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

Abstract

The characteristic rheological properties of self-compacting concrete mixes allow for their placing in variants other than the traditional one – from the top of a formwork. This paper presents the results of a study on the effect of different casting directions – from the top and bottom of a form – of a self-compacting mix on the bond phenomenon between reinforcement steel and high-performance selfcompacting concretes (HPSCC). Three HPSCC mixes having different contents of a mineral additive in the form of silica fume (replacing 0, 5 and 10% of the cement's mass) have been used to produce test elements. The elements had dimensions of 160 × 480 × 1600 mm. Ribbed steel reinforcing bars with diameters of 16 mm were used and fixed in formworks perpendicularly to the concrete placing direction. Bond tests were carried out following the pull-out method. This article analyses the impact of the concrete casting direction on: the compressive strength of the HPSCCs, the bond stiffness, the bond strength, the effect of the distance from the casting point and the top-bar effect. The tests on the elements cast in the bottom-up direction revealed an improvement of both the compressive strength and the rebar-to-concrete bond properties in the upper parts of the elements as compared to the traditional concreting. Placing from the bottom of the form offered uniformity of the bond conditions along the heights of the specimens studied in this experiment. Given the wider and wider implementation and a lack of any normative references for these concretes and alternative casting directions, the authors argue in favour of taking into account the concrete placing direction in the standard recommendations regarding the assessment criteria of the bond condition quality.