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Early age carbonation curing for precast reinforced concretes

Carbonation is considered detrimental to reinforced concrete. However if carbonation is performed at early age through curing, the process could be beneficial. This paper is to present a study on a unique process that is developed for early age carbonation curing of precast reinforced concrete to maximize the performance improvement and the carbon storage capacity. The process includes vibration casting, in-mold curing, off-mold preconditioning, carbonation curing and subsequent hydration. It was found that a carbon uptake of 16% based on the cement content could reduce concrete pH to 9.2 on the surface and maintain pH of 13.0 at core immediately after 12 h carbonation. The subsequent hydration was able to increase the pH on surface over 12.3 which was comparable to hydration reference. The carbonated concrete had shown more resistance to permeation by having a higher electrical resistivity on surface and was not more vulnerable to weathering carbonation. The off-mold preconditioning in open air caused no shrinkage cracking because of the controlled evaporation rate. The process makes concrete a sandwich structure with carbonate-rich surface which is responsible for strength gain, carbon storage and durability enhancement and is suitable for precast reinforced concrete production.

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Carbonation is considered detrimental to reinforced concrete. However if carbonation is performed at early age through curing, the process could be beneficial. This paper is to present a study on a unique process that is developed for early age carbonation…
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Variations of sorptivity with rheological properties of concrete cover in self-consolidating concrete

Transport properties of the concrete cover can influence the durability of concrete. Concrete cover of conventional vibrated concrete has greater porosity because of the looser packing density of coarse aggregate against the surface of the formwork, which is referred to as the “wall effect”. In the case of self-consolidating concrete (SCC), the volume of coarse aggregate is lower, and the packing density of the aggregate can depend on the flow properties of the SCC under its own weight. The extent of the wall effect on the quality of the concrete cover can vary with the rheological properties of the concrete. The work presented in this paper seeks to evaluate the effect of changes in rheological properties of SCC on the sorptivity of the concrete cover that can be affected by the degree of consolidation of the SCC near formed surfaces as well as changes that can result from water migration and changes in the packing of solid particles in the vicinity of formed surfaces. The sorptivity of the concrete cover is also compared to that of the bulk concrete. In total, 17 SCC mixtures covering a wide range of rheological properties were investigated. Good correlation between initial plastic viscosity of SCC determined by the modified Bingham model and the sorptivity measured during the first 6 h of testing is established. It is likely that the initial plastic viscosity has a marked influence on the volume of the largest capillary pores of concrete, which can significantly affect transport properties and durability. Test results indicate that the sorptivity of the concrete cover in SCC is similar to that obtained in the interior bulk concrete.

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Transport properties of the concrete cover can influence the durability of concrete. Concrete cover of conventional vibrated concrete has greater porosity because of the looser packing density of coarse aggregate against the surface of the formwork, which is referred to…
Read more

Analysis of the debonding process of CFRP-to-timber interfaces

The use of Fiber Reinforced Polymers (FRP) in the strengthening of timber structures is quite recent and few studies have discussed the debonding between these materials. The analysis of the Mode II debonding process between FRP composites and timber elements may be of great importance because this mode is predominant in the case, for instance, of the bending of beams. Knowing the appropriate bond-slip model to use on the estimation of the performance of FRP-to-timber interfaces is greatly relevant. Under such circumstances, a detailed knowledge of all the states that CFRP-to-timber interfaces are subjected to is important as well. The current work gives answers to these aspects proposing an analytical solution based on a tri-linear bond-slip model that is capable of describing precisely the full-range debonding behavior of FRP-to-timber interfaces. Thus, the purpose of this study is to contribute to existing knowledge with an analytical solution capable of describing the full-range debonding process between a FRP composite and a substrate. The analytical solutions herein proposed are also compared with the results obtained from several experiments based on single-lap shear tests. Comparisons at different load levels and different bonded lengths are presented. The slips, strains in the CFRP composite and bond stress distributions within the bonded interface are emphasized in the text. The complete load-slip response of CFRP-to-timber interface is also analyzed. Each state of the debonding process is described and each one is identified in the load-slip curve.

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The use of Fiber Reinforced Polymers (FRP) in the strengthening of timber structures is quite recent and few studies have discussed the debonding between these materials. The analysis of the Mode II debonding process between FRP composites and timber elements…
Read more

Recent Construction and Building Materials Articles

This paper presents a framework for employing X-ray computed tomography (CT) for the assessment of hot mix asphalt (HMA) fatigue damage. The analyses were carried out on asphalt beams in order to quantify the damage created by four-point bending load. A new algorithm has been developed for calculating the thresholding levels of the images acquired before and after testing. The thresholding level prior to testing was estimated using laboratory air voids. To determine the post-testing thresholding level, the proposed algorithm matches 16-bit image histograms obtained before and after the test. This process is implemented only for the portion of the histogram that represents the aggregate colour intensities that remain unchanged during the testing.

The results and analysis reveal that the developed technique is a valid method for successfully quantifying and evaluating HMA fatigue damage in large specimens. Because of the high degree of precision they provide, 16-bit images are recommended for this type of analysis. It was found that the distributions of air voids and damage are not uniform in asphalt beams and that they vary significantly throughout the length of a single beam as well as from beam to beam. This study also confirms the effectiveness of X-ray CT for quantifying HMA fatigue damage in asphalt beams following crack propagation.

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This paper presents a framework for employing X-ray computed tomography (CT) for the assessment of hot mix asphalt (HMA) fatigue damage. The analyses were carried out on asphalt beams in order to quantify the damage created by four-point bending load….
Read more