Abstract

This work reports results obtained by studying the behavior of reinforced concrete beams under a tension-testing regime, in which the beams were exposed to an environment with high CO2 concentration. Beam molding followed by humid cure required 7 days and the subsequent application of a two-point load with the lowest possible frame dimensions in a controlled atmosphere 80% of which was concentrated CO2; relative humidity was 60ƒ¦5%. Carbonation depth results, cracks, as well as the cover¡¦s effectiveness in reinforcement protection, were analyzed with respect to beam serviceability.

Keywords: beam, carbonation, concrete, crack, durability.

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Resumo

This work reports results obtained by studying the behavior of reinforced concrete beams under a tension-testing regime, in which the beams were exposed to an environment with high CO2 concentration. Beam molding followed by humid cure required 7 days and the subsequent application of a two-point load with the lowest possible frame dimensions in a controlled atmosphere 80% of which was concentrated CO2; relative humidity was 60ƒ¦5%. Carbonation depth results, cracks, as well as the cover¡¦s effectiveness in reinforcement protection, were analyzed with respect to beam serviceability.

Palavras Chave: beam, carbonation, concrete, crack, durability.

Abstract

Limestone filler is an extended supplementary material used in the formulation of European and Latin American cements. Also limestone filler is being used to produce self-consolidating concretes. In a sulfate environment, the presence of limestone filler in cement increases the deterioration risk and can lead to the formation of thaumasite. This paper presents results of expansion test conducted in accordance with ASTM C 1012 and the microstructural analysis on cement pastes with sulfate-resistant portland cement (SRPC) (C3A < 2%) containing 12 and 18 % of limestone filler. Specimens were exposed to sulfate solution for three years at 20 ºC and 5 ºC. The evolution of attack was determined using XRD analysis on the material obtained from near surface layers of the specimens. Complementary, IR studies were carried out. Results show that SRPC containing 18 % limestone filler was more susceptible to sulfate attack than SRPC and SRPC with 12 % of limestone filler. The attack was characterized by the inward front leading first to the formation of ettringite, later formation of gypsum and finally thaumasite formation. All specimens present greater deterioration at low temperature

Keywords: limestone filler, sulfate attack, sulfate resistant portland cement, Thaumasite, XRD-analysis.

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Resumo

Limestone filler is an extended supplementary material used in the formulation of European and Latin American cements. Also limestone filler is being used to produce self-consolidating concretes. In a sulfate environment, the presence of limestone filler in cement increases the deterioration risk and can lead to the formation of thaumasite. This paper presents results of expansion test conducted in accordance with ASTM C 1012 and the microstructural analysis on cement pastes with sulfate-resistant portland cement (SRPC) (C3A < 2%) containing 12 and 18 % of limestone filler. Specimens were exposed to sulfate solution for three years at 20 ºC and 5 ºC. The evolution of attack was determined using XRD analysis on the material obtained from near surface layers of the specimens. Complementary, IR studies were carried out. Results show that SRPC containing 18 % limestone filler was more susceptible to sulfate attack than SRPC and SRPC with 12 % of limestone filler. The attack was characterized by the inward front leading first to the formation of ettringite, later formation of gypsum and finally thaumasite formation. All specimens present greater deterioration at low temperature

Palavras Chave: limestone filler, sulfate attack, sulfate resistant portland cement, Thaumasite, XRD-analysis.

Abstract

The aim of the study is to investigate the microstructural changes resulting from the use of mineral admixtures in portland cement as well as to assess the effect of carbonation on the hydration products formed. Six different types of concrete were developed, constituted by binary mixtures of portland cement, each with the following contents (in partial replacement of cement) and admixtures types: silica fume 10 %, metakaolin 10 %, rice-husk ash 10 %, fly-ash 25 %, blast-furnace slag 65 % and normal portland cement (no-admixtures). After a period of 28 days in the moist room, followed by a complementary period of approximately 100 days in laboratory environment, the concretes underwent a CO2 attack (5 % content) for two weeks and samples of these concretes were analyzed by means of scanning electron microscopy (SEM) and X-ray diffractometry (XRD). As main results, it was verified that: among the concretes, those produced with rice-husk ash and silica fume yielded the highest relative compactness; in the morphology study, it was noted that in addition to the occurrence of calcite (main carbonation product), there was C-S-H carbonation and the formation of C-A-S-H, the latter resulting from the employment of admixtures with a high alumina content (as metakaolin, fly-ash and blast-furnace slag, for example). In the XRD analysis, the significant occurrence of the compound calcite was verified, which was not verified in some SEM analyses, which did not always clearly reveal that compound.

Keywords: carbonation, concrete, durability, microstructure, mineral admixture.

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Resumo

The aim of the study is to investigate the microstructural changes resulting from the use of mineral admixtures in portland cement as well as to assess the effect of carbonation on the hydration products formed. Six different types of concrete were developed, constituted by binary mixtures of portland cement, each with the following contents (in partial replacement of cement) and admixtures types: silica fume 10 %, metakaolin 10 %, rice-husk ash 10 %, fly-ash 25 %, blast-furnace slag 65 % and normal portland cement (no-admixtures). After a period of 28 days in the moist room, followed by a complementary period of approximately 100 days in laboratory environment, the concretes underwent a CO2 attack (5 % content) for two weeks and samples of these concretes were analyzed by means of scanning electron microscopy (SEM) and X-ray diffractometry (XRD). As main results, it was verified that: among the concretes, those produced with rice-husk ash and silica fume yielded the highest relative compactness; in the morphology study, it was noted that in addition to the occurrence of calcite (main carbonation product), there was C-S-H carbonation and the formation of C-A-S-H, the latter resulting from the employment of admixtures with a high alumina content (as metakaolin, fly-ash and blast-furnace slag, for example). In the XRD analysis, the significant occurrence of the compound calcite was verified, which was not verified in some SEM analyses, which did not always clearly reveal that compound.

Palavras Chave: carbonation, concrete, durability, microstructure, mineral admixture.

Abstract

Unexpected or premature concrete deterioration due to alkali-aggregate reactivity is a widespread problem worldwide. The most common management actions for concrete structures affected by ASR can generally be grouped into activities to (1) control moisture access to the concrete by improving drainage systems or applying physical barriers or a variety of waterproof coatings, (2) slow down the process of ASR through chemical treatments such as the use of lithium-based compounds, (3) restrain expansion forces using physical containment, post-tensioning, encapsulation, and (4) try to accommodate the deleterious effect of AAR expansion by releasing stresses using slot-cutting. The effectiveness of the above methods has been shown to vary widely from one application to another; however, it is generally recognized that most of the above remedial measures are temporary solutions that may help to save some time and money until the deleterious process of AAR expansion has stopped.

Keywords: Alkali-aggregate reaction, alkali-silica reaction, remedial measures, repair of concrete.

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Resumo

Unexpected or premature concrete deterioration due to alkali-aggregate reactivity is a widespread problem worldwide. The most common management actions for concrete structures affected by ASR can generally be grouped into activities to (1) control moisture access to the concrete by improving drainage systems or applying physical barriers or a variety of waterproof coatings, (2) slow down the process of ASR through chemical treatments such as the use of lithium-based compounds, (3) restrain expansion forces using physical containment, post-tensioning, encapsulation, and (4) try to accommodate the deleterious effect of AAR expansion by releasing stresses using slot-cutting. The effectiveness of the above methods has been shown to vary widely from one application to another; however, it is generally recognized that most of the above remedial measures are temporary solutions that may help to save some time and money until the deleterious process of AAR expansion has stopped.

Palavras Chave: Alkali-aggregate reaction, alkali-silica reaction, remedial measures, repair of concrete.

Abstract

Reinforcement corrosion is attracting research interest in many areas due to the economical consequences of the damage generated by the process. Several proposals can be found on prediction of the time to reinforcement corrosion and service life duration. In this paper a proposal is made for using the electrical resistivity to calculate both the initiation and propagation periods. For the time period to corrosion onset, the electrical resistivity serves to model the porosity and its connectivity and therefore can be used to calculate transport processes. Due to the reaction of chlorides and carbon dioxide with cement phases, the resistivity has to be factorised by a “reaction factor”, r, accounting for it. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a relationship with the corrosion current. An equation is presented to predict service life. Using this equation, minimum resistivity values can be established according to cover thickness and in function of exposure classes. The model is under calibration with real data of carbonation and chloride penetration in specimens an in cores drilled from real structures.

Keywords: Concrete resistivity, reaction factor, service life.

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Resumo

Reinforcement corrosion is attracting research interest in many areas due to the economical consequences of the damage generated by the process. Several proposals can be found on prediction of the time to reinforcement corrosion and service life duration. In this paper a proposal is made for using the electrical resistivity to calculate both the initiation and propagation periods. For the time period to corrosion onset, the electrical resistivity serves to model the porosity and its connectivity and therefore can be used to calculate transport processes. Due to the reaction of chlorides and carbon dioxide with cement phases, the resistivity has to be factorised by a “reaction factor”, r, accounting for it. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a relationship with the corrosion current. An equation is presented to predict service life. Using this equation, minimum resistivity values can be established according to cover thickness and in function of exposure classes. The model is under calibration with real data of carbonation and chloride penetration in specimens an in cores drilled from real structures.

Palavras Chave: Concrete resistivity, reaction factor, service life.

Abstract

Polymer-modified mortars and concretes have been widely used as construction materials since 1960. However, it has been reported that these materials show lower strengths when saturated in water or when submitted to high humidity conditions. In an attempt to understand this behavior, the present study evaluated mortars modified with hydroxyethyl cellulose (HEC) and poly(ethylene-co-vinyl acetate) - EVA - during wetting and drying. The results have shown that the mechanical strengths (especially flexural tensile and bond strengths) of the mortars are affected by the water absorption. However, this phenomenon seems to be partially or completely reversible.

Keywords: mechanical behavior, polymer-modified cement-based material, water immersion and drying.

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Resumo

Polymer-modified mortars and concretes have been widely used as construction materials since 1960. However, it has been reported that these materials show lower strengths when saturated in water or when submitted to high humidity conditions. In an attempt to understand this behavior, the present study evaluated mortars modified with hydroxyethyl cellulose (HEC) and poly(ethylene-co-vinyl acetate) - EVA - during wetting and drying. The results have shown that the mechanical strengths (especially flexural tensile and bond strengths) of the mortars are affected by the water absorption. However, this phenomenon seems to be partially or completely reversible.

Palavras Chave: mechanical behavior, polymer-modified cement-based material, water immersion and drying.

Abstract

Reinforced concrete structures, when submitted to fire or exposed to high temperatures, might suffer important microstructural changes, which in turn affect their macroproperties, such as compressive strength and porosity. These changes are both chemical and physical in nature, involving loss of water, thermal expansion/contraction and the modification of the crystalline arrangements of some of its constituents. Their combined action might significantly reduce the resistance of the structural member, inclusive to the point of collapse. Researches in the field of concrete fire resistance are usually focused on the monitoring of the external signs of degradation that are visually noticeable, such as the micro-cracks, expansions and spalling. The chemical and physical changes in the microstructure are less examined, although they are the primary reasons behind the degradation. The research discussed in this paper was designed to look into the determination of the residual properties of mortar and cement pastes after heating exposure at up to 800oC. The changes in the microstructure were monitored with XRD, SEM, DTA and dilatometry tests. The results obtained confirmed that high density concretes are more vulnerable to spalling and that the degree of deterioration can be correlated to changes in the x-ray difratograms.

Keywords: high density concrete, high temperatures, residual properties, spalling.

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Resumo

Reinforced concrete structures, when submitted to fire or exposed to high temperatures, might suffer important microstructural changes, which in turn affect their macroproperties, such as compressive strength and porosity. These changes are both chemical and physical in nature, involving loss of water, thermal expansion/contraction and the modification of the crystalline arrangements of some of its constituents. Their combined action might significantly reduce the resistance of the structural member, inclusive to the point of collapse. Researches in the field of concrete fire resistance are usually focused on the monitoring of the external signs of degradation that are visually noticeable, such as the micro-cracks, expansions and spalling. The chemical and physical changes in the microstructure are less examined, although they are the primary reasons behind the degradation. The research discussed in this paper was designed to look into the determination of the residual properties of mortar and cement pastes after heating exposure at up to 800oC. The changes in the microstructure were monitored with XRD, SEM, DTA and dilatometry tests. The results obtained confirmed that high density concretes are more vulnerable to spalling and that the degree of deterioration can be correlated to changes in the x-ray difratograms.

Palavras Chave: high density concrete, high temperatures, residual properties, spalling.