Strengthening flat slabs without shear reinforcement against punching shear by concrete overlay

Research in this paper presents a theoretical study of increasing in punching shear capacity of the strengthened flat slab by concrete overlay. The parametric study is based on comparison of three different relevant standards design models and presents results how Eurocode 2 (EN 1992-1-1), Model Code 2010 and draft of second generation of Eurocode 2 (prEN 1992-1-1) take into account strengthening by concrete overlay. A reference specimen is represented by a fragment of a flat slab supported by circular column. Influence of concrete toppings depends on thickness and also on reinforcement ratio. In Eurocode 2 and new generation of Eurocode 2 the increase of punching shear resistance of the slab with concrete topping can be taken into account only by reinforcement ratio and thickness of the slab considering the perfect connection and bond between the original slab and new layer of concrete overlay. Model Code 2010 is based on Critical shear crack theory and the reinforcement ratio in concrete topping was considered in equation of moment of resistance and punching shear resistance is calculated by considering the rotation and deformation of the slab. Estimation of results by parametric study are compared by non-linear model from Atena software.

Theoretical prediction of punching shear capacity of flat slabs without shear reinforcement strengthened by concrete topping

The main reasons for strengthening flat slabs are the change of the use of a building, increase in the value of loads, degradation of the concrete cover layer, or insufficient reinforcement. This paper is focused on the assessment of punching shear capacity of the strengthened flat slabs without shear reinforcement. One of the possibilities how to enhance punching shear capacity is the addition of reinforced concrete topping. The main goal of this paper is to compare the possibilities for calculation of the increase in the punching shear capacity by investigation of the influence of different thicknesses of concrete toppings and different reinforcement ratio. A reference specimen is represented by a fragment of a flat slab with the thickness of h = 200 mm supported by circular column with the diameter of 250 mm. Three different thicknesses (50 mm, 100 mm, 150 mm) of concrete toppings were considered together with three different reinforcement ratios for each thickness of concrete overlay. Theoretical predictions of the punching shear resistance of flat slabs were evaluated by design guidelines according to the relevant standards: Eurocode 2 (EN 1992-1-1), Model Code 2010 and draft of the second generation of Eurocode 2 (prEN 1992-1-1). The differences in the influence of reinforcement ratio are significant. In Model Code 2010 the reinforcement ratio in concrete topping was considered in equation of moment of resistance. This is unlike in both of the mentioned Eurocodes, where the reinforcement ratio was assumed as a geometric average value of the original reinforcement ratio in the slab before strengthening and of the reinforcement ratio of concrete topping. All the predicted theoretical calculations are based on the perfect connection and bond between the original and new layer of concrete. These predictions should be verified by experimental investigation, which is going to be prepared shortly. By the additional increase in the thickness of concrete topping or in the amount of added reinforcement the attention should be payed to the limitation of the punching shear resistance by the value of the maximum punching shear resistance in the compression concrete strut.

Consideration of the Heating of High-Performance Concretes during Cyclic Tests in the Evaluation of Results

Material-efficient, highly load-bearing members made of high-performance compressive concretes are often exposed to cyclical loads because of their slender construction, which can be relevant to the design. When investigating the fatigue behaviour of high-performance concretes in pressure swell tests, however, the specimen temperature rises strongly owing to the elevated loading rate at frequencies higher than 3 Hz. This leads to a negative influence on the achieved number of load cycles compared to tests carried out at slow speeds and calculated values, for example, according to fib Model Code 2010. This phenomenon, which was already observed, must be considered when generating design formulae or Wöhler lines for component design, as the test conditions with high constant load frequencies as well as sample storage in a climate chamber at constant conditions are prerequisites that cannot be expected in real material applications. Therefore, laboratory testing influences must be eliminated in order to avoid underestimating the material. Instead of adjusting the test conditions to prevent or control temperature development, as was the case in previous approaches, this article shows how the temperature effects can be corrected when analysing the results, considering both the applied stress and the maximum temperature reached. For this purpose, a calculation method was developed that was validated on the basis of a large number of fatigue tests. Thus, in the future, the application of one temperature sensor to the test specimen can effectively advance the extraction of values for Wöhler curves, even with high test frequencies.

Reliability-based assessment of the partial factor for shear design of fibre reinforced concrete members without shear reinforcement

Fibre reinforced concrete (FRC) is increasingly used for structural purposes owing to its many benefits, especially in terms of improved overall sustainability of FRC structures relative to traditional reinforced concrete (RC). Such increased structural use of FRC requires safe and reliable models for its design in ultimate limit states (ULS). Particularly important are models for shear strength of FRC members without shear resistance due to the potential of brittle failure. The fib Model Code 2010 contains a model for the shear strength of FRC members without shear reinforcement and the same partial factor accepted for RC structures is accepted for FRC elements. This approach, however, is potentially on the unsafe side since the uncertainties of some design-determining mechanical properties of FRC (i.e., residual flexural strength) are larger than those for RC. Therefore, in this study, a comprehensive reliability-based calibration of the partial factor γc for the shear design of FRC members without shear reinforcement according to the fib Model Code 2010 model is performed. As a first step, the model error δ is assessed on 332 experimental results. Then, a parametric analysis of 700 cases is performed and a relationship between the target failure probability βR and γc is established. The results demonstrate that the current model together with the prescribed value of γc = 1.50 does not comply with the failure probabilities accepted for the different consequences of failure of FRC members over a 50-year service life. Therefore, changes to the shear resistance model are proposed in order to achieve the target failure probabilities.

fib Bulletin 100. Design and assessment with strut-and-tie models and stress fields: from simple calculations to detailed numerical analysis.

Following the long-standing tradition of fib in promoting the use of consistent design methods, strut-and-tie models were formally incorporated in Model Code 1990 to serve as the design basis for discontinuity regions. This choice was largely acknowledged as a sound approach for design and was thereafter followed in many national standards. For Model Code 2010, some update and revision of the previous provisions was performed, but the scope of the method was particularly broadened by introducing its complementary use with the stress field method. Since Model Code 2010, significant new knowledge has been generated in this topic. Particularly, the use of software implementing the theoretical ground of the stress field method is becoming increasingly popular and efficient, allowing for design, optimisation and assessment of structures in a simple, transparent and accessible manner. In this Bulletin, the current state-of-the-art of the strut-and-tie models (STM) and the stress field models (SFM) is presented. Reference is not only made to classical rigid-plastic solutions, but also to solutions considering compatibility of deformations, such as elastic-plastic approaches or models allowing investigation of serviceability behaviour and deformation capacity of concrete structures. It is shown in the Bulletin that all models share the same ground and fundamental hypotheses. Their results are presented in a unitary and consistent manner by means of compression fields in the concrete and stresses in the reinforcement. The consistency amongst these approaches and their potential use in practice is also explored by means of the Levels-of-Approximation (LoA) approach as described in Model Code 2010. Another effort in this Bulletin has been devoted to provide comparisons of the solutions according to strut-and-tie and stress fields to tests, in order to discuss on their pertinence and limitations. This perspective is also completed with practical examples presented of structures actually designed with this technique and where the potential of these methods can be appreciated in a clear manner. Finally, a number of special topics are also covered in the Bulletin, related to numerical optimisation, verifications at serviceability states, minimum reinforcement or the applicability of the methods under cyclic or reversal actions. This Bulletin not only aims to give state-of-the-art rules and methods to design according to these techniques, but also to provide an outlook of how these methods could be implemented in future standards. This material also serves as the background document for the revision of the current provisions of Model Code 2010 in the new Model Code 2020.

Applicability of Existing Crack Controlling Criteria for Structures with Large Concrete Cover Thickness

Widely used crack width calculation models and allowable crack width limits have changed from time to time and differ from region to region. It can be identified that some crack width calculation models consist with limitations for parameters like cover thickness. The current Norwegian requirement for cover thickness is larger than these limitations. The applicability of existing crack width calculation models and the allowable crack width limits must be verified for structures with large cover thickness. The background of crack width calculation models in Eurocode, Model Code 2010, Japanese code, American code and British code have been examined. By comparing the experimental crack widths with the predictions of the aforementioned models, the existing codes can be identified as requiring modification. Considering the durability aspect, it can be identified a long-term study proving that the allowable crack width can be increased with the increase in cover thickness. When considering the aesthetic aspect, the authors suggest categorizing the structures based on their prestige level and deciding the allowable crack widths accordingly. The paper proposes potential solutions for future research on how to improve both crack width calculation methods and allowable crack width limits to be used effectively in structures with large cover thickness.

Shear Capacity and Behaviour of Bending Reinforced Concrete Beams Made of Steel Fibre-Reinforced Waste Sand Concrete

Inthis paper, we report the results of our research on reinforced concrete beams made of fine aggregate fibre composite, with the addition of steel fibres at 1.2% of the composite volume. The fine aggregate fibre composite is a novel construction material, in which the aggregate used is a post-production waste. Twenty reinforced concrete beams with varying degree of shear reinforcement, in the form of stirrups with and without the addition of steel fibres, tested under loading. The shear capacity results of reinforced concrete beams made of the fine aggregate fibre composite being bent by a transversal force, as well as the cracking forces causing the appearance of the first diagonal crack, are discussed. The stages of functioning of such elements are described. Furthermore, the effect of the steel fibres on the reduction of diagonal cracking is analysed. Computation of the shear capacity of the tested elements is performed, based on the Model Code 2010 and RILEM TC-162 TDF standards, for two variants of the compression strut inclination angle θ that measured during testing, and the minimum(in accordance with the Model Code 2010 standard). We found that the SMCFT method part of Model Code 2010 showed the best compatibility with the experimental results. The tests and analyses performed demonstrate that the developed novel fibrecomposite—the properties of which are close to, or better than, those of the ordinary concrete—can be used successfully for the manufacturing of construction elements in the shear capacity aspect. The developed fine aggregate fibrecomposite could serve, in some applications, as an alternative to ordinary concrete.

Durabilidade das estruturas de concreto armado: estudo comparativo entre prescrições normativas acerca de controle de fissuração

A abertura excessiva de fissuras nas estruturas de concreto armado compromete suas funções essenciais, como capacidade de suporte e proteção contra agentes agressivos. Assim, os regulamentos destinados à durabilidade, usualmente, abordam o comportamento da fissuração no concreto. É possível observar que no Brasil, a normalização responsável pela durabilidade do concreto armado apresenta algumas inconsistências, relacionadas a parâmetros de projeto, como a classificação de agressividade ambiental e a estimativa da abertura de fissura. Neste sentido, este trabalho tem como objetivo efetuar uma análise crítica entre a ABNT NBR 6.118, o Eurocode 2 e fib Model Code 2010, no que tange aos requisitos de durabilidade para estruturas de concreto armado, tais como as classes de exposição, qualidade do concreto e aço, a espessura de cobrimento, desempenho em serviço e abertura de fissuras. Em paralelo, são efetuadas análises referentes às influências desses parâmetros na estimativa da abertura de fissuras e na vida útil de projeto, bem como a apresentação de diretrizes a serem incorporadas nos processos de projeto brasileiros, para se obter um melhor desempenho das estruturas. Para tal, foram efetuadas uma análise qualitativa e uma quantitativa dos preceitos normativos. Sendo a primeira, responsável por evidenciar as diferenças presentes nos parâmetros analisados, e a segunda, responsável pelo estudo de uma viga bi-apoiada, submetida à variação dos requisitos que interferem na estimativa da abertura de fissuras. Foram verificadas diferenças entre a norma nacional e os padrões internacionais estudados, todavia, percebe-se que a norma apresenta prescrições que se adequam aos limites de durabilidade, sendo tão rigorosa quanto os códigos internacionais. Finalmente, observa-se que esse trabalho auxilia no estudo das propriedades do concreto armado e dos requisitos relacionados à durabilidade das estruturas, apresentando a influência das diferenças encontradas nos padrões que estimam a durabilidade das estruturas de concreto armado.

Experimental Investigations on Interface between Ordinary and Lightweight Aggregate Concretes Cast at Different Times

Experimental investigations on 12 push-off specimens with dimensions of 600 × 300 × 180 mm (200 × 180 mm shear plane) were presented. Models reflected the connection between ordinary concrete (NWC) substrate and lightweight aggregate concrete (LWAC) overlay. The main purpose of the study was to investigate behaviour of the interface between concretes cast at different times. Two different interface conditions were considered: Smooth and rough (obtained by graining). In the selected elements, additional reinforcement consisting of one ∅8 bar was injected. The elements were tested under load control. The failure of the specimens without interface reinforcement was violent and resulted from breaking of the adhesive bond. Specimens with shear reinforcement failed in a ductile manner, however, due to the low reinforcement area, the residual load capacity was much lower than the load recorded just before cracking. It was found that mechanical roughening of the surface can lead to degradation of the concrete structure. As a result, the load-carrying capacities of elements with smooth interface proved to be higher than the ultimate loads of elements with deliberately roughened contacts. Comparative analysis showed that the existing design procedures ACI 318-19, Eurocode 2, Model Code 2010, and AASHTO can lead to safe but conservative estimation of the actual resistance of the concrete interface.