Improving durability through probabilistic design

In order to obtain a more controlled durability and long-term performance of concrete structures in chloride containing environment special care is needed in the design phase of reinforced concrete structures. The recent development of probability-based procedures has proven to give a more realistic basis for both durability design and condition assessment of reinforced concrete structures. Although there is still a lack of relevant data, this approach has been successfully applied to several new concrete structures, where requirements to a more controlled durability and service life have been specified. Since parameters both for concrete durability and environmental exposure typically show a high scatter, a probability-based approach has shown to give a very powerful basis for durability analysis. This approach is primarily being applied for obtaining a more controlled durability and long-term performance of new concrete structures, but it also provides a very valuable basis for condition assessment of existing concrete structures in chloride containing environment. In the present paper, the probability-based durability analysis is briefly described and used in order to demonstrate the importance and sensitivity of the various parameters affecting and controlling the durability of concrete structures in chloride containing environment. The results show that this procedure provides valuable information concerning the design options, making the decision process more reliable.(undefined

Inovação em betões: nova normalização e produção de betões especiais

Universidade do Minho (UM).Parque de Exposições de Braga (PEB)

Avaliação dos ensaios correntes para a medição da durabilidade do betão

Medidas prescritivas como a razão água/material cimentício máxima, dosagem mínima de material cimentício e recobrimento mínimos são apresentadas pela NP ENV 206 [1] como medidas para assegurar a durabilidade do betão. Estes parâmetros variam muito num só elemento de betão dependendo de diversos factores como as condições de cura, a mão de obra, a qualidade do betão, entre outros [2]. Há a necessidade de implementar normas baseadas no desempenho em que a qualidade do betão é determinado adequadamente. São necessários ensaios de durabilidade que avaliam o desempenho do betão. O trabalho desenvolvido baseia-se na avaliação de cinco ensaios diferentes [3]: a resistência à compressão, a resistividade eléctrica, a permeabilidade ao oxigénio, o ensaio de migração do ião de cloro “CTH Rapid Method”, a absorção capilar e a permeabilidade ao oxigénio. Verifica-se que, os resultados tornam-se mais fiáveis quanto maior a idade e melhor a qualidade do betão. A comparação do factor de sensibilidade dos ensaios indica que a permeabilidade ao oxigénio é extremamente sensível a variações na qualidade do betão

Technological evolution of concrete : from ancient times to ultra high performance concrete

Since the fall of the Roman Empire, concrete only suffered significant advances with the discovery of portland cement. The technological knowledge that the Romans held was empirical, based on the principle of trial and error, and allowed the discovery of new materials which properties could be reproduced but not explained. The ability to go beyond empiricism was only acquired by the scientific development that occurred mainly from the twentieth century onwards. In fact, no other binder materials were used other than those used by the Romans by the beginning of the nineteenth century. Only in the late eighteenth century was there a concrete renaissance. Nowadays, we can produce high-performance concrete with relative ease, maintaining the versatility of conventional concrete with the durability and strength of natural stone. However, it can be easily shaped, armed, hard-working pre and post tensioned with cables and mixed with different kind of fibers. In this paper, an overview of the use of concrete from ancient times to the most sophisticated ultra-high performance concrete in use is presented

Betão autocompactável : influência dos superplastificantes (carboxilatos modificados) na fluidez e deformabilidade das pastas ligantes

O objectivo deste estudo é clarificar o comportamento das pastas no betão autocompactável, estudando o efeito de superplastificantes da família dos carboxilatos modificados em pastas feitas com cimento CEM I 42,5 R e com misturas de cimento CEM I 42,5 R e cinzas volantes, fazendo variar o tipo (composição química) e as dosagens do superplastificante

Quality control based on electrical resistivity measurements

The electrical resistivity of concrete is one of the main parameters controlling the initiation and propagation of reinforcement corrosion. It is common knowledge that concrete electrical resistivity is mainly dependent on the w/c ratio (pore connectivity), volume and type of cement, temperature and the moisture. This research work studies the effect of specimen shape and temperature of measurement on electrical resistivity measurements of concrete using the four-point Werner electrode. In addition, the estimation of future values based on early age measurements is also studied. Research has indicated that there is a strong relationship between electrical resistivity and durability indicators at a given age. A relationship for the estimation of the electrical resistivity using early age measurements is suggested. Furthermore, it was observed that temperature has a significant influence on the electrical resistivity of concrete. Based on test results, a relationship similar to the Arrhenius equation is also suggested that can be used for conversion of electrical resistivity measurements to a reference temperature. Based on these equations, the electrical resistivity and compressive strength of concrete at 28 days is predicted using values of electrical resistivity of up to 7 days. Errors depend on the equation used but are approximately 5 % for estimates up to 28 days

Software for probability-based durability analysis of concrete structures

In recent years, much research work has been carried out in order to obtain a more controlled durability and long-term performance of concrete structures in chloride containing environments. In particular, the development of new procedures for probability-based durability design has been shown to provide a more realistic basis for the analysis. Although relevant data is still lacking, this approach has been successfully applied to several new concrete structures, where requirements for a more controlled durability and service life have been specified. A probability-based durability analysis has also become an important and integral part of condition assessment of existing concrete structures in chloride containing environments. In order to facilitate the probability-based durability analysis, simple software has been developed, where the probabilistic approach is based on a Monte Carlo simulation. In the present paper, the software for the probability-based durability analysis is briefly described and applied in order to demonstrate the importance and sensitivity of the various durability parameters affecting and controlling the durability of concrete structures in chloride containing environments

Estimating long-term durability parameters based on electrical resistivity measurements

Concrete's electrical resistivity is one of the main parameters controlling the initiation and propagation of reinforcement corrosion. It is common knowledge that concrete electrical resistivity is mainly dependent on the porosity, temperature and the moisture content. This research work studies the possible relationship between concrete electrical resistivity and compressive strength of concrete. It is intended to evaluate the possibility of predicting the strength gain of concrete at a given time using the resistivity measurements. Furthermore, in order to take account of temperature effect, the variation of resistivity due to temperature is studies. Results obtained indicate that there is a strong relationship between compressive strength at a given age and the electrical resistivity. A relationship for prediction of the compressive strength using electrical resistivity of concrete is suggested that allows the estimation of the compressive strength based on the electrical resistivity value. Furthermore, it was observed that temperature has a significant influence on the electrical resistivity of concrete. Based on test results, a relationship similar to the Arrhenius equation is also suggested that can be used for prediction of electrical resistivity with temperature. It is suggested that a method, similar to the maturity method for predicting the strength gain with time and temperature, can be used successfully to account for prediction of electrical resistivity with time and temperature. Based on this method, the electrical resistivity and compressive strength of concrete at 28 days is predicted using values of electrical resistivity of up to 4 days. Errors in estimates were less than 15 %. If values of up to 7 days are used, the error associated with the estimates is less than 5 %

Probability-based durability analysis of concrete structures in marine environment

[Tese de doutoramento em Engenharia Civil]A majority of concrete structures in marine environment shows signs of degradation due to the corrosion of the reinforcement in the presence of chlorides. In some cases, the degradation is visible within a few years of construction completion. Despite the fact that the marine environment is particularly sever, other factors affect the premature degradation due to corrosion, such as: poor construction quality as a result of poor workmanship; inadequate standards based on prescriptive measures; and poor design as a result of insufficient information with regards to the parameters that influence the degradation process. While standards are gradually improving, with the tendency to become performance based in future, much information is needed to assist the designer in the decision making process. This research work is aimed at providing designers with adequate means for probability – based durability analysis of concrete structures in marine environment. A durability model for the reinforcement corrosion, based on the second Law of Fick, is developed that takes into account the time dependant parameter for diffusion coefficient and the influence of temperature. The model has been developed primarily for providing a general basis for durability analysis of concrete structures in marine environment. Such a durability analysis can be applied for obtaining a more controlled durability and the long-term performance durability design of new concrete structures, as well as, an improved basis for condition assessment of existing concrete structures in marine environment. The model has been developed to satisfy a balanced compromise of practicality and robustness. The existing models are either difficult to use due to the complexity of calculations, and the fact that they require sophisticated tests for collecting the necessary data, or are too simplistic to provide reliable results. The proposed model requires few basic data, either available or easily obtainable, without compromising its scientific robustness and validity. As an extension of the model a probability - based software is proposed for the durability analysis. The parameters of the model have been discussed and the sensitivity of the model towards these parameters is analysed. Results show that the proposed software provides valuable information indicating the effect of each parameter on the outcome and the extent in which these parameters should be stochastically defined. Further, the proposed model was validated comparing the results from the assessments of the structures with the outcome of the probabilistic simulation of the service life of the structure. The effect small changes in parameters have on the overall performance can be rapidly evaluated. Through the use of the proposed model, the designer can appreciate the consequences of design options on the concrete and the structure. The importance of correct temperature use in the model is demonstrated with two examples, one at 10 ºC and another at 20 ºC. An increase (more than double) in the time to reach the serviceability limit was observed. The results from this investigation demonstrate that the analysis of the durability performance of the structures can be expanded and improved by using the proposed model, in relation to the traditional condition assessment.A maior parte das estruturas de betão armado em ambiente marítimo demonstram sinais de degradação resultante da corrosão das armaduras na presença de cloretos. Nalguns casos, a degradação é visível poucos anos após a construção da estrutura. Se bem que o ambiente marítimo é extremamente severo, existem outros factores que afectam a degradação prematura do betão, tais como: a fraca qualidade da construção resultante da falta de mão de obra especializada; normas baseadas em requisitos prescritivos; projectos desadequados devido à falta de informação em relação aos parâmetros que influenciam o processo da degradação. Enquanto que as normas, de uma forma geral, estão a melhorar, com a tendência a basearem-se cada vez mais em requisitos de desempenho, ainda há necessidade em auxiliar o projectista na especificação da durabilidade da estrutura. Este trabalho de investigação está centrado em fornecer aos projectista meios adequados para uma análise probabilística da durabilidade da estruturas de betão armado em ambiente marítimo. Um modelo de durabilidade para a corrosão da armadura baseado na segunda Lei de Fick é desenvolvido tendo em conta a dependência do coeficiente de difusão com o tempo e com a temperatura. O modelo foi desenvolvido essencialmente para servir como um suporte para a análise da durabilidade de estruturas de betão armado em ambiente marítimo. Uma análise desta natureza pode ser aplicada tanto ao controlo da durabilidade e do desempenho a longo prazo de novas estruturas de betão armado, como à avaliação do estado de degradação de estruturas existentes em ambiente marítimo. O modelo proposto é desenvolvido por forma a conciliar a facilidade de uso com a robustez científica. Modelos existentes ou são demasiado complicados para serem utilizados de uma forma prática, i.e. requerem normalmente ensaios sofisticados para recolher os dados necessário, ou, são excessivamente simplificados para fornecerem resultados fiáveis. O modelo proposto requer poucos dados básicos que, estão facilmente disponíveis, ou poderão ser obtidos com relativa simplicidade, sem contudo comprometer a robustez e validade científica. Como extensão natural do modelo, é proposto um software baseado na metodologia probabilística para a análise da durabilidade. Os parâmetros do modelo foram estudados, e a sensibilidade do modelo a variação destes é analisada. Os resultados demonstram que o software proposto fornece informações valiosas no que respeite à influência de cada parâmetro na análise da durabilidade, e na forma como estes devem ser quantificados estocasticamente. O modelo proposto foi validado com resultados obtidos de estruturas existentes. Através das simulações probabilísticas o efeito de alterações dos parâmetros do modelo no desempenho da estrutura pode ser eficazmente avaliado. Recorrendo ao software, o projectista pode avaliar a consequência de opções de projecto no betão e na estrutura. A importância de utilização de temperaturas prevalecentes no local da obra é demonstrada através de exemplos analisados a 10 ºC e 20 ºC. Os resultados desta investigação realçam que a análise do desempenho de durabilidade das estruturas pode ser expandida e melhorada com recurso ao modelo proposto quando comparado com a avaliação da durabilidade tradicional.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/4724/2001. European Social Fund (ESF) - 3rd Community Support Framework. Research Council of Norway