Marco Pepe

A Conceptual Model for Designing Recycled Aggregate Concrete for Structural Applications

This Thesis proposes a conceptual formulation for controlling the resulting mechanical properties of Recycled Aggregate Concretes (RACs) via generalised mix-design rules intended at covering the specific features of Recycled Concrete Aggregates (RCAs). As a matter of fact, the RCAs are characterised by a higher porosity and water absorption capacity than ordinary aggregates and, hence, general mix-design rules for ordinary structural concrete cannot be applied to RACs as such. Therefore, as a further step that goes beyond the currently available experimental and empirical investigations, the research presented in the present thesis proposes a rational mix design method for predicting the compressive strength evolution of RACs by considering both the mixture composition and the key properties of RCAs (i.e.; the amount of the attached mortar and indirectly the water absorption capacity). The formulations proposed in this thesis are based upon the results of several investigations carried out for characterising both RCAs and RACs. In fact, several tests were performed on different kinds of recycled aggregates, taking into account their origin, the processing procedure adopted for crushing the concrete demolition debris and their resulting size grading. The analysis of the results obtained in these tests led to proposing a comprehensive conceptual formulation that links the main engineering properties of aggregates to the porosity, particle density and attached mortar content of RCAs. Moreover, several concrete batches were produced for investigating the influence that three key parameters (viz. aggregate replacement ratio, nominal water-to-cement ratio and initial moisture condition of aggregates) have on the relevant properties for structural concrete. The behaviour of structural concrete is then analysed at a “fundamental” level, with the aim of scrutinising the physical properties and the mechanical behaviour of RACs, by taking into account the actual mixtures composition. Particularly, it is based on observing the hydration process developing inside RACs during the setting and hardening phase, and how these are influenced by the presence of RCAs. The activities described in this work were carried out at the laboratories of the University of Salerno (IT), the Federal University of Rio de Janeiro (BR), as part of the “EnCoRe” Project (FP7PEOPLE-2011-IRSES n. 295283; www.encorefp7.unisa.it) funded by the European Union within the Seventh Framework Programme, and Delft University of Technology (NL).

Recycled Aggregate Concretes

This chapter aims at reporting the main information about the mechanical properties of concrete made with recycled concrete aggregates. The first section proposes an overview about concrete technology and summarises well known rules and practices currently adopted in the field of concrete technology. The second section proposes a summary of the State-of-the-Art knowledge about RAC. Finally, all the applied research methodology and the performed experimental activities are described.

Concrete Industry: Waste Generation and Environmental Concerns

In the second half of the last century, the world production of goods and services increased disproportionately, due to a strong demographic growth, expansion of urban areas and a great development of industrialization. Apart from undoubted benefits in terms of improved living standards, this phenomenon also resulted in an growing pressure on natural balances and resources of the Earth. However, the above mentioned process led to a remarkable increase in waste generation. This chapter introduces the main issues related to the construction and demolition waste with a particular focus on the concrete waste generated by the demolition.

Rheological Behavior at Fresh State of Structural Recycled Aggregate Concrete

At the end of their service life, concrete members can be demolished and reused as alternative constituents in the production of new concrete mixtures. This process leads to producing a sustainable construction material often referred to as Recycled Aggregates Concrete (RAC), in which ordinary aggregates are (partially or totally) replaced by Recycled Concrete Aggregates (RCAs). However, due to the intrinsic characteristics of RCAs (i.e., higher porosity in comparison with ordinary aggregates) the definition of reliable relationships, capable of predicting the relevant properties of RACs, is still considered an open issue. Moreover, in last years, the scientific community mainly focused on understanding the mechanical behavior of RAC at hardened state and only few studies addressed the properties at the fresh state.

Tension stiffening behavior of self compacting high strength fiber reinforced concrete incorporating river gravels

This study proposes a comprehensive analysis on the structural performance of reinforced Self Compacting High Strength Fiber Reinforced Concrete members. Particularly, it summarizes the results of an experimental investigation aimed at analyzing the tension stiffening behavior of concrete mixtures produced with different kinds of natural aggregates: crushed granite and two types of amazon river gravels (derived from two different Brazilian regions). The mixtures were proportioned in order to achieve 60 MPa of compressive strength and, moreover, beyond three plain concrete mixtures, 35 mm length steel fibers were added to these mixtures, as spread reinforcing element, in volumetric fractions equal to 0.5% and 1.0%. The results derived from this type of test furnish a comprehensive analysis on the crack formation and propagation on concrete elements as well as distributed cracking mechanisms. The results indicate that the adherence between the steel bar and the concrete is not affected by the presence of the river gravels and that the fiber reinforcement controls crack width leading to a great number of cracks and, this effect is more pronounced for higher volume fractions.

Concrete with Recycled Aggregates: Experimental Investigations

The mechanical behaviour of Recycled Aggregate Concrete (RAC) is investigated by reporting the main results of experimental tests intended at understanding the influence of Recycled Concrete Aggregates (RCAs) on the resulting mechanical properties of concrete. The focus is placed on the higher porosity of RCAs and their higher water absorption capacity. Consequently, the role of the initial moisture conditions of RCAs at mixing is also unveiled and its consequences on both the hydration reaction and the time evolution of compressive strength are highlighted. The influence of processing procedures intended at reducing the aforementioned porosity is also discussed.

Cement Replacement: Experimental Results for Concrete with Recycled Aggregates and Fly-Ash

The experimental activity reported in this chapter was aimed at enhancing the knowledge about the mechanical behaviour and durability of concretes made with Recycled Concrete Aggregates (RCAs) and coal Fly Ash (FA) and their possible use for structural purposes. To this end, starting from a reference concrete composition, twelve mixtures were designed by replacing part of the ordinary constituents (i.e. cement, sand and coarse aggregates) with the FA and RCAs. The time evolution of the compressive strength, as well as the splitting strength, were measured with the aim to monitor the mechanical performance, whereas the durability performance was scrutinised by measuring water permeability, carbonation depth and chloride-ions ingress. The obtained results unveil the influence of both RCAs and FA on the resulting concrete performance and highlight that their combined use can lead to a synergistic effect in terms of the relevant physical and mechanical properties of structural concrete.

State of Knowledge on Green Concrete with Recycled Aggregates and Cement Replacement

Since the construction industry is characterized by a huge demand for both energy and raw materials, it is fully concerned by the need for enhancing sustainability, which is certainly the main challenge for all industrial sectors in the twenty-first century. Therefore, several solutions are nowadays under investigation to reduce the environmental impact of concrete production. They often consist of partially replacing the ordinary concrete constituents with recycled ones, in view of the objective of reducing both the demand of raw materials and the amount of waste to be disposed in landfills. The most recent advances in this field are summarized in this chapter, which is intended at drawing the line of the current state of knowledge on “sustainable” structural concrete.

Generalised Mix Design Rules for Concrete with Recycled Aggregates

A conceptual formulation for controlling the resulting mechanical properties of Recycled Aggregate Concretes (RACs) is proposed via a set of generalised mix-design rules intended at covering the specific features of Recycled Concrete Aggregates (RCAs). As a matter of fact‚ the RCAs are characterised by a higher porosity and water absorption capacity than ordinary aggregates and‚ thus‚ general mix-design rules for ordinary structural concrete cannot be applied to RACs as such. Therefore‚ the formulations proposed herein are intended at generalising those rules taking into account the key properties of RCAs‚ as they are possibly influenced by the alternative processing procedures‚ which can be applied when turning demolition debris into concrete aggregates. Particularly‚ these formulations aim at predicting both the final value and the time evolution of compressive strength of RACs depending on their production procedure and mixture composition. The proposed formulations are calibrated and validated on the results of various experimental campaigns covering the effect of several aspects and parameters‚ such as the processing procedures‚ the source for RCAs‚ the actual aggregate replacement ratio‚ the water-to-cement ratio‚ the water absorption capacity and the initial moisture condition of coarse recycled aggregates. Design charts of the proposed formulations show ease of the method as well as the potential of employing this rational design method for RAC.

Inverse Identification of the Bond-Slip Law for Sisal Fibers in High-Performance Cementitious Matrices

The use of Natural Fibers (NFs) in Fiber-Reinforced Cementitious Composites (FRCCs) is an innovative technical solution, which has been recently employed also in High-Performance FRCCs. However, NFs are generally characterized by complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. This paper presents the results of pull-out tests carried out on sisal fibers embedded in a cementitious mortar. These results are considered for identifying the bond-slip law that describes the interaction between the sisal fibers and the cementitious matrix. A theoretical model, capable of simulating the various stages of a pull-out test, is employed as part of an inverse identification procedure of the bond-slip law. The accuracy of the resulting simulations demonstrates the soundness of the proposed theoretical model for sisal fibers embedded in a cementitious matrix.