INTERFICIAL TRANSITION ZONES IN RECYCLED AGGREATE CONCRETE

Using different mixing approaches and techniques

INTRODUCTION

RAC or Recycled Aggregate Concrete isone of the most important use of wasteconcrete and is being used throughout theworld .

It is manufactured by using previouslyprepared concrete as aggregate withfresh cement matrix.

It is sustainable and environment friendly as it recycles the concrete which would have been discarded as waste and had to be dumped off.

It reduces the construction cost and when used properly gives similar performance as virgin aggregate concrete.

INTRODUCTION (cont.)

Some advantages of using RAC are: Keeping concrete debris out of landfills saves landfill space. Using recycled material as gravel reduces the need for gravel mining. Recycling one ton of cement could save 1,360 gallons (5.14 m3) water, 900 kg of CO2. Using recycled concrete as the base material for roadways reduces the pollution involved in trucking material.

INTRODUCTION(cont.)

The strength of RAC greatly depends on the micro structural interfaces present in it.

There are two interfaces in RAC:

1. Old interfacial Transition Zone(Old ITZ):It is the zone formed between natural aggregate and old cement matrix.

2. New Interfacial Transition Zone(New ITZ):It is the zone formed between Recycled Coarse Aggregate(RCA) and new cement matrix.

The study of interfaces is important as they are a prime source of internal micro cracks which greatly affect the strength and durability of concrete.

SOME TERMS AND DEFENITIONS NORMAL MIXING APPROACH(NMA)

It is traditional mixing approach in which all the ingredients are mixed all together.

It has been used for a long time and is effective for the production of virgin aggregate concrete.

TWO STAGE MIXING APPROACH(TSMA)

In this method, the RCA is firstly mixed with cement paste, which is followed by adding sand to the mixture, thus making a two stage mixing.

The two-stage mixing can help to form a layer of cement slurry on the surface of RCA to fill up the initial damage such as micro-cracks and voids, leading to an improved interfacial zone at the pre-mix stage.

TSMA VS. NMA

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NAC(NMA)

RAC(NMA)

RAC(TSMA)

SCANNING ELECTRON MICROSCOPY(SEM)

It is a type of microscopy which uses electron produce an detailed image of any 3-D body.

It uses an electron gun to produce a beam of electrons which hit the surface and then get to the detectors which forms a detailed, magnified, digital image of the specimen.

With SEM technique, one can identify the phase distribution, such as C–S–H

or CH, in concrete.

ATOMIC FORCE MICROSCOPY(AFM)

It is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit.

It is another increasingly popular tool for studying cement-based materials which is often used to assess concrete surface roughness.

NANOINDENTATION METHOD

Nanoindentation is a variety of indentation hardness tests applied to small volumes.

In nanoindentation small loads and tip sizes are used, so the indentation area may only be a few square micrometres or even nanometres. This presents problems in determining the hardness, as the contact area is not easily found. Instead, an indenter with a geometry known to high precision (Berkovich

tip) is employed.

This helps to find out the Young’s modulus for points very near to each other thus giving an accurate picture about the variation of strength of material .

For the present study the indendation scheme followed is as shown in figure:

For Old ITZ:

For New ITZ:

RESEARCH SCOPE AND ADVANTAGES

This research mainly focuses on studying two properties of ITZs: the elastic modulus and the thickness.

Knowing the elastic modulus distribution within ITZs can help to improve our knowledge of the failure mechanism and cracking progresses in RAC.

And knowing the thickness can help to develop methodologies to improve the quality of RAC by minimizing the ITZ volume.

This study presents two gaps in the current understanding of RAC behavior by:

1. Obtaining a more accurate understanding of local mechanical properties on nanoscales for ITZs in RAC

2. Revealing the differences in nanoscale mechanical properties between old and new ITZs in RAC

CONCLUSIONS

For the old ITZ, the indentation modulus increases as the distance from original aggregate surface increases. For the new ITZ with TSMA, there is no obvious trend indicating that the indentation modulus increases or decreases with the increasing distance from the old paste matrix surface, while in the new ITZ with NMA the modulus distribution appears to decrease significantly from the old paste matrix surface.

It indicates that the TSMA can effectively reduce the size and effect of water layers and CH crystals formed around the RCA. Therefore, the amount of porosity is reduced and the enhanced hydration provides a source for production of C–S–H.

It is observed that the volume fractions of porosity and CH phases in the new ITZ with TSMA are relatively lower than those of the old ITZ and new ITZ with NMA, which can explain the improvement in RAC compressive strength.

CONCLUSIONS (cont.) For the new ITZ with TSMA, the

cement slurry permeates into the porous old paste matrix and fills up the voids and cracks, which improve the microstructure of the new ITZ.However, for the new ITZ with NMA, there is a significantly loose microstructure that contains a large volume of porosity and CH crystals.

It is interesting to note that there is a stronger and denser zone existing in the vicinity of the new ITZ in RAC. The EDX analysis exhibits that the dense zone with high indentation modulus is most likely the calcium carbonate crystals which covers the surface of RCA

GRID NANOINDENTATION

Old ITZ New ITZ

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Natural aggregate

OldITZ

Old Pastematrix

Old Pastematrix

New ITZ

NewPastematrix

Contour map of modulus in GPa (old ITZ) Contour map of modulus in GPa (new ITZ)

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Contour map of hardness in GPa (old ITZ) Contour map of hardness in GPa (new ITZ)

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Natural aggregate

OldITZ

Old Pastematrix

Old Pastematrix

New ITZ

NewPastematrix

Perpendicular to old ITZ, at old pastematrix side

Perpendicular to new ITZ, at new pastematrix side

SEM images of old ITZ and new ITZ in RAC at 90 days.