Service Life Prediction of Lining Concrete of Subsea Tunnel under Combined Compressive Load and Carbonation

Qingdao Jiaozhou Bay subsea tunnel is the second self-built tunnel in China with the designed service life over 100 years.The durability of lining concrete are one of an important factors to determinate the service life of tunnel.Considering the main environmental loads and mechanical loads of subsea tunnel,the durability properties of lining concrete under combined action of compressive load and carbonation has been studied through the critical compressive load test,accelerated carbonation test,natural carbonation test and capillary suction test.The tests results show that critical compressive load apparently accelerates the carbonation and deteriorates the anti-permeability of concrete.Under the combined action of critical compressive load and carbonation,the durability of lining concrete decreases.Based on the carbonization life criteria and research results,for the high-performance concrete with proposed mix ratio,the predicted service life of lining concrete for Jiaozhou bay subsea tunnel is about 80 years which fails to reach the required service life.It is necessary to adopt other measurements simultaneously to improve the durability of lining concrete.

Durability of Lining Concrete of Subsea Tunnel under Combined Action of Freeze-thaw Cycle and Carbonation

Through the fast freeze-thaw cycle test,accelerated carbonation test,and natural carbonation test,the durability performance of lining concrete under combined action of freeze-thaw cycle and carbonation were studied.The experimental results indicate that freeze-thaw cycle apparently accelerates the process of concrete carbonation and carbonation deteriorates the freeze resistance of concrete.Under the combined action of freeze-thaw cycle and carbonation,the durability of lining concrete decreases.The carbonation depth of lining concrete at tunnel openings under freeze-thaw cycles and tunnel condition was predicted.For the high performance concrete with proposed mix ratio,the lining concrete tends to be unsafe because predicted carbonation depth exceeds the thickness of reinforced concrete protective coating.Adopting other measurements simultaneously to improve the durability of lining concrete at the tunnel openings is essential.

Failure Patterns and Energy Analysis of Shaft Lining Concrete in Simulated Deep Underground Environments

The failure patterns and energy evolution of three types of shaft lining concrete subjected to static and dynamic loading were reported.The energy and damage characteristics of concrete were determined by means of a uniaxial hydraulic servo machine,acoustic emission (AE) equipment,a split Hopkinson pressure bar (SHPB) and an ultrasonic wave analyser.The experimental results indicate that the confluence of multiple cracks forms a penetrating cross section in normal high-strength concrete (NHSC) under the condition of static loading,while the elastic energy that surges out at failure can cause tremendous damage when subjected to dynamic loading.A single crack was split into multiple propagation directions due to the presence of fibres in steel fibre-reinforced concrete (SFRC);adding fibre to concrete should be an effective way to dissipate energy.The non-steam-cured reactive powder concrete (NSC-RPC) designed in this paper can store and dissipate more energy than normal concrete,as NSC-RPC exhibits a strong ability to resist impact.Applying NSC-RPC to the long-service material of a shaft lining structure in deep underground engineering is quite effective.

Damage on lining concrete in highway tunnels under combined sulfate and chloride attack

The combined effect from sulfate and chloride is one of the important reasons to cause the damage of lining concrete in highway tunnels. To investigate the effect of chloride ions on the corrosion of lining concretes under sulfate attack, ultrasonic detecting, compression test and X-ray Diffraction （XRD） were performed on the concretes to obtain the ultrasonic velocity, corrosion thickness, compression strength and corrosion products. The ultrasonic results, compression strength and XRD patterns confirmed that the existence of chloride certainly depressed the corrosion damage on the lining concretes under sulfate attack, and the depressing effect increased with the content of chloride in the composite solution. The corrosion damage on the concretes experienced three stages independent of the composition of corrosive solution： initial slower enhancement on the strength, stabilization period and linear degradation period. The existence of chloride mainly affected the final degradation stage and obviously decreased the corrosion thickness.

Frost heaving force considering synchronous damage to tunnel lining and surrounding rocks under freeze—thaw cycles

In areas with seasonal freezing,when the tunnel lining concrete is saturated with water infiltrating the interior,the lining and the surrounding rocks will simultaneously freeze.However,the current calculation of the frost heaving force fails to consider the synchronous damage to the lining and surrounding rocks under freeze-thaw cycles.Therefore,as per the elastic calculation model of the frost heaving force and model of steady-state heat transfer of circular tunnels,this study introduces the frost heaving rate of lining and surrounding rocks.First,the analytical solution of frost heaving force is obtained for simultaneous frost heaving of lining and surrounding rocks under any steady-state temperature field.Then,based on the fracture theory and meso-damage mechanics,the damage variables of lining and surrounding rocks under freeze-thaw cycles are extracted,representing their elastic modulus and porosity.Finally,the formula of frost heaving force for synchronous damage to the lining and surrounding rocks at any steady-state temperature field is obtained.The calculation results demonstrate that the lower the temperature inside the lining,the greater the frost heaving force.With the increasing number of freeze-thaw cycles,frost heaving force tends to gradually increase initially,reaching a peak value at 85 freeze-thaw cycles,decreasing to 80%of the peak value at 140 cycles before reaching a constant value.The lining participates in frost heaving,increasing the frost heaving force.The initial increase rate of frost heaving force is 15.7%.Changing the fitting coefficients s1 and s2 of the lining and surrounding rocks can effectively control the magnitude of the frost heaving force in the tunnels.

Compressive strength prediction of sprayed concrete lining in tunnel engineering using hybrid machine learning techniques

Sprayed concrete lining is a commonly employed support measure in tunnel engineering,which plays an important role in construction safety.Compressive strength is a key performance indicator of sprayed concrete lining,and the traditional measuring method is time-consuming and laborious.This paper proposes various hybrid machine learning algorithms to accomplish the advanced prediction of compressive strength of sprayed concrete lining based on the mixture design.Two hundred and five sets of experimental data were collected from a water conveyance tunnel in northwestern China for model construction,and each set of data was made up of six basic input variables(i.e.,water,cement,mineral powder,superplasticizer,coarse aggregate,and fine aggregate)and one output variable(i.e.,compressive strength).In order to eliminate the correlation between input variables,a new composite indicator(i.e.,the water-binder ratio)was introduced to achieve dimensionality reduction.After that,four hybrid models in total were built,namely BPNN-QPSO,SVR-QPSO,ELM-QPSO,and RF-QPSO,where the hyper-parameters of BPNN,SVR,ELM,and RF were auto-tuned by QPSO.Engineering application results indicated that RF-QPSO achieved the lowest mean absolute percentage error(MAPE)of 3.47% and root mean square error(RMSE)of 1.30 and the highest determination coefficient(R^(2))of 0.93 in the four hybrid models.Moreover,RFQPSO had the shortest running time of 0.15 s,followed by SVR-QPSO(0.18 s),ELM-QPSO(1.19 s),and BPNN-QPSO(1.58 s).Compared with BPNN-QPSO,SVR-QPSO,and ELM-QPSO,RF-QPSO performed the most superior performance in terms of both prediction accuracy and running speed.Finally,the importance of input variables on the model performance was quantitatively evaluated,further enhancing the interpretability of RF-QPSO.

Numerical analysis and capacity evaluation of composite sprayed concrete lined tunnels

Spray-applied membranes for waterproofing of sprayed concrete tunnels have led to the possibility of shear transfer between primary and secondary linings through the membrane interface,with the potential for reducing overall lining thickness.Laboratory tests have shown a reasonable degree of composite action in beam specimens.In this study,a numerical model previously calibrated against such tests is applied to a whole tunnel,considering soil–structure interaction and staged lining construction.The model shows composite action,and load sharing between the lining layers is expected in the tunnel as in the beams.Parametric studies over the practical range of interface stiffness values show that composite action is maintained,although at high interface stiffness,excessive bending may be imposed on the secondary lining,requiring additional reinforcement.An effcient composite shell design with minimal additional rein-forcement is achievable if the secondary lining thickness is reduced as compared to current practice.Robustness of the system,measured in terms of the interface’s ability to transfer stress under unequal loading causing distortion on the tunnel,is found to be generally ade-quate.However,adjacent construction in close proximity may provide insuffcient margin on membrane tensile de-bonding,particularly if the membrane is partially or fully saturated.

Multiscale structural analysis inspired by exceptional load cases concerning the immersed tunnel of the Hong Kong-Zhuhai-Macao Bridge

This paper contains an assessment of the added value of multiscale material models for concrete in the context of macroscopic struc-tural analysis of(steel-reinforced)concrete structures.Two examples are discussed.They are inspired by the possibility of car accidents inside the immersed tunnel of the Hong Kong-Zhuhai-Macao Bridge(HZMB).The first example deals with vehicles crashing into the tunnel wall.The high-dynamic strengthening effect of concrete is studied based on an engineering mechanics model.The structural nature of the dynamic strength increase factor(DIF)is demonstrated by means of high-dynamic strength values measured on mortar cylinders of different size.Furthermore,the evolution of the DIF as a function of hardening of concrete at material ages beyond 28 days is studied.A validated multiscale model for concrete renders a customized analysis for the specific concrete used for the aforementioned tunnel possible.It is found that the DIF decreases with progressive hardening of concrete at material ages beyond 28 days.The second example is inspired by tunnel fires as may happen after car accidents.The study refers to thermal stresses in steel-reinforced concrete beams sub-jected to sudden heating.The thermal expansion coefficient of the concrete of the tunnel is quantified by means of a multiscale model.It is used as input for linear thermo-mechanical Finite-Element simulations of steel-reinforced concrete beams.The essential macroscopic simulation results are temperature distributions and associated stress fields.They are employed for top-down quantification of micro-scopic stress states inside the cement paste and the aggregates.This allows for quantifying two sources of microstructural stress fluctu-ations:(i)the macro-to-micro stress concentration and(ii)the mismatch of microscopic thermal expansion coefficients.In both examples,the multiscale models for concrete have increased the informative content of the structural simulations.