Quoc-Bao Bui

Evaluation of one-way shear behaviour of reinforced concrete slabs: experimental and numerical analysis

Abstract The shear design of concrete slabs is still an unsolved problem. The following study presents experimental and numerical investigations on the shear behaviour of reinforced concrete slabs (without shear reinforcement) under concentrated loads. The small thick slabs of 10 cm were tested in this study. Experimental tests were conducted to quantify the shear strength and the associated failure modes. The influence of several variables was addressed such as the influence of boundary conditions, four supported side slabs instead of two supported side slabs, the influence of loading plate length. A series of eight tests on six slabs were presented. The experiments are firstly used to evaluate the pertinence of Eurocode 2 and Model Code 2010 formulations using the levels of approximation LoA I and LoA II for the shear design of reinforced concrete slabs without shear reinforcement in comparison with the French approach, and secondly validated numerical modelling using the non-linear finite element method. The proposed numerical model showed good agreement with the experimental results in terms of slab behaviour.

Redistribution of Moment at Beam-Column Joints in RC Structures: Comparison Between an Experimental Study and Eurocode 2

The current approaches authorized by Eurocode 2 for the design of reinforced concrete (RC) structures at the structural scale are: linear-elastic analysis, linear-elastic with limited redistribution of moments, plastic and non-linear analysis. Numerous experiments on RC beams were performed which provided interesting information for the models proposed in the standards. However, at the RC frame level where the beam-column are present, although several experiments have already been carried out in the literature, this topic is still to be investigated, due the important number of parameters: interior joint, exterior joint, stiffness ratio between beam and column, steel reinforcement type, … This paper presents firstly an experimental study on an H-form RC frame structure (two vertical columns and one horizontal beam). Several loading-unloading cycles were carried out. Displacement fields during the test were measured by the image correlation technique. Then, experimental results were compared with that of the analytical models proposed in Eurocode 2 and an advance FE code (CAST3M). The comparisons show that analytical methods presented in Eurocode 2 underestimated about 20–30% of the ultimate capacity of the structure; while the non-linear analysis with CASTEM code – when material characteristics and boundary conditions are correctly identified - could provide good results comparing to the experiments.

Tentative to use wastes from thermal power plants for construction building materials

Thermal power plants (TPP) generates wastes (bottom and fly ashes) which become a serious environmental problem in Vietnam. Indeed, although in several countries fly ash can be used for cement industry, fly ash from actual TPP in Vietnam does not have enough good quality for cement production, because the fly ash treatment phase has not yet included in the generations of existing Vietnamese TPP. That is why bottom ash and fly ash purely become wastes and their evacuation is an urgent demand of the society. This paper presents an investigation using fly and bottom ashes in the manufacturing of construction materials. The main aims of this study is to reduce environmental impacts of fly and bottom ashes, and to test another non-conventional binder to replace cement in the manufacture of unburnt bricks. Several proportions of fly ash, bottom ash, cement, gravel, sand and water were tested to manufacture concretes. Then, geopolymer was prepared from the fly ash and an activator. Specimens were tested in uniaxial compressions. Results showed that the cement concrete tested had the compressive strengths which could be used for low rise constructions and the material using geopolymer could be used for non-load-bearing materials (unburnt bricks).

Shear Parameters of Rammed Earth Material: Results from Different Approaches

Rammed earth (RE) is a construction material which is manufactured by compacting the soil in a formwork, in different layers. Several recent studies have investigated this material. The seismic performance of RE buildings is an important topic which needs to be carefully investigated. The complex numerical model seems a performant approach to investigate the seismic performance of a whole building. To correctly establish the model, the shear parameters of the material, which are the cohesion and the friction angle, should be identified. This paper first presents experimental studies on the shear parameters of RE through the direct shear tests, at two different scales. The differences of the results at different scales are analyzed. Then, the obtained experimental values are used in a numerical model to simulate the shear behavior of RE walls which are loaded by a constant vertical stress and pushed horizontally on the top. From the obtained results, the values for numerical models are recommended.

Nondestructive Vibrational Tests and Analytical Solutions to Determine the Young’s Modulus of Rammed Earth Material

Rammed earth (RE) is a construction material which is manufactured from the soil. The soil is dynamically compacted at its optimum water content, inside a formwork to build a monolithic wall. The RE wall is composed of several layers, about 10–12 cm thick. In the last decades, RE material has been the focus of numerous scientific researches because of sustainable properties of this material: low embodied energy, positive hygrothermal behavior and a particular esthetic aspect. In several situations, nondestructive methods are needed to assess the mechanical characteristics of RE material, for both old and new RE constructions. This paper presents how in situ vibrational measurements can be used to identify the dynamic behavior of RE walls and to determine the Young’s modulus of the RE walls measured. To determine Young’s modulus from the dynamic characteristics, an analytical model based on Timoshenko’s beam theory is presented, both for flexural and torsional modes. Then, the proposed analytical model is verified with measurements on several walls having different cross-sectional forms: rectangle and L-shape. The walls’ natural frequencies were identified from the dynamic measurements by using the Frequency Domain Decomposition method. In parallel, for comparison, the Young’s modulus of the RE material studied was also determined by classical static measurements (on the walls, prismatic and cylindrical specimens). The displacements were measured by using the Image Correlation technique. The comparisons showed that the results from the proposed analytical method provided high accuracies and better than that obtained by measurements on the usual specimens (prismatic and cylindrical).

Using Wastes from Thermal Power Plants for Manufacturing of Low Strength Construction Materials

Thermal power plants (TPP) generates wastes (bottom and fly ashes) which become a serious environmental problem in Vietnam. Indeed, although in several countries fly ash can be used for cement industry, fly ash from actual TPP in Vietnam does not have enough good quality for cement production, because the fly ash treatment phase has not yet included in the generations of existing Vietnamese TPP. That is why bottom ash and fly ash purely become wastes and their evacuation is an urgent demand of the society. This paper presents an investigation using fly and bottom ashes in the manufacturing of construction materials. The aim of this study is to propose a possibility to reduce environmental impacts of fly and bottom ashes, and manufacture construction materials having low embodied energy by using less cement. Several proportions of fly ash, cement, gravel, sand and water are tried to manufacture low strength concretes which can be used for non-load-bearing walls. Specimens are tested in uniaxial compressions. Results show that with a reasonable cement amount (4–8% by weight), by replacing cement by fly ash or replacing sand by bottom ash at 10-30%, the obtained materials can be used for non-bearing materials or low strength structural materials.

A Numerical Modeling of RC Beam-Column Joints Compared to Experimental Results

Beam-column joints are commonly considered critic regions for RC frames subjected to earthquake. That is why assessing the beam-column joint capacity is an important topic, especially for structures constructed before the modern seismic design codes, or for buildings in post-seismic situations. Among the in-situ structural assessment methods, the vibrational testing is currently mentioned. The authors have developed an analytical method to assess the damage evolution of a structure in function of its dynamic characteristics. The method consists of two main steps and the first one necessitates a robust model which can reproduce the static behavior of the studied structure. For this purpose, the authors try and assess the relevancy of existing numerical models to choose the most relevant for the second step. This paper presents an assessment of the CDP (Concrete Damage Plasticity) model implemented in the Abaqus software. First, an experimental study on a RC beam-column frames is presented. Unloading-reloading cycles were performed during the tests and the displacement fields were recorded by using the image correlation technique. The experimental data are used to assess the relevancy of the CDP model, but these data can be useful also for the further studies to verify and improve the accuracy of the numerical or analytical models.