GEOGRID REINFORCED CONCRETE

1

C.SAI CHANDRA

B110831CE

Contents

• Geogrids

• Applications in present scenario

• Factors favouring geogrids to use with concrete

• Test setup and measurement instrumentation

• Results and graphs of test

• Observations and analysis

• Conclusions

• recommendations

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Geogrid

• A geosynthetic material

• made of polymer materials such as

a. polyester

b. polyethylene

c. Polypropylene

• characterized by

bands of narrow elements in grid-like pattern

large voids between those bands

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Functioning of geogrids(CLk)

• capture the aggregates - interlock the aggregates -

create a mechanically stabilized earthwork

• redistributes the load over wider area and reduce the

vertical stress

• Provides

Lateral Restraint

Improved Bearing Capacity

Tension Membrane Effect(clk)

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Types of geogrids

• Based on shape:

1. Uniaxial geogrids - Wall, slope applications

2. Biaxial geogrids - roads

3. Triangular geogrids - trafficked surfaces

• Based on method of manufacture:

1. Punched and drawn geogrids

2. Coated yarn geogrids

3. Laser welded geogrids

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Fig.1 Uniaxial geogrid Fig.2 Biaxial geogrid

Fig.3 Triangular geogrid 6

Geometry of geogrids

Applications

Geogrids in pavements

• Shifts the failure envelop from weaker subgrade to stronger

base material

• Enhances the bearing capacity of subgrade without any soil

treatment

• Reduces the structural cross section for given service life

7Fig 4 Fig 5

Geogrids in steep slopes

• Improves the soil retention on slip plane surface

• Its tensile strength carries loading forces imposed of

failure wedge

• Provides the possibility of slopes of our desired

steepness.

8Fig 6 Fig 7

Geogrids in retaining walls

• Resist the force of unstable soil wedge on retaining walls

• Permit to construct retaining walls of suitable heights by using

geogrids of proportional length and size.

9Fig 8

Geogrids in concrete

Factors favoring geogrids to be used with

concrete:

• Resistant to chemicals

• Inert to aqueous solutions of acids, alkalis and salts

• No nutritional values -

not attacked by micro organisms

• Corrosion resistant

• Good tensile strength

• Temperature resistant

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Flexural test on geogrid reinforced beams

Specimen details:

• Cross section 150 x 150 mm

• Span length 530 mm

• Triangular notch

o Width=8mm

o Depth=4.5mm

o Made across the beam’s bottom surface

• A geogrid layer at 50mm above the bottom surface

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Numbers of specimens tested: 21

• 12 Normal strength concrete blocks(35Mpa)

3 with no reinforcement(serves as control)

3 with uniaxial geogrid

3 with biaxial geogrid

3 with triangular geogrid

• 9 high strength concrete blocks(45Mpa)

3 with no reinforcement

3 with biaxial geogrid

3 with triangular geogrid

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Specimen fabrication

o PCC mix

Portland cement

natural sand(fine aggregates)

medium sized limestone(NMAS 9.5mm)

coarse limestone aggregates(NMAS19mm)

Normal strength beams

o Mix ratio coarse:medium:fine:cement=1.7:1.2:2:1

o Water cement ratio=0.52

High strength beams

o Mix ratio coarse:medium:fine:cement=1.5:0.8:1.7:1

o Water cement ratio=0.43 13

Details of geogrids used

Table 1 uniaxial geogrid click 14

Table 2 biaxial geogrid(CLICK)

table 3 triaxial geogrid15

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fig 9 dimensions of geogrids

Testing setup and measurement instrumentation

• Flexural testing is done according to ASTM

• Monotonic loading by hydraulic UTM

• Displacement control at constant cross head rate of

0.002mm/sec

• Data acquisition system to collect data

• Clip-on gauge to measure crack mouth opening displacement(CMOD)(CLK)

• Transducers to measure the horizontal and vertical displacements at notch

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Fig.10 longitudinal section showing

loading position and reinforcement layout

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Fig.11 Testing and measurement instrument setup(CLICK)19

Results and analysis

• load vs vertical displacement of normal strength beams

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Load vs. vertical deflection of high strength

beam:

21

22

Continued…….

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Summary of test results

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Pcontrol=max load of unreinforced specimen

Pmax = load at first peak in reinforced specimen

Pp = post peak load

Continued………….

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∆max = deflection at max load

∆control= deflection at post peak load

Continued……….

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CMOD = crack mouth opening displacement

Table 4 brief summary of test results

observations

• Repeatability is observed in the behavior of replicates

• Small variability is due to slight difference in

Fabrication

Testing

Consistent behavior of geogrid(CLK)

• Load of failure is maximum for replicate in which

load propagation started at notch

• Unreinforced beams failed by brittle failure(clc)

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Continued………………..

• In reinforced beams the first drop is due to

Inability of concrete in taking the load after cracking

Debonding between the concrete and geogrid

• New rise is due to load taken by geogrid

• Slope of new rise is less due to less elasticity is

modulus

• Further series of drops are due to tear in the one or

more ribs at once(clk)

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Continued……………

• Inclusion of geogrid layer increased the maximum load and deformation of initial peak(CLK)

Increment in strength and deformation at initial peak

of reinforced beams over unreinforced beams is

o 20% and 40% - uniaxial

o 12% and 25% - biaxial

o 28% and 48% - triaxial

• Post peak load capacity is more than first peak capacity in uniaxial geogrids(CLK)

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Continued………….

• More multiple peaks are observed in uniaxial geogrid reinforced beams(CLK)

• Uneven distribution of junctions in triaxial geogrids

causes it to act as one whole reinforcement

• Single peak is observed in triaxial reinforced beams

• Change in post peak behavior of triaxial high strength beam indicates(clk)

• ‘correlation between concrete strength and mechanical characteristics of reinforcement’

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Failure mechanism

32Fig.8 failure mechanisms

Failure mechanisms observed in beams

• Fig a – immediate brittle failure leading to specimen

Separation in control specimens

• Fig b – crack initiation and propagation in uniaxial geogrid beams

• Fig c – delayed failure due to geogrids holding the specimen intact

• Fig d – failure mode just before the total failure in uniaxial geogrid beams

• Fig e – geogrid junction resisting load at crack

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Continued……………

• Fig f – rib failure; failure mode of geogrids

• Fig g – rib failure in biaxial geogrids

• Fig h – rib failure in triaxial geogrids

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Load verses CMOD Graphs

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observations

• CMOD measures the resistance of the beam to growing a crack

• CMOD values are more related to concrete strength till the failure of bottom concrete occurs

• The type and property of the geogrid influence the CMOD value after the load transfer

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Flexural strength

• Flexural strength calculated as modulus of rupture R is calculated with formula

where P=max total load in Kn

l= span length

b = specimen width

d = specimen height

• Increment in flexural strength in normal and high strength beams is

20% - uniaxial geogrids

12% and 0% - biaxial geogrids

28% and 6% - triaxial geogrids

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Fracture energy

• Area under load CMOD graph

• Geogrid reinforcements increased the fracture energy

• Highest increase is seen in uniaxial type due to more

ductility

• High strength specimens attained less energy

compared to low strength specimens

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Conclusions

• All types of geogrids provided

Ductile post cracking behavior

High fracture energy

High flexural strength

Large deflection

• Physical and mechanical properties of geogrids have

impact on peak and post peak behavior

• Post peak behavior in descending order

uniaxial>biaxial>triaxial geogrids

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continued………….

• A correlation exists between concrete strength, tensile

properties of geogrids.

• Provide considerable benefits when used as non

structural reinforcement under light loading

conditions

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Recommendations

• Still more parameters like

methods to improve the interlocking

use of multiple geogrid layers

effects of junction location

behavior under cyclic loading

are to be investigated to reveal complete significance

of geogrids in concrete

• Studies on reduction of ballast thickness of railtracks

by using geogrids can be carried

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