Abstract
The effects of a surface nano-silica treatment, carried out with the sol gel method, on the post-cracking behaviour of polypropylene macro-synthetic fibre reinforced concrete are experimentally investigated here for the first time. The present study extends previous experimental and analytical investigations on the corresponding improvement of the bonding properties of a single synthetic macro fibre, performed by means of pull-out test. Scanning electron microscopy is adopted here to explore the changes in the morphological characteristics of polypropylene macro synthetic fibres, before and after mixing in the concrete matrix. A comparative analysis, carried out with three-point bending tests on notched beam specimens, is used to evaluate the effects of the nano-silica treatment on the concrete post cracking behaviour. Increase in concrete toughness and residual post-cracking strength is recorded due to improved adhesion between fibres and the concrete matrix and to the consequent increase in the frictional shear stress generated during the fibre pull-out, especially for large crack opening. As shown by the SEM images, the nano-treatment favours the bonding of the concrete hydration products to the surface of the treated fibres, thus ensuring strengthening of the interface transition zone. In addition, the links between the nano-silica coating and the concrete hydration products improve the frictional shear stress and thus the overall energy absorption, as denoted by the increase of the residual strength during the post-cracking phase.
Highlights
Concrete is one of the most widespread materials used for buildings and constructions because of its many benefits in terms of high performance, economic purposes and versatility
The strengthening effects observed in the pull-out response of a single treated fibre may only partially extend to the macroscopic flexural behaviour of macro synthetic fibre reinforced concrete (MSFRC) [26] depending on fibre dosage and other random factors, such as the fibres distribution and orientation [27,28,29], which may reduce the overall effects of the advantages observed in the pull-out of a single treated fibre
The enhancement of the post cracking behaviour of the MSFRC by the use of the nano-silica treatment can be explained by a local modification of the Interface Transition Zone (ITZ) depending on the products bonded to the surface of the treated fibres, which can be observed from the images here reported
Summary
Concrete is one of the most widespread materials used for buildings and constructions because of its many benefits in terms of high performance, economic purposes and versatility. As reported in some experimental works [22], the mechanical properties of fibrous reinforced cementitious composites can be improved by increasing the local strength and micro hardness of the ITZ. This purpose can be achieved by reducing the porosity of the ITZ, by adding nano-fillers or polymers to the concrete mixture [23,24] or by performing chemical treatments on the outer surface of the fibre [25] that increase the functionality and the specific contact surface area, in order to produce an anchoring effect. The strengthening effects observed in the pull-out response of a single treated fibre may only partially extend to the macroscopic flexural behaviour of macro synthetic fibre reinforced concrete (MSFRC) [26] depending on fibre dosage and other random factors, such as the fibres distribution and orientation [27,28,29], which may reduce the overall effects of the advantages observed in the pull-out of a single treated fibre
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