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

Abstract

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.

Effect of self-compacting concrete placement technology on the load-bearing capacity of the concrete-concrete and steel-concrete bond in layered elements

“…In the technological variant where the previously laid layer was mechanically disturbed, no average decrease occurred for a delay of 15 minutes, while for delays of 30 and 60 minutes, it was 6.0% and 11.4%, respectively. Similar results were observed in experiments conducted on beam elements (14). Additionally, studies employing mixes with different thixotropy ratios (16,17) reported that for a delay of 60 minutes, the residual strength of the joint ranged from 56% to 91% -for the inclined shear test, 15% to 76% -for the direct shear test, and 56% to 80% -for the flexural strength test.…”

“…Additionally, it was found that applying a second layer with a thickness significantly exceeding that recommended [100 mm] (15) was not adequate to achieve a shear effect between the two layers during a delay time of 60 minutes. Considering both the outcomes of this research and findings in the literature (13,14,16,17), the recommendation is to place successive layers of SCC mix in beam and wall elements with a thickness of ≥100 mm, allowing a delay of up to 30 minutes.…”

Multilayer Casting of Eco-Efficient Self-Compacting Concrete with Reduced Binder Content

“…The authors drew analogous conclusions in their previous publication based on self-compacting concrete with a binder content of 450 kg/m 3 [ 16 ]. That mixture exhibited stronger thixotropic behaviour and the variant of casting the mixture onto an undisturbed surface resulted in greater deterioration of the load-bearing capacity of the joint between successive layers.…”

“…A considerable number of studies with new generation concretes [ 3 , 5 , 16 , 17 ] were conducted on the load-bearing capacity of the joint resulting from delays using different destructive tests. A reduction in mechanical strength of more than 40% was reported on the cylindrical core samples using a simple compression test [ 5 ].…”

“…Furthermore, in other studies on self-compacting mixtures with different levels of structural build-up at rest [ 3 , 4 ], for the delay time of 60 min, the residual bond strength varied in the range of 56–91% (slant shear test), 15–76% (direct shear test) and 56–80% (flexural strength test). Tests on three variants of fresh surface conditioning [ 16 ] indicated that the slightest reduction in bond strength between neighbouring layers occurring due to mechanical disturbance of prior layer and for a delay time of 45 and 60 min was 6% and 5%, respectively.…”