Bond behaviour of deformed steel reinforcement in lightweight foamed concrete

[摘要] ENGLISH ABSTRACT: Lightweight foamed concrete is a low density concrete that utilizes the entrapment of air generatedfrom a protein based foam mix constituent. It is produced with the base constituents:water, cement and fly-ash, whereafter stable foam is added to change the density.Lightweight foamed concrete is a relatively new building material with many economic advantages,but it does not reach some of the typical engineering properties of normal weight concrete.The latter is thoroughly researched, characterized and documented in design standardsused throughout engineering practice, whereas the paucity of literature on lightweight foamedconcrete adds to the slow progress in its development.Recent progress in mix design and density control of lightweight foamed concrete have led tothe notion for its use in structural application i.e. steel reinforced lightweight foamed concrete.This study characterizes the bond behaviour of deformed steel reinforcement in lightweightfoamed concrete, quantifies several of its engineering properties and presents prediction models.The concretes used for testing were a benchmark normal weight concrete and lightweight foamedconcretes with casting densities of 1200, 1400 and 1600 kg/m3 and they were evaluated forcompressive strength, Youngs modulus, indirect tensile splitting strength and wedge splittingfracture energy.Pull-out and beam-end tests were used to quantify the characteristic bond behaviour. Thedesign of these tests comprises of the loading arrangements, specimen preparation, slip measuringtechniques, equipment configuration for accurate slip control and a method for determining adesign bond stress correlated to a physical occurrence.Even though compressive strengths suitable for structural application were achieved for some ofthe lightweight foamed concretes, its other strength properties and bond behaviour results didnot reach the predicted or measured values of normal weight concrete.Compared to normal weight concrete, the fracture energy of lightweight foamed concrete is atleast an order of magnitude smaller. The intrinsic brittleness of lightweight foamed concretebecomes more with increased density and is shown to have a considerable effect on the bondbehaviour. The beam-end tests on the 1600 kg/m3 concrete showed early onset of bond deteriorationas a result of internal cracks, intensified by its brittle nature.Evaluation of the bond-slip envelopes show the denser lightweight foamed concrete performingwell when considering bond stress magnitude, but lacking ductility in failure. The leastdense lightweight foamed concrete showed excellent ductility during failure, but lacked sufficientbonding stress.A new concept, named the Bond Integrity (BI), was introduced to quantify early bond disturbancessuch as internal cracks. It was formulated from the measured rate of change in applied moment and rate of change in the bar tensile force during the beam-end test.Prediction models for the design bond stress of lightweight foamed concrete were derived fromthe stress conditions at the steel-concrete bond interface, taking into consideration two failuremodes; pull-out and splitting failure.The outcome of this study not only quantifies but also emphasizes the lack of sufficient bondingof deformed steel in lightweight foamed concrete. The inadequate crushing strength of theleast dense concrete and the brittleness observed for the denser concrete, both diminishes bondintegrity and permits significant bond development. It can be concluded that lightweight foamedconcrete is not suitable for structural application and that further development of the materialis required to increase strength, reduce brittleness and introduce innovative reinforcing systemsthat will lead to improved bond performance.Apart from providing standardized and novel results, together with detailed guidelines for theexperimental setup and quality control, this study may well become the future basis for furtherdevelopment of lightweight foamed concrete as a structural material.

[发布机构] Stellenbosch University