Predicting Early Age Temperature Evolution in Massive Structures from Non-standard Characterization Test

Assessment of mechanical properties in unirradiated and irradiated zircaloys and steels with non-standard tests and finite element calculations

New experimental approach to accelerate the development of internal swelling reactions (ISR) in massive concrete structures

At early age, temperature in massive concrete structures may reach over 70°C because hydration is an exothermic chemical reaction. Temperature evolution (increasing followed by decreasing temperature) can lead to damage risks in the short and long term. Firstly, cracking due to self-restrained strains (essentially thermal strains) can occur (which increase the transport properties and so the kinetics of degradation); and secondly, delayed ettringite formation can appear. In addition, if autogenous and thermal strains are restrained, compressive stresses and then tensile stresses increase, which can cause crossing cracks. However, this paper will not deal with these phenomena. In the first part, sensitivity to delayed ettringite formation and early age cracks by self-restrained strains are studied with regards to the environmental conditions by a numerical approach. This part shows that the external temperature has a significant impact on the maximal temperature reached, but that the temperature difference between the core and the surface is mainly impacted by the wind velocity. Then, a parametric study on the effect of the variation of thermal properties at early age has been achieved and shows that it needs to be taken into account. Finally, visco-elastic mechanical calculations show the impact of thermal property variation on the stresses generated by self restraint.

Taking the Zigaojian tunnel, Hangzhou–Huangshan high-speed railway, China, as background, the rock mass structure effect on smooth blasting quality was studied. Four rock mass structures were determined on the basis of the information collected on the tunnel site. Smooth blasting finite element models were established using LS-DYNA. The accuracy of the numerical calculation model was verified by comparing the overbreak and underbreak between the numerical simulation and monitoring. Orthogonal numerical test was used to study the rock mass structure effect through single factor and main effect analysis methods. With the decrease in rock mass integrity, the smooth blasting overbreak of tunnels with massive integrity structure, massive structure, layered structure, and cataclastic structure increased. For massive integrity structure and cataclastic structure, the peripheral hole spacing should be emphatically considered. Meanwhile, in massive structure and layered structure, the included angle and spacing of structural planes had a great influence on the smooth blasting quality. The research results could provide a reference to improve the quality of similar tunnel smooth blasting.

To optimize the hydration process of blended cement, cement clinker and supplementary cementitious materials (SCMs) were ground and classified into several fractions. Early hydration process of each cementitious materials fraction was investigated by isothermal calorimeter. The results show fine cement clinker fractions show very high hydration rate, which leads to high water requirement, while fine SCMs fractions present relatively high hydration (or pozzolanic reaction) rate. Cement clinker fractions in the range of 8–24 μm show proper hydration rate in early ages and continue to hydrate rapidly afterward. Coarse cement clinker fractions largely play “filling effect” and make little contribution to the properties of blended cement regardless of their hydration activity (or pozzolanic activity). The hydration process of blended cement can be optimized by arranging high activity SCMs, cement clinker, and low activity SCMs in fine, middle, and coarse fractions, respectively, which not only results in reduced water requirement, high packing density, and homogeneous, dense microstructure, but also in high early and late mechanical properties.

Numerical analysis of the thermal active restrained shrinkage ring test to study the early age behavior of massive concrete structures

Link prediction in dynamic networks is an important task with many real-life applications in different domains, such as social networks, cyber-physical systems, and bioinformatics. There are two key processes in dynamic networks: network structural evolution and network temporal evolution, where the former represents interdependency between entities and their neighbors in the network at each timestamp, while the latter captures the evolving behavior of the entire network from the current timestamp to the next. Structural evolution generally assumes that a node is more likely to co-evolve with its neighbors in the near future. Temporal evolution focuses on the trend of network evolution as a whole, based on the accumulation of historical data. It is thus essential to use characteristics of both structural and temporal evolutions to emulate complex behaviors of dynamic networks. However, very few existing work considered both processes. In addition, real-life networks are often very sparse with limited observed links. A missing link between two nodes does not always imply that the two nodes do not have a relation in reality, especially when they share many common neighbors. Most existing methods only focus on the first-order proximity of networks, which is usually insufficient to capture the relationships among nodes.

Chemical shrinkage of cement pastes and mortars at very early age: Effect of limestone filler and granular inclusions

Weight reduction of automotive components by tailoring materials is the state of the art. This basically has resulted in the development of advanced joining methods like clinching, friction stir welding, self-pierced riveting etc. to assemble similar or dissimilar materials, with significant change in sheet properties. In the present work, the main aim is to predict the temperature evolution during Self-Pierced Riveting (SPR) of sheets by Finite Element (FE) analyses. Load evolution is also predicted. Generally temperature estimation during SPR is not attempted. The influence of a few selected SPR parameters has been studied on the temperature and load evolution through FE simulations. The relationship between these parameters and the temperature and the load evolution are revealed. Later a neural network model is developed to predict the temperature rise during SPR. The same has been validated at 20 intermediate levels and the predictions are accurate. Thus a hybrid FEM-ANN model for SPR has been developed to predict the SPR outputs efficiently.

This study investigated the material properties of air-cured ultra-high-performance fiber-reinforced concrete (UHPFRC) at early ages. To assess the material properties of UHPFRC at early ages, compressive and three-point bending tests were respectively performed at the 10 stages of early ages on o 100 × 200 mm cylinders and 100 × 100 × 400 mm specimens made from UHPFRC with a 2% steel fiber volume ratio. Based on the regression analysis of the compressive and flexural test results for each age, model equations were proposed that can predict the compression strength, modulus of elasticity and flexural strength at early ages. In addition, to determine the tensile softening curves of UHPFRC at early ages, the inverse finite element (FE) analysis was applied using the poly-approximation method; based on the results, model equations are presented that can predict the tensile softening curve. The load-variable curves obtained from the simulation using the proposed tensile softening model fit the measurements from the flexural tests relatively well. Key words: UHPFRC, air curing, material property, tensile softening, early age.

Validation of the fully-analytical solution with temperature superposition for the nuclear waste repository

Numerical modeling of the effects hydration and number of hydrogen bonds on the mechanical properties of the tropocollagen molecule.

Creep testing of concrete since setting time by means of permanent and repeated minute-long loadings

This paper describes how a new class of polycarboxylate superplasticizer with a grafted pendant group of polyethylene oxide has been developed and extensively used in the construction field because of its outstanding water reducing ratio, high workability retention at small dosages and excellent compatibly with higher dosage rates of industrial by-products. Nevertheless, a drawback of this new generation superplasticizer is their effects in delaying the rate of cement hydration, especially when they are used at higher dosages and in low temperatures environment. These aspects limit their industrial application because of the slow mechanical strength development at the early age. In order to overcome this drawback, a new polycarboxylate superplaticizer (SSP) has been developed by means of optimizing the comb copolymer structure so as to meet ultra-early strength development requirements of construction projects. The results of mortar or concrete tests clearly indicate that SSP has the capacity to considerably accelerate the development of early age mechanical strength in conditions of low curing temperature, in comparison with the ordinary superplasticizers. The compressive strength of concrete containing SSP can reach 500% and 480% at an early age of 24 hours and 30 hours, respectively, when compared with ordinary polycarboxylate polymers at the same dosages. Engineering experimental results proved that the SSP can meet the requirements of concrete admixtures for high-speed railway construction projects.

Mechanical properties of cement pastes and mortars at early ages