A leitura como um processo cognitivo
Figueiredo, Olívia Maria
1999-01-01
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12 records were found.
their reinforcement, clearly showing the effect of carbonation of concrete.
This structure represents a construction solution particularly used in the 40s to 50s of the
twentieth century in several Lisbon areas. It was used for giving access to the patio and for
evacuation in case of emergency and, normally, was located in the back of the building.
The complete replacement of the carbonated concrete cover was achieved by a high quality
polymer cement mortar (PCM), of styrene-butadiene polymer dispersion.
This paper describes the different activities undertaken during the rehabilitation, the
problems encountered during the works and, finally, the lessons learnt from this project.
Este registo pertence ao Repositório Científico do LNEC
Este registo pertence ao Repositório Científico do LNEC
Polymer modification of mortar and concrete leads to reduced water absorption, lower ionic mobility, reduced thickness of the interfacial transition zone, lower availability of Ca(OH)2 and higher tensile strength and ductility and thus may have an inhibiting effect on alkali-silica reaction (ASR) and related damage. However, the existing studies are inconclusive and cover only a limited range of polymers. In this study, polymer-cement mortars (PCMs) based on styrene-butadiene, acrylic and epoxy polymer additions were subjected to an accelerated alkali reactivity test and their expansion compared with that of an unmodified cement mortar (CM). Despite their lower capillary water absorption in standard laboratory climate, the weight increase of PCMs during exposure to the 80 ºC NaOH solution bath of the alkali reactivity test was higher t...
Due to the improvement of several properties, such as lower water absorption and increasing resistance to
microcracking, polymer modification of mortar and concrete potentially leads to an inhibiting effect on alkali–
silica reaction (ASR). In this study, polymer–cement mortars (PCMs) were subjected to accelerated alkali
reactivity tests and their performance compared to that of an unmodified cement mortar (CM). Expansion
of PCMs was overall higher, but SEM observations revealed apparent lower microcracking intensity of the
cement paste, compared to the unmodified CM. Subsequent stiffness damage tests (SDTs) quantified the
existing damage after alkali reactivity tests and confirmed the lower microcracking intensity in PCMs.
At the National Laboratory of Civil Engineering (LNEC), Portugal, and with financial support from Fundação para a Ciência e a Tecnologia (FCT) given to the research project PTDC/ECM/101810/2008 – Polymer-modified Cement Mortars for the Repair of Concrete Structures, an ongoing study aims to assess the influence of polymers on the degradation mechanism of cement-based materials due to ASR. For polymer dosages in excess of 10% of the mass of cement a continuous network of polymer film starts to form amongst cement hydrates, effectively transforming the cement matrix into a polymer-cement co-matrix (Figure 1). The effect of this polymer network on the expansion and microcracking resulting from ASR was assessed for a variety of polymer-modified cement-based mortars (PCMs) using acrylic-styrene, styrene-butadiene and epoxy polymers and perf...
É reconhecida a importância do aprofundamento dos conhecimentos sobre a fluência em tração do betão, atendendo à variedade de aspetos apresentados por este mecanismo e por poder influenciar a fissuração do betão.
O betão é um material que apresenta uma rotura frágil e é muito suscetível a fissurações. A sua baixa capacidade de deformação e baixa resistência à tração têm uma influência negativa no tempo de vida das estruturas de betão. Por este motivo, entre outros, não se podem negligenciar as propriedades em tração do betão para o dimensionamento de estruturas, nomeadamente os efeitos do tempo na rotura por tração.
O presente artigo pretende descrever o ensaio de determinação da fluência em tração do betão que se encontra em fase de implementação no LNEC. Com base em campanhas experimentais, indicam-se os principais aspetos a consid...
This paper compares the factors known to influence the deleterious alkali-silica reaction in concrete with the properties that polymers tend to modify in cementitious materials. A discussion on the potential ASR-influencing mechanisms of polymer additions is provided, along with a critical review of the existing literature on the subject. The influence of the potentially significant differences in mechanical properties and sorptivity between polymer-modified and unmodified cementitious materials on the expansion results of alkali reactivity tests is also discussed. According to the available results, the influence of polymers on ASR-related expansion seems to depend on both polymer type and dosage, suggesting the existence of concurrent, not yet well understood mechanisms by which polymers may influence ASR-related expansion. As polyme...
