Paulo Cachim

Fatigue behavior of fiber-reinforced concrete in compression

Abstract An experimental program has been carried out to evaluate the performance of plain concrete and fiber-reinforced concrete under compressive fatigue loading. Two types of hooked-end steel fibers (30 mm length and 60 mm length) have been tested and their performance compared. The displacements and the acting load were measured during the tests so that several material parameters could be identified and assessed. Wohler diagrams were determined, cyclic creep curves were plotted and the evolution of the secant stiffness was also appraised for the tested materials. The correlation between the secondary creep rate and the fatigue life was investigated. The monotonic stress–strain envelope was compared with fatigue deformations at failure and a good agreement was found between them.

Mechanical properties of cement mortars with superabsorbent polymers

This paper deals with the mechanical properties of mortars with internal curing, by means of water-entrainment with super absorbent particles (SAP). The use of SAP in high performance concrete is focusing on the mitigation of autogenous deformation, arising from self-desiccation. The effect of SAP in mortars subjected to several curing conditions and different water/cement ratio was analysed. The curing conditions ranged between 30 and 100%RH at 20°C temperature. Water/cement ratio ranged between 0.25 and 0.35 for both reference and internal cured mixtures. Tensile and compressive strength tests were performed at different ages. Results include weight loss measurements with time and its relation to environmental conditions and strength.

The mechanical response of turbostratic carbon nanotubes filled with Ga-doped ZnS: I. Data processing for the extraction of the elastic modulus.

The mechanical response of hybrid carbon nanotubes to applied uniaxial compressive forces has been evaluated inside a transmission electron microscope. The initially crooked nanocolumnar materials had an average elastic modulus of 0.53 GPa, measured in situ via a device based on an atomic force microscope cantilever. To extract this property it was necessary to curtail several sources of error (contact sliding, electronic interferences, etc) and develop the methodology herewith outlined. Since the present study was carried out with a commercially available sample holder, these mechanical studies are pertinent to all those working with one-dimensional structures such as nanorods and nanowires.

Glued Composite Timber-Concrete Beams. I: Interlayer Connection Specimen Tests

The composite interaction in timber-concrete beams is usually achieved with mechanical devices, such as shear connectors or plates driven into the materials. However, this connecting system shows some inherent inconveniences, such as the need for drillings and the limited mechanical improvement owing to interface slip. Using adhesives instead of mechanical connectors might attenuate these drawbacks. However, engineers are hesitant to apply adhesives for structural purposes, mostly because of the different thermohygrometric behavior of the adhesive and the connected material(s). This is particularly concerning timber because its hysteretic response to moisture may cause interface delamination. Therefore, as a preliminary stage for the test of composite beams, an extensive experimental campaign on small-size specimens was performed. The study focused on the shear strength at the interface and the role of some factors. Several series were considered by changing those parameters, and the results were compared. Both prefabricated and cast-on-site concrete specimens were considered. The results show that this system looks suitable for structural application, at least under steady dry conditions.

The mechanical response of turbostratic carbon nanotubes filled with Ga-doped ZnS: II. Slenderness ratio and crystalline filling effects

Using a sample holder with an integrated force sensor, a collection of carbon nanotubes filled with Ga-doped ZnS, and spanning a broad window of lengths and diameters, has been mechanically studied inside a transmission electron microscope. The successful evaluation of the filled nanostructures was seen to depend on their slenderness ratio. Upon controlled removal of the encapsulated sulfide, the system considerably changed its response to uniaxial compressive stress. This report follows part 1 of the study which was instrument-focused and laid the ground to achieve consistent results with a novel type of nanomechanics setup for one-dimensional nanostructures (Costa et al, 2009 Nanotechnology. 40:405706).

Copper Slag Mortar Properties

Copper slag is a copper industry waste that is essentially used as an abrasive for metals to remove coatings or paintings, due to its hardness, sieve distribution and rough surface, mainly in the naval industry. After the wearing process, copper slag retains most of its original properties, and remaining an environmental problem. Chemical composition of copper slag presents a high Fe, Si and Al oxide content what makes it a pozzolanic material. Thus, it is possible to use it as a partial replacement for sand and after triturating it into very small particles as a replacement for cement. After a chemical, physical, mineralogical, and environmental characterization of copper slag, its use as a partial replacement of sand on mortars was investigated. The experimental program was developed to evaluate the effect of several percentages of sand substitution in mortars. For mortars 25, 50 and 75 % of sand replacement by volume was studied. A standard 1/3 cement/sand ratio, by weight, with cement type CEM 42.5 I, was used. The type of sand and the effect of washing the slag were also investigated. Strength was measured at 7, 28, 60 and 90 days. Results obtained until now indicate that a reduction of strength occurs when the slag content increases. Additionally, it was observed a delay of the hardening process by the addition of the copper slag. As a conclusion, the use of copper slag as a sand replacement seemed to be a practical option.

Glued Composite Timber-Concrete Beams.II: Analysis and Tests of Beam Specimens

This paper reports an experimental program on composite timber-concrete beams with glued interface. The test results are compared to those provided by an analytical model based on the assumption of a fully composite behavior. Both cast-on-site and prefabricated composite timber-concrete beams were produced, aimed at simulating rehabilitation situations or the possibility of a partial or of a full prefabrication of the composite element. Timber-concrete beams with dowel-type shear connectors were also produced and tested for comparison. The results show that the strength is similar, but the glued beams displayed a larger stiffness and, consequently, smaller deflections, which may be of relevance in design. The results also show that, at least under stabilized and dry room conditions, the prevailing mode of failure is tension in timber and, when shear failure occurs, it is mostly conditioned by the shear strength of the concrete or the timber, not by that of the adhesive. As the system depends on the shear strength of the interface, a preceding program consisting in tests on small-size composite specimens was undertaken and is reported in a companion paper.

Numerical modelling of fibre-reinforced concrete fatigue in bending

A numerical model for the analysis of concrete under fatigue loading is presented. It is based on viscoplasticity theory, replacing time-dependent with cycle-dependent behaviour. The analysis is performed in a two-step procedure. Initially, loads are monotonically increased until they reach their maximum level of the cyclic process, which is done with a standard non-linear analysis. Then, with the loads fixed at the maximum level, the cycles are applied to the structure and a cycle-dependent analysis is executed. Some examples that illustrate the model behaviour are presented.

Fire resistance tests on timber beam-to-column shear connections

Purpose This paper aims to present the results of an extensive experimental programme on the fire behaviour of timber beam-to-column shear connections, loaded perpendicularly to the grain. Design/methodology/approach The experimental programme comprised tests at normal temperature and loaded fire resistance tests on beam-to-column connections in shear. Twenty-four full-scale tests at normal temperature were performed covering nine different connection typologies, and 19 loaded fire resistance tests were conducted including 11 connections typologies. Findings The results of the fire resistance tests show that the tested typologies of steel-to-timber dowelled connections reached more than 30 and even 60 minutes of fire resistance. However, aspects such as a wider gap between the beam and the column, reduced dowel spacing, and the presence of reinforcement with self-drilling screws all have a negative influence on the fire resistance. Originality/value The experimental programme addressed the fire behaviour of timber beam-to-column shear connections loaded perpendicularly to the grain in a systematic way testing a wide range of common connection typologies significantly enlarging their experimental background.

O papel do metacaulim na proteção dos concretos contra a ação deletéria de cloretos

The objective of this study is to evaluate the protective capacity of concretes produced with metakaolin in relation to the transportation and penetration of chlorides. Thus, from a commercial concrete of fck equal to 30 MPa, more two other concretes were produced by replacing 10% of cement by metakaolin, by weight. In one of them, it was kept the same water/binder of the initial reference mix design (w/b = 0.60), and in the other concrete the compressive strength remained fixed. In all three mixes, the same range of concrete consistency was maintained, with a slump equal to (100 + 10) mm. The front of chlorides in the cement matrix was evaluated by spraying a solution of silver nitrate, after an attack of 8 weekly cycles of wetting and drying using a solution containing chlorides. To obtain an indicative of the internal structure of the concretes, it was carried out the test of water absorption by immersion, which permitted an evaluation of the concrete open porosity, as well as it was performed the analysis of concrete samples by means of XRD and SEM. These studies aimed to verify the potential of metakaolin in fixing chlorides in the form of Friedels salt, besides providing microstructural analysis of the concretes. It was concluded with this work that the incorporation of metakaolin decreases the diffusivity of chlorides to the extent that this mineral addition produces refinement of the concrete pore structure and also because it induces the formation of Friedels salt, which becomes it an effective agent in preventing the corrosion of reinforcement in chloride-rich environments.