Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach

Abstract

Owing to the shortage of space for land reclamation in Hong Kong, it is difficult to dispose of tons of masonry waste generated daily from construction activities. Adoption of recycled aggregate from concrete waste thus becomes a burning issue. The Hong Kong SAR Government has set up a recycling plant in Tuen Mun Area 38 aiming at turning concrete waste into recycled aggregate with a practice note and specifications issued for controlling the quality of recycled aggregate. However, the use of recycled aggregate concrete to high grade applications is rarely reported because of its poorer compressive strength and high variability in mechanical behavior. This paper proposes a new approach in mixing concrete, namely "two-stage mixing approach (TSMA)", intended to improve the compressive strength for recycled aggregate concrete and hence lower its strength variability. Based upon experimental works, improvements in strength to recycled aggregate concrete were achieved. The effect can be attributable to the porous nature of the recycled aggregate and hence the pre-mix process can fill up some pores and cracks, resulting in a denser concrete, an improved interfacial zone around recycled aggregate and thus a higher strength when compared with the traditional mixing approach.

Enhancement of Recycled Aggregate Concrete Properties by a New Treatment Method

“…For that reason, recycling CDW as a new aggregate source for concrete has received much more attention due to its feasibility, as well as its environmental and economic benefits [4]. Using recycled concrete aggregate (RCA) for concrete structures in many countries is still controversial and not easy to implement because of the lower qualities of RAC [5] and more complicated microstructure [6] such as containing large flat Ca(OH)2 crystals (CH crystal, about 20-25% of the volume of solids in the hydrated paste) which accumulated in the voids and in the cement paste [7]; a huge amount of pores and cracks; especially when it holds two interfacial transition zone (ITZ) systems including: new ITZ between RCA and new cement paste; old ITZ between old natural aggregate and old adhered mortar [6], which causes negative effects on performance of RAC [8]. Which is why, numerous studies have been conducted to evaluate and improve the mechanical and durability properties of RAC.…”

Corrosión por cloruros del acero de refuerzo embebido en concreto con agregado grueso reciclado y materiales cementantes suplementarios

“…(SCM or RCA) have different micro-structure. In the case of RCA concrete, it has greater porosity than conventional concretes (Gómez, 2002;Tam et al, 2005;Etxeberría et al, 2007). On the other hand, it has been demonstrated that concrete electric resistivity is associated with the microstructure of cement matrix (pores distribution) and with the corrosion rate of reinforced steel (Tumidajski et al, 1996;Tumidajski, 2005;Polder et al, 2000).…”

The influence of recycled concrete aggregates in pervious concrete

“…In RCA, the existing cementitious material on the transition zone is more porous, has small cracks and therefore the binding properties it affords are weak, critically influencing mechanical resistance. In addition, these pores and cracks tend to require more water, increasing the final amount of the liquid present at the transition zone between the new mixture and RCA [26]. Optical microscopy was used to identify the different transition zones.…”