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J.M. Robberts

Bio: J.M. Robberts is an academic researcher from University of Pretoria. The author has contributed to research in topics: Finite element method & Slab. The author has an hindex of 2, co-authored 4 publications receiving 21 citations.

Papers
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Journal Article
TL;DR: In this paper, the authors investigated the economic value of SVF slabs in South Africa by comparing the direct construction cost to that of two other large span slab systems, namely coffer and post-tensioned slabs.
Abstract: Large span concrete flat-slab systems with internal spherical void formers (SVF) have been used in Europe for over a decade. They are bi-axially reinforced concrete flat-slab systems with a grid of internal spherical void formers. This paper addresses three issues associated with SVF slab systems: their shear resistance, their short-term elastic deflections and their economical value in a South African context. Due to the ''loss'' (or reduction) of aggregate interlock required for shear resistance in SVF slabs, the design requirements of the reinforced concrete design code are affected. Research at the Technical University of Darmstadt (TUD) in Germany proved a shear resistance reduction factor of 0,55 to be conservative, while research at the University of Pretoria suggests a greater factor of 0,85 when taking into account the shear capacity of the permanent steel cages that hold the spheres in position in some SVF slab systems. Laboratory tests at the TUD, supported by theoretical calculations, further showed reduced deflections for SVF slabs compared to solid slabs. Stiffness is not reduced as much as the selfweight, resulting in smaller overall deflections for SVF slabs compared to those of solid slabs with the same thickness. In this paper the economical value of SVF slabs in South Africa will be investigated by comparing the direct construction cost to that of two other large span slab systems, namely coffer and post-tensioned slabs.

13 citations

Journal Article
TL;DR: In this article, a smeared crack model based on nonlinear fracture mechanics was developed which allows for either linear or bilinear softening and assumes shear retention dependent on the strain normal to a crack.
Abstract: A smeared crack model, based on non-linear fracture mechanics, was developed which allows for either linear or bilinear softening and assumes shear retention dependent on the strain normal to a crack. A mesh objectivity verification study proves that the proposed crack modelling method is mesh objective. The crack model and its computational procedure is verified for a benchmark concrete gravity dam model and an existing concrete gravity dam by comparing the results with those of numerical investigations obtained by other researchers. Furthermore, an existing concrete gravity dam in South Africa is analysed and evaluated with regard to dam safety in terms of the maximum overflow level. A higher imminent failure flood is predicted in the analysis than that obtained by classical strength-based methods. The study proves the usefulness and applicability of the proposed crack model and implementation procedure in predicting crack response and evaluating the safety of concrete gravity dams. A sensitivity study on the material fracture properties and fracture parameters is included for the purpose of investigating the uncertainties often encountered in this type of analysis. The influence of the fracture properties and parameters on the cracking response and the overall structural behaviour is discussed.

9 citations

Journal Article
TL;DR: A simplified finite element model for the analysis of time-dependent deflections of reinforced concrete slabs is presented to reduce the computational effort required for more rigorous approaches to non-linear material behaviour.
Abstract: The aim of this paper is to present a simplified finite element model for the analysis of time-dependent deflections of reinforced concrete slabs. The proposed method aims to reduce the computational effort required for more rigorous approaches to non-linear material behaviour. Established, simple theories of cracking, shrinkage and creep are described and adapted for implementation in finite element software developed by the first author. Results are compared to several experimental studies by other researchers to evaluate the accuracy of the proposed model.

1 citations

Journal Article
TL;DR: In this paper, a simplified finite element model for the analysis of time-dependent deflections of reinforced concrete slabs is presented, which aims to reduce the computational effort required for more rigorous approaches to non-linear material behaviour.
Abstract: The aim of this paper is to present a simplified finite element model for the analysis of time-dependent deflections of reinforced concrete slabs. The proposed method aims to reduce the computational effort required for more rigorous approaches to non-linear material behaviour. Established, simple theories of cracking, shrinkage and creep are described and adapted for implementation in finite element software developed by the first author. Results are compared to several experimental studies by other researchers to evaluate the accuracy of the proposed model.

1 citations


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TL;DR: In this paper, the bending behavior of a composite slab made from precast concrete with steel and CRS reinforcement was investigated, and the results showed that the CRS can stabilise the hollow core and act as additional reinforcement which enhances the load bearing capacity by 112% and the initial stiffness of the slabs by 24%.

26 citations

Journal ArticleDOI
TL;DR: In this paper, the authors presented a research on flexural behaviour of hollow monolithic reinforced concrete slabs and analyzed their flexural capacity and stiffness using DIANA software, which can be applied for analyzing the stress state of the examined structures.
Abstract: The article presents a research on flexural behaviour of hollow monolithic reinforced concrete slabs. It focuses on the results of experimental investigation into full-size hollow reinforced concrete slabs and analyses their flexural capacity and stiffness. The self-weight of the slabs directly depends on the shape and number of hollows. An increase in the hollowness of a slab significantly reduces the load caused by self-weight. This allows increasing the estimated length of the slab under the same payload. An increase in the amount of hollows of the slab changes the stiffness of the slab cross-section that has a direct impact on slab deflection. Considering the shape of the slab cross-section, theoretical calculations of the flexural capacity and deflection of experimental slabs were made. The design of a new type of slabs and variations in different parameters of the slab experience difficulties in conducting a large amount of experimental tests. Therefore, the initial analysis may apply to numerical simulation. The paper describes the principles of designing a numerical model. The calculations were made using DIANA software. The stiffness and flexural capacity of the hollow slabs were established employing numerical simulation compared to the results of experimental investigations. The findings indicate that numerical simulation can be applied for analysing the stress state of the examined structures.

25 citations

Journal ArticleDOI
Jianbo Li, Xin Gao, Xing-an Fu, Chenglin Wu, Gao Lin 
TL;DR: In this paper, a nonlinear extended scaled boundary finite element method (X-SBFEM) was developed incorporating the cohesive fracture behavior of concrete, which consists of an iterative procedure to accurately model the traction distribution within the fracture process zone (FPZ) accounting for the cohesive interactions between crack surfaces.
Abstract: Fracture mechanics is one of the most important approaches to structural safety analysis. Modeling the fracture process zone (FPZ) is critical to understand the nonlinear cracking behavior of heterogeneous quasi-brittle materials such as concrete. In this work, a nonlinear extended scaled boundary finite element method (X-SBFEM) was developed incorporating the cohesive fracture behavior of concrete. This newly developed model consists of an iterative procedure to accurately model the traction distribution within the FPZ accounting for the cohesive interactions between crack surfaces. Numerical validations were conducted on both of the concrete beam and dam structures with various loading conditions. The results show that the proposed nonlinear X-SBFEM is capable of modeling the nonlinear fracture propagation process considering the effect of cohesive interactions, thereby yielding higher precisions than the linear X-SBFEM approach.

16 citations

Journal ArticleDOI
TL;DR: A hybrid data-driven method which employs the extreme gradient boosting machine and the particle swarm optimization metaheuristic for predicting long-term deflection results from the input data can be a promising tool to assist civil engineers in forecasting deflections of reinforced-concrete members.
Abstract: During the life cycle of buildings and infrastructure systems, the deflection of reinforced-concrete members generally increases due to both internal and external factors. Accurate forecasting of long-term deflection of these members can significantly enhance the effectiveness of structural maintenance processes. This research develops a hybrid data-driven method which employs the extreme gradient boosting machine and the particle swarm optimization metaheuristic for predicting long-term deflections of reinforced-concrete members. The former, a machine learning technique, generalizes a non-linear mapping function that helps to infer long-term deflection results from the input data. The later, a swarm-based metaheuristic, aims at optimizing the machine learning model by fine-tuning its hyper-parameters. The proposed hybridization of machine learning and swarm intelligence is constructed and verified by a dataset consisting of 217 experiments. The experiment results, supported by statistical tests, point out that the hybrid framework is able to attain good predictive performances with average root-mean-square error of 11.38 (a reduction of 17.4%), and average coefficient of determination of 0.88 (an increase of 6.0%) compared to the non-hybrid model. These results also outperform those obtained by other popular techniques, including Backpropagation Neural Networks and Regression Tree in several popular benchmarks, such as root-mean-square error, mean absolute percentage error, and the coefficient of determination R2. This is backed up by statistical tests with the level of significance $$\alpha = 0.05$$ . Therefore, the newly developed model can be a promising tool to assist civil engineers in forecasting deflections of reinforced-concrete members.

16 citations

13 Sep 2013
TL;DR: In this paper, the experimental results of BubbleDeck slab subjected to static loadings are presented, and the effects of various factors to the behaviors of bubbledeck slab are considered, such as the concrete strength, the shape and diameter of plastic balls, the size of reinforcing mesh at top and bottom.
Abstract: The new prefabricated construction technology using BubbleDeck slab is recently applied in many industrial projects in the world. BubbleDeck slab uses hollow spherical balls made by recycled plastic and therefore it is an innovatory method of virtually eliminating the concrete part in the middle of conventional slab which does not contribute to the structural performance. This hence reduces significantly the structural selfweight. In this paper, the experimental results of BubbleDeck slab subjected to static loadings are presented. The effects of various factors to the behaviors of BubbleDeck slab are considered, such as the concrete strength, the shape and diameter of plastic balls, the size of reinforcing mesh at top and bottom. In order to demonstrate the superiority and advances of the mentioned technology, the improving of the plastic ball’s shape by using hollow elliptical balls for better load-bearing capacity in BubbleDeck is also presented in details. The research results show the effectiveness and feasibility of the application of BubbleDeck in the construction works in Ho Chi Minh City, Vietnam.

10 citations