Structural implications of ultra-high performance fibre-reinforced concrete in bridge design

The present research represents a theoretical and experimental contribution to the understanding of the structural behaviour of elements made of ultra-high performance fibre-reinforced concrete (UHPFRC). UHPFRC is investigated as an advanced cementitious material offering particular potential in innovative bridge design. The optimised material composition results in high compressive strength and non-negligible tensile strength and ductility, provided by multi-microcracking. This allows significant tensile forces to be sustained by elements in bending even without the use of ordinary reinforcement. Thanks also to the material's resistance to environmental degradation, very thin structural elements can be constructed. This research focuses primarily on the bending behaviour and design of thin UHPFRC beams and slabs. Punching-shear is also investigated as a possible failure mode of thin UHPFRC slabs. One of the main differences between other concretes and UHPFRC is that the latter requires mechanical models capable of taking tensile behaviour into account for rational structural application. Analytical and numerical models are developed in this study to simulate the non-linear bending response of UHPFRC beams and slabs. This permits the assessment of element behaviour at service states and prediction of failure loads. Theoretical research on both bending and punching-shear failure is supported by experimental research on beams and slabs made of BSI® UHPFRC with 2.5 % volume of 20-mm long steel fibres. The analytical model for beams in bending takes both material multi-microcracking and macrocrack propagation with tensile softening into consideration. Multi-microcracking is modelled as a pseudo-plastic tensile behaviour, while the macrocrack is simulated based on the assumptions of the fictitious crack model. The results are in good agreement with experimental data and simulations obtained from a developed finite element model. Using theoretical results and experimental data it is demonstrated that pre-peak behaviour and bending strength are mainly governed by multi-microcracking. The propagation of the macrocrack provides only a minor additional contribution to bending strength, but, in the case of thin beams, plays an important role in providing ductility in bending. Theoretical results demonstrate that, due to the presence of pronounced pseudo-plastic deformations, size effect on bending strength is much less significant for UHPFRC than for other quasi-brittle materials, which corresponds to experimental observations. It is however shown that, even if the pseudo-plastic phase is less pronounced, thin elements develop behaviour similar to that of elements with high pseudo-plastic tensile deformations, owing to the low stress decrease in tensile softening. Nonetheless, in the absence of pseudo-plastic tensile deformations, the behaviour of thick elements approaches that of typical quasi-brittle materials, with a more pronounced size effect. In the case of thin statically indeterminate beams and slabs, it is shown that a high level of tensile ductility can allow sufficient internal force redistribution to occur, leading to a significant increase in load-bearing capacity. Moreover, high rotations can be sustained after cracking while almost constant bending strength is maintained, resulting in a plastic-like behaviour. It is demonstrated that the theory of plasticity can thus be applied: a formulation is proposed to predict the resistant plastic moment, enabling easy estimation of the bending failure load for thin elements. The analysis results show good agreement with test results for slabs of different thicknesses. However, due to the remarkable size effect on ductility in bending, the rotation capacity of UHPFRC elements thicker than approximately 100 mm is limited, and the theory of plasticity does not apply. Experimental and theoretical research on the punching-shear failure of thin UHPFRC slabs demonstrates the influence of structural parameters on achieved shear resistance. A proposal is made for considering fibre contribution in shear resistance as a structure-dependent parameter, relating the critical shear crack opening to slab rotation. This approach results in more accurate predictions for thin elements with larger deformations as compared to current code predictions that overestimate resistances for such elements. With a view to the structural application of UHPFRC in bridge design, the concept of ribbed deck slab is studied. Based on the theoretical and experimental results it is demonstrated that thin UHPFRC slabs (40-60 mm) without ordinary reinforcement can be effectively used in this concept: sufficient bending and punching-shear resistances to locally applied traffic loads can be ensured. With prestressed ribs, UHPFRC ribbed slabs attain high load-bearing capacity, while structural dead weight is significantly decreased. This concept could open up new vistas in the design of new structures and offers effective possibilities for the structural repair or widening of existing bridges.

Poinçonnement des planchers-dalles avec colonnes superposées fortement sollicitées

Roberto Guidotti

Flat slabs are commonly used in buildings due to their easiness of construction and economy. In order to keep these advantages, columns are usually not continuous through the slabs in multi-storey buildings. In these cases, the slabs are subjected to large compressive stresses at the support area of the columns, which can exceed the uniaxial compressive strength of the concrete of the slab. This critical zone is in addition subjected to large shear forces and bending moments due to the loads applied on the slab. This leads to a series of potential failure modes: crushing of the concrete of the slab between columns, flexural failures or punching shear failures. Most research has previously focused on the influence of bending of the slab on the column strength. However, no works have provided in-depth investigation of the strength of the slab when large column loads are applied. In this research, an extensive experimental program has shown that the stresses applied at the support area of the columns can be significantly larger than the uniaxial compressive strength of concrete. The test results have clearly shown that no special confinement or load transfer devices are required between columns for most cases (moderate column loads). In addition, two phenomena have been observed. The first one is a reduction on the flexural strength as column loads are applied. The second corresponds to a significant increase on the punching shear strength and deformation capacity with column loading. Existing theoretical approaches for flat slab behaviour and strength are shown not to be directly applicable for slabs subjected to large column loading. In this research, the principles of two general theories (the theory of plasticity and the critical shear crack theory) are thus used to investigate such cases. The theory of plasticity allows calculating a plastic failure envelope accounting for bending and column loading, whereas the critical shear crack theory, which in this work as been further investigated theoretically, is used to derive a failure criterion accounting for punching shear failure in presence of column loading. The results for both theories are finally presented in terms of a single interaction diagram between column loading and slab loading (bending and shear of the slab). The theoretical approaches require however the help of rather refined numerical tools for estimating the strength of a flat slab. In order to use the theoretical approaches for design, a simplified approach has been developed, allowing to calculate the strength as well as the deformation capacity of flat slabs. These tools were implemented in a design method for slab-column joints in multi-storey building. This design approach allows to derive simplified interaction diagrams that can be compared with the loading history of the structural element analysed.

EPFL2010

Thin UHPFRC slabs without conventional reinforcement as light-weight structural elements

Aurelio Muttoni, Dario Redaelli, Ana Spasojevic

Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) is a material characterised by significantly improved mechanical and durability properties compared to other concretes. In recent years, an important research effort has been made to identify appropriate structural forms and to provide practical design rules allowing efficient UHPFRC structures to be conceived. This paper presents the results of an experimental and theoretical study on the structural behaviour of thin UHPFRC slabs without conventional reinforcement. It is shown that these elements can provide high bending resistance, and, applied in an appropriate structural arrangement, lead to strong and light-weight structures that cannot be built with ordinary reinforced concrete. The favourable behaviour of thin UHPFRC members results from material mechanical properties: high compressive strength and significant ductility in tension of thin UHPFRC members lead to a ductile bending response, even when conventional reinforcement is not provided. It is demonstrated that the theory of plasticity can be safely used to assess the bending failure load for thin UHPFRC slabs. A practical expression is proposed to define the resistant plastic moment, allowing an easy estimation of the bending load-bearing capacity of thin statically indeterminate UHPFRC elements. The results are compared with experimental data on slabs of various thicknesses, showing good agreement.

fib Symposium London 20092009

Structural performance of complex core systems for FRP-balsa composite sandwich bridge decks

Michael Osei-Antwi

Based on current fiber-reinforced polymer (FRP) composite construction principles, FRP decks fall into two categories: pultruded decks and sandwich decks. Sandwich decks comprise face sheets and either honeycombs or foams reinforced with internal FRP webs for shear resistance. The honeycomb structure and the webs cause debonding between the upper face sheets and the core due to the uneven support of the former. An alternative material that has high shear capacity and can provide uniform support for the upper face sheet is balsa. Balsa panels have therefore been proposed as the core material for sandwich decks in this research work. Balsa panels are produced by adhesively bonding dissimilar balsa blocks, resulting in a non-homogenous and anisotropic material. These inherent characteristics are not taken into account in the current shear behavior of balsa, thus making it unreliable. Balsa also exhibits high ductility when subjected to compressive loads, however, the shear ductility required by engineers to design safe sandwich structures is lacking. Furthermore, currently existing GFRP-balsa sandwich bridge concepts can only be applied to short-span bridges due to high cost and manufacturing challenges in the case of sandwich slab bridges. In hybrid sandwich deck-steel girder bridges, low bending stiffness in the bridge direction and low composite action in the deck have been the drawbacks. The purpose of this research is to develop novel concepts for lightweight, stiffer and stronger sandwich decks, using balsa cores, which can be fabricated with fewer manufacturing challenges and offer longer spans than existing decks. Balsa panels were experimentally investigated to establish their shear properties and shear ductility at the three orthotropic shear planes. The influence of shear plane, balsa density and adhesive joints on the shear properties was quantified. Two new GFRP-balsa sandwich bridge concepts (complex core systems) have been proposed for long-span bridges. In the first concept, the sandwich core comprises high- and low-density balsa cores and an FRP arch reinforced at the core interface. Sandwich beams based on this concept were experimentally investigated to evaluate their structural performance. The beams demonstrated high bending stiffness and strength and were lightweight. Crack initiation and propagation in the balsa blocks of the complex balsa core could finally be explained. A new analytical model to predict the bending behavior of the new sandwich beams was developed. The second bridge concept involves integrating timber inserts into the balsa core of a sandwich deck. GFRP-balsa sandwich beams, with timber inserts, were numerically investigated to evaluate their structural performance. High stress concentrations occurred in the face sheets and cores at the balsa/timber core joints which were eliminated by changing the core joints from butt to scarf. An optimum angle of termination of scarf joints, based on low stress concentrations at the joints, low costs and manufacturing challenges, was recommended in the design of GFRP-balsa sandwich decks. An existing analytical model for predicting axial stress concentrations in face sheets at butt core joints was extended to scarf joints and a new analytical model was developed to predict shear stress concentrations in the cores at both butt and scarf joints. Finally, the results obtained from the experimental work, the new models and the two proposed sandwich bridge concepts were implemented in the design of new GFRP-balsa sandwich slab bridges and decks. The structural limits of the new bridges were established and the potential of the new GFRP-balsa sandwich deck to replace a reinforced concrete deck was explored. Taking manufacturing limits into account (800-mm slab thickness) and using the proposed complex core system, sandwich bridge slabs of up to approximately 19 m can be constructed. Furthermore, GFRP-balsa sandwich decks bonded on steel girders can reach spans of up to 30 m. The findings of this research work therefore establish that deteriorated reinforced concrete decks with long spans of even up to 30 m can be replaced by GFRP-balsa sandwich decks. Also, based on this work, in the future, existing reinforced concrete decks can be widened using the proposed sandwich decks due to their lightweight, which has been extensively demonstrated. This will lead to cost saving since the bridge substructure will no longer need to be reconstructed.