Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete （SRUHSC） columns and steel reinforced concrete （SRC） beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirIup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete （SRC） frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.

Evaluation of the Shear Strength of Perfobond Rib Connectors in Ultra High Performance Concrete

Since the previous strength prediction models for the perfobond rib connector were proposed based upon the results of push-out tests conducted on concretes with compressive strength below 50 MPa, push-out test is performed on perfobond shear connectors applying ultra high performance concretes with compressive strength higher than 80 MPa to evaluate their shear resistance. The test variables are chosen to be the diameter and number of dowel holes and, the change in the shear strength of the perfobond rib connector is examined with respect to the strength of two types of UHPC: steel fiber-reinforced concrete with compressive strength of 180 MPa and concrete without steel fiber with compressive strength of 80 MPa. The test results reveal that higher concrete strength and larger number of holes increased the shear strength, and that higher increase rate in the shear strength was achieved by the dowel action. The comparison with the predictions obtained by the previous models shows that the experimental results are close to the values given by the model proposed by Oguejiofor and Hosain [1].

Residual Mechanical Properties and Explosive Spalling Behavior of Ultra-High-Strength Concrete Exposed to High Temperature

In order to explore the characteristics of ultra-high-strength concrete exposed to high temperature,residual mechanical properties and explosive spalling behavior of ultra-high-strength concrete( UHSC) and high strength concrete( HSC) exposed to high temperatures ranging from 20 ℃ to 800 ℃ were determined. The microstructure of the specimens after exposure to elevated temperature was analyzed by means of scanning electron microscope( SEM) and mercury intrusion porosimetry( MIP). The residual compressive strengths of UHSC and HSC were first increased and then decreased as temperature increased. After exposure to 800 ℃,the compressive strengths of UHSC and HSC were 24. 2 % and 22. 3 % of their original strengths at 20 ℃,respectively. The residual splitting tensile strengths of both UHSC and HSC were consistently decreased with the temperature increasing and were approximately 20% of their original strengths after 800 ℃. However,the residual fracture energies of both concretes tended to ascend even at 600 ℃. The explosive spalling of UHSC was more serious than that of HSC. Moisture content of the specimens governs the explosive spalling of both concretes with a positive correlations,and it is more pronounced in UHSC. These results suggest that UHSC suffers a substantial loss in load-bearing capacity and is highly prone to explosive spalling due to high temperature. The changes in compressive strength are due to the changes in the density and the pore structure of concrete. The probability and severity of explosive spalling of UHSC are much higher than those of HSC due to the higher pore volume in HSC.

Seismic Performance of Steel Reinforced Ultra High-strength Concrete Columns

The seismic performance of steel reinforced ultra-high-strength concrete columns(SRSHC) with various shear-span ratios(λ) were studied through a series of experiments.The concrete compressive cube strength value of experimental specimens ranged from 92.9 MPa to 108.1 MPa.The main experimental variables affecting seismic performance of specimens were axial load ratio and stirrup reinforcement ratio.The columns(λ=2.75) subjected to low cyclic reversed lateral loads failed mainly in the flexural-shear mode failure and columns(λ≤2.0) subjected to low cyclic reversed lateral loads failed mainly in the shear mode failure.Shear force-displacement hysteretic curves and skeleton curves were drawn.Coefficient of the specimen displacement ductility was calculated.Experimental results indicate that ductility decreases with axial pressure ratio increasing,and increases with stirrup reinforcement ratio increasing.Limit values of axial pressure ratio and minimum stirrup reinforcement ratio of columns are proposed to satisfy definite ductility requirement.The suggested values provide a reference for engineering application and for the amendment of the current Chinese design code of steel reinforced concrete composite structures.

MODIFICATION THE CEMENTIOUS MATERIAL OF ULTRA-HIGH-STRENGTH SLEEPER CONCRETE

This paper presents investigation results on the natural ultra-fine mineral flour of crystalline silica fume (CSF) and porous quartz sand stone (PQSS) which can modify cement mortar strength under hydrothermal synthesis reaction (HSR) in the autoclave-cured condition. The replacement of cement by CSF and PQSS can signifi cantly increase the Jflerural and compressive strength which reach 22MPa and 150MPa respectively and de-crease the porosity oj the cement mortar. The ratio oj fine aggregation, standard sand to cementions material has sig nificant influence on the mortar strength. The mechanisms involved in cement and natural mineral flour and the HSR are presented. CaO/SiO2 ratio ranges from 3. 20 to 1. 11. the main hydrate phase is C2SH and there is not Tober-morite through X-Ray diffraction qualitative analysis. The new and ultra-high strength cementious material as basic material of sleeper concrete can he used in prestressed reinforcement sleeper concrete.

Seismic Behavior of Steel Reinforced Ultra High Strength Concrete Column and Reinforced Concrete Beam Connection

To investigate the seismic behavior of connections composed of steel reinforced ultra high strength concrete (SRUHSC) column and reinforced concrete (RC) beam, six interior strong-column-weak-beam connection specimens were tested subjected to reversal cyclic load. Effects of applied axial load ratio and volumetric stirrup ratio on ductility, energy dissipation capacity, strength degradation and rigidity degradation were discussed. It was found that all connection specimens failed in bending in a ductile manner with a beam plastic hinge. The ductility and energy dissipation capacity increased with the decrease of applied axial load ratio or increase of volumetric stirrup ratio. The displacement ductility coefficient and equivalent damping coefficient lay between those of steel reinforced ordinary concrete connection and those of reinforced concrete connection. The applied axial load ratio and volumetric stirrup ratio had less influence on the strength degradation and more influence on the stiffness degradation. The stiffness degraded sharply with the decrease of volumetric stirrup ratio or increase of applied axial load ratio. The experimental results indicate that SRUHSC column and RC beam connection exhibited better seismic performance and can provide reference for engineering application.

Experimental Study on Flexural Behavior of Post-Tensioning Bonded Partially Prestressed Ultra-High Strength Concrete Beams

This paper presents the results of four partially prestressed ultra-high strength concrete beams in flexure. The test results are used to evaluate the effects of prestressing tendon depth and area on flexure behavior of specimen beams. The test results indicate that: the cracking load,yielding load,peak load and stiffness postcracking of specimen beams are enhanced by reducing prestressing tendon depth or increasing prestressing tendon area, and the flexural ductility is improved by increasing prestressing tendon depth or reducing prestressing tendon area. The effect of complex reinforcement index considering the strength of the equivalence principle and the reinforcement position on loading levels under serviceability limit state,flexural strength and displacement ductility factor are studied. The influence coefficient of prestressing tendon kpis introduced in the complex reinforcement index. As the complex reinforcement index increases, the loading levels under serviceability limit state and flexural strength increases linearly,and the displacement ductility factor decreases linearly. The test results also verify the conventional beam flexural theory based on the plane cross-section assumption for predicting ultimate flexural strength of partially prestressed ultra-high strength concrete beams is valid. After the introduction of the coefficient kp,the calculation method of cracks in code for design of concrete structure in china are appropriated for the specimen beams.

Design of Eco-friendly Ultra-high Performance Concrete with Supplementary Cementitious Materials and Coarse Aggregate

Aiming to investigate the mix design of eco-friendly UHPC with supplementary cementitious materials and coarser aggregates, we comprehensively studied the workability, microstructure, porosity, compressive strength, flexural strength, and Young’s modulus of UHPC. Relationship between compressive strength and Young’s modulus was obtained eventually. It is found that the compressive strength, flexural strength, and Young’s modulus of UHPC increase by 19.01%, 10.81%, and 5.99%, respectively, when 40 wt% cement is replaced with supplementary cementitious materials. The relationship between compressive strength and Young’s modulus of UHPC is an exponential form.

Different Curing Systems on Mechanical Properties of Ultra-High Performance Concrete with Coarse Aggregate

High durability and high tensile strength makes ultra-high performance concrete( UHPC) an ideal material for bridges,while its early shrinkage in the construction of cast-in-situ mass concrete leads structure crack-easily,which restricts the application of UHPC in deck system. Whether reasonable amount of coarse aggregate can influence the strength of UHPC and improve the shrinkage performance or reduce the cost is still in doubt. Besides,in order to improve its constructability and workability, whether autoclaved curing system of UHPC can be changed remains to be further researched. In response to these circumstances, a systematic experimental study on the strength of UHPC mixed with coarse aggregate in different ratios has been presented in this paper. The three curing systems,namely standard curing,180-200 ℃/1. 1 MPa autoclaved curing,and hot water curing were tested to reveal the relationship between UHPC's properties and curing systems,and the UHPC ' s microstructure was also preliminarily studied by scanning electron microscope( SEM). The experimental research can draw the following conclusions. Under the condition of the same mix ratio, autoclaved curing guarantees the highest compressive strength,followed by hot water curing and standard curing. The compressive strength of concrete increases with the temperature in the range of 25 to 90 ℃ hot water curing,and high temperature in precuring period can speed up the strength development of UHPC,but the sequence of precuring period does not obviously affect the results. In 90 ℃ hot water and autoclaved curing,the strength is over 150 MPa,and it has little relation with gravel ratio. While the value increases first and then decreases in a lower temperature curing with the increasing of gravel amount,even only about 80 MPa at room temperature. The strength increases moderately along with the increase of the curing age by standard curing,especially in the initial stage.

The Effect of Various Polynaphthalene Sulfonate Based Superplasticizers on the Workability of Reactive Powder Concrete

A superplasticizer is a type of chemical admixture used to alter the workability(viscosity)of fresh concrete.The workability of fresh concrete is often of particular importance when the water-to-cement(w/c)ratio is low and a particular workability is desired.Reactive Powder Concrete(RPC)is a high-strength concrete formulated to provide compressive strengths exceeding 130MPa and made of primarily powders.RPC materials typically have a very low w/c,which requires the use of a chemical admixture in order to create a material that is easier to place,handle and consolidate.Superplasticizer are commonly used for this purpose.Superplasticizers are developed from different formulations,the most common being Polycarboxylate Ether(PCE),Polymelamine Sulfonate(PMS),and Polynaphthalene Sulfonate(PNS).This study investigates the effect of various PNS based superplasticizers on the rheological performance and mechanical(compressive strength)performance of a RPC mixture.Six distinctive types of PNS based superplasticizers were used;three of various compositional strengths(high,medium,low range)from a local provider,and three of the same compositional strengths(high,medium,low)from a leading manufacturer.The properties investigated were the individual superplasticizers’viscosity,the concrete workability,determined through a mortar spread test,the concrete viscosity,and the compressive strength of the hardened RPC mixtures measured at 7,14,and 28 days.Two separate RPC mixtures were prepared,which contained two different water-to-cementitious ratios,which consequently increases the dosage of superplasticizer needed,from 34.8L/m3 to 44.7L/m3.The results show that the name brand high range composition produced the overall highest spread,lowest viscosity,and a highest compressive performance.However,the local provider outperformed the name brand in the mid and low range compositions.Lastly,the rheology assessment also confirmed that the name brand high range,and RPC fabricated with the name brand high range,developed the lowest viscosities.

Experimental on C50 Ultra-high-early Strength Concrete

In this paper,C50 ultra-high-early-strength concrete was prepared with rapid-harden sulphoaluminate cement,alkali carbonate early strength agent,inorganic tribasic acid retarder and polycarboxylate water reducing agent. The properties testing results of C50 ultra-high-early-strength concrete indicated that the compressive strength at 12 h exceeded 37 MPa and at 3 d exceeded 59 MPa. Compared to the C50 ordinary reference concrete,the durability of C50 ultra-high-early strength concrete was investigated. The results indicated that C50 ultra-high-early-strength concrete had the same chloride ion permeability resistance,carbonization resistance and abrasion resistance to the C50 ordinary concrete. As to the sulfate resistance,the ultra-high-early-strength concrete was much better than the ordinary concrete.

Analysis of Compressive Strength Development of Ultra-high Performance Concrete

A numerical procedure was presented for evaluating the compressive strength development of ultra-high performance concrete(UHPC) with cement-silica fume-slag binder.This numerical procedure started with initial packing behavior of designed UHPC using a random sequential packing method.Furthermore,synergistic effect of combined mineral admixtures was addressed with respect to hydration heat.Accordingly,hydration degree of cement and reaction degrees of mineral admixtures were determined based on a blended cement hydration model.Finally,a compressive strength evolution model was proposed and the evolution of compressive strength of three mixes with different binder recipes was compared.The results showed that the both initial packing behavior of UHPC mixes and synergistic effect of mineral admixtures are critical for predicting the properties of UHPC.A remarkable void fraction of 0.2042 was observed for UHPC mix designed by optimization algorithms under random packing.Furthermore,a negative synergistic effect of the combination of silica fume and slag was obtained with regarding to compressive strength.In addition,for a given mineral admixture replacement of 20%,the formulation of mineral admixture should be carefully tailored where the UHPC incorporating 5%silica fume and 15%slag shows the highest compressive strength.

海砂超高性能混凝土试验

采用未淡化的海砂制备超高性能混凝土(UHPC)和普通混凝土,研究了不同氯离子含量的海砂对UHPC抗压强度、孔结构、快速氯离子渗透性以及内置钢筋耐久性的影响,并与普通混凝土进行分析比较。结果表明,海砂中的氯离子含量对UHPC抗压强度并不会产生较大的消极影响;海砂UHPC的临界孔半径约为2 nm,与海砂普通混凝土不同,孔隙率随海砂中氯含量的增加而增加;即使海砂氯离子含量高达0.636%,海砂UHPC的氯离子渗透性仍可忽略不计;海砂UHPC中钢筋在28 d后处于钝化状态并趋于稳定。

高强钢管超高性能混凝土短柱轴压承载力性能试验研究

为研究高强钢管内填超高性能混凝土(UHPC)的钢管混凝土短柱轴压性能,该文设计22根高强钢管UHPC短柱进行轴压试验,从破坏模式、荷载-纵向应变关系对试件的轴压性能进行对比分析,旨在通过径厚比、混凝土强度、截面类型等变化来探究高强钢管UHPC短柱的实际承载力性能差异.试验结果表明:高强钢管UHPC短柱的轴压性能受径厚比、混凝土强度影响较大;试件承载力随钢管壁厚增加而增加,但相同钢管直径和不同钢管直径增幅呈现不同规律.该文从延性、核心混凝土强度提高程度两个角度进行分析并提出高强钢管混凝土试件的相关参数设计建议.最后,将高强钢管UHPC短柱的轴压试验承载力结果与国内外规范GB 50936—2014、AIJ计算承载力结果进行对比,并基于极限平衡理论推导出高强圆钢管UHPC试件的轴压承载力计算公式,同时结合该文试验数据对其进行验证,证明了公式的准确性.

高强钢管超高性能混凝土界面黏结滑移性能试验

目的为了研究高强钢管超高性能混凝土的黏结滑移性能,方法考虑径厚比、长径比和钢纤维掺量3个参数的影响,设计并制作15根构件进行试验研究,通过推出试验,得到轴向荷载与加载端位移的荷载-位移曲线,以及高强钢管与超高性能混凝土之间的黏结强度,根据试验结果,对影响黏结强度的参数进行定性定量分析。结果结果表明:随着高强钢管长径比和径厚比的增加,黏结强度减小;随着钢纤维掺量增加,试件黏结强度先增加后减小,即当钢纤维掺量从1%增加到2%时,黏结强度增加,但当钢纤维掺量从2%增加到3%时,黏结强度反而降低;试件荷载-滑移曲线表现为两种形式,一是有明显的峰值点,峰值点后曲线急速下降(部分出现尾部上升的现象),二是曲线无明显峰值点,拐点后曲线持续缓慢增长。高强钢管超高性能混凝土的黏结破坏分为黏结阶段、滑动阶段和抗摩擦阶段。结论以长径比、径厚比、含钢率和套箍系数为参数,结合试验数据并对其进行回归分析,得到适用于高强钢管超高性能混凝土黏结强度的计算模型,并将计算结果与试验结果进行对比,计算值与试验值吻合较好,可为实际工程提供一定参考。

BFRP筋与超高性能纤维混凝土黏结性能试验研究

为探究玄武岩纤维增强复合材料(BFRP)筋与超高性能纤维混凝土(UHPFRC)之间的黏结性能,通过中心拉拔试验,对BFRP-UHPFRC试件破坏模式、黏结-滑移曲线进行分析,且探讨了筋材直径、单一纤维及钢-PVA混杂纤维对黏结强度的影响。结果表明,试件破坏模式包括BFRP筋拔出破坏与拉断破坏。BFRP筋直径的增大将导致黏结强度降低。随着单一纤维掺量增加,黏结强度呈先增大后减小的趋势。而相较于单一纤维,钢-PVA混杂纤维显著提高了试件的黏结强度,当掺入1%钢纤维和0.5%PVA纤维时,黏结强度最高(25.35 MPa)。此外,通过灰狼算法优化的支持向量回归模型实现对黏结强度的预测,预测值与实际值拟合较好。

粗集料对超高强混凝土徐变的影响

徐变是混凝土固有的时变特性,对混凝土结构和构件的受力及长期服役性能有着重大影响。以粗集料为切入点,研究粗集料的掺量、粗集料最大粒径以及粗集料与基体性质协调性等因素对超高强混凝土的力学性能和徐变性能的影响。结果表明:掺粗集料的超高强混凝土徐变明显小于不掺粗集料的混凝土,掺粗集料可有效降低超高强混凝土的徐变,但减小粗集料的最大粒径对徐变无明显抑制效果。此外,超高强混凝土并非强度越高徐变越小,其徐变的大小主要受超高强混凝土强度与弹性模量的协同发展程度的影响。通过修正FIB MC2010模型,建立了适用于超高强混凝土的徐变预测模型。

UHPC抗冲击性能的试验研究

抗冲击性能是混凝土结构重要技术特征。针对超高性能混凝土(UHPC)抗冲击性能,采用不同掺量的钢纤维和橡胶颗粒,试验研究了钢纤维和橡胶掺量对UHPC的工作性能、强度和抗冲击性能的影响,并对钢纤维和橡胶颗粒掺量对UHPC的影响机理进行了分析。试验研究结果表明:随着钢纤维掺量的增加,UHPC的坍落度和扩展度均不断降低,含气量增大,强度和抗冲击性能明显增大,钢纤维可有效提高UHPC的抗冲击性能,掺量宜控制在3%之内;随着橡胶颗粒掺量的增加,UHPC的抗压强度不断降低,但坍落度、扩展度、抗折强度及抗冲击性能呈现先增大后降低趋势,掺入适量的橡胶颗粒可有效提高UHPC的抗折强度和抗冲击性能,掺量宜控制在10%之内。通过采用钢纤维和橡胶颗粒,可配制出抗冲击性能良好的UHPC。

高强型钢超高性能混凝土短柱轴压性能试验及其有限元分析

为研究高强型钢超高性能混凝土短柱的轴压性能,对试件进行了轴心受压试验,设计参数包括配钢率、箍筋间距和箍筋形式。观察了试件的轴压破坏过程,获得了破坏形态及轴向荷载-位移曲线,分析了承载能力、变形能力、刚度等轴压性能指标,以及超高性能混凝土、纵筋、箍筋和型钢的应变发展规律。在试验研究基础上,采用ABAQUS建立了高强型钢超高性能混凝土短柱轴压性能有限元分析模型,计算结果与试验结果吻合较好,进而进行了参数分析。结果表明:试件发生的是典型轴压破坏,中部表面出现“锯齿形”裂缝;轴向荷载-位移曲线会出现2次荷载峰值;箍筋间距减小时,试件的承载能力和变形能力均提高,但刚度退化速率没有显著变化;配钢率增大时,试件的承载能力提高,变形能力先增大后减小,刚度退化变缓;型钢强度增大时,试件的承载能力和变形能力均提高,提高速度先快后慢;超高性能混凝土抗压强度增大时,试件的承载能力提高,变形能力降低;超高性能混凝土受拉能够达到峰值应变;纵筋在极限点之前发生受压屈服;箍筋在极限点时应变不足屈服应变的1/3,但在破坏点前达到受拉屈服;型钢翼缘在极限点前后发生屈服,腹板屈服晚于翼缘。

可恢复功能装配式高强再生混凝土剪力墙抗震性能试验研究

为推动高强再生混凝土构件应用于可恢复功能建筑结构,提出一种边缘构件配置弱粘结超高强纵筋的装配式高强再生混凝土剪力墙。对4个剪跨比为2.2的装配式高强再生混凝土剪力墙进行低周反复荷载试验,分析了边缘配置弱粘结超高强纵筋、墙体有无钢纤维的装配式高强再生混凝土剪力墙的损伤破坏形态、滞回性能、刚度退化、残余变形、裂缝宽度等抗震与可恢复性能指标。结果表明:在大变形条件下,边缘配置弱粘结超高强纵筋的装配式高强再生混凝土剪力墙水平承载力持续增大,混凝土损伤程度较轻,抗侧刚度退化缓慢,残余变形与裂缝宽度较小。基于试验结果,建议了可恢复功能装配式高强再生混凝土剪力墙在1%位移角下的抗震承载力计算方法,其计算结果与试验结果符合较好。