Peter Marti

Tension chord model for structural concrete

Based on a simple tension chord model, problems of cracking, minimum reinforcement, tension stiffening and deformation capacity of structural concrete members are treated in a unified manner. Key f...

SIZE EFFECT IN DOUBLE-PUNCH TESTS ON CONCRETE CYLINDERS

Results of two series of experiments with a total of 42 double-punch tests on plain concrete cylinders are reported. The specimen size, which was the only tests variable, ranged from 8 to 128 times the maximum aggregate size. The observed size effect is in good agreement with Bazants nonlinear fracture mechanics approach.

DESIGN OF CONCRETE SLABS FOR TRANSVESRSE SHEAR

To supplement conventional design methods, this paper reviews mechanisms of transverse shear transfer in reinforced concrete slabs. A truss-model-based design procedure is developed for transversely reinforced slabs, and its application is illustrated for the case of a large transfer plate in a high-rise building.

HARMONIZED TEST PROCEDURES FOR STEEL FIBER-REINFORCED CONCRETE

A circular slab test method is described as an alternative to modulus of rupture and square slab tests for steel fiber-reinforced concrete. Results of 20 modulus of rupture, 12 square slab, and 24 circular slab tests are compared based on a general theoretical approach that accounts for the random fiber distribution and the successive softening by fiber pullout. Existing requirements for modulus of rupture and square slab tests are reviewed, and harmonized procedures for these as well as the circular slab tests are proposed. It is suggested that an effective flexural tensile strength and a fracture energy parameter be used to characterize the strength and toughness of steel fiber-reinforced concrete. Furthermore, an acceptance criterion is introduced, aimed at excluding materials with drastic softening.

Strength of Masonry Subjected to Combined Actions

A sandwich model is presented to predict the strength of masonry wall elements subjected to combined in-plane forces and moments. The validity of the model is demonstrated by comparisons with eighteen large-scale tests on clay brick and calcium-silicate block walls. While the experimental verification has been limited to clay brick and calcium-silicate block masonry the sandwich model is also applicable to concrete block masonry.

BENDING OF STAY CABLES

The bending stresses in stay cables subjected to varying axial forces and imposed end rotations are investigated. It is shown that these stresses can be effectively reduced by a suitable rotation of the anchorages with respect to the chord lines and by guiding the cables at their end, providing a sufficiently stiff support at a certain distance from the anchorage. Recommendations for appropriate guide lengths and stiffness are given.

HOW TO TREAT SHEAR IN STRUCTURAL CONCRETE

Recent developments of strut-and-tie model, compression field, and limit analysis approaches are reviewed. It is shown how the different methods supplement each other, permitting a consistent dimensioning and detailing of new, and a rational evaluation of existing, structural concrete members. It is demonstrated that shear forces provide the key to understanding the flow of the internal forces both in beam and frame and in plate and shell structures. While it is emphasized that strut-and-tie models and stress field considerations should be part of the tool box of every designer, it is pointed out that deformation and failure mechanism considerations are particularly useful for the evaluation of existing structures. A brief presentation of a newly developed tension chord model is included. Based on this model, a variety of related subjects (bond, minimum reinforcement, and deformation capacity) can be treated in a unified manner.

Reinforced Concrete Slab Shear Prediction Competition: Experiments

This paper describes 28 large-scale tests that were conducted to investigate the shear strength and deformation capacity of orthogonally reinforced concrete slabs. Test parameters included the slab thickness, the in-plane and the transverse reinforcement ratios, and the deviation of the principal shear (and moment) direction from the direction of the in-plane reinforcement. Eight of the 28 tests were used for an international competition to predict the expected load-deformation response. Information on the test concept, the test specimens, and the test procedures is given. Findings show that with a sufficient transverse reinforcement, ductile flexural failures instead of brittle shear failures occurred. Contrary to the specimens without transverse reinforcement, the shear strength of thick slabs was not reduced compared to thin slabs; that is, no size effect was observed for the specimens with transverse reinforcement. Results also indicate that a deviation of 45 degrees of the principal moment direction from the reinforcement direction resulted in a significant decrease of the cracked slab stiffness of the orthogonally reinforced concrete slabs. The tests with transverse reinforcement showed no reduction in strength, while the tests without transverse reinforcement did show a strength reduction.

Effect of Fabric Webs on the Static Response of Spindle-Shaped Tensairity Columns

Tensairity is a lightweight structural concept comprising struts and cables stabilized by a textile membrane which is inflated by low pressurized air. This paper addresses the effect of fabric webs inside the membrane hull on the static response of spindle-shaped Tensairity columns to axial compression. Two full-scale spindle-shaped columns, one without and one with webs, were fabricated and tested. The columns were subjected to axial compressive loading for various levels of internal air pressure to quantify its effect on the global structural response. It was found that the stiffness and the load bearing capacity for both columns increased with increasing air pressure. The experimental results also revealed the benefits of including fabric webs in the spindle configuration in terms of axial stiffness and buckling load. Comparisons with an analytical solution and finite-element predictions showed good correlation for the axial stiffness in the case without webs. For the case with web deviations between predicted and experimental results indicated that structural detailing and imperfections in the manufacturing process strongly influence the performance of Tensairity columns with internal webs.

Temporary Corrosion Protection and Bond of Prestressing Steel

This study investigates the performance of different temporary corrosion protection methods for prestressing steel. Four types of laboratory corrosion tests with three emulsifiable oil products in three concentrations resulted in the selection of one product that showed by far the best corrosion protection behavior. Field tests during the winter on 16 transverse deck post-tensioning tendons of 2 road bridges demonstrate the practical suitability of the selected product. Using this product, a pullout test with a long embedment length was performed on a post-tensioned seven-strand tendon with a plastic duct and compared with a reference test using untreated strands. A bond shear stress reduction by a factor of approximately 2.5 was observed compared with the untreated strands. This reduction does not appear to significantly influence the load-deformation response of post-tensioned concrete members and suggests that the emulsifiable oil does not need to be removed before grouting of the tendons. However, since the bond reduction caused by emulsifiable oils in grouted tendons may lead to a somewhat soft load-deformation response of post-tensioned concrete members in the decompressed state, a lower-bound estimate of the corresponding stiffness can be obtained if necessary by assuming a completely unbonded behavior of the post-tensioning tendons.