic polyurethane (TPU) and amorphous poly alpha olefin (APAO) compound modified asphalt binder

• The effect of TPU/APAO on the asphalt mixture performance was explored. • APAO could enhance the rutting resistance of TPU modified asphalt mixture. • TPU has positive impact on the cracking resistance of asphalt mixture. • 2 % TPU + 6 % APAO modified asphalt mixture had superior road performance. The using of waste polymer modified asphalt binder in pavement construction is cost effective and beneficial to the environment. In this paper, the modified asphalt binders are prepared by adding thermoplastic polyurethane (TPU, 0 %, 2 %, 4 %, 6 %, by weight of asphalt binder) and amorphous poly alpha olefins (APAO, 0 %, 2 %, 4 %, 6 %). The conventional physical, rotational viscosity and dynamic shear rheometer (DSR) tests were conducted on the modified binders and the results indicated that the addition of APAO could improve the rutting resistance the TPU modified asphalt binder, and the modified asphalt binder with 2 % TPU and 6 % APAO showed the best rutting resistance among the studied binders. To assess the effect of APAO dosage on the performance of TPU modified asphalt mixture, mechanical performance tests including Marshall stability, dynamic stability, indirect tensile, immersion Marshall, freeze–thaw splitting and long-term aging tests were carried out on asphalt mixtures with various TPU/APAO content combinations. The test results proved that 2 % TPU + 6 % APAO modified asphalt mixture showed better resistance to rutting and moisture damage, the superior long-term aging damage resistance, and the excellent anti-freeze–thaw damage ability, when compared to the unmodified asphalt mixture. According to the indirect tensile test results at −10 °C, despite the indirect tensile strength ( ITS ) value of 2 % TPU modified asphalt mixture decreased after APAO added, the ITS value of asphalt mixture modified with 2 % TPU and 6 % APAO was still improved 5.8 % than base asphalt mixture. Based on the comprehensive analysis of the data, 2 % TPU + 6 % APAO modified asphalt mixture has superior road performance, and the performance characteristic of TPU/APAO compound modified asphalt mixture indicated that it could be applied for warmer regions.

Investigating the Mechanical and Durability Characteristics of Fly Ash Foam Concrete.

Although fly ash foam concrete (FAFC) is lightweight, heat-retaining, and insulating, its application options are constrained by its weak construction and short lifespan. The effects of various dosage ratios of the foaming agent (i.e., hydrogen peroxide), silica fume, and polypropylene fiber on the dry density, compressive strength, thermal insulation performance, pore structure parameters, and durability of FAFC were analyzed in this study, which sought to address the issues of low strength and low durability of FAFC. According to the findings, there is a negative correlation between the amount of hydrogen peroxide (as the foaming agent) and compressive strength, and, as the silica fume and polypropylene fiber (PP fiber) content rise, the strength will initially rise and then fall. The distribution of pore sizes gradually shifts from being dominated by small pores to large pores as the amount of foaming agent increases, while the porosity and average pore size gradually decrease. When the hydrogen peroxide content is 5%, the pore shape factor is at its lowest. The pore size distribution was first dominated by a small pore size and thereafter by a large pore size when the silica fume and PP fiber concentration increased. Prior to increasing, the porosity, average pore size, and pore shape factor all decreased. Additionally, the impact of PP fiber on the freeze–thaw damage to FAFC was also investigated at the same time. The findings indicate that the freeze–thaw failure of FAFC is essentially frost heave failure of the pore wall. The use of PP fiber is crucial for enhancing FAFC’s ability to withstand frost. The best frost resistance is achieved at 0.4% PP fiber content. In conclusion, the ideal ratio for overall performance was found to be 5% hydrogen peroxide content, 4% silica fume content, and 0.1% polypropylene fiber content. The results obtained could be applied in different fields, such as construction and sustainable materials, among others.

Desert silty sand modified by anionic PAM and ordinary portland cement: Microfabric reinforcement and durability

The modification of desert silty sands has been a great challenge due to its relative uniform particle dimension and non-cohesion. This paper presents a study on utilizing a new additive, anionic polyacrylamide (PAM) with ordinary Portland cement (OPC) to modify the desert silty sand. Unconfined compression strength (UCS) tests were conducted to investigate the strength and durability (resistance to cyclic wet-dry and freeze–thaw damage) of modified sands at the maximum dry density; the scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy tests were carried out for microstructural analysis. Results reveal that the PAM fraction has a significant influence on the mechanical performance of the modified desert silty sand, and a threshold PAM dosage exists, beyond which adding excessive PAM will inhibit the strength and durability further. The microstructural analysis reveals that the physical stick-up of the PAM-to-sand particles leads to the formation of the spatial cobweb-like reticular structure. The Ca2+ cation released by OPC served as a cationic bridge, which in turn enhance the PAM adsorption efficiency. However, excessive PAM would play a negative role, chiefly because of the excessive accumulation between the hydrated production and sand particles. This study indicates that the combination of cement and PAM has the virtues of both cement-stabilized and PAM microfabric-reinforced sand. This binary PAM–OPC modification approach plays a more efficient role if a rational PAM incorporation (0.1–0.3 % in this study) is selected.

Mechanical strength model of engineered cementitious composites with freeze–thaw damage based on pore structure evolution

A freeze-thaw cycle test, nuclear magnetic resonance (NMR) T2 spectrum test, uniaxial tensile test, and uniaxial compression test were designed to study the service conditions of engineered cementitious composites (ECC) in a freeze-thaw (FT) environment in cold areas. The degradation law of the mechanical properties and damage characteristics of the pore structure of ECC with three water-binder ratios (M1: W/B = 0.24, M2: W/B = 028, and M3: W/B = 0.32) are discussed. The results demonstrate that the number of micropores and mesopores in the ECC specimens increases with an increase in the number of freeze-thaw cycles (FTs), thus gradually increasing the porosity. The uniaxial tensile strength (UTS) and uniaxial compressive strength (UCS) decrease gradually with increasing number of FTs. After FTs300, the porosity of the three groups increases by 43.83%, 59.39%, and 59.16%; the loss ratios of UTS are 26.16%, 24.47%, and 19.63%; and the loss ratios of UCS are 31.52%, 27.98%, and 27.30%, respectively. Based on the mechanical properties and pore structure test results of ECC, relation models of the UTS and UCS of freeze-thaw damaged (FTD) ECC with porosity were constructed according to the simplified center hole model of ECC and the elastic mechanics theory. To further disclose the FT evolution laws of uniaxial tensile/compressive strengths (UT/CS), the relationship models of UT/CS and number of FTs with the degree of FTD were deduced. The constructed models and test results were compared with those of some classical porosity-strength models. It was found that the constructed models met the requirements of satisfactory reliability and applications.

Improvement of CO2-Cured Sludge Ceramsite on the Mechanical Performances and Corrosion Resistance of Cement Concrete.

The application of CO2 curing on sludge ceramsite may improve its mechanical properties, and then increase the corresponding corrosion resistance. In this study, the influence of CO2-cured sludge ceramsite on the strength and long-term properties of cement concrete is investigated. CO2 curing time ranges from 0 h to 2 d. The cylinder compressive strength and water absorption rate of CO2-cured sludge ceramsite are first determined. Additionally, the flexural and compressive strengths, the chloride permeability and the freeze—thaw damage, as well as the corresponding thermal conductivity of cement concrete, are tested. Furthermore, the corrosion resistance of reinforcement inner-sludge-ceramsite cement concrete is measured. Finally, the scanning electron microscope photos of sludge ceramsite are obtained. Results show that the cylinder compressive strength of CO2-cured sludge ceramsite is 15.1, ~34.2% higher than that of sludge ceramsite. Meanwhile, the water absorption rate of CO2-cured sludge ceramsite is 39.6, ~82.4% higher than that of sludge ceramsite. The compressive strength and the flexural strength of cement concrete with CO2-cured sludge ceramsite are 11.4 and 18.7, ~21.6% and ~31.5% higher than the cement concrete with sludge ceramsite, respectively. The resistance of NaCl freeze—thaw cycles, determined by comparing the mass loss rate and the loss rates of mechanical strengths, is effectively improved by CO2 curing, while the thermal conductivity of cement concrete is decreased by CO2 curing. The corrosion resistance of inner reinforcement is improved by the application of CO2 curing on sludge ceramsite.