Creep and Fracture in Concrete: A Fractional Order Rate Approach

The paper analyses the interaction between strain-softening and time-dependent behaviour in the case of quasi-static fracture of concrete. A viscous element based on a fractional order rate law is coupled with a micromecanical model for the fracture process zone. This approach makes it possible to include a whole range of dissipative mechanisms in a single rheological element. Creep fracture in mode I conditions is analysed through the finite element method, the cohesive (or fictitious) crack model and a new space and time integration scheme. The comparison with creep tests executed on three-point bending conditions shows a good agreement

Enhanced concrete crack closure with hybrid shape memory polymer tendons

YesThe paper presents a new healing system that uses pre-tensioned hybrid tendons to close cracks in cementitious structural elements. The tendons comprise an inner core, formed from aramid fibre ropes, and an outer sleeve made from a shape memory PET. During the manufacturing process, the inner core of a tendon is put into tension and the outer sleeve into compression, such that the tendon is in equilibrium. A set of tendons are then cast in a cementitious structural element and heat activated once cracking occurs. This triggers the shrinkage potential of the PET sleeve, which in turn releases the stored strain energy in the inner core. The tensile force thereby released applies a compressive force to the cementitious element, in which the tendons are embedded, that acts to close any cracks that have formed perpendicular to the axis of the tendons. Details of the component materials used to form the tendon are given along with the tendon manufacturing process. A set of experiments are then reported that explore the performance of three different tendon configurations in prismatic mortar beams. The results from these experiments show that the tendons can completely close 0.3 mm cracks in the mortar beams and act as effective reinforcement both before and after activation. A nonlinear hinge-based numerical model is also described, which is shown to be able to reproduce the experimental behaviour with reasonable accuracy. The model is used to help interpret the results of the experiments and, in particular, to explore the effects of slip at the tendon anchorages and the amount of prestress force that remains after activation. It is shown that, with two of the tendon configurations tested, over 75% of the prestress potential of the tendon remains after crack closure.UK-EPSRC (Grant No. EP/P02081X/1, Resilient Materials 4 Life, RM4L)

Automated rib location and optimization for plate structures

For a given loading, the stiffness of a plate or shell structure can be increased significantly by the addition of ribs or stiffeners. Hitherto, the optimization techniques are mainly on the sizing of the ribs. The more important issue of identifying the optimum location of the ribs has received little attention. In this investigation, a methodology has been developed for the automatic determination of the optimum locations of the ribs for a given set of design constraints

Uncertainty on application of bi-linear approximation to tension softening material near creep rupture region

Tension softening and time-dependence of fracture are coupled phenomena and these have to be taken into account in order to identify the realistic behaviour of concrete. Tension softening behaviour of concrete is generally approximated to bi-linear diagram for simplicity. A thorough study is made of the applicability of this commonly used bi-linear approximation for the static tension-softening response of concrete to predicting its time-dependent response. It is found that the bi-linear approximation predicts a totally wrong creep response in the tertiary region close to creep rupture. The reason for this severe inadequacy is discussed