Fundamental Issues Towards Unified Design Theory of Recycled and Natural Aggregate Concrete Components

In the past 20 years,recycled aggregate concrete(RAC),as a type of low-carbon concrete,has become a worldwide focus of research.However,the design methodology for RAC structural components remains a challenge.Consequently,demands for a unified design of natural aggregate concrete(NAC)and RAC components have been presented.Accordingly,this study analyses the necessity of a unified design theory and provides an in-depth demonstration of the strength determination,compressive constitutive relationship,and design method of concrete components.The coefficient of variation of RAC strength is found to be generally higher than that of NAC strength.The compressive and tensile strengths of RAC can be defined and determined using the same method as that used for NAC.The uniaxial compressive constitutive relationship between NAC and RAC has a unified mathematical expression.However,the elastic modulus of RAC decreases,and its brittleness exhibits an increasing trend compared with that of NAC.Finally,to unify the design formulae of RAC and NAC components for bearing capacity,modification factors for RAC components are proposed considering safety and reliability.Additionally,the feasibility of the proposed unified time-dependent design theory is demonstrated in terms of conceptual design and structural measures considering the effects of strength degradation and reinforcement corrosion.It is believed that this study enriches and develops the basic theory of concrete structures.

Multi-Scale Investigation on Concrete Prepared with Recycled Aggregates from Different Parent Concrete

Recycled aggregates(RA)are frequently obtained from various unknown sources,which caused variation in properties among recycled aggregates concrete(RAC).This paper investigated the macro and microscopic properties of RAC prepared with RAs originated from different parent concretes with 90-day strength ranging from 30 MPa to 100 MPa.These parent concretes were prepared in advance and crushed to produce RA of distinct qualities.The attached mortar content can reach up to 69%in the concrete with highest strength grade.The microscopic investigation on different RAC was conducted with the X-ray Micro-Computed Tomography scanning technique and image process.Experimental results showed that the properties of RA derived from various parent concrete varied because of the amount of residual mortar.The development of mechanical properties and chloride-ion penetration of RACs all followed a binomial trend with the strength grades of the parent concrete because of the different quantity and quality of RAs.

混凝土结构低碳近似概率设计法探索

为应对国家“双碳”目标与建筑业低碳发展挑战,探索并提出了一套以概率理论为基础、以分项系数表达的混凝土结构低碳设计实用方法.面向碳中和目标,建立了低碳极限状态方程,定义了低碳指标以度量混凝土结构满足碳排放限值的水准.依据碳排放的不确定性特点,定义了“恒碳”“活碳”,分别在结构隐含碳与建筑全寿命碳边界内厘清2种低碳设计范式,引入碳排放分项系数将概率化设计简化为基于确定性碳排放评价结果的“近似概率设计法”.开展案例试算,将中国碳中和目标分解为混凝土结构碳排放限值,展示建筑类型对碳排放分项系数的影响特点.结果表明,碳排放分项系数与“恒碳”“活碳”变异性大小、所占比例直接相关,在10–1误差范围内,不同建筑类型有望采取统一分项系数,如目标低碳失效概率为0.05时,“恒碳”和“活碳”分项系数可分别取1.1和1.8.引入3类影响系数,以匹配不同建筑类型碳排放限值的差异化分配需求,适应不同地区、不同碳排放调控情景下的发展需求,助力我国建筑业低碳转型.

Effect of Waste Marble Powder on Physical and Mechanical Properties of Concrete

The emission of greenhouse gases from cement production is an obstacle for sustainable development of construction industry.The use of waste materials in constructions instead of cement could be a feasible solution to green construction.Waste marble powder with good cementing property can be used in concrete partially replace cement.In this research,the effects of using waste marble powder on the physical and mechanical properties of concrete have been studied.Three groups of particle size and five levels of replacement ratio(5%,10%,15%,20%,25%)for each group have been designed.During the different stage of hydration process,the microstructure,phase composition and thermal properties of marble-cement paste have been investigated,the influence of particle size of marble powder on these properties has been discussed.The compressive strength and stressstrain relation were tested for different series prepared by partially replacing cement at proportions of 5%–25%separately.From the data analysis,it was observed that the using of waste marble powder would affect the hydration products and further affect the mechanical properties of concrete,the addition of marble powder that partially replace the cement at particular proportions in concrete is feasible.

Finite Element Analysis on the Uniaxial Compressive Behavior of Concrete with Large-Size Recycled Coarse Aggregate

To model the concrete with complex internal structure of concrete with large sized aggregates the effect of internal structure on uniaxial compression behavior are studied.Large-sized recycled aggregates behave differently in the concrete matrix.To understand the influence on concrete matrix,a finite element model was developed to model recycled aggregate concrete composed of multiple randomly distributed irregular aggregates and cement mortar.The model was used to calculate the effect of large-size recycled coarse aggregate(LRCA)on the strength of recycled aggregate concrete and simulate the compressive strength of cubes and prisms.The factors such as the strength of new concrete,the strength of old concrete,the defective element content,the shape of LRCA,the incorporation ratio of LRCA and the size of LRCA that can affect the strength of concrete are analyzed in this paper.Results showed that the influence of various factors on concrete strength are in the following desend-ing order:(i)strength of newly poured concrete;(ii)original strength of recycled aggregates;and(iii)defects.It can be seen that the cracking of the phase material elements starts along the bonding zones between gravel and mortar or the new and old mortar,then spreads to mortar and finally to LRCA.The cracking tendency is most significant in LRCA,which means that the fracturing is related to the fracture of the LRCA.After evaluating the variations in strength and quality of the recycled concrete,the influences on concrete strength and quality were studied.The results showed that the proposed concrete model with LRCA was successfully applied to studying the uniaxial compressive behavior of concrete with large-size recycled coarse aggregate.

Crack Propagation and Failure Characteristics of Modeled Concrete with Natural and Brick Aggregates

The failure characteristics of recycled concrete containing brick aggregates are still indistinct,especially how the angular aggregates effect the crack propagation.Based on the concept of modeled concrete,the development of cracks in concrete containing the natural aggregate and brick aggregate under a compression loading was studied.The strain distribution was analyzed with the Digital Image Correlation(DIC).The modeled aggregates include circular and squared ones,and the squared modeled aggregates were placed in different orientations,including 0°,22.5°and 45°.The results show that when the aggregate is placed at 45°,the upper and lower vertices of the aggregate lead to the highest critical strain concentration,therefore,cracks are easy to propagate from these areas and the strength of the corresponding modeled concrete is the lowest.When the modeled natural aggregate is placed at the orientation of 0°,the strain concentration first appears at the interface on both lateral sides of the aggregate.The brick aggregate has a lower elastic modulus and strength than the surrounding mortar.As a result,cracks always propagate through the brick aggregate,which is the primary reason for the low strength of the corresponding concrete.

工程渣土资源化基础问题与低碳技术路径

本文针对国内工程渣土排放量大但高附加值利用率低、资源化过程碳排放高等难题,探究了技术层面的基础问题,并提出了可行的创新技术路径:阐明了工程渣土的定义、成分与危害,并将现有资源化处置技术概括分成4类;梳理了国内近20年工程渣土的4类处置与资源化技术发展脉络与地区差异,分析了技术的主要研发与应用方向,明晰并界定了技术术语;进一步探明了工程渣土的基本性质,厘清了直接再利用、简单处理与改良、固化以及烧制4类技术的科学原理.根据国内工程渣土的处置现状,深入剖析了工程渣土资源化利用率低在技术层面的基础问题,分析了资源化处置的减排潜力.结合我国的碳达峰、碳中和战略,提出了多路径多场景利用、协同处置、数字化技术赋能等创新性解决路径与高附加值策略,并建议了具有潜力的低碳新技术以及工程渣土综合利用的低碳技术路径.

废弃混凝土再生原理与再生混凝土基本问题

从混凝土的历史沿革入手,本文首先阐述了砂石骨料与胶凝材料经科学设计实现原料到产品的制备合成原理;而后针对服役后废弃混凝土的分离、分解和分级,剖析了混凝土再生的路径与原则,揭示了废弃混凝土加工成再生骨料和再生粉料演变机理;进一步分析了再生原料制备新混凝土面临的基本问题以及多尺度性能改善机制,并对比传统与现代配合比设计方法,提出了基于机器学习的再生混凝土设计与制备模式,提炼了再生混凝土性能设计与应用的三方面准则.最后,围绕全再生骨料混凝土、碱激发再生骨料混凝土和3D打印再生混凝土等技术,对再生混凝土的创新与未来发展提出了建议.本文旨在充分发挥废弃混凝土的资源化再利用价值与低碳潜力,构建混凝土循环再生新范式,助力建筑业绿色低碳转型.

Chloride diffusion in concrete with carbonated recycled coarse aggregates under biaxial compression

Chloride attack on concrete structures is affected by the complex stress state inside concrete,and the effect of recycled aggregates renders this process more complex.Enhancing the chloride resistance of recycled concrete in a complex environment via carbonization facilitates the popularization and application of recycled concrete and alleviates the greenhouse effect.In this study,the chloride ion diffusion and deformation properties of recycled concrete after carbonization are investigated using a chloride salt load-coupling device.The results obtained demonstrate that the chloride ion diffusivity of recycled concrete first decreases and then increases as the compressive load increases,which is consistent with the behavior of concrete,in that it first undergoes compressive deformation,followed by crack propagation.Carbonation enhances the performance of the recycled aggregates and reduces their porosity,thereby reducing the chloride diffusion coefficient of the recycled concrete under different compressive load combinations.The variation in the chloride ion diffusivity of the carbonized recycled aggregate concrete with the load is consistent with a theoretical formula.

Where are demolition wastes going:reflection and analysis of the February 6,2023 earthquake disaster in Turkey

On 6 February 2023 at 09.17 BST,an earthquake measuring 7.8 on the Richter scale struck the southern border of Turkey near Syria,causing massive casualties and building damage.Badly damaged buildings need to be demolished,bringing a large amount of demolition waste,which,if not properly disposed of,can be a burden on the environment.In this study,damage to buildings in the quake-hit areas of Turkey is investigated,including reinforced structures and masonry structures.Based on this,the amount of demolition waste produced and the proportion of waste components are estimated roughly.Ultimately,the paper puts forward the strategy of recycling demolition waste after the earthquake and the application scenario planning of recycled products.Conclusively,the amount of demolition waste generated after the earthquake ranges from 450 to 920 million tons,providing new ideas for post-disaster reconstruction work.Besides,post-disaster waste management,safe demolition and environmentally friendly disposal and recycling technologies for demolition and construction wastes will bring good economic and environmental benefits,help the reconstruction of disaster areas,and provide a model for the resource utilization of construction and demolition waste worldwide.

Axial compressive behavior of GFRP-timber-reinforced concrete composite columns

This paper investigated the compressive behavior of a novel glass fiber reinforced polymer(GFRP)-timber-reinforced concrete composite column(GTRC column),which consisted of reinforced concrete with an outer GFRP laminate and a paulownia timber core.The axial compression tests were performed on 13 specimens to validate the effects of various timber core diameters,slenderness ratios,and GFRP laminate layers/angles on the mechanical behaviors.Test results indicated that with the increase in the timber core diameter,the ductility and energy dissipation ability of the composite column increased by 52.6%and 21.6%,respectively,whereas the ultimate load-bearing capacity and initial stiffness showed a slight decrease.In addition,the GFRP laminate considerably improved the ultimate load-bearing capacity,stiffness,ductility and energy dissipation capability by 212.1%,26.6%,64.3%and 3820%,accordingly.Moreover,considering the influence of timber core diameter,an ultimate load-bearing capacity adjustment coefficient was proposed.Finally,a formula was established based on the force equilibrium and superposition for predicting the axial bearing capacity of the GTRC columns.

Impact assessment of river sand resource shortage under different policy scenarios in China

More than half of the annual global concrete materials were produced in China due to the rapid developing construction industry,which partly led to the shortage of river sand.However,mining rate exceeds the natural replenishment rate of river sand recently,resulting in depletion of natural river sand accumulation.The increasing demand of river sand influences lots of aspects including altered landforms,increasing carbon emissions,ecological deterioration,international trades and disputes.To face the river sand resource shortage in China and to propose possible coping strategies,the data of river sand for construction in China and other related data were collected,and it is suggested that effective policy measures should be taken right now to protect river sand and strictly manage sand mining.Professional solutions for river sand shortage can be summarized as“5Rs”principle,which includes reduce,recycle.reuse,replace and recover.System dynamic model is established to predict the trend of river sand shortage and it was predicted that the gap between river sand supply and demand will come up to 63%.The impact of three policy scenarios is tested in the model,and the gap can be reduced to 35%by single policy scenario,while the scenario with all policy measures is able to reduce the contradiction between supply and demand to 4%.Suggestions are proposed from the aspects of structural and material technology,policy measures and international alliances.Attention should be paid to the shortage of river resources,to realize the sustainable development of the construction industry and other related industries,and to promote the harmonious coexistence of human and nature.

Editorial:Low-carbon Materials and Green Construction

Dear colleagues,Welcome to the inaugural issue of Low-carbon Materials and Green Construction(LMGC).This journal is a new international journal concerned with the materials and construction in civil engineering to cope with the challenges raised by climate change,together with resources and energy consumption,which is dedicated to the widest dissemination of high-quality original research and insights.

混凝土结构低碳设计理论前瞻

国家“双碳”战略和建筑业低碳发展共识,呼唤混凝土结构的低碳设计理论,本文通过简述混凝土行业低碳转型的历史与趋势,提出气候变化挑战下混凝土结构-环境共生系统的设计新需求;在混凝土结构-环境动态耦合关系的量化表征中,剖析了低碳设计从考虑可持续性的结构可靠性设计,到以双向设计为特征的结构可持续性设计的演进,将现有混凝土结构可持续性相关分析与设计方法融入低碳设计范畴,为气候变化下混凝土结构生命周期内的低碳排放与可靠服役性能提供保障;为实施低碳设计调控,结合本团队十余年的理论探索与实践,构筑混凝土结构3R^+(C)减碳技术体系,挖掘现有典型低碳技术的减排潜力与推广前景,进一步,提出未来研究方向与建议,推动混凝土结构低碳设计创新发展与应用。

Deformation field and crack analyses of concrete using digital image correlation method

The study on the deformation distribution and crack propagation of concrete under axial compression was conducted by the digital image correlation (DIC) method. The main parameter in this test is the water-cement (fT/C) ratio. The novel analysis process and numerical program for DIC method were established. The displacements and strains of coarse aggregate, and cement mortar and interface transition zone (ITZ) were obtained and verified by experimental results. It was found that the axial displacement distributed non-uniformly during the loading stage, and the axial displacements of ITZs and cement mortar were larger than that of coarse aggregates before the occurrence of macrocracks. The effect of W/C on the horizontal displacement was not obvious. Test results also showed that the transverse and shear deformation concentration areas (DCAs) were formed when stress reached 30%-40% of the peak stress. The transverse and shear DCAs crossed the cement mortar, and ITZs and coarse aggregates. However, the axial DCA mainly surrounded the coarse aggregate. Generally, the higher W/C was, the more size and number of DCAs were. The crack propagations of specimens varied with the variation of W/C. The micro-crack of concrete mainly initiated in the ITZs, irrespective of the W/C. The number and distribution range of cracks in concrete with high W/C were larger than those of cracks in specimen adopting low W/C. However, the value and width of eraeks in high W/C specimen were relatively small. The W/C had an obvious effect on the characteristics of concrete deterioration. Finally, the characteristics of crack was also evaluated by comparing the calculated results.

Review of recent developments in cement composites reinforced with fibers and nanomaterials

The quest for high-performance construction materials is led by the development and application of new reinforcement materials for cement composites.Concrete reinforcement with fibers has a long history.Nowadays,many new fibers associated with high performance and possessing eco-environmental characteristics,such as basalt fibers and plant fibers,have received much attention from researchers.In addition,nanomaterials are considered as a core material in the modification of cement composites,specifically in the enhancement of the strength and durability of composites.This paper provides an overview of the recent research progress on cement composites reinforced with fibers and nanomaterials.The influences of fibers and nanomaterials on the fresh and hardened properties of cement composites are summarized.Moreover,future trends in the application of these fibers or of nanomaterial-reinforced cement composites are proposed.

Punching shear behavior of recycled aggregate concrete slabs with and without steel fibres

A study on the punching shear behavior of 8 slabs with recycled aggregate concrete(RAC)was carried out.The two main factors considered were the recycled coarse aggregate(RCA)replacement percentage and the steel fibre volumetric ratio.The failure pattern,load-displacement curves,energy consumption,and the punching shear capacity of the slabs were intensively investigated.It was concluded that the punching shear capacity,ductility and energy consumption decreased with the increase of RCA replacement percentage.Research findings indicated that the incorporation of steel fibres could not only improve the energy dissipation capacity and the punching shear capacity of the slab,but also effectively improve the integrity of the slab tension surface and thereby changing the trend from typical punching failure pattern to bending-punching failure pattern.On the basis of the test,the punching shear capacity formula of RAC slabs with and without steel fibres was proposed and discussed.

Research on recycled concrete and its utilization in building structures in China

Large quantities of construction and demolition(C&D)building waste are being generated as a result of rapid urbanization and natural disasters in China.An increasing awareness of environmental protection is escalating C&D waste disposal concerns.This paper presents a brief introduction to current shaking table test research in China on structures built with recycled aggregate concrete(RAC).Test structures include a cast-in situ frame model,a precast frame model and a block masonry building.The test results prove that it is feasible to use RAC as a structural material in seismic areas,with recommended modifications and proper design,especially in low-rise structures.This paper also presents several successful applications of RAC in civil and structural engineering projects in China,which will serve to promote RAC as a global ecological structural material.

Behaviors of recycled aggregate concrete-filled steel tubular columns under eccentric loadings

The paper investigates the behaviors of recycled aggregate concrete-filled steel tubular(RACFST)columns under eccentric loadings with the incorporation of expansive agents.A total of 16 RACFST columns were tested in this study.The main parameters varied in this study are recycled coarse aggregate replacement percentages(0%,30%,50%,70%,and 100%),expansive agent dosages(0%,8%,and 15%)and an eccentric distance of compressive load from the center of the column(0 and 40 mm).Experimental results showed that the ultimate stresses of RACFST columns decreased with increasing recycled coarse aggregate replacement percentages but appropriate expansive agent dosages can reduce the decrement;the incorporation of expansive agent decreased the ultimate stresses of RACFST columns but an appropriate dosage can increase the deformation ability.The recycled coarse aggregate replacement percentages have limited influence on the ultimate stresses of the RACFST columns and has more effect than that of the normal aggregate concretesteel tubular columns.

Experimental study on behavior of mortar-aggregate interface after elevated temperatures

A push-out test program was designed and conducted to study the meso-scale behavior of mortaraggregate interface for concrete after elevated temperatures ranging from 20℃ to 600℃ with the concept of modeled concrete （MC） and modeled recycled aggregate concrete （MRAC）. The MCs and MRACs were designed with different strength grade of mortar and were exposed to different elevated temperatures. Following that the specimens were cooled to room temperature and push-out tests were conducted. Failure process and mechanical behaviors were analyzed based on failure modes, residual load-displacement curves, residual peak loads and peak displacements. It is found that failure modes significantly depended on specimen type, the elevated temperature and the strength grade of mortar. For MC, major cracks started to propagate along the initial cracks caused by elevated temperatures at about 80% of residual peak load. For MRAC, the cracks appeared at a lower level of load with the increasing elevated temperatures. The cracks connected with each other, formed a failure face and the specimens were split into several parts suddenly when reaching the residual peak load. Residual load-displacement curves of different specimens had similarities in shape. Besides, effect of temperatures and strength grade of mortar on residual peak load and peak displacement were analyzed. For MC and MRAC with higher strength of new hardened mortar, the residual peak load kept constant when the temperature is lower than 400℃ and dropped by 43.5% on average at 600℃. For MRAC with lower strength of new hardened mortar, the residual peak load began to reduce when the temperatures exceeded 200℃ and reduced by 27.4% and 60.8% respectively at 400℃ and 600℃. The properties of recycled aggregate concrete （RAC） may be more sensitive to elevated temperatures than those of natural aggregate concrete （NAC） due to the fact that the interracial properties of RAC are lower than those of NAC, and are deteriorated at lower temperatures.