Coupling model for calculating prestress loss caused by relaxation loss,shrinkage,and creep of concrete

The calculation model for the relaxation loss of concrete mentioned in the Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts(JTG D62—2004) was modified according to experimental data. Time-varying relaxation loss was considered in the new model. Moreover, prestressed reinforcement with varying lengths(caused by the shrinkage and creep of concrete) might influence the final values and the time-varying function of the forecast relaxation loss. Hence, the effects of concrete shrinkage and creep were considered when calculating prestress loss, which reflected the coupling relation between these effects and relaxation loss in concrete. Hence, the forecast relaxation loss of prestressed reinforcement under the effects of different initial stress levels at any time point can be calculated using the modified model. To simplify the calculation, the integral expression of the model can be changed into an algebraic equation. The accuracy of the result is related to the division of the periods within the ending time of deriving the final value of the relaxation loss of prestressed reinforcement. When the time division is reasonable, result accuracy is high. The modified model works excellently according to the comparison of the test results. The calculation result of the modified model mainly reflects the prestress loss values of prestressed reinforcement at each time point, which confirms that adopting the finding in practical applications is reasonable.

Creep and Shrinkage Effects on the Bond-Slip Characteristics and Ultimate Strength of Composite Slabs

Composite one-way concrete slabs with profiled steel sheeting as permanent formwork are commonly used in the construction industry. The steel sheeting supports the wet concrete of a cast-in-situ reinforced or post-tensioned concrete slab and, after the concrete sets, acts as external reinforcement. In this type of slab, longitudinal shear failure between the concrete and the steel sheeting is the most common type of failure at the ultimate load stage. Design codes require the experimental evaluation of the longitudinal shear capacity of each type of steel decking using full-scale tests. This paper presents the results of the short-term testing up to failure of two types of profiled steel decking that are commonly used in the construction industry in Australia. Fourteen full-scale, simply-supported slabs were tested in four-point bending with shear spans of either span/4 or span/6. Four slabs were tested at age of 28 days and the other 10 slabs were subjected to drying shrinkage and various levels of sustained loads for a period of at least 6 months prior to testing to failure. The effects of creep and drying shrinkage on the load carrying capacity and deformation of the slabs at ultimate loads are presented and discussed. The bond-slip relationship of each slab is determined from the test data and the values of maximum longitudinal shear stress calculated using different methods are described and compared.

Health monitoring and comparative analysis of time-dependent effect using different prediction models for self-anchored suspension bridge with extra-wide concrete girder

The structural health status of Hunan Road Bridge during its two-year service period from April 2015 to April 2017 was studied based on monitored data.The Hunan Road Bridge is the widest concrete self-anchored suspension bridge in China at present.Its structural changes and safety were evaluated using the health monitoring data,which included deformations,detailed stresses,and vibration characteristics.The influences of the single and dual effects comprising the ambient temperature changes and concrete shrinkage and creep(S&C)were analyzed based on the measured data.The ANSYS beam finite element model was established and validated by the measured bridge completion state.The comparative analyses of the prediction results of long-term concrete S&C effects were conducted using CEB-FIP 90 and B3 prediction models.The age-adjusted effective modulus method was adopted to simulate the aging behavior of concrete.Prestress relaxation was considered in the stepwise calculation.The results show that the transverse deviations of the towers are noteworthy.The spatial effect of the extra-wide girder is significant,as the compressive stress variations at the girder were uneven along the transverse direction.General increase and decrease in the girder compressive stresses were caused by seasonal ambient warming and cooling,respectively.The temperature gradient effects in the main girder were significant.Comparisons with the measured data showed that more accurate prediction results were obtained with the B3 prediction model,which can consider the concrete material parameters,than with the CEB-FIP 90 model.Significant deflection of the midspan girder in the middle region will be caused by the deviations of the cable anchoring positions at the girder ends and tower tops toward the midspan due to concrete S&C.The increase in the compressive stresses at the top plate and decrease in the stresses at the bottom plate at the middle midspan will be significant.The pre-deviations of the towers toward the sidespan and pre-lift of the midspan girder can reduce the adverse influences of concrete S&C on the structural health of the self-anchored suspension bridge with extra-wide concrete girder.

Dilatometric Analysis of Irreversible Volume Change during Phase Transformation in Pure Iron

One assumption underlying the conventional dilatometric analysis based on the lever rule is that the volume of the specimen changes isotropically during phase transformation,which conflicts with the irreversible length change shown in actual measurements.The contribution of this irreversible effect to the dilation data of pure iron upon heating and cooling was respectively quantified via conversion equations based on lattice parameters.A model considering the elastic strain and creep deformation was established for both the interpretation of the irreversible volume change and the discrepancy between the results measured by a dilatometer and a micrometer.