Ktisis Collection: Άρθρα/Articles

Is concrete a poromechanics material? - a multiscale investigation of poroelastic properties

Wed, 29 Oct 2003 22:58:59 GMT

Title: Is concrete a poromechanics material? - a multiscale investigation of poroelastic properties

Authors: Constantinides, Georgios; Ulm, Franz Josef; Heukamp, Franz H.

Abstract: There is an ongoing debate, in Concrete Science and Engineering, whether cementitious materials can be viewed as poromechanics materials in the sense of the porous media theory. The reason for this debate is that a main part of the porosity of these materials manifests itself at a scale where the water phase cannot be considered as a bulk water phase, but as structural water; in contrast to water in the gel porosity and the capillary porosity. The focus of this paper is two-fold: (1) to review the microstructure of cementitious materials in the light of microporomechanics theory by starting at the scale where physical chemistry meets mechanics, and which became recently accessible to mechanical testing (nanoindentation); (2) to provide estimates of the poroelastic properties (drained and undrained stiffness, Biot coefficient, Biot modulus, Skempton coefficient) of cementitious materials (cement paste, mortar and concrete) by means of advanced homogenization techniques of microporomechanics. This combined experimental-theoretical microporomechanics approach allows us to deliver a blueprint of the elementary poroelastic properties of all cementitious materials, which do not change from one cementitious material to another, but which are intrinsic properties. These properties result from the intrinsic gel porosity of low density and high density C-S-H, which yield a base Biot coefficient of 0.61 < b ≤ 0.71 and a Skempton coefficient of B = 0.20 - 0.25. While the base Biot coefficient decreases gradually at larger scales, because of the addition of non-porous solid phases (Portlandite, ..., aggregates), it is shown that the Skempton coefficient is almost constant over 3-5 orders of magnitude.

A biomechanical analysis of articular cartilage of the human elbow and a potential relationship to osteochondritis dissecans

Fri, 29 Oct 1993 22:58:59 GMT

Title: A biomechanical analysis of articular cartilage of the human elbow and a potential relationship to osteochondritis dissecans

Authors: Constantinides, Georgios; Athanasiou, Kyriacos A.; Schenck, Robert C

Abstract: A cadaveric study of the articular surfaces of the radiocapitellar joint was undertaken to identify a biomechanical mechanism of osteochondritis dissecans of the humeral capitellum. The articulating radial head and capitellum of fresh anatomic specimens were dissected and tested for intrinsic mechanical properties. Significant differences exist in the mechanical properties and thickness of cartilage topographically in the capitellum and radial head, and between the two surfaces. The medial portion of the radial head is the softest of all radiocapitellar osteochondral segments tested. Comparing medial to lateral sites of the capitellum, there is a trend of decreased stiffness; lateral segments are softer than medial ones. The central section of the radial head is significantly stiffer than the lateral capitellum. There is no significant difference between the stiffness of the radial head sites and the medial capitellum. The disparity in the mechanical properties of the central radial head and lateral capitellum would increase strain in the lateral capitellum. During high- valgus stress activities such as throwing, this increased strain may be a factor in the initiation and localization of the dissecans lesion observed in osteochondritis dissecans of the elbow.

On the use of nanoindentation for cementitious materials

Tue, 29 Oct 2002 22:58:59 GMT

Title: On the use of nanoindentation for cementitious materials

Authors: Constantinides, Georgios; Ulm, Franz Josef; Van Vliet, Krystyn J.

Abstract: Recent progress in experimental and theoretical nanomechanics opens new venues in materials science for the nano-engineering of cement-based composites. In particular, as new experimental techniques such as nanoindentation provide unprecedented access to micro-mechanical properties of materials, it becomes possible to identify the mechanical effects of the elementary chemical components of cement-based materials at the scale where physical chemistry meets mechanics, including the properties of the four clinker phases, of portlandite, and of the C-S-H gel. In this paper, we review some recent results obtained by nanoindentation, which reveal that the C-S-H gel exists "mechanically" in two different forms, a low-density form and a high-density form, which have different mean stiffness and hardness values and different volume fractions. While the volume fractions of the two phases depend on mix proportions, the mean stiffness and hardness values do not change from one cement-based material to another; instead they are intrinsic properties of the C-S-H gel.

Biomechanical properties of hip cartilage in experimental animal models

Sat, 29 Oct 1994 22:58:59 GMT

Title: Biomechanical properties of hip cartilage in experimental animal models

Authors: Constantinides, Georgios; Athanasiou, Kyriacos A.; Agarwal, Animesh

Abstract: The material properties of normal adult articular cartilage were determined in the femoral head and acetabulum of baboons, dogs, and bovines, and were compared with those of normal human hip cartilage. In situ creep and recovery indentation experiments were performed using an automated creep indentation apparatus. To curvefit the entire creep curve, a numerical algorithm based on biphasic finite element methods and nonlinear optimization was developed. This effort represents the first successful use of 100% of the creep indentation curve to obtain the mechanical properties of normal articular cartilage. The results show that material properties of articular cartilage exhibit significant topographical variations in the femoral head and acetabulum, and between these two bone structures. Furthermore, significant differences exist in the mechanical properties of hip cartilage among the 4 species. Specifically, in all species the smallest aggregate modulus is found in the inferior aspect of the femoral head. Among all species, human hip cartilage is the stiffest in all test sites; bovine tissue is the softest. Human tissue has the smallest Poisson's ratio and permeability in all test sites. The aggregate modulus of human hip cartilage is closely resembled by that of baboon hip cartilage. Anatomically, canine and baboon hips exhibit similar characteristics to the human hip joint; the bovine hip joint is distinctly different. Based on this study's data, the baboon represents the most appropriate animal model of normal human hip articular cartilage.