Degradation of permeability resistance of high strength concrete after combustion

To evaluate the remaining durability of concrete materials after combustion, the permeability of high strength concrete (HSC) after combustion was studied. The transport behavior of chloride ion, water and air in concrete after combustion and the effect of temperature, strength grade, and aggregation on the permeability of HSC after combustion are investigated by chloride ion permeability coefficient (D_c), water permeability coefficient (D_w) and air permeability coefficient (D_a). The experiment results show that all three permeability coefficients commendably reflect changes of permeability. The permeability coefficient increases with the evaluation temperature. After the same temperature, the permeability coefficient of HSC is lower than that of normal strength concrete (NSC). However, the degree of degradation of permeability coefficient of HSC is greater than that of NSC. The permeability resistance of HSC containing limestone is better than that of HSC containing basalt. Combining changes of compressive strength and permeability, the remaining durability of concrete materials after combustion is appropriately evaluated.     

Permeability testing of drilling core sample from pavement

The permeability coefficient of pavement material is a very important parameter in designing the drainage of pavement structures and is also used to evaluate the quality of road construction. New equipment is used to measure the permeability coefficient of the pavement drilling core sample and relevant testing methods are introduced. Testing drilling core samples from a certain highway of Yunnan province has been proven to be feasible. The test results are also analyzed. __________ Translated from Journal of Kunming University of Science and Technology (Science and Technology), 2007, 32(1): 72–76 [译自:昆明理工大学学报(理工版)]     

Unified solution of limit loads of thick wall cylinder subject to external pressure considering strain softening

Based on the unified strength theory [1], a unified strength criterion for strain softening materials, such as concrete or rock, was derived, and the elastic and plastic limit loads of a thick-walled cylinder made of these materials subject to external pressure were also given. In addition, the influence of some factors on the limit loads of such cylinders as the ratio of the external radius to internal radius, r _b/r _a, the coefficient b, which reflects the effect of medium principal stress and the normal stress of the relevant surface on the material destroy degree, the ratio of tensile strength to compressed strength of the material, α, and the damage variable β were discussed in detail. Some examples were given and some meaningful results were obtained. __________ Translated from Journal of Hunan University (Natural Science), 2006, 33(2): 1–5 [译自: 湖南大学学报 (自然科学版)]     

Carbonation depth prediction of fly ash concrete subjected to 2-and 3-dimensional CO2 attack

Based on accelerated carbonation test, 1D, 2D and 3D carbonation of high performance concrete with different fly ash contents (0%, 15%, 20%, 40%, 60%) were studied. The interaction coefficients of 2D and 3D carbonation are defined. The time-dependent coefficient n of carbon dioxide diffusion and the interaction coefficients K of 2D and 3D carbonation are analyzed. The mathematical model which includes n and K is deduced. The experiments indicate that 2D and 3D carbonation have obvious interaction. Interaction coefficients obey the attenuation function through time t. The mathematical model of 2D and 3D carbonation life prediction based on Fick scattering law can be solved by Fourier expansion. The life prediction of concrete of Sutong Bridge beam shows that the 1D, 2D, 3D carbonation prediction life are 1595 a, 882 a and 758 a, respectively, 2D and 3D carbonation life predicted are reduced clearly. __________ Translated from Journal of Southeast University (Natural Science Edition), 2007, 37(4): 645–650 [译自: 东南大学学报(自然科学版)]     

Biaxial tensile-compressive experiment of concrete at high temperatures

Biaxial tension-compression experiments of concrete of five stress ratios at high temperatures were carried out using the large static-dynamic triaxial test system in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. The stress ratios σ ₁/σ ₃ are 0, 0.1, 0.25, 0.50, and 0.75. The temperatures are 20°C, 200°C, 300°C, 500°C, 600°C. The mechanical behavior of concrete under biaxial tension-compression at high temperatures is analyzed. It is found that both the tensile strength and strain diminished with the increase in temperature under each stress ratio. Based on the test results, the relationship between tensile strengths and stress ratios and temperature is proposed. In addition, the failure criterion of concrete under biaxial stress state of tension-compression at high temperatures is established. Translated from Journal Dalian University of Technology, 2006, 46 (1): 54–58 [译自: 大连理工大学学报]     

Experimental research on seismic behavior of recycled concrete frame under varying cyclic loading

A study is conducted on the seismic behavior of one natural concrete frame and two recycled concrete frames with 100% recycled coarse aggregate whose scales are 1:2 entirely, and a comparative study is conducted under low-reversed cyclic lateral loading and different vertical loading. This work aims to estimate the failure mechanism, hysteresis loops, displacement ductility, deterioration of strength and stiffness and energy dissipation of recycled concrete frames under low-reversed cyclic loading as well as the influence of different vertical loading. Analysis on the basis of the experiment proves that it is entirely feasible to apply recycled concrete to practical engineering for the sake of its good seismic behaviors. Theoretical base is provided for further study and practical application of recycled concrete structure. __________ Translated from Journal of Tongji University (Natural Science), 2007, 35(8): 1013–1018 [译自: 同济大学学报(自然科学版)]     

Correlation between tension softening relation and crack extension resistance in concrete

Changes of the material fracture toughness with crack propagation can be described by a crack extension resistance curve, one of the fundamental fracture criteria in crack mechanics. Recently, experimental observation of the fracture behavior in concrete was used to develop a new fracture criterion, the crack extension G _R resistance curve, to analyze crack propagation during the entire concrete fracture process. The variation of the crack extension resistance is mainly associated with the energy consumption in the fracture process zone ahead of the stress-free crack tip. The crack extension resistance is then a function of the softening curve, which is a basic mechanical property in the fracture process zone. The relationship between the softening curve and the crack extension G _R resistance curve is then analyzed based on results of three-point bending beams tests. The results indicate that the characteristic points of the crack extension resistance G _R curve is closely related to the characteristic point on used tension softening curve. __________ Translated from Journal of Tsinghua University (Science and Technology), 2008, 48(3): 316–320 [译自: 清华大学学报(自然科学版)]     

高强度混凝土的耐火性能研究

高强度混凝土(HSC)具有强度高,空隙率低,抗渗性好,耐久性好等优点,在建筑工程特别是高层建筑中被广泛采用。但是,与普通混凝土相比,高强混凝土的耐火性能较差,特别是火灾中的抗爆裂性能较差。该文从工程应用的角度出发,结合国外的研究成果,对高强混凝土(HSC)的耐火性能进行探讨,将为高强度混凝土抗火设计提供参考。     

高强度混凝土的耐火性能研究

高强度混凝土（HSC）具有强度高，空隙率低，抗渗性好，耐久性好等优点，在建筑工程特别是高层建筑中被广泛采用。但是，与普通混凝土相比，高强混凝土的耐火性能较差，特别是火灾中的抗爆裂性能较差。本文从工程应用的角度出发．结合国外的研究成果，对高强混凝土（HSC）的耐火性能进行探讨，将为高强度混凝土抗火设计提供参考。     

Limit of axial force ratio and requirement for stirrups of RC columns with special shape

Thousands of columns with special shape are analyzed by nonlinear numerical methods. The ductility is calculated to investigate the limit of the axial force ratio and circumstantial requirement for stirrups of an reinforced concrete (RC) column with special shape, in the point of view of the characteristic value for providing stirrup. The limit of the axial force ratio of columns with special shape in relation to the characteristic value of the stirrup is obtained. Then, the effect of stirrup arrangement on the ductility of the RC column is discussed in case of buckling of the longitudinal reinforcement and constraint concrete columns. The complete requirement for stirrups of RC column with special shape is given. Translated from Journal of Tianjin University (Natural Science Edition), 2006, 39(3): 295–300 [译自: 天津大学学报 (自然科学版)]     

Effect of concrete creep and shrinkage on tall hybrid-structures and its countermeasures

This paper aims to study the different vertical displacements in tall hybrid-structures and the corresponding engineering measures. First, the method to calculate the different vertical displacements in tall hybrid-structures is presented. This method takes into account the effects of construction process by applying loads sequentially story by story. Based on the concrete creep and shrinkage calculation formula in American Concrete Institute (ACI) code, with the assumption that loads are increased linearly in members, the creep and shrinkage effects of members are analyzed by adopting two parameters named average load-aged coefficient and average age-last coefficient. The effects of steel ratio on members creep are analyzed by age-adjusted module method (AEMM). The effects that core-tube were constructed in advance to outer steel frame were also considered. Then, based on the sample calculation, the measures to effectively reduce the different vertical displacements in hybrid-structures are proposed. This method is simple and practical in the calculation of different vertical displacements in tall and super-tall hybrid-structures. __________ Translated from Journal of Hunan University (Natural Sciences), 2008, 35(1): 1–5 [译自: 湖南大学学报 (自然科学版)]     

Experimental study on fire protection methods of reinforced concrete beams strengthened with carbon fiber reinforced polymer

In this paper, two reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) and attached with thick-painted fire resistant coating were tested for fire resistance following the standard fire testing procedures. The experimental results show that the specimen pasted with the insulated layer of 50 mm in thickness could resist fire for 2.5 h. It is also demonstrated that the steel wire mesh embedded in the insulated layer can effectively prevent it from cracking and eroding under firing. __________ Translated from Journal of Tongji University (Natural Sciences), 2006, 34(11): 1452–1456 [译自: 同济大学学报 (自然科学版)]     

Field test on temperature field and thermal stress for prestressed concrete box-girder bridge

A field test was conducted to investigate the distribution of temperature field and the variation of thermal stress for a prestressed concrete (PC) box-girder bridge. The change of hydration heat temperature consists of four periods: temperature rising period, constant temperature period, rapid temperature fall period and slow temperature fall period. The peak value of hydration heat temperature increases with the increasing casting temperature of concrete; the relation between them is approximately linear. According to field tests, the thermal stress incurred by hydration heat may induce temperature cracks on the PC box-girder. Furthermore, the nonlinear distribution of temperature gradient and the fluctuation of thermal stress induced by exposure to sunlight were also obtained based on continuous in-situ monitoring. Such results show that the prevailing Chinese Code (2004) is insufficient since it does not take into account the temperature gradient of the bottom slab. Finally, some preventive measures against temperature cracks were proposed based on related studies. The conclusions can provide valuable reference for the design and construction of PC box-girder bridges. __________ Translated from Journal of Huazhong University of Science and Technology (Urban Science Edition), 2007, 24(4): 83–97 [译自: 华中科技大学学报 (城市科学版)]     

混凝土的渗水、吸水特性研究

论述了混凝土的渗透性对耐久性能的影响,研究了不同水灰比混凝土各龄期的渗水、吸水性能;并且探讨了混凝土强度与吸水性能、渗水性能的相关关系。试验结果表明：水灰比越大,龄期越短,混凝土的吸水性系数和渗水性系数越大;混凝土强度与吸水性能、渗透性能的相关性较好。     

Spatial embedded reinforcement of 20-node block element for analysis PC bridges

The formula for the contribution of prestressed reinforcement on embedded reinforcement element is derived according to the mechanical behavior of PC bridges and the foundational principle of finite element method. Mechanical concept is definite and examples validate the calculation results. Reinforcement element model allows generating a finite element mesh without taking into consideration the layout of reinforcements. Furthermore, the prestressing tendon may pass through the concrete elements in an arbitrary manner. It is an effective approach that the no-node loads are diverted from the tendons to the adjacent concrete elements. A useful arithmetic analysis of the spatial curved tendon PC Bridges is provided. __________ Translated from Journal of Tongji University (Natural Science), 2007, 35(4): 455–460 [译自: 同济大学学报(自然科学版)]     

Influence of bacteria on the compressive strength, water absorption and rapid chloride permeability of fly ash concrete

This paper presents the results of an experimental investigation carried out to evaluate the influence of Sporoscarcina pasteurii bacteria on the compressive strength and rapid chloride permeability of concrete made without and with fly ash. Cement was replaced with three percentages (10, 20 and 30) with fly ash by weight. Three different cell concentration (0, 10³,10⁵,10⁷ cells/ml) of bacteria were used in making the concrete mixes. Tests were performed for compressive strength, water absorption and rapid chloride permeability at the age of 28 days. Test results indicated that inclusion of S. pasteurii in fly ash concrete enhanced the compressive strength, reduced the porosity and permeability of fly ash concrete. Maximum increase (22%) in compressive strength and four-times reduction in water absorption was observed with 10⁵ cells/ml of bacteria. This improvement in compressive strength was due to deposition on the bacteria cell surfaces within the pores.Calcite deposition in concrete observed nearly eight times reduction in chloride permeability of fly ash concrete. The present work highlights the influence of bacteria on the properties of concrete made with supplementing cementing material such as like fly ash. Usage of bacteria like S. pasteurii improves strength and durability and strength of fly ash concrete through self-healing effect.     

Application of cable-supported spatial grid in dry coal shed

This paper presented a new structural style cable-supported spatial grid, which was applied in large span dry coal sheds. The influence of configuration of cable on the force of cable and beam, the ratio of beam force to cable force and the deflection of span was investigated, and a rational configuration of cable was obtained. The results show that the cable-supported spatial grid can maximize the use of material strength, and have the advantages of low usage of steel, large span and sufficient headroom. __________ Translated from Journal of Hunan University (Natural Sciences), 2007, 34(2): 29–32 [译自: 湖南大学学报 (自然科学版)]     

Influence of initial curing conditions and exposure environments on chloride migration in concrete using electrochemical method

To investigate the influence of initial curing conditions and exposure environments on chloride ions’ migration in concrete, the specimens that were cured in different conditions and placed in several marine environments were studied. The amount of chloride ions passing through the specimen’s section was monitored in the accelerated chloride migration test. The effective chloride diffusion coefficients (D _e) were calculated from the steady-state on the basis of Nernst-Planck’s equation. The results indicated that the initial curing conditions and the wet-dry interval of external environments have coupling influence on effective diffusion coefficient of chloride ion significantly. D _e reflected the density and pore connectivity of concrete. The co-effects of the initial curing condition and following exposure environment should be taken into account in durability design for concrete structure located in marine environment.     

Moment-curvature relationship of FRP-concrete-steel double-skin tubular members

Tests were conducted on 3 specimens to study the flexural behavior of fiber reinforced polymer (FRP)-concrete-steel double-skin tubular members (DSTMs). The strip method was used to calculate the section momentcurvature curves of the 3 specimens and 12 models. A theoretical formula is presented for the flexural strength of DSTMs. The test results show that the tension zone of the specimen FRP tubes was in hoop compression while the compression zone was in hoop tension. The load-carrying capacity did not decrease even when the mid-span deflection reached about 1/24 of the span length. The tests, simulation and theoretical analysis resulted in a simplified formula for the flexural strength of DSTMs and a tri-linear moment-curvature model was expressed as a function of the section bending stiffness for DSTMs. __________ Translated from Journal of Tsinghua University (Science and Technology), 2008, 47(12): 2105–2110 [译自: 清华大学学报(自然科学版)]     

Behavior of steel fiber-reinforced high-strength concrete at medium strain rate

Impact compression experiments for the steel fiber-reinforced high-strength concrete (SFRHSC) at medium strain rate were conducted using the split Hopkinson press bar (SHPB) testing method. The volume fractions of steel fibers of SFRHSC were between 0 and 3%. The experimental results showed that, when the strain rate increased from threshold value to 90 s⁻¹, the maximum stress of SFRHSC increased about 30%, the elastic modulus of SFRHSC increased about 50%, and the increase in the peak strain of SFRHSC was 2–3 times of that in the matrix specimen. The strength and toughness of the matrix were improved remarkably because of the superposition effect of the aggregate high-strength matrix and steel fiber high-strength matrix. As a result, under impact loading, cracks developed in the SFRHSC specimen, but the overall shape of the specimen remained virtually unchanged. However, under similar impact loading, the matrix specimens were almost broken into small pieces. __________ Translated from Journal of Southeast University (Natural Science Edition), 2007, 37(5): 892–897 [译自: 东南大学学报(自然科学版)]