Concrete Recycling

Research and Practice

Francois de Larrard, Horacio Colina, Francois de Larrard, Horacio Colina

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781351052801
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

François de Larrard is Scientific Director of the R&D centre of the LafargeHolcim group and Scientific Director of the French national project Recybéton. He formerly spent almost thirty years at IFSTTAR (formerly LCPC). He has been granted both the Robert l'Hermite medal and the G.H. Tattersall award by RILEM, and is author of two books, including Concrete Mixture-Proportioning which is also published by Taylor & Francis.

Section IV

Properties of hardened recycled concrete

P. Rougeau

CERIB

Previous sections have presented the peculiarities of recycled aggregate (RA), especially their microstructure and porosity. Their intrinsic characteristics are determined by the nature of natural aggregate (NA) and the amount and characteristics of the old cementitious paste. Recycled aggregate concrete (RAC) can have specific hardened properties depending on the nature and substitution rate of RA.

The questions investigated in this section for each hardened property are as follows:

•Does the RA play a specific role?

•What are the mechanisms involved?

•Are the usual tests and methodologies adapted to RAC?

•What are the required conditions to produce RAC with similar properties than NAC?

This section is subdivided into five chapters:

•The first one (Chapter 9) concerns the microstructure of RAC with a focus of the interfacial transition zone between the old and the new cement paste in RAC and the transport of water from the oversaturated RA to the cement paste.

•The second one (Chapter 10) describes the influence of the introduction of RA on the instantaneous properties of concrete. The relations between compressive and tensile strength and elastic modulus considering the substitution rate of RA are investigated.

•The third one (Chapter 11) concerns delayed deformations and fatigue properties, particularly shrinkage and creep deformations.

•The fourth one (Chapter 12) deals with durability-related properties: properties related to the corrosion risks of the reinforcement (carbonation, chloride migration, gas permeability, porosity), the resistance to freeze/thaw cycles, the risks linked to alkali/silica reactions and to the presence of sulfates (ettringite/thaumasite formation).

•Finally, the fifth one (Chapter 13) deals with the behavior under fire. The spalling of RAC and the fire resistance of two beams are studied.

Chapter 9

Microstructure of recycled concrete

E. Garcia-Diaz and G. Le Saout

Centre des Matériaux des Mines d’Alès, IMT Mines Ales

A. Djerbi

IFSTTAR, Marne-la-Vallée

CONTENTS

9.1Introduction

9.2State of the art

9.3RECYBÉTON’s outputs

9.3.1Mortars’ microstructure

9.3.1.1Materials and method

9.3.1.2Results

9.3.2Concrete microstructure

9.3.2.1Materials and method

9.3.2.2Results

9.4Research needs

9.5Conclusion

Abstract

Interfacial Transition Zone (ITZ) between old and new cement paste in recycled mortars and concretes is characterized. The microstructure is investigated during the first phase of hardening (2 and 28 days) for mortars and up to 1 year for concretes. Mortars and concretes made with over-saturated fine and coarse recycled aggregates develop more porous ITZ with lower anhydrous profiles. Transport of water from the over-saturated recycled aggregates to the cement paste, and water lens formation by micro-bleeding effect, are expected to explain this phenomenon. On the other hand, mortars made with dried fine recycled aggregates develop “denser” ITZ with anhydrous profile similar to that observed on mortars made with dried natural fine aggregates with the same targeted average W/C ratio. Transport of portlandite from the new cement paste to the old cement paste is observed during the first 28 days of hardening. This phenomenon is stronger for mortars made with over-saturated fine aggregates. The reduction of W/C ratio to obtain C25, C35 and C45 recycled concretes improves slightly the porosity profiles. But concretes based on 100% of recycled coarse aggregates have higher porous ITZ than concretes based on 30% of recycled fine and coarse aggregates. The low porosity of the ITZ of the C45 concrete based on 30% of fine and coarse recycled aggregates could be the consequence of a positive curing effect, with a contribution of the recycled aggregate water to cement hydration because of the partial desaturation during the hardening of the cement paste with a low initial water cement ratio (W/C close to 0.41). A treshold value for paste porosity which separates porous networks wealkly interconnected (low increase of permability with porosity) and porous network very interconnected (high increase of permeability with porosity) has been identified. The value is closed to 17%–18% of porosity.

9.1Introduction

Recycled aggregates (RAs) are composed of a mixture of natural aggregates (NAs) roughly coated with hardened cement paste or mortar. The presence of this carbonated and porous old paste could modify mechanisms governing the formation of interfacial transition zone (ITZ) between aggregate and cement paste. The consequence could be a specific microstructure formation between the old and new cement paste. The objective of this chapter is the first to characterize by scanning electron microscopy (SEM) the interphase area between the old and new paste by semiquantitative measurement on mortars and concretes. Dried and oversaturated aggregates have been used to appreciate the effect of water transfer between RA and new cement paste on this ITZ. Then, the porosity of the concrete microstructures is correlated with intrinsic gas permeability to identify the area of weakly interconnected network improving durability.

9.2State of the Art

The ITZ is a cement paste zone between the aggregate and the homogeneous bulk paste. Several authors (Barnes et al. 1978; Crumbie 1994; Kjellsen et al. 1998; Langton and Roy 1980; Monteiro et al. 1985; Olivier et al. 1995; Scrivener et al. 1988; Scrivener and Gartner 1987; Scrivener and Pratt 1996; Zimbelman 1985) have studied in detail the microstructure of the ITZ and its forming mechanisms. This zone is characterized by a microstructure gradient: the porosity increases in the ITZ from the bulk cement paste to the aggregate surface. The microstructure gradient is mainly the consequence of the “wall effect” exerted by the coarse aggregate on the fine cement particles. Because of this “wall effect”, the initial cement content decreases and the water content increases from the bulk cement paste to the coarse aggregate surface. The range of order of the depth of the disturbed area by the “wall effect” is approximately of several tenths microns and corresponds to the initial depth of the ITZ. During the hardening and because of a filling of the porosity by the hydrated products, the depth of the ITZ generally decreases. At the same time, a mechanism of transpor...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Committee
• Foreword
• Acknowledgements
• Editors
• Contributors
• Introduction
• SECTION I Recycled aggregates
• SECTION II Binder incorporating recycled concrete fraction
• SECTION III Recycled concrete from production to hardening stages
• SECTION IV Properties of hardened recycled concrete
• SECTION V Mix design of recycled concrete
• SECTION VI Reinforced recycled concrete
• SECTION VII Experimental construction sites
• SECTION VII Sustainability of concrete recycling
• SECTION IX Implementation of concrete recycling
• Conclusion
• Appendix
• References
• Index