Cristina Zanotti

Towards sustainable repairs: substrate-repair interface mode-I fracture analysis

An experimental programme for the optimisation of concrete structural and non-structural repair was performed. The programme focused on the development of sustainable durable repairs throughout the enhancement of substrate-repair bond and compatibility, as well as the development of efficient solutions for crack control. As part of the programme, mode-I fracture characterisation of a substrate-repair interface was addressed. The repair material was a mortar reinforced with polyvinyl alcohol (PVA) fibres. Experimental tests, crack growth resistance analysis and major results are presented and discussed in this paper. Mode-I crack growth resistance tests were performed by means of the contoured double cantilever beam (CDCB) test. Tensile fracture analysis was supported by splitting (indirect) tension tests.

Strengthening of a Bridge Pier with HPC: Modeling of Restrained Shrinkage Cracking

After more than fifty years from the opening of the largely discussed “Autostrada del Sole” Highway in 1964, the infrastructure system in Italy appears marked by the passing of time, similarly to what observed in several other countries worldwide. The great heterogeneity of the Italian landscape has determined a great variety of construction types, such as large span concrete bridges over the northern rivers and large arch concrete bridges over the valleys of the central region. Increment of vehicle traffic and new seismic regulations are setting new requirements to adapt the existing infrastructure, which should be otherwise replaced. Moreover, reinforced concrete (RC) aging and deterioration have led to structural and material degradation, including severe cracking and corrosion. Specialized materials such as High Performance Concrete (HPC) could represent a viable convenient solution for repairing, strengthening and retrofitting of RC structures as both structural capacity and durability can be refurbished. However, alongside high mechanical performance, HPC is characterized by a high cracking sensitivity at very early age, due to its high stiffness and shrinkage. Restrained shrinkage cracking, particularly significant in repaired structures where the existing concrete generates a considerable restraint against the free movement of the repair material, may represent a limit to the effective application of these materials. For this reason, shrinkage compatibility of HPC with the existing concrete substrate needs to be experimentally and numerically assessed. A study is herein presented where, based on experimental tests, different numerical models are developed and compared to assess and eventually minimize the risk of shrinkage cracking in bridge piers strengthened with HPC.

Bond Strength of Steel FRC Repairs to Concrete Substrate: Investigation on Adhesion Strength, Friction, and Bond Enhancing Mechanisms

Interventions on reinforced concrete structures are increasingly required to compensate the current infrastructure deficit worldwide. Nonetheless, structural and non-structural repairs have exhibited poor effectiveness due to debonding and overall lack of durability. Fiber Reinforced Concrete (FRC) is drawing increasing interest for concrete repair in light of fibers’ ability to enhance concrete durability. Sporadic studies have also shown the potential of fibers to improve concrete-concrete bond, a crucial repair property. However, the information available so far is limited and more comprehensive investigations are required to characterize concrete-FRC bond. A study is presented on the effect of 13 mm long steel fiber reinforcement added to a repair mortar on its shear bond strength to a concrete substrate. 0%, 0.5% and 1% fiber volume fractions are compared and two substrate treatments are applied, namely: substrate left as cast or sandblasted. Cohesion strength and friction coefficients, two parameters inherently characterizing substrate-repair bond, are assessed by means of Modified Slant Shear Cylinder (MSSC) test with variable bond plane inclination, corresponding to variable interfacial shear-normal stress ratio. The beneficial effect of steel fibers on substrate-repair bond and its correlation to substrate treatment is investigated. Cohesion and friction variations are statistically verified through a permutation technique. For plain repair mortars, results are compared with available predictive models.

Flexural Behavior and Single Fiber-Matrix Bond-Slip Behavior of Macro Fiber Reinforced Fly Ash-Based Geopolymers

A study on flexural response and single fiber-matrix bond-slip behavior in fly ash-based geopolymer reinforced with different types of macro steel fibers is presented. Although direct correlations between bond-slip behavior and composites’ flexural response cannot be drawn, investigation of the fiber-matrix bond-slip behavior allows a fundamental understanding of some of the mechanisms involved in the flexural response. Three types of fibers (straight, end-deformed & length deformed), two curing regimes (ambient & heat) under flexural and single fiber pullout conditions are discussed. The performance of geopolymer composites is compared with Portland cement composites. The quantitative effect of fiber geometry is analyzed by introducing the “fiber deformation ratio”. End deformed steel fiber exhibits a better performance than straight and length-deformed fibers both in flexural and pullout tests. In the single fiber tests, steel fibers perform better for lower fiber deformation ratios, where the full fiber pull-out mechanism can be exploited; for higher deformation ratios, the strong bearing forces developed, combined with the high adhesion strength of the geopolymer-steel fiber interface, lead to more brittle failure mechanisms, such as fiber breakage or matrix failure. These mechanisms are partially mitigated in the composite (group effect), where end-deformed steel fibers exhibit the most ductile flexural response.