Mechanical Properties Of Carbon Fiber Reinforced Coral Concrete Under Various Stress Conditions

Concrete is one of the most used building materials in civil engineering.In order to safeguard the maritime sovereignty and develop the marine economy,promoting the development of the national"Belt and Road"initiative,accelerating the construction of maritime power and islands in the South China Sea,these have become a national strategy.As the islands and reefs in the South China Sea are far away from the mainland,they lack the traditional concrete raw materials such as sand,stone,and freshwater.Therefore,the production of new concrete based on materials available on islands and satisfying the requirements has become the key technology to speed up the construction of islands and reefs.Coral concrete is a new type of marine concrete that can be made from the materials available on the islands and reefs and meets the requirements of engineering applications.However,previous studies have shown that coral concrete has shortcomings such as high brittleness and poor crack resistance.At the same time,coral concrete is mainly used in road engineering,protection engineering,and building engineering on islands;these engineering put forward higher requirements for the crack resistance,impact resistance and abrasion resistance of coral concrete.Therefore,it is necessary to improve these shortcomings of coral concrete,and adding carbon fiber（CF）,a kind of high-performance microfiber commonly used in concrete,is an effective measure.The basic physical and mechanical properties,flexural properties and uniaxial mechanical properties of concrete are the basis of its application.In practical engineering members,concrete is usually subjected to low-speed impact loads（such as pavement,breakwater,precast concrete piles,etc.）,combined compression-shear stresses（such as deep beams,airstrips,high-speed pavement,brackets,etc.）,multi-axial stresses（such as pre-stressed concrete reactor pressure vessel,end of beam,area of frame joint,shear wall,etc.）,so the mechanical response of concrete under various stress conditions is the basic theoretical problem and the basis of analysis and design of concrete structure.Based on this background,this paper prepared more than 1,000 specimens and studied the basic physical and mechanical properties of carbon fiber reinforced coral concrete（CFRCC）and its mechanical properties under various stress conditions.The main work,innovations,and conclusions are as follows:1.The microstructure and basic physical and mechanical properties of CFRCC were systematically studied.Results show that adding CFs into coral concrete can significantly improve its restrained shrinkage cracking performance,effectively delay the propagation of microcracks and transfer the stress at the tip of microcracks,enhance the compressive strength and tensile strength,but reduce the early strength development speed.However,an excessive CF dosage will weaken the enhancement of CFs.The pore system of coral aggregate is well developed,and the interfacial transition zone between aggregate and mortar has good bonding performance and seldom damages.The conversion formula of cube compressive strength at different ages and the calculation formulas of axial compressive strength,splitting tensile strength,and elastic modulus were established.2.Drop weight impact test was carried out.Results show that the addition of CFs can transform the failure pattern from obvious brittleness to relatively good ductility and improve both the initial and final impact resistance of coral concrete.The calculation formula of impact energy and the statistical distribution model of impact resistance were established.3.Four-point bending test was carried out.Results show that the bending damage process can be divided into three phases,namely microcrack initiation phase,macrocrack evolution phase,and main crack propagation phase.Adding CFs into coral concrete can delay the latter two phases,improve its ultimate flexural strength,peak deflection,flexural toughness,and energy consumption capacity,and also increase the number of macrocracks when it fails.4.Uniaxial compression test of prism specimen was carried out,and the corresponding failure process was analyzed.Results show that both the ductility and axial compression toughness ratio of coral concrete are lower than that of ordinary concrete.Adding CFs into coral concrete can improve its peak stress,peak strain,residual stress,ultimate strain,ductility,axial compression toughness,elastic modulus,and ratio of elastic modulus to peak secant modulus,and also increase the number of cracks and the angle between the main crack and horizontal direction.The calculation formulas for peak stress,peak strain,initial elastic modulus,as well as the phenomenological damage constitutive model were established.5.Combined compression-shear test was carried out.Results show that the greater the normal stress ratio,the more tortuous the main shear cracks,the more the concrete spalling on the free surface,and the more obvious the friction traces on the shear failure surface.As either the normal stress ratio or CF dosage increases,the shear performance improves,but a too high normal stress ratio or an excessive CF dosage will weaken the enhancement.The friction coefficient on the shear failure surface decreases approximately linearly with the increase of the normal stress ratio,and increases with the increase of the compressive strength.The failure criterion and whole shear load-displacement curve damage model for CFRCC under combined compression-shear stresses were established.6.Biaxial compression equal stress proportion loading test was carried out.Results show that as the stress ratio increases,the failure pattern changes from columnar failure to flaky failure.The addition of CFs increases the number of cracks and the angle between the cracks and main principal stress direction,improves the biaxial compressive strength₃ and peak strain₃,and also slightly improves the initial elastic modulus₃₀.With the increase of stress ratio,₃ and₃ increases first and then decreases,₂,₂/₃,and₃₀ increases approximately linearly.The new biaxial strength failure criterion,strain failure criterion,and2D orthotropic damage incremental constitutive model were established.7.Triaxial compression equal stress proportion loading test was carried out.Results show that when oblique shear failure occurs,there are friction traces on the failure surface.The addition of CFs changes the local failure pattern.When₁/₃>0.15～0.20,the stress-strain curve has an obvious"stress plateau."In this condition,the ultimate strength of CFRCC should be the strength of the starting point of the stress plateau.The triaxial compressive strength and peak strain₃ first increase and then decrease with the increase of the intermediate principal stress₂,and increase with the increase of the minimum principal stress₁.The initial elastic modulus₃₀ increases with the increase of₂,₁,and compressive strength.Adding CFs into coral concrete can improve its triaxial mechanical performance,but the greater the stress ratio,the weaker the enhancement.The new four-parameter strength failure criterion and 3D orthotropic incremental constitutive model were established.The research in this paper is helpful to further understand the basic physical and mechanical properties of coral concrete and CFRCC and their mechanical properties under various stress conditions.The results can provide a reference for related standard formulation,practical engineering application,and further research of coral concrete and CFRCC,and can also provide basic mechanical model for simulation analysis of coral concrete members.