The Transport Properties of Concrete under the Simultaneous Coupling of Fatigue Load and Environment Factors

A set of coupling experimental instrument was designed to study the transport properties of chloride ion in concrete under simultaneous coupling action of fatigue load and environmental factors. Firstly the water-saturated performance of modem concrete was investigated, then diffusion performance of chloride ion under different stress levels and different temperature were studied respectively; meanwhile, the time- dependent behavior of the chloride ion diffusion in concrete was also researched. The results showed that the saturation degree of concrete can reach as high as 99%. Besides, diffusion coefficient of chloride ion increased with increasing of the stress level and temperature, and when the stress level and temperature are at 0.6 and 60 ℃ respectively, the diffusion coefficient is 6.3 ×10 -14 m2/s, moreover the diffusion coefficient of chloride ion in concrete decreased with time under the simultaneous coupling action of fatigue load and environment factors.

Coupling effects of influence factors on probability of corrosion initiation time of reinforced concrete

The coupling model of major influence factors such state affecting the chloride diffusion process in concrete is as environmental relative humidity, load-induced crack and stress discussed. The probability distributions of the critical chloride concentration Cc, the chloride diffusion coefficient D, and the surface chloride concentration Cs were determined based on the collected natural exposure data. And the estimation of probability of corrosion initiation considering the coupling effects of influence factors is presented. It is found that the relative humidity and curing time are the most effective factors affecting the probability of corrosion initiation before and after 10 years of exposure time. At the same exposure time, the influence of load-induced crack and stress state on the probability of corrosion initiation is obvious, in which the effect of crack is the most one

Influence of concrete differential aging time on the construction phases analysis of the Fenghua River Bridge

The Fenghua River Bridge is a major structure on the highway between Hengzhang and Guojiachi, which is to be built with a four-span prestress concrete (PC) box girder and symmetrical cantilever castings. In this paper, a finite element method (FEM) model is set up to study the effects of concrete differential aging time on the construction phases of the Fenghua River Bridge by calculating the vertical displacement of the folding segment of the middle span and the longitudinal bending moment of Pier 12#. In the model, the girders are classified into 150 changing sections based on the desgn scheme, and their construction is to be carried in 16 phases respectively to build 12 blocks connected by a side folding segment and a middle folding segment, covered with a second dead load and in completion for 20 years. It is found that the internal forces and deformations of the concrete structures at the aging time of 60 d are quite different from those of 0 d aging time while the behaviors of the structures of 120 d aging time is nearly the same as those of 60 d aging time― the differences are so small that can be neglected, suggesting that the creep develops obviously about one month after the cement is hardened and the development fades later on.

Discrete Numerical Study on Type II Fracture of Partially Detached Concrete Panels in Cold Region

When the upper chord beam of the beam-string structure(BSS)is made of concrete-filled steel tube(CFST),its overall stiffness will change greatly with the construction of concrete placement,which will have an impact on the design of the tensioning plans and selection of control measures for the BSS.In order to accurately obtain the bending stiffness of CFST beam and clarify its impact on the mechanical properties of composite BSS during con-struction,the influence of some factors such as height-width ratio,wall thickness of steel tube,elasticity modulus of concrete,and friction coefficient on the bending stiffness are analyzed parametrically by the numerical simula-tion technology based on an actual project.The calculation formula of the equivalent bending stiffness of CFST is also established through mathematical statistical simulation.Then,the equivalent bending stiffness is introduced into the construction and use stages of the composite BSS,respectively,and the mechanical properties such as prestress-tensioning control value,structural deformation,and internal force of key members are comparatively analyzed when adopting two different construction plans.Moreover,the optimal construction plan of concrete placementfirst and then prestress-tensioning is proposed.

Consideration of Viscoelasticity in Time Step FEM-Based Restraint Analyses of Hardening Concrete

Concrete structures may suffer considerable restraint stresses during their hardening period. This is caused by several deformation impacts, especially temperature field changings due to hydration heat and volume changes due to autogenous shrinkage. Mainly affected are massive concrete members, but also the application of new concrete types or the erection of outstanding constructions requires further investigations in this context. 3D-FEM analyses of hydration heat induced temperature development in combination with the well known shrinkage give sufficient results for the deformation impact. The according elastic restraint stresses can be determined with consideration of the concrete’s rising elastic modulus and the restraint degree of the system. But due to duration of the heat flow process, the height of restraint stresses is strongly dependent from the viscoelasticity of the concrete. The viscoelastic effects consist of many components constituted by changing material properties influencing themselves. In practice, different simplified approaches are available for considering this in calculations. Their implementation in time step analyses is not generally admitted and requires expertise. In contrast, present research develops material models needing specific input parameters for every use case. This contribution focuses on a practicable approach considering the superposition of the viscoelastic behaviour of every stress increment in time step FEM analysis. The differentiation between the pure viscoelastic material behaviour (as it is given in the codes for idealistic conditions like creep or relaxation) and the according viscoelastic system response (addicted to the systems variable restraint degree) allows the transfer of this model into practice. As one application example of this approach, the compatibility check and the FEM-based recalculation of the monitoring program of a massive power plant slab will be presented.

Time-dependent water permeation behavior of concrete under constant hydraulic pressure

In the present work, a concrete permeability testing setup was designed to study the behavior of hydraulic concrete subjected to constant hydraulic pressure. The results show that when concrete is subjected to high enough constant hydraulic pressure, it will be permeated, and after it reaches its maximum permeation rate, the permeability coefficient will gradually decrease towards a stable value. A time-dependent model of permeability coefficient for concrete subjected to hydraulic pressure is proposed. It is indicated that the decrease of the permeability coefficient with permeation time conforms well to the negative-exponential decrease model.

Application of Fractal Theory in Brick-Concrete Structural Health Monitoring

In order to monitor and forecast the deformation of the brick-concrete building, by taking a brick-concrete building as research object, fiber grating sensors were used to collect the monitoring data and double logarithmic curve of limit value characteristic and monitoring data were obtained based on the fractal theory. Constant dimension fractal method cannot be used to analyze the data directly. With the method of variable dimension fractal, we accumulate data, and the double logarithmic curve is smooth. Piecewise fractal dimensions are close. The outer interpolation method is used to calculate the fractal dimension of the next point and then back calculate the vertical displacement. The relative errors are calculated by comparing the forecast values and monitoring values, and the maximum relative error is 5.76%. The result shows that the fractal theory is suitable to use in the forecast of the deformation and the accuracy is good.

Time Dependent Chloride Diffusion Coefficient in Concrete

In order to study how to correctly apply the two different time dependencies to service life prediction models, the definition of parameters in the time dependent equation using the long-term test method and the short-term test method, and its relationship were discussed. The experimental results show that, as the two time dependencies have different meanings, they have different manners when handled in service life prediction models. Time dependency of chloride diffusion coefficients in concrete was obtained by fitting experimental data. A large number of accurate experimental data were required to fit the formula of time dependency to ensure the accuracy of service life prediction model.

Effect of curing, capillary action, and groundwater level increment on geotechnical properties of lime concrete: Experimental and prediction studies

Lime concrete and lime treatment are two attractive techniques for geotechnical engineers.However,researches have rarely been carried out to study the effects of moisture and capillary action due to increasing groundwater level on geotechnical properties of lime concrete.The aim of this study is to investigate the effects of curing time and degree of saturation on some of geotechnical properties of lime concrete such as unconfined compressive strength(UCS),secant modulus(ES),failure strain,brittleness index(IB),and deformability index(ID) using unconfined compression tests.First of all,geotechnical and chemical properties of used materials were determined.After curing times of 14 d,28 d,45 d,and 60 d in laboratory condition,the specimens were exposed to saturation levels ranging from 0 to 100%.The results showed that the moisture and curing time have significant effects on the properties of lime concrete.Based on the results of scanning electron micrograph(SEM) test,it was observed that the specimen was characterized by a rather well-structured matrix since both the filling of a large proportion of the coarse-grained soil voids by clay and the pozzolanic activity of lime led to retaining less pore water in the specimen,increasing the UCS and ES,and consequently resisting against swelling and shrinkage of the clay soil.Moreover,due to the pozzolanic reactions and reduction of water,by increasing the curing time and decreasing the degrees of saturation,UCS,ES,and IBincreased,and IDdecreased.Based on the experimental results,a phenomenological model was used to develop equations for predicting the properties in relation to the ratio of degree of saturation/curing time.The results showed that there was a good correlation(almost R2> 90%) between the measured parameters and the estimated ones given by the predicted equations.

An Experimental Study on the Behavior of Shear Keys According to the Curing Time of UHPC

Precast segmental construction has been recently developed to reduce the construction cost and shorten the construction term as compared to the cast-in-place method in a will to establish the design and erection system of structures using Ultra High Performance Concrete (UHPC). However, this method requires the presence of segmental joints to transfer the loads between neighboring segments, which stresses the importance of securing structural safety and serviceability. Therefore, need is for research on the behavior of the segmental joint for the structures erected by the precast segmental construction method. To that goal, this paper presents an experimental study on the behavior of shear keys with respect to the curing time of UHPC in the segmental joint. Analysis is done on the load-displacement relation according to the curing time of the shear keys and on the failure modes of the shear keys according to the cracking pattern at failure.

Hydration Process and Crack Tendency of Concrete Based on Resistivity and Restrained Shrinkage Crack

Hydration process, crack potential and setting time of concrete grade C30, C40 and C50 were monitored by using a non-contact electrical resistivity apparatus, a novel plastic ring mould and penetration resistance methods, respectively. The results show the highest resistivity of C30 at the early stage until a point when C50 accelerated and overtook the others. It has been experimentally confirmed that the crossing point of C30 and C50 corresponds to the final setting time of C50. From resistivity derivative curve, four different stages were observed upon which the hydration process is classified; these are dissolution, induction, acceleration and deceleration periods. Consequently, restrained shrinkage crack and setting time results demonstrated that C50 set and cracked the earliest. The cracking time of all the samples occurred within a reasonable experimental period thus the novel plastic ring is a convenient method for predicting concrete＇s crack potential. The highest inflection time（t_i） obtained from resistivity curve and the final setting time（t_f） were used with crack time（t_c） in coming up with mathematical models for the prediction of concrete＇s cracking age for the range of concrete grade considered. Finally, an ANSYS numerical simulation supports the experimental findings in terms of the earliest crack age of C50 and the crack location.

Micro-scale FEM Calculation of Concrete Temperature during Production and Casting

A micro-scale finite element method(FEM) was proposed to precisely calculate the heat conduction between mortar and aggregate, and thus to accurately predict the non-uniformity of concrete pouring temperature. The concrete temperature field during vibration was also precisely calculated by accurate description of heat absorption characteristics of different parts of concrete when vibration. Based on the above method, the prediction model was used to predict the pouring temperature of a practical engineering. The comparison between actual results and simulated values shows that this method can be adopted to accurately predict the non-uniformity of concrete pouring temperature and the influence of mechanized vibration on concrete pouring temperature, and thus accurately predict pouring temperature. The control of casting temperature is crucial for preventing concrete fracture. The study provides a new method for predicting the pouring temperature of concrete structures, which has great practical value in engineering application.

Random Time Dependent Resistance Analysis on Reinforced Concrete Structures

The analysis method on random time dependence of reinforced concrete material is introduced,the effect mechanism on reinforced concrete are discussed,and the random time dependence resistance of reinforced concrete is studied.Furthermore,the corrosion of steel bar in reinforced concrete structures is analyzed.A practical statistical method of evaluating the random time dependent resistance,which includes material,structural size and calculation influence,is also established.In addition,an example of predicting random time dependent resistance of reinforced concrete structural element is given.

In-situ Monitoring the Setting Behavior of Foamed Concrete Using Ultrasonic Pulse Velocity Method

The applicability of ultrasonic pulse velocity （UPV） method to in-situ monitor setting and hardening process of foamed concrete （FC） was systematically investigated. The UPVs of various FC pastes were automatically and continuously measured by a specially designed ultrasonic monitoring apparatus （UMA）. Ultrasonic tests were performed on FC mixtures with different density （300, 500, 800 and 1 000 kg/m3）, and different fly ash contents （0%, 20%, 40% and 60%）. The influence of curing temperatures （20, 40, 60 and 80~C） was also studied. The experimental results show that three characteristic stages can be clearly identified during the setting process of an arbitrary FC paste： dormant stage, acceleration stage, and deceleration stage. Wet density, fly ash content, and curing temperature have great impact on setting behavior. A stepwise increase of the wet density results in shorter dormant stage and larger final UPV. Hydration reaction rate is obviously promoted with an increase in curing temperature. However, the addition fly ash retards the microstn,lcture formation. To aid in comparing with the ultrasonic results, the consistence spread test and Vicat needle test （VNT） were also conducted. A correlation between ultrasonic and VNT results was also established to evaluate the initial and final setting time of the FC mixtures. Finally, certain ranges of UPV with reasonable widths were suggested for the initial and final setting time, respectively.

Preparation for Retarding and High Early Strength Concrete

The primary objective of this research was to determine optimum dosage of mixing concrete containing plasticizers and fly ash, consistent with desirable structural grade concrete properties. Factorial tests were also conducted to investigate the four main factors： water-cementing materials ratio, water content, content of superplasticizers （SP） and fly ash content. It was found that the requirement for setting time played the dominant role in shrinkage and anti-cracking, and fly ash played a critical role in workability and reducing heat of hydration but showed insignificant effects on slump, early strength and initial setting time of concrete.

Fracture model for predicting concrete cover-cracking induced by steel corrosion based on interface bond state

Time-to-cracking of the concrete cover induced by the steel corrosion is one of the critical problems faced by engineers, operators and asset managers in making strategies for the maintenance and repairs of reinforced concrete （RC） structures affected by corrosion. In this paper, a theoretical model for predicting the time-to-cracking is derived by assuming the bond between the steel bar and the concrete as a linear combination of perfectly smooth and bonded. The model takes into account the characteristics of existing exiguous flaws and initial cracks in the concrete before the load acting on RC structures. The validity of the proposed model is prehminarily verified by comparing the obtained results with the available experimental results. A remarkable advantage of the proposed method is its apphcation to the prediction of the service life of RC structures, made of the deformed steel bars as well as the round bars. By determining an experimental constant a, which is related to the interface bond state between the steel bar and the concrete, the service life of RC structures can be predicted using the proposed scheme. Analysis of major factors affecting the time-to-cracking demonstrates that the length of the initial crack affects the service life of RC structures significantly. Moreover, the larger cover thickness and the smaller diameter of the steel bar within a certain range are beneficial to prolonging the time-to-cracking.

Experimental study on retarding mortar and its application in prestressed concrete

Based on a large number of orthogonal tests and theoretical analyses, the retarding mortar which meets the requirements of retard-bonded prestressed concrete was prepared. Initial setting time of the retarding mortar may vary from several hours to 15 d at 5 ℃-35 ℃ due to quantities and average curing temperature. And its 28 d compressive strength is above 35 MPa. Thus the influence of quantities on setting time and 28 d compressive strength, and the relationship between setting time and average curing temperature were investigated. The optimum quantities were obtained by studying the interaction of admixtures, and the retarding mechanism was discussed. Based on 52 retard-bonded prestressed strands by manual work from 24 retard-bonded prestressed concrete T-beams, static friction drag, change factor κ and friction factor μ were obtained from the test when retard-bonded prestressed strands were tensioned. Application of the retarding mortar will be vast in practical concrete projects.

Workability Tests on Fresh Concrete Formulated with Eco-friendly Admixture

Extra-cellular compounds, secreted by microorganisms into their surroundings, can be integrated in concrete composition as admixtures. These substances are important in biofilm formation and some of them can be used as corrosion inhibitor of concrete reinforcement. This paper deals with products made with biological surface active compounds/agents allowing the development of more eco-friendly concrete. The influence of this environmentally friendly bio admixture on setting time, workability, bending and compressive strengths of various mortar based materials made of CEM I, CEM III and CEM V was studied. Mechanical tests were carried out to highlight the influence of admixture in workability and hardening of samples containing the biological product with ratio in the range of 0-2.5%. It was demonstrated that the presence of the new bio-compound admixture in mortar decreases their compressive strength after 28 days of standard curing, in spite of remaining higher than standard minimal strength. Furthermore, Vicat needle experiments have shown a tendency of this admixture to decrease the setting time. A discussion was finally proposed in order to correlate the setting times and the decrease of the mortar compressive strength, corresponding in fact to a hardening delay. This setting time delay could be linked to a delay of the admixtured mortar to increase its resistance. The slump results highlight the action of bio-admixture as a plasticizer on mortars because it increases their workability for a same water-cement ratio. This effect seems variable according to the added amount.

Comparison between Response Spectrum and Time History Method of Dynamic Analysis of Concrete Gravity Dam

Dam structure built to store water has failed with resulting loss of life, social, economic and environmental losses due to seismic vibrations. These vibrations are dynamic in nature. These vibrations must be reduced with proper application of engineering principles and for estimating the behavior of concrete gravity dam dynamic analysis plays an extraordinary role. This paper presents the dynamic time history analysis and response spectrum method of a concrete gravity dam by using STAAD-PRO. Here Finite Element Approach is used to analyze the dam. A concrete gravity dam model is prepared in STAAD-PRO to perform the time history analysis and response spectrum analysis and a comparison is done between both these methods. Concrete gravity dam is a large structure which retains a very large amount of water on its upstream side and it is very crucial for a dam to survive against vibrations of earthquake. So it is a matter of study to check the behaviour of a dam during and after the application of the loading.