Compressive Strength of Basic Magnesium Sulfate Cement Coral Aggregate Concrete(MCAC)on Non-Destructive Testing

Basic magnesium sulfate cement coral aggregate concrete(MCAC)is a new type of concrete consisting of basic magnesium sulfate cement,coarse coral aggregate,coral reef sand and seawater.The rebound hammer(RH),the ultrasonic pulse velocity(UPV)and the compressive strength(fcu)tests of 14 sets of cube specimens of the MCAC after 28 d of aging were conducted.The impact of the content and length of sisal fiber on the relationship between the fcu-RH and the fcu-UPV was determined.A mathematical model was established to predict the strength of the MCAC using the UPV,RH,and comprehensive UPV/RH methods and to obtain the curves of test strength.The applicability of the test strength curves of ordinary portland concrete(OPC),light-weight aggregate concrete(LAC),and coral aggregate concrete(CAC)to MCAC was assessed.The results showed that the test strength curves of OPC,LAC and CAC were inappropriate to determine the strength of MCAC using non-destructive method.The relative standard error of the curves of test strength of the RH method and the comprehensive method met the specifications,whereas that of the UPV method did not.

Research on Flexural Behavior of Coral Aggregate Reinforced Concrete Beams

Through the flexural behavior test of coral aggregate reinforced concrete beams(CARCB) and ordinary Portland reinforced concrete beams(OPRCB), and based on the parameters of concrete types, concrete strength grades and reinforcement ratios, the crack development, failure mode, midspan deflection and flexural capacity were studied, the relationships of bending moment-midspan deflection, load-longitudinal tensile reinforcement strain, load-maximum crack width were established, and a calculation model for the flexural capacity of CARCB was suggested. The results showed that with the increase in the reinforcement ratio and concrete strength grade, the crack bending moment(Mcr)and ultimate bending moment(Mu) of CARCB gradually increased. The characteristics of CARCB and OPRCB are basically the same. Furthermore, through increasing the concrete strength grade and reinforcement ratio, Mcr/Mu could be increased to delay the cracking of CARCB. As the load increased, crack width(w) would also increase. At the beginning of the loading, w increased slowly. And then it increased rapidly when the load reached to the ultimate load, which then led to beam failure. Meanwhile, with a comprehensive consideration of the effects of steel corrosion on the loss of steel section and the decrease of steel yield strength, a more reasonable calculation model for the flexural capacity of CARCB was proposed.

Influence of steel corrosion to flexural behavior of coral aggregate concrete beam

To study the flexural behavior and calculation model,8 coral aggregate concrete(CAC)beams with different types of steel were designed.The flexural behavior of CAC beam was tested.The failure mode,bearing capacity,the maximum crack width(ws)and average crack spacing(lm)were studied.A calculation model for the bearing capacity of CAC beam was proposed.The results indicated that with the steel strength increased,the cracking moment(Mcr)and ultimate moment(Mu)of CAC beam increased,and the development of the ws gradually slowed,which effectively inhibited the formation of cracks and improved the flexural behavior of CAC beam.For CAC structures in the ocean engineering,it is recommended to use organic new coated steel to extend its effective service life.In addition,considering the influence of steel corrosion,a calculation model for the Mcr,Mu,lm and ws of CAC beam was established.

Effect of Carbonation and Drying-Wetting Cycles on Chloride Diffusion Behavior of Coral Aggregate Seawater Concrete

Based on seawater immersion,drying-wetting cycles,carbonation and drying-wetting cycles for coral aggregate sea-water concrete(CASC)with different strength grades,the effect of carbonation and drying-wetting cycles on chloride diffusion be-havior of CASC is studied.The results show that the free surface chloride concentration(Cs),free chloride diffusion coefficient(Df)and time-dependent index(m)of CASC in the drying-wetting cycles is obviously higher than that in seawater immersion.The Df and m of CASC of carbonation and drying-wetting cycles is higher than that in the drying-wetting cycles.Carbonation increases the Df and m of CASC,which is against CASC to resist chloride corrosion.The corrosion possibility of CASC structures in different ex-posed areas is as follows:splash zone(carbonation and drying-wetting cycles)>tidal zone(drying-wetting cycles)>underwater zone(seawater immersion).Besides,the chloride diffusion rate of C65-CASC is 17.8%-63.4%higher than that of C65-ordinary aggre-gate concrete(OAC)in seawater immersion(underwater zone).Therefore,anti-corrosion measures should be adopted to improve the service life of CASC structure in the oceanic environment.

Three-Dimensional Mesoscopic Investigation on the Impact of Specimen Geometry and Bearing Strip Size on the Splitting-Tensile Properties of Coral Aggregate Concrete

The use of coral aggregate concrete(CAC)as a novel construction material has attracted significant attention for the construction of reef engineering structures.To investigate the static splitting-tensile behaviors of CAC under the influence of two factors,namely specimen geometry and bearing strip size,a three-dimensional(3D)mesoscale modeling approach with consideration for aggregate randomness in shape and distribution was adopted in this study.We established 12 different specimen models with two specimen shapes(i.e.,a cube with an edge length of 150 mm and a cylinder with dimensions ofφ150 mm×300 mm)and six strip widths(i.e.,6,9,12,15,18,and 20 mm)for calculation.The effects of specimen geometry and strip width on the splitting-tensile properties of CAC,such as failure processes,final failure patterns,and splitting-tensile strength(fst),are analyzed and discussed systematically.The results indicate the high reliability of the developed mesoscale modeling approach and reveal the optimal computational parameters for simulating and predicting the splitting-tensile properties of CAC.The fstvalues of CAC are associated with both the specimen geometry and width of the bearing strip.The fstvalues of the cube model are slightly higher than those of the cylinder model for the same bearing strip size,representing geometry effects that can be explained by differences in fracture area.Additionally,the fstvalue of CAC gradually increases with the relative width of the bearing strip ranging from 0.04 to 0.13.Based on the elastic solution theory,the variation area of CAC fstvalues with the relative width of the bearing strip was determined preliminarily,which has great significance for studying the tensile performance of CAC.

Effect of carbonation-drying-wetting on durability of coral aggregate seawater concrete

Based on the drying-wetting cycles experiment and the carbonation-drying-wetting cycles experiment for coral aggregate seawater concrete(CASC)with different strength grades,the effects of carbonation-drying-wetting on the durability of CASC are studied with the surface state,mass loss rate,relative dynamic elastic modulus,ultrasonic wave velocity and cube compressive strength as indices.Results show that the mass loss rate of CASC increases gradually with the increase in cycle times in the drying-wetting and carbonation-drying-wetting cycles.The mass loss rate increases relatively slowly at the initial stage but it increases remarkably after 10 cycles.The relative dynamic elastic modulus and ultrasonic wave velocity decrease gradually with the increase in cycle times.After 6 cycles,the decrease rate of the relative dynamic elastic modulus and ultrasonic wave velocity of CASC tends to be flat and the surface is slightly damaged.Compared with the initial 28 d cube compressive strength,the cube compressive strength of CASC decreases by 8.8%to 11.0%.Drying-wetting cycles and carbonation can accelerate seawater erosion on CASC,and drying-wetting cycles result in salting-out and accelerate the destruction of concrete.Therefore,the carbonation-drying-wetting accelerates the destruction of CASC.

A New 3D Meso-mechanical Modeling Method of Coral Aggregate Concrete Considering Interface Characteristics

On the basis of the three-dimensional(3D)random aggregate&mortar two-phase mesoscale finite element model,C++programming was used to identify the node position information of the interface between the aggregate and mortar elements.The nodes were discretized at this position and the zero-thickness cohesive elements were inserted.After that,the crack energy release rate fracture criterion based on the fracture mechanics theory was assigned to the failure criterion of the interface transition zone(ITZ)elements.Finally,the three-phase mesomechanical model based on the combined finite discrete element method(FDEM)was constructed.Based on this model,the meso-crack extension and macro-mechanical behaviour of coral aggregate concrete(CAC)under uniaxial compression were successfully simulated.The results demonstrated that the meso-mechanical model based on FDEM has excellent applicability to simulate the compressive properties of CAC.

Effects of Mineral Admixtures on Chloride Diffusion in Environment-Friendly Coral Aggregate Concrete

Coral materials can replace concrete aggregates and achieve material self sufficiency for reducing the construction costs of island projects.This paper studies the effects of different mineral admixtures on the properties of coral aggregate concrete(CAC).The chloride concentration of CAC after different erosion times is measured using the potentiometric method,and the porosity of the CAC is ca lculated using thermogravimetric and drying methods.The chloride concentration of the CAC presents a two-phases dis tribution.The peak chloride concentration fol-lowed a power function,increasing with the erosion time.The chloride diffusion coefficient of CAC is 7.9%-37.5%larger than that of ordinary aggregate concrete,and the addition of 15% fly ash and 5%silica fume can significantly reduce the chloride diffusion coefficient,with a maximum reduction of 45.0%.The porosity obtained via the thermogravimetric and drying methods is well correlated.The porosity has a strong negative correlation with the compressive strength and a strong positive correlation with the chloride diffusion coefficient.

Prevention and Control of Chlorella for C30 Concrete Surface with Coral Aggregate

C30 coral aggregate concrete with chlorella control effect was prepared by adding nano-TiO_(2) and hydrophobic material,and the effects of nano-TiO_(2) and hydrophobic material on the basic properties of C30 coral aggregate concrete and chlorella control effect under different experimental conditions were compared.The experimental results show that nano-TiO_(2) and hydrophobic materials have a certain degree of influence on the basic properties of concrete,but the influence is not significant.Under long-term immersion,nano-TiO_(2) and hydrophobic materials can inhibit the growth of Chlorella vulgaris.The maximum fluorescence value of concrete is decreased by 53.6% after adding TiO_(2),and the maximum fluorescence value of concrete is prolonged by 20%(1 day).The maximum fluorescence value of concrete is decreased by 67.7% after adding hydrophobic materials,and the maximum fluorescence value of concrete is also prolonged by 20%(1 day);Under the condition of simulated tidal water,the inhibition effect of Nano-TiO_(2) on the growth degree and growth rate of Chlorella vulgaris is weakened,at this time the maximum fluorescence value of concrete mixed with nano-TiO_(2) is decreased by 50.5%,and the maximum fluorescence value is only prolonged by 14.3%;while the inhibition of hydrophobic materials on the growth degree and growth rate of Chlorella vulgaris is enhanced significantly,and the maximum fluorescence value of concrete with hydrophobic materials is decreased by 80.3%;the maximum fluorescence time is prolonged by 114.3%.

Dynamic Impact Compressive Behavior Investigation of Sisal Fiber-reinforced Coral Seawater Concrete

Split Hopkinson pressure bar(SHPB)was used to investigate the dynamic compressive properties of sisal fiber reinforced coral aggregate concrete(SFCAC).The results showed that,with the increase of strain rate,the dynamic compressive strength,peak strain and toughness index of SFCAC are all greater than its static properties,indicating that SFCAC is a kind of rate-sensitive material.When the sisal fiber was blended,the failure mode showed obvious ductility.At high strain rates,the SFCAC without sisal fiber specimen was comminuted,and the SFCAC showed a"cracked without breaking"state.The results indicated that the sisal fiber played a significant role in reinforcing and strengthening the properties of concrete.The finite element software LS-DYNA was used to simulate two working conditions with strain rates of 78 and 101 s-1.The stressstrain curves and failure patterns obtained were in good agreement with the experimental results.

基于声发射技术的高强全珊瑚钢筋混凝土板破坏特性研究

为了探究全珊瑚钢筋混凝土板破坏特性,设计了3种不同配筋率(0.85%、1.13%和1.41%)的高强全珊瑚钢筋混凝土板构件.结合声发射技术,开展了全珊瑚混凝土板构件四点弯曲加载试验,揭示了配筋率对高强全珊瑚钢筋混凝土板力学性能的影响.基于声发射参数速率过程理论,建立了损伤定量评估模型,并进一步提出了可用于工程实践的损伤评估准则.结果表明:随配筋率增大,高强全珊瑚混凝土板构件起裂荷载、起裂时间与峰值荷载均增大,但中心挠度降低了11.29%;单位时间声发射参数变化均存在2个峰值区,可作为板构件的起裂评判和破坏预兆;根据声发射参数累计值变化可将高强全珊瑚混凝土板构件破坏过程划分为轻度(<0.35)、中度(0.35~0.66)、高度(0.66~0.84)和重度(>0.84)损伤4个程度.所建立模型适用于不同配筋率的全珊瑚混凝土板构件损伤评估.

PVA纤维增强珊瑚混凝土静动态力学性能试验研究

为增强珊瑚混凝土的力学性能,对其进行了5种聚乙烯醇纤维体积分数(0、0.5%、1%、1.5%和2%)条件下的静态力学试验,并采用直径为100 mm的分离式霍普金森杆研究其在4种应变率下的冲击压缩性能。结果表明:掺入适量纤维能明显提高混凝土静态抗压强度与劈裂抗拉强度;动态力学试验表明珊瑚混凝土具有明显的应变率效应,加入纤维后能提高动态增强因子对应变率的敏感性,增强效果在纤维掺量为1.5%时趋于饱和;纤维在混凝土中起到了阻裂与耗能的作用,能明显地改善混凝土的破坏形态与冲击韧性。

粗骨料粒径对珊瑚混凝土力学性能的影响

为了探究粗骨料粒径对珊瑚混凝土的静态力学性性能,采用RMT-150实验机结合材料力学相关知识,对连续级配粗骨料粒径为5~16mm(J0)及单一级配粗骨料粒径为5~10mm(J1)、10~16mm(J2)、16~20mm(J3)和20~25mm(J4)进行静态抗压抗拉试验,并且对比分析实验结果,探究不同粗骨料粒径珊瑚混凝土的力学性能。研究结果表明:粗骨料粒径为5~10mm的时候珊瑚混凝土的抗压抗拉强度达到最佳,随着骨料粒径的增大,珊瑚混凝土的抗压抗拉强度呈现先增加后减小的趋势。适当的粗骨料粒径能够提升粗骨料混凝土的静态抗压抗拉强度。

珊瑚黏液缓释剂的制备及其对太阳长棘海星群体聚集的诱导效果

太阳长棘海星(Acanthaster cf. solaris)灾害暴发是导致南海珊瑚礁退化的主要原因之一,开发太阳长棘海星灾害防控技术具有重要意义。本研究搭建了太阳长棘海星行为学室内研究平台,发现鹿角杯形珊瑚(Pocillopora damicornis)对太阳长棘海星的诱导聚集效果要强于丛生盔形珊瑚(Galaxea fascicularis)和澄黄滨珊瑚(Porites lutea)(P <0.05)。同时,收集鹿角杯形珊瑚黏液,利用羟丙基甲基纤维素等试剂制备了珊瑚黏液缓释剂,发现其对太阳长棘海星有明显诱导聚集效果(P <0.01),且硬度较低、缓释速率较快的缓释剂团块对太阳长棘海星的诱导聚集效果更明显。本研究开发了1种珊瑚黏液缓释剂的制备方法,证明了鹿角杯形珊瑚黏液能够显著诱导太阳长棘海星发生群体聚集,为太阳长棘海星的群体聚集机制的解析和灾害防控技术的创新发展提供了科学依据。

CFRP-PVC管约束珊瑚海水海砂混凝土柱轴压性能试验研究及有限元分析

为研究碳纤维增强聚合物-聚氯乙烯复合管(CFRP-PVC)约束珊瑚海水海砂混凝土(CSSC)柱的轴心受压性能,对6个试件进行轴心受压加载试验和有限元参数分析,重点研究了PVC管内、外表面CFRP层数和脱空缺陷对该组合柱轴压性能的影响。结果表明,CFRP-PVC管可以有效约束CSSC,使其处于三轴受压状态,当内、外壁均粘贴CFRP时可以有效减缓突然破坏的特征。相同情况下,管内壁粘贴1层CFRP时具有更好的延性和承载力,管外壁粘贴2层CFRP时约束应力提高系数高达121.5%,脱空缺陷仅对延性有显著影响。基于ABAQUS软件建立了CFRP-PVC约束CSSC柱有限元模型,准确还原了组合柱在轴向荷载作用下的破坏过程和CFRP损伤形态。当组合柱套箍系数小于0.3或长径比大于14.54时,荷载-位移曲线会失去强化段,提高外层CFRP层数会小幅提高强化段刚度。提出了适用于CFRP-PVC约束CSSC的承载力计算方法,误差为1.2%。

珊瑚骨料种类和混杂纤维对珊瑚混凝土强度与断裂性能的影响

海水珊瑚骨料混凝土(SCAC)由珊瑚礁砂石、海水等海洋地域性原材料与胶凝材料混合制备而成,在海洋国家的国防和基础设施建设中展现出缩短工期、降低成本、减少能源消耗的优势。但SCAC性脆易裂,珊瑚骨料的制约作用随着混凝土强度等级的提高而更加显著。本文探究了两种不同类型的珊瑚骨料对SCAC性能的影响,并将柔性的聚乙烯醇(PVA)纤维和刚性的钢纤维混杂,以期提升SCAC的强度、韧性与断裂性能。结果表明,含低强珊瑚骨料的SCAC抗压强度比含高强珊瑚骨料的SCAC抗压强度降低了30.8%(从55.6降至38.5 MPa),掺入纤维可以在一定程度上缓解骨料自身缺陷对SCAC强度的影响。此外,提出了一种新的本构模型来描述SCAC的应力-变形关系,模型预测结果与试验数据吻合较好。相比于不掺纤维的对照组,掺入0.1%、0.2%和0.3%PVA纤维的SCAC的断裂能分别提高了10%、49%和88%。在纤维用量相同时,混杂纤维组的断裂能比单掺纤维组高46%。

Mix design optimization of seawater sea sand coral aggregate concrete

With the resource shortage in coastal areas,the construction of concrete structures using freshwater and river sand has brought great economic and environmental costs.The use of seawater,sea sand,and coral aggregates in concrete mixes has become an alternative solution for coastal and marine structures,especially for offshore structures and artificial islands.In this study,378 seawater,sea sand,and coral aggregate concrete(Coral-SWSSC)specimens were prepared,and their mechanical properties were investigated comprehensively through compressive tests to explore the optimized mix design at different grades.Results showed that the mechanical properties of Coral-SWSSC were strongly correlated with the water-to-binder ratios,coastal particle gradings,and pretreatment method of coastal particles.Based on the experimental results,mix proportion designs of CoralSWSSC were proposed for concrete from C20 to C50 grades,and the failure mechanism of Coral-SWSSC at different grades was discussed according to their respective failure modes.The findings of the current study provide knowledge on the optimized design of Coral-SWSSC,which can be used to promote the application of Coral-SWSSC in offshore,marine,and ocean engineering.

Effects of carbon-sequestering coral aggregate on pore structures and compressive strength of concrete

CO_(2)sequestration/storage shows considerable impacts on the pore structures and compressive strength of concrete.This paper presents a study in which coral aggregates were presoaked in Ca(OH)_(2) slurries with different solid-to-liquid ratios(i.e.0.2,0.4,and 0.6 g/mL)followed by accelerated carbonation.The effects of CO_(2)sequestration on the particle size distribution,cylinder compressive strength,water absorption,and apparent density of coral aggregate were investigated.The evolution of pore structures in coral aggregate concrete after CO_(2)sequestration was also studied.Additionally,the effect of CO_(2)sequestration on the development of compressive strength of coral aggregate concrete was explored.The results showed that CO_(2)sequestration affected the properties of coral aggregate.Moreover,the porosity of CaCO_(3) formed by CO_(2)sequestration was the highest in the concrete.With the increase of solidto-liquid ratio,the porosity of cement pastes and the CaCO_(3) increased,and more big pores existed in the cement pastes and CaCO_(3).Furthermore,the compressive strength of coral aggregate concrete when the solid-to-liquid ratio was 0.2 g/mL increased compared with that before CO_(2)sequestration,but the compressive strength reduced when the ratio increased to 0.6 g/mL.

海洋骨料混凝土材料与结构性能研究进展

针对陆地砂石资源日益匮乏的现状,远离陆地的深远海岛礁建设需要利用海水、海砂和珊瑚等海洋资源制备混凝土,以实现就地取材从而避免远途运输并降低工程造价。本文主要介绍了“海洋骨料混凝土”的概念以及国内外关于海洋骨料混凝土所做的研究工作,包括材料的工作、力学、抗氯离子渗透、抗冻融、抗收缩和断裂性能。结果表明,海洋骨料混凝土的力学以及耐久性能与普通混凝土相当。随后,介绍了海洋骨料混凝土与纤维增强复合材料的黏结性能以及结构构件的抗压和抗弯性能方面的研究进展,纤维增强复合材料可以制作成筋材、管材、片材以及编织网用于海洋骨料混凝土结构中,使得其抗压与抗弯性能明显提升,但海洋环境下混凝土的碱性对纤维增强复合材料的耐久性影响显著。

珊瑚骨料透水混凝土强度和透水性能

采用珊瑚骨料制备混凝土是海洋工程建设的重要内容。为掌握珊瑚骨料制备透水混凝土的关键技术,通过测试28 d抗压强度、弯拉强度、连续孔隙率和透水系数,研究了不同预湿状态珊瑚骨料等体积取代部分碎石和矿物掺合料对透水混凝土强度和透水性的影响。结果表明:透水混凝土强度随着预湿珊瑚骨料掺量的增加而降低,其透水性不断增加;掺加10%未预湿珊瑚骨料增加了透水混凝土强度,降低了透水性,继续增加其掺量,降低了透水混凝土强度,增加了透水性;双掺粉煤灰和硅灰增加了珊瑚骨料透水混凝土强度,降低了透水性。珊瑚骨料透水混凝土的强度与透水系数和连续孔隙率存在着线性关系。