Mechanical Behavior of Recycled PET Fiber Reinforced Concrete Matrix

(Paper Code: 15FR07000455)

Dr. C. Marthongand

Dr. D. K. SarmaNational Institute of Technology Meghalaya,

India

WASET, ICMMCE 2015, July 20-21, 2015, Paris, France

Concrete is known to be weak in tensile strength and crack resistance.

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Introduction

Day by day new solutions to overcome the limitations of concrete are being developed.

Fibers are normally used as an alternative material which helps in enhancing the flexural and tensile strength of the concrete.

The most commonly used fibers as concrete reinforcing materials are steel, glass and polymeric fibers.

Polymeric fibers used as concrete reinforcements are made of nylon, aramid, polypropylene, polyethylene, polyester etc.

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PET fibers are generally used as a discrete reinforcement in the substitution of steel fiber for enhancing the tensile strength as well as improving the ductile property of concrete.

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Polyethylene terephthalate (PET) fibers

PET fibers can be produced by hand cutting from PET bottles or through mechanical slitting.

So far no guideline defining the shape and dimensions of the PET fibers

ACI Committee 544, 1996 defined various types of steel fibers viz. straight, deformed , crimped, flattened sleet sheet fibers, Machined chip and Melt Extract.

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Significance of the study

Straight slit sheet fibers normally results in a weak bond with the concrete matrix and they can slip out at low loads.

To improve the anchorage effect two additional PET fibers with varying geometry (flattened end slit sheet and deformed slit sheet) were considered.

The influence of fiber geometry on the mechanical properties of concrete specimens was investigated.

Parameter investigated1. Workability2. Compressive strength3. Tensile strength4. Flexural strength5. Ultrasonic pulse velocity test

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Specimens design

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Specimen Size (mm) Testing Methodology Characteristics

SP1Beam: 100x100x400

Flexural and Ultrasonic pulse velocity (UPV) Concrete with 0%

fiberCylindrical: 150x300 Compressive and

splitting tensile

SP2Beam: 100x100x400 Flexural and UPV Concrete with 0.5%

straight slit sheet fiber

Cylindrical: 150x300 Compressive and splitting tensile

SP3

Beam: 100x100x400 Flexural and UPV Concrete with 0.5% flattenedend slit sheet fiber

Cylindrical: 150x300 Compressive and splitting tensile

SP4Beam: 100x100x400 Flexural and UPV Concrete with 0.5%

deformed slit sheet fiber

Cylindrical: 150x300 Compressive and splitting tensile

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Cement and Aggregates: OPC of 53 grades conforming to IS: 12269 (1987) Maximum size of coarse aggregate was 12.5 mm. River sand was used as fine aggregate. Aggregates tested as per relevant codes (IS: 2386(a,b), 1963).

PET fibers: Maintaining the same cross-sectional area (7.2 cm2 approx.). Three fibers were used (a) Straight (b) flattened-end and (c) deformed slit sheet.

Casting and Curing of Specimens: Mix for ordinary concrete of 25 MPa at 28 days Water cement ratio (w/c) of 0.5. Fiber volume fractions of 0.5% A conventional step of mixing was adopted.

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Materials use and casting of specimens

Testing Methodology

1. Fresh concrete properties: Slump test as per guidelines of IS: 1199 (1959).

2. Hardened concrete samples are tested for compressive,

tensile and flexural strength after 28 days curing. 3. All tests were carried out in a hydraulic compression

testing machine of capacity 1000 kN. 4. Ultrasonic scanning of the specimens to assess the

homogeneity and integrity of PET fibers concrete

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Test Results

Fresh Concrete Properties

For a w/c of 0.5%, a medium degree of workability was achieved for concrete with and without fibers.

Thus, it can be concluded that the shape of the fiber has a small significant effect on the workability of concrete.

A comparable slump values were also observed for fiber concrete of different geometry.

SP1: Reference concrete specimens SP2: Straight slit fiber concrete SP3: Flattened-end slit fiber concrete SP4: Deformed slit fiber concrete

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Compressive strengthThe compressive strength test was carried out in accordance to IS: 516 (1959).

fc = compressive strength (MPa)P = maximum crushing load resisted by the specimen before failure D = the diameter of the cylindrical specimen (mm).

The increase in strength for specimens SP3 and SP4 was attributed to the good mechanical anchorage effect of fiber in the concrete matrix. A typical wedge failure mode was observed for specimens SP3 and SP4. It reveals that shape of fibers has a significant effect on the failure of the. specimens. 9ICMMCE 2015

Tensile strengthIn accordance to IS: 5816 (1999) an indirect tensile strength test was carried out

2ct

Tf

LD

fct = Splitting tensile strength (MPa)T = Maximum splitting tensile load (kN), D and L= diameter and length of cylindrical specimen (mm)

The increase in tensile strength of fiber concrete is due to the fiber bridging properties in the concrete matrix. The reference specimens containing no fibers suddenly splitted once the concrete cracked. The PET fiber concrete specimen exhibited cracking but did not fully separate.This suggests that PET fiber reinforced concrete has the ability to dissipate more energy. 10ICMMCE 2015

Flexural strength

2b

PLf

bd

fb = flexural strength (MPa),P = maximum flexural load (kN) L, b and d = supported length, width and failure point depth of the specimens (mm)

All loads in the flexural test were applied till the specimens attained approximately the same displacement level or collapsed whichever occurred earlier.

Specimen SP1 failed suddenly and collapsed.

All fiber concrete specimens could sustain higher displacement level. The plot also depicts that curves of SP3 and SP4 lies above SP2.

This shows that fiber with varying cross-sections created an anchorage effect in the concrete matrix.

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Energy dissipation

The energy absorbed by the specimen is represented by the area formed by the load vs. displacement curves (Shannag and Ziyyad, 2007).

The ability of a structural member to resist the fracture when subjected to static or to dynamic or impact loads depends to a large extent on its capacity to dissipate its energy.

The plot for fiber concrete (SP2 to SP4) are bigger than that of SP1

This shows that higher amounts of energy were dissipated by PET fiber concrete.

SP3 and SP4 dissipated more energy compared to SP2 which reflect a better performance in energy dissipation capability.

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Ultrasonic test

Test were carried out to investigate the structure of the PET fiber concrete, ultrasonic pulse velocity (UPV) test was conducted on the prismatic specimens.

UPV values of fiber concrete as well as reference concrete specimens are in the range of 3.5 km/sec to 4.5 km/sec, which indicates the quality of concrete falls in the “good” scale as per quality assessment of IS: 13311 (1992).

However, it was noted that the inclusion of PET fibers led to a 8-11% decrease on the ultrasonic pulse velocity due to their lower specific gravity.

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Concluding remarks

The following conclusions were drawn in the study:

•The workability of fresh concrete was slightly decreased with the inclusion of 0.5% PET fibers. However, geometry of the fiber has a small effect on workability of concrete. •The addition of 0.5% PET fiber in concrete enhanced the compressive strength, improved the tensile and ductile property of the specimens and varied with the fiber geometry.

•Slightly lower UPV values due to the fibers' lower density and also to the low bond strength between the PET fibers and cement matrix, which may have led to a discontinuous microstructure.

•Nevertheless, the use of 0.5% of PET fibers was capable of producing concrete specimens with UPV values that fall within the “good” quality grading (3.5 km/s to 4.5 km/s) as per IS: 13311 (1992). 14ICMMCE 2015

Thank you