Effect of bottom-up placing on bond properties of high-performance self-compacting concrete

2020

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Self-compacting concrete (SCC) should generally be placed continuously, but it is not uncommon for contractors to be forced to use interruptions in concrete works due to delivery delays. The multilayer casting of SCC can cause weak bond conditions in the contact area of subsequent layers. Methods of preventing cold joint or lift line formation for normal concretes are not suitable for self-compacting concretes. This article provides research on the effect of multilayer casting technology on the bond strength between two layers of SCC. Three technological variants of connecting successive layers of SCC mixture on beam elements were analyzed: The free flow of the mixture, dropping the mixture from a greater height, and mechanical disturbance of the first layer. Three delay times were applied: 30, 45, and 60 min between two layers of SCC. In general, the research revealed that, regardless of the multilayer casting variant, the bond strength between two layers decreased as the delay time was extended. The best performance and the lowest drop in bond strength were obtained for samples with a mechanically disturbed first layer, independent of the delay time. This method gave similar results to a reference element made without a break in concreting. It was also demonstrated that current recommendations and standard guidelines for multilayer casting appear to be insufficient for ensuring an adequate bond between layers.

Section: Introductionmentioning

confidence: 99%

Effect of Placement Technology on the Bond Strength Between Two Layers of Self-Compacting Concrete

2020

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“…When comparing the results of tests on SCC concrete presented in this article with the findings obtained in identical tests on HPSCC concrete [26], observations can be made on a slightly wider spectrum of cases. The concrete placing from the bottom of the form both for SCC and HPSCC generally led to the improvement of bond characteristics (bond stiffness, bond strength, top-bar effect, the effect of the rebar distance from the casting point).…”

Section: Comparision To Hpsccmentioning

confidence: 89%

“…The main factor contributing to enhanced bond conditions is the more sealed structure of self-compacting concretes and high-performance self-compacting concretes than normal concrete thanks to the use of mineral additives and microfillers. The studies which have been carried out up to this moment [2][3][4]26] demonstrate that the microstructure of concretes can be significantly tightened by their more effective deaeration during the concrete placing from below. The improvement of the bond phenomenon within an entire element, namely the elimination or significant reduction of the zone of poor bond conditions, raises doubts regarding the very strict design recommendations in the case of the anchorage length of reinforcing bars.…”

Section: Future Researchmentioning

confidence: 99%

“…It is observed that there is a growing interest in this technology within the construction industry, so it would seem appropriate to investigate the effect and its impact on the parameters of the hardened concrete, including its bond to reinforcing bars. Authors have already started working on the project concerning high-perfomance self-compacting concrete (HPSCC) [26]. The authors found that the casting direction has an influence on the phenomenon of the steel-concrete bond in the case of both HPSCC mixtures containing a filler in the form of silica fume and those based entirely on cement.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Placing Self-Compacting Concrete Bottom up on the Quality of Bond Conditions

2020

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A self-compacting concrete (SCC) mixture, due to its special rheological properties, may be placed differently than in traditional variants. The paper presents the results of a study on the effect of a bottom-up placing direction on the quality of bond conditions between steel and self-compacting concrete. Elements with overall dimensions of 160 × 480 × 1600 mm consisting of elementary samples with dimensions of the bond test basic modules were constructed. Ribbed steel rebars with diameters of 16 mm were used and located in formworks perpendicularly to the concrete placing direction. Bond tests were carried out according to the pull-out method. The bottom-up placing contributed to the uniformity of the bond conditions quality in the test elements and an overall improvement of the bond properties, especially in its top part. Given the increasing implementation of alternative methods of mix placing and the promising results, the topic should be further investigated.

“…No comprehensive research has yet been conducted into the effect of the bottom-up placing of the self-compacting concrete mixture on its bonding to steel rebars; neither is this technology considered in the standard guidelines. Preliminary studies were presented in [ 9 , 10 ]. It was observed that, in general, the bottom-up placing of the concrete mixture improved the quality of the hardened concrete in terms of the bond strength and properties of bond stiffness, bond strength, top-bar effect, and the proximity of rebar to the casting point effect in the upper regions of the element.…”

Section: Introductionmentioning

confidence: 99%

X-ray CT Investigation of Bond Mechanism in Reinforced SCC Elements of Different Placement Technology

2021

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The effect of different placing methods of the self-compacting concrete (SCC) mix—from the top and from the bottom of the form—on the bond failure mechanism was investigated within the scope of this paper. Existing studies regarding the known mechanisms of bond failure do not consider the bottom-up method of concrete placing, which improves the quality of the concrete microstructure around reinforcing bars. Background tests were performed on panel elements with dimensions of 800 × 480 × 160 mm. Ribbed steel reinforcing bars with a diameter of 16 mm were used in the tests, which were placed horizontally in the forms. A pull-out method was used to investigate the bond strength. X-ray computed tomography (CT) was used as a novel and non-destructive technique that allowed a 3D insight into the bond between the rebar and the concrete after the ultimate bond stress had been reached. The results provided a clear description of the phenomena occurring during the fresh state of concrete in the vicinity of rebars (bleeding, plastic settlement, vertical density variation) and showed their significance for bond mechanisms. Finally, it was demonstrated that placing the mix from the bottom of the form resulted in the same bond failure mechanism for both bars located at the top and the bottom of the panel elements. This was translated into identical bond properties throughout the element with regard to bond stiffness and bond strength. It was found that the described and known mechanisms of bond failure are only an idealized description of the performance of the reinforcing bar-concrete joint. The analysis of the steel–concrete interface (SCI) imaging indicated that, in reality, the forming bond failure mechanisms were a complex process that could be affected by many factors.