High Prestress Research Articles

Finite Element Modeling and Artificial Neural Network Analyses on the Flexural Capacity of Concrete T-Beams Reinforced with Prestressed Carbon Fiber Reinforced Polymer Strands and Non-Prestressed Steel Rebars

The use of carbon fiber reinforced polymer (CFRP) strands as prestressed reinforcement in prestressed concrete (PC) structures offers an effective solution to the corrosion issues associated with prestressed steel strands. In this study, the flexural behavior of PC beams reinforced with prestressed CFRP strands and non-prestressed steel rebars was investigated using finite element modeling (FEM) and artificial neural network (ANN) methods. First, three-dimensional nonlinear FE models were developed. The FE results indicated that the predicted failure mode, load-deflection curve, and ultimate load agreed well with the previous test results. Variations in prestress level, concrete strength, and steel reinforcement ratio shifted the failure mode from concrete crushing to CFRP strand fracture. While the ultimate load generally increased with a higher prestressed level, an excessively high prestress level reduced the ultimate load due to premature fracture of CFRP strands. An increase in concrete strength and steel reinforcement ratio also contributed to a rise in the ultimate load. Subsequently, the verified FE models were utilized to create a database for training the back propagation ANN (BP-ANN) model. The ultimate moments of the experimental specimens were predicted using the trained model. The results showed the correlation coefficients for both the training and test datasets were approximately 0.99, and the maximum error between the predicted and test ultimate moments was around 8%, demonstrating that the BP-ANN method is an effective tool for accurately predicting the ultimate capacity of this type of PC beam.

Seismic response investigation of prestressed anchor cable supporting rock slope with weak interlayer in Qinghai-Tibet Plateau, China

Earthquake-induced rock landslides in the eastern mountains of the Tibetan Plateau, especially landslides with weak interlayers pose a significant threat to major construction projects. Prestressed anchor cable is one of the main reinforcement methods of rock slopes. This paper combines shaking table model tests and numerical simulation to study the reinforcement effect and dynamic response characteristics of prestressed anchor cables applied to rock slopes with weak interlayers under strong earthquakes. The research results show that prestressed anchor cables can effectively reinforce slopes with weak interlayers. A small cable inclination, a small spacing and a high prestress are recommended in the seismic reinforcement design of prestressed anchor cable. In addition, the characteristics of slope progressive damage and prestress loss under the earthquake are found by the shaking table test. The results have been applied in hazard prevention and control of rock slopes on the Chengdu-Lanzhou Railway at the eastern Qinghai-Tibet Plateau.

Experimental investigation on rockburst characteristics of highly stressed D-shape tunnel subjected to impact load

Rockburst has always been a challenge for the safe construction of deep underground engineering. This study investigated the rockburst characteristics in highly-stressed D-shape tunnels under impact loads from rock blasting and other mining-related dynamics disturbances. The biaxial Hopkinson pressure bar was utilized to apply varying biaxial prestress and the same impact loads to cube specimens with D-shape hole. High-speed camera and digital image correlation (DIC) were used to capture the failure process and strain field of specimen. The test results demonstrate that the D-shape hole specimen experience rockburst under coupled static stress and impact load. Under this circumstance, the rockburst mechanism of the D-shaped hole specimens involves spalling in sidewall induced by impact load, indicating dynamic tensile failure. The high static prestress provides the initial stress field, while the impact load disrupts the stress equilibrium, result in the stress or strain concentration in the sidewall of the D-shape hole, inducing rockburst. Moreover, the rockburst process can be divided into (1) calm stage, (2) crack initiation, propagation, and coalesce stage, (3) spalling stage and (4) rock fragments ejection stage. Impact load triggers rockburst occurrence, while vertical stress further determines the rockburst characteristics. The influence range and magnitude of strain concentration zone and displacement deformation of the tunnel surrounding rock increases with increasing vertical stress, thus inducing more severe rockburst.

Shear strengthening of RC beams with prestressed NSM CFRP: Influencing factors and analytical model

This study proposes the application of prestressed near-surface-mounted (NSM) carbon-fiber-reinforced polymer (CFRP) technique in the field of the shear-strengthening of bridges for the first time. Fourteen reinforced concrete (RC) beams shear strengthened with prestressed NSM CFRP were tested under static load. The effect of the CFRP prestressing level, spacing, angle, and end-anchorage measures on the shear-strengthening behavior was evaluated. The experimental results demonstrate that the ultimate shear capacity of prestressed NSM CFRP shear-strengthened beams increased by 65–127% when compared to that of the reference beams, and the width of shear cracks was effectively suppressed. The failure mode of prestressed NSM CFRP shear-strengthened beams without end-anchorage measures was web concrete cover separation, which can be suppressed using a CFRP U-jacket and through-beam screw. Increasing the CFRP prestressing level and percentage enhanced the ultimate shear capacity and cracking resistance of the strengthened beams. However, an excessively high CFRP percentage and prestress level combination resulted in large shear crack angles and decreased the shear contribution of CFRP and concrete. Finally, an analytical model based on the modified compression field theory (MCFT) was proposed to predict the flexural-shear load response of strengthened beams, which was in agreement with the experimental results.

Excavation compensation and bolt support for a deep mine drift

To overcome large deformation of deep phosphate rock roadways and pillar damage, a new type of constant-resistance large-deformation negative Poisson's ratio (NPR) bolt that can withstand a high pre-stress of at least 130 KN was developed. In the conducted tests, the amount of deformation was 200–2000 mm, the breaking force reached 350 KN, and a high constant-resistance pre-stress was maintained during the deformation process. A stress compensation theory of phosphate rock excavation based on NPR bolts is proposed together with a balance system for bolt compensation of the time-space effect and high NPR pre-stress. Traditional split-set rock bolts are unable to maintain the stability of roadway roofs and pillars. To verify the support effect of the proposed bolt, field tests were conducted using both the proposed NPR bolts and split-set rock bolts as support systems on the same mining face. In addition, the stress compensation mechanism of roadway mining was simulated using the particle flow code in three dimensions (PFC3D)-fast Lagrangian analysis of continua (FLAC3D) particle-flow coupling numerical model. On-site monitoring and numerical simulations showed that the NPR excavation compensation support scheme effectively improves the stress state of the bolts and reduces the deformation of the surrounding rock. Compared to the original support scheme, the final deformation of the surrounding rock was reduced by approximately 70%. These results significantly contribute to domestic and foreign research on phosphate-rock NPR compensation support technology, theoretical systems, and engineering practices, and further promote technological innovation in the phosphate rock mining industry.

Numerical and experimental evaluation of a variable-stiffness wedge anchorage for basalt-fiber-reinforced polymer tendons

The small-sized variable-stiffness wedge (VSW) anchorage, a second-generation solution, was numerically optimized to anchor basalt-fiber-reinforced polymer tendons, building upon the previous first-generation anchorage. The static, fatigue, and relaxation characteristics of the anchorage were experimentally evaluated. Results revealed that VSWs effectively mitigated radial stress concentration within the tendons compared to constant stiffness wedges. Bonding optimization techniques involving cross-splitting and sandblasting proved advantageous in accurately assessing the tensile properties of round tendons. The second-generation anchorage demonstrated an 87% anchoring efficiency, surpassing that of a longer commercial steel-wedge anchorage significantly. The tendon-anchorage assembly can sustain 2 million cyclic loadings under a maximum fatigue stress of 0.44 ffpk (standard value for tendon tensile strength) and a stress range of 0.04 ffpk; moreover, sliding between wedges and tendons resulted in negligible stress reduction across various stress ranges. A logarithmic function effectively modeled the relationship between relaxation time and load retention in tendons with R-squared values consistently above 0.94 throughout; furthermore, the million-hour relaxation rate closely aligned with observed tendon behavior. The application of an initial tensile stress of 0.5 ffpk to tendons allowed for simultaneous high strength and minimal prestress loss.

Enhanced properties of nanocrystalline alumina ceramic with compressive prestress and coherently aligned nanograins

AbstractPrestress strategy is effective to inhibit crack advancement and toughen structural materials such as concrete and glass. However, it is difficult to be utilized in ceramic because of the solid‐state processing. In this work, nanocrystalline alumina ceramic with high compressive prestress and coherently aligned nanograins were successfully prepared by the ultra‐high pressure sintering technology. The as‐prepared ceramics exhibited remarkably enhanced mechanical properties, including hardness of 25.9 GPa and fracture toughness of 3.2 MPa·m1/2, which were respectively 36% and 51% higher than corresponding parameters of sapphire. Ultra‐high pressure generated compressive prestress as large as several hundred megapascal, and coherent nanograins in the dimensions of 10–15 nm, which markedly contributed to the simultaneously enhancement in toughness and hardness for ceramic materials. The prestressing strategy provides a new opportunity for fabricating advanced ceramics with high failure resistance.

Research on the influence of high temperature oxidation environment on the properties of C/C composite material

In this paper, the influence of extreme service environment on the mechanical properties of C/C composites was analyzed. The mechanical tests for ordinary C/C samples and C/C samples with anti-oxidation coating were carried out under various high temperature and oxidation conditions. The properties of C/C composites with anti-oxidation coating were focused on. It was pointed out that the material properties were mainly affected by the coupling factors of high temperature, pre-stress, oxidation environment and duration. The influence laws of these factors for the high temperature strength of C/C composites with anti-oxidation coating were further discussed and summarized.

Numerical Analysis of Fracture Behaviour for Cracked Joints in Corrugated Plate Girders Repaired by Stop-Holes.

The efficient crack eliminated stop-hole measure was proposed to repair and reduce the stress concentration associated fracture risk of the corrugated plate girders by setting it at the critical joint of flange plate with tightened bolts and gaskets under preloading. To investigate the fracture behaviour of these repaired girders, parametric finite element analysis was conducted, focusing on the mechanical feature and stress intensity factor of crack stop-hole in this paper. The numerical model was verified against experimental results first, and then the stress characteristics due to the presence of crack open-hole were analysed. It was found that the moderate-sized open-hole was more effective than the over-sized open-hole in the reduction of stress concentration. For the model with prestressed crack stop-hole through bolt preloading, the stress concentration was nearly 50% with the prestress around open-hole increased to 46 MPa, but such a reduction is inconspicuous for even higher prestress. Relatively high circumferential stress gradients and the crack open angle of oversized crack stop-holes were decreased owing to additional prestress effects from the gasket. Finally, the shift from the original tensile area around the edge of the crack open-hole that was prone to fatigue cracking to a compression-oriented area is beneficial for the reduction of stress intensity factor of the prestressed crack stop-holes. It was also demonstrated that the enlargement of crack open-hole has limited influence on the reduction of stress intensity factor and crack propagation. In contrast, higher bolt prestress was more beneficial in consistently reducing the stress intensity factor of the model with the crack open-hole, even containing long crack.

Development of superconducting model dipole magnets beyond 12 T with a combined common-coil configuration

The superconducting magnet group at IHEP (the Institute of High Energy Physics, Chinese Academy of Sciences, China) has been engaging in the R&D of high field accelerator magnets since 2014 for the pre-study of the next-generation high energy particle accelerators like CEPC-SPPC (Circular Electron Positron Collider-Super Proton Proton Collider), FCC (Future Circular Collider), etc. A superconducting model dipole magnet named LPF1 was first fabricated with NbTi and Nb3Sn in a combined common-coil configuration in 2017 and tested in 2018, a main field of 10.23 T was reached within two 10 mm apertures. The magnet was reassembled with higher pre-stress in 2019 to investigate the influence of the pre-stress loading level on the performance of common-coil dipole magnets. The main field of this upgraded magnet named LPF1-S was increased to 10.71 T within two 12 mm apertures. By improving the key processing technologies in Rutherford cable fabrication, coil winding, Nb3Sn & NbTi splices soldering and coil impregnation, the recently fabricated model dipole magnet named LPF1-U attained a 12.47 T main field in two 14 mm apertures in 2021. To fully take advantage of Nb3Sn and NbTi superconductors in different field regions, LPF1-U was still designed and developed with a combined coil configuration with Nb3Sn for the inner two coils and NbTi for the outer four coils. Additionally, graded coil topology and different coil LSS (length of straight section) settings were adopted in the optimization of LPF1-U layouts. Bladder and key technology was used for the support structure, and fiber optic sensors based on FBG (fiber Bragg grating) were utilized to monitor the stress distribution and variation in the structures and coils through the whole process from magnet assembly to cool-down and excitation. During the performance test, a relatively large number of trainings were carried out and LPF1-U became more stable after the thermal cycle. A peak field of 12.7 T in the coils (12.47 T main field in apertures) corresponds to an operating current of 6865 A and a load line ratio of 90% was finally attained. It is worth mentioning that LPF1-U is the first hybrid—Nb3Sn, NbTi—common coil dipole magnet that has achieved a main field of more than 12 T in two apertures. The design, fabrication and performance analysis of the LPF series dipole magnets are presented in this paper.

The residual stress and martensitic transformation of 304 stainless steel in pre-stress grinding: Influence and control on chloride induced SCC

In order to investigate the effect of pre-stress grinding on SCC resistance and its mechanism, a series of experiments and simulations were conducted. In the beginning, the SCC behaviors of 304 stainless steel on machined surface were studied. Then the SCC-relevant parameters of the ground surface were characterized in terms of residual stress and microstructures. Following that, a calculation model for evaluating the surface residual stress considering martensitic transformation was proposed. The results suggest that the proper pre-stress in grinding helps reduce the SCC length and density, transform the crack forms and decrease the martensitic content. The higher pre-stress is beneficial to reducing the tensile residual stress and decreasing the depth of the tensile stress layer, thus inhibiting the initiation and propagation of SCC. The release of the pretension provided by the pre-stress during grinding introduces the compressive effect, which reduces the surface stress. The reduction is approximately equal to the value of the pre-stress applied. By validation, the proposed theoretical model has the accuracy required for prediction. Besides pre-stress, smaller cutting depth, lower feed speed, faster linear speed, and grinding wheels with smaller grain sizes are recommended to obtain higher SCC-resistant machined surfaces in pre-stress grinding.

On the Vibration-Damping Properties of the Prestressed Polyurethane Granular Material

Granular materials promise opportunities for the development of high-performance, lightweight vibration-damping elements that provide a high level of safety and comfort. Presented here is an investigation of the vibration-damping properties of prestressed granular material. The material studied is thermoplastic polyurethane (TPU) in Shore 90A and 75A hardness grades. A method for preparing and testing the vibration-damping properties of tubular specimens filled with TPU granules was developed. A new combined energy parameter was introduced to evaluate the damping performance and weight-to-stiffness ratio. Experimental results show that the material in granular form provides up to 400% better vibration-damping performance as compared to the bulk material. Such improvement is possible by combining both the effect of the pressure-frequency superposition principle at the molecular scale and the effect of the physical interactions between the granules (force-chain network) at the macro scale. The two effects complement each other, with the first effect predominating at high prestress and the second at low prestress. Conditions can be further improved by varying the material of the granules and applying a lubricant that facilitates the granules to reorganize and reconfigure the force-chain network (flowability).

Heat-induced explosive spalling of self-prestressing, self-compacting concrete slabs

A novel concrete mix has been developed that can achieve high levels of self-prestressing through the controlled expansion of the concrete sample with cast-in carbon fibre reinforced polymer (CFRP) bars. Experiments have shown that the mechanical properties and durability of the mix are not adversely affected by the mix’s self-expanding nature. However, the behaviour of this mix at elevated temperatures is largely unknown, raising legitimate concerns regarding the fire performance of the resulting self-prestressed concrete elements. The research presented in this paper investigates the behaviour of concrete elements manufactured from self-prestressed, self-compacting concrete (SPSCC) when exposed to severe heating – such as would likely be experienced during a building fire. Nine specimens, with dimensions (600 mm × 200 mm × 45 mm) were tested under one-sided exposure to an experimentally simulated ISO 834 standard heating regime. The results showed that the SPSCC samples were acutely prone to explosive spalling under these conditions. The results also suggest that the comparatively higher moisture content of SPSCC samples, as compared with conventional concrete mixes of similar composition and mechanical properties, appeared to be the most critical factor for heat-induced concrete cover spalling. Higher prestress (compressive) forces also appeared to exacerbate the spalling likelihood of SPSCC samples. The addition of 2 kg/m3 of polypropylene (PP) fibres led to the complete elimination of spalling in SPSCC samples. The self-prestress levels in samples with PP fibres were 30 % less than those without PP fibres, for reasons which require additional investigation. Differential thermal expansion between the internal CFRP bars and the concrete was observed to restrain the elongation and thermal curvature of the samples during heating, up until the point where the bars debonded from the surrounding concrete due to elevated temperature and (presumed) increased tensile stress in the tendon anchorage zones. The results provide compelling evidence supporting the need to include PP fibres within high-performance, self-prestressing, self-compacting concrete slabs.

Study on large deformation and failure mechanism of deep buried stratified slate tunnel and control strategy of high constant resistance anchor cable

Aiming at the significant deformation problem of deep buried stratified soft rock tunnel, taking the No.3 inclined shaft of Muzhailing tunnel as background, the deformation and failure mechanism of the tunnel and the control strategy of high constant resistance anchor cable are studied. The deformation and failure mechanism of stratified tunnel surrounding rock is studied by physical model test, including the tunnel's temperature field, strain field, and displacement field from excavation to failure. Lateral stress significantly influences the deformation of the surrounding rock of the horizontal stratified tunnel. The surrounding rock of the tunnel forms a dumbbell-shaped tensile stress zone under the action of stress. The high tensile stress of the roof and floor leads to dislocation and layer separation, which causes floor heave failure and temperature change. The indoor tensile test verified the super strength characteristics and large deformation of the high constant resistance anchor cable. Based on the excavation compensation method and the coupling control mechanism of high constant resistance long and short anchor cables, a new high prestress constant resistance coupling control strategy is proposed. The numerical simulation and field test studies according to the new design are presented, which show that the high constant resistance anchor cable coupling support significantly reduces the tensile stress zone, effectively controls the deformation of tunnel surrounding rock, and avoids the damage of supporting structure and the cracking and deformation of the lining. The research results can provide a reference for constructing and supporting deep-buried, stratified soft rock tunnels.

Study on failure mechanism of deep soft rock roadway and high prestress compensation support countermeasure

The problem of surrounding rock stability control of deep soft rock has become increasingly prominent. Taking Shiyakou Coal Mine as the engineering background, the deformation and failure characteristics of roadway is obtained through on-site investigation and drilling peep. X-ray diffraction and electron microscope scanning experiments analyze the mineral composition and microstructure of surrounding rock, and the microscopic mechanism of roadway failure is obtained. The relationship between roadway deformation, rock inclination and position of the weak layer was analyzed by the numerical simulation. And the deformation and failure mechanism of soft rock cross-cut roadway is obtained. By analyzing the action mechanism of the high-stress compensation support theory, the countermeasure of high prestress es compensation support based on Constant-resistance large deformation (CRLD) bolt was put forward. The analysis of field test and monitoring results show that the control effect of the new support scheme is good and has guiding for similar projects.

Roof-cutting and energy-absorbing method for dynamic disaster control in deep coal mine

Deep mining roadways are often under high-stress conditions and strong mining-induced disturbances. In the traditional longwall mining method, stress concentration and energy accumulation take place in the surrounding rock of the roadway, often causing dynamic disasters. To this end, the roof-cutting and energy-absorbing (RCEA) control method is proposed. Directional roof presplitting is used to transfer the roof stress and release the energy accumulated in the rock. Importantly, a roadway is automatically formed and coal pillars are eliminated. Moreover, new constant resistance energy-absorbing bolt and cable with high prestress and high elongation are used to control the roadway roof, which are independently developed. As a result, the self-supporting capacity of the rock is improved, and the energy released by the rock deformation is effectively absorbed by the support system. To verify the fidelity of the RCEA method, the numerical comparison test between the automatically formed roadway (AFR) method and the gob side entry driving method is carried out. Compared with the latter, the energy per unit length of AFR is reduced by 4.59*10 6 J, and the maximum energy density is reduced by 36.6%, which shows that AFR has profound effect on the energy release. Subsequently, the mechanical tests on the new bolt (cable) are carried out. The forces at fracture of the new bolt and cable are 1.63 times and 2.80 times that of the common bolt, and the maximum energy absorbed per unit length are 3.05 times and 2.89 times that of the common bolt. The new bolt (cable) has characteristics of high strength and high energy absorption. Finally, the strength-energy combined support design method and the field application are carried out. Field monitoring results show that the RCEA method can effectively release the energy accumulated in the rock and ensure the safety and stability of the roadway.

Highly prestressed NPR cable coupling support technology and its application in the deep roadway

The stability control of deep roadways has always been a difficult problem related to the safe production of mines. This study took the -950 level substation roadway of the Wanfu Coal Mine in China as the project background to study a new support technology with negative Poisson’s ratio (NPR) cables as the core. Firstly, based on the mechanical properties of the NPR cable with a large constant resistance, a high elongation, and uniform full-length energy absorption, the highly prestressed NPR cable coupling support technology was proposed. Its mechanism was revealed, i.e., by strengthening the surrounding rock with a high prestress and releasing its deformation energy with a large constant resistance and a large deformation, the “stress compensation” effect of the NPR cables can be brought into full play so that coupling of the “support-surrounding rock” can be achieved. Secondly, the numerical model of the second-generation NPR cable was established by FLAC3D, which can realise the constant resistance and large deformation of the NPR cable and achieve the sequential breaking of the NPR steel strands during the deformation of the NPR cable. Numerical tensile tests verified its effectiveness. Finally, the NPR cable support scheme was designed through theoretical calculations and numerical simulations. After field application, the roadway deformation was constrained within 80 mm, effectively preventing engineering failures. This study may serve as guidance for similar projects.

Excavation compensation method and key technology for surrounding rock control

The change in the stress state of the underground rock caused by excavation is the root cause of the deformation instability. Compensating for the stress change is one of the keys to effectively control the instantaneous deformation of rockbursts and the slow deformation of soft rocks in deep underground engineering. At present, the Platts pressure arch theory and the New Austrian Tunneling Method are commonly used in ground control, but their applicability to high-stress deep rock masses is limited. The strength and elongation of the traditional support materials are insufficient, the prestress is low, and it is difficult to effectively control the large deformation of deep rock masses. In this work, the excavation effect and support compensation effect of the rock are analyzed from the mechanics perspective, and an excavation compensation method suitable for controlling large deformation of deep rocks is proposed. The excavation effect verification experiments are carried out to examine the large deformation failure characteristics. Based on the excavation compensation method, a high prestress excavation compensation control technology is developed by using a new Negative Poisson's ratio (NPR) material. This technology is successfully applied to deep buried hard rocks and soft rocks to realize the effective control of the large deformation, which proves the applicability of the excavation compensation method and technology.

Strength weakening effect of high static pre-stressed granite subjected to low-frequency dynamic disturbance under uniaxial compression

This study aimed to elucidate the strength weakening effect of high static pre-stressed rocks subjected to low-frequency disturbances under uniaxial compression. Based on the uniaxial compressive strength (UCS) of granite under static loading, 70%, 80%, and 90% of UCS were selected as the initial high static pre-stress (σp), and then the pre-stressed rock specimens were disturbed by sinusoidal stress with amplitudes of 30%, 20%, and 10% of UCS under low-frequency frequencies (f) of 1, 2, 5, and 10 Hz, respectively. The results show that the rockburst failure of pre-stressed granite is caused by low-frequency disturbance, and the failure strength is much lower than UCS. When the σp or f is constant, the specimen strength gradually decreases as the f or σp increases. The experimental study illustrates the influence mechanism of the strength weakening effect of high static pre-stress rocks under low-frequency dynamic disturbance, that is, high static pre-stress is the premise and leading factor of rock strength weakening, while low-frequency dynamic disturbance induces rock failure and affects the strength weakening degree.

Experimental study of hemispheric rolling cable-strut joints for cable dome applications

Cable domes achieve structural stiffness and loading capacity through tension cables and compression struts. Friction from cable-strut joints contributes a major prestress loss during tensioning. This paper presents a novel hemispheric rolling cable-strut joint which provides smooth load transmission and high prestress efficiency. It comprises a hemispheric cover plate, a bottom plate, a roller, and top and bottom bearings. The bearings embedded in the roller reduce friction transmission from the tensioned cable; thus, prestress loss is minimized. A theoretical analysis of load transmission and prestress loss is conducted for the cable-strut joint, followed by an experimental study. The experiments test cable-strut joints with different types of bearings, using a fixed-roller joint as a reference, and are shown to validate the theoretical approach. A potential method to improve the prestress efficiency during tensioning is discussed. Prestress loss rates are compared for the test joints and an optimum bearing type is recommended in consideration of both friction coefficients and service time. The hemispheric rolling cable-strut joint accommodates complex cable and strut configurations with clear load path and minimal prestress loss. Our results show that it provides a high-performance solution that can be used to improve the structural stability and stiffness of next generation long-span spatial structures.