CHAPTER 4 CONCRETE CORE TREPANNING TECHNIQUE 4.1 …shodhganga.inflibnet.ac.in/bitstream/10603/15821/9/09... · 2015-12-04 · core drilling machine, with diamond tipped cutting tool

53

CHAPTER 4

CONCRETE CORE TREPANNING TECHNIQUE

4.1 INTRODUCTION

Assessment of in-situ stresses in concrete structures which are under

biaxial stress state is complex to handle. Concrete core trepanning technique

is developed to assess the in-situ stress under biaxial stress state. The

proposed technique employs a three element strain gage rosette to measure the

strain release due to core-drilling. The reliability of the proposed technique

was evaluated through laboratory studies. Numerical analysis was carried out

using finite element method for evaluating the efficacy of the method. The

details of the studies carried out to evaluate the proposed concrete core

trepanning technique are presented in this chapter.

4.2 CONCRETE CORE TREPANNING TECHNIQUE

Concrete core trepanning technique was developed (Parivallal et. al.,

(2001), Kesavan et. al., (2005)) for in-situ stress evaluation in concrete

structural elements in uniaxial stress condition. This developed technique is

furthered to measure in-situ stresses in concrete under bi-axial stress state. In

this proposed technique, three electrical resistance strain gages namely SG1,

54

SG2 and SG3 of 30mm gage length were fixed at the center of the intended

core. Here also 30mm length of gage with 50mm diameter core was used. The

gages used were of three element staked strain gages rosette of 0 /45 /90

orientation as shown in Figure 4.1.

Figure 4.1. Strain gage rosette configuration

An annular hole of 50mm diameter with 50mm depth was formed by

diamond core drilling. On drilling the annular hole around the core, the strain

gage measures the change in strain due to core drilling. A standard concrete

core drilling machine, with diamond tipped cutting tool of 50mm diameter,

was used in this method. Portable strain measuring equipment with a

resolution of 1micro-strain was used to measure the strain. The elastic strain

relief due to core drilling was recorded till cutting to a depth of 50mm, in the

increments of 10mm. The in-situ principal strains, stresses and direction of

principal stresses were determined from the measured strains and by using

elastic modulus (EC) and Poisson’s ratio ( ) of concrete from Equation (4.1),

Equation (4.2) and Equation (4.3) respectively:

SG1 ( a)

SG2 ( b)

45mm

SG3( c)

50mm

1, 1

2, 2

X

Y

55

222,1 2

21

2 cabcaca (4.1)

222,1 2

11

12 cabcacaCE (4.2)

ca

cab22tan (4.3)

where, 1,2 are major and minor principal strains; 1,2 are major and minor

principal stresses; EC and are elastic modulus and Poisson’s ratio of

concrete; a, b, c are the released strain from the strain gages SG1, SG2 and

SG3 respectively; and is the direction of principal stress(see Figure 4.1).

The reliability of this technique for in-situ stress assessment was

established in the laboratory, by conducting experimental investigations on

concrete specimens with known stress/strain field. Numerical analysis was

also carried out using finite element method for evaluating the efficacy of the

method.

4.3 NUMERICAL ANALYSIS

A Finite element model of 45mm diameter and 10mm depth was

created using ANSYS to model the core (see Figure 4.1). Since the core was

drilled at the increments of 10mm (up to a maximum depth of 50mm), five

models of 45mm diameter with depths of 10mm, 20mm, 30mm, 40mm and

50mm were created and analysed. The 3D model was meshed with 20-noded

solid element SOLID95. The element was defined by 20 nodes having three

degrees of freedom per node (ie. translations in the nodal x, y, and z

directions). For the analysis, assuming that the existing stress state

corresponds to x = -2 N/mm2 and y = -3 N/mm2, the same stress state was

considered in the analysis. The stress distribution across the depth was

56

assumed to be uniform. Concrete of M40 grade with Modulus of elasticity

(EC) of 31623 N/mm2 and Poisson’s ratio ( ) of 0.17 was used in the analysis.

The introduction of a hole into a stressed body relaxes the stresses over the

region. This occurs because every perpendicular to a free surface (hole surface

in this case) is necessarily a principal axis on which the shear and normal

stresses are zero. The elimination of these stresses on the hole surface changes

the stress in the immediately surrounding region, causing the local strains on

the surface of the test specimen to change correspondingly. The released

stress was applied as pressure load on the elements lying on the surface of the

core. The relieved radial stress variation was calculated from the Equation

(4.4) and applied on the core model with opposite polarity as shown in

Figure 4.2.

2)2cos1(

2)2cos1( yx

r(4.4)

Where x, y existing stress, and = angle measured to the node with respect

to x axis. All the three translations were not allowed at the end of the core

were given as the boundary condition. Loading and boundary conditions were

applied to the model and all five models with different depths were analysed

using ANSYS.

For the evaluation of in-situ stresses, it was assumed that the gages

SG1, SG3 and SG2 were aligned along x axis, y axis and 45 to the x axis

respectively. From the analysis, strain distribution on the surface of the model

was obtained. Figures 4.3 to 4.7 show the strain Contours for different core

depths of 10mm, 20mm, 30mm, 40mm and 50mm respectively.

57

Figure 4.2 Typical model showing the boundary condition and loading

x y

Figure 4.3 Contours of released strain for core depth of 10mm

x y


58

x y


x y


x y


59

From the analysis, the released strains were obtained for the three

gages. For Gage-1 and Gage-3 the strain variation along the gage orientations

a and c can be directly obtained from x and y respectively. For Gage-2, the

strain variation was obtained by transforming the x, y and xy along the

direction of Gage-2 by using Equation (4.5).

2sin2

2cos22

xyyxyxb (4.5)

Released strain variations along the three gages for different core depths of

10mm, 20mm, 30mm, 40mm and 50mm are given in Figures 4.8 to 4.12.

0

20

40

60

80

100

-30 -20 -10 0 10 20 30Distance in mm

SG1SG2SG3

Figure 4.8 Released strain variation along the gages - 10mm core depth

60

0

20

40

60

80

100

-30 -20 -10 0 10 20 30Distance in mm

SG1SG2SG3


0

20

40

60

80

100

120

-30 -20 -10 0 10 20 30Distance in mm

SG1SG2SG3


61

0

20

40

60

80

100

-30 -20 -10 0 10 20 30Distance in mm

SG1SG2SG3


0

20

40

60

80

100

-30 -20 -10 0 10 20 30Distance in mm

SG1SG2SG3


From the relieved strain variations, strain response for the gages SG1,

SG2 and SG3 (ie. a, b and c) were obtained by averaging the strain variation

for the gage length of 30mm. From the evaluated strain for gages SG1, SG2

62

and SG3, the principal strain / stresses and direction of principal stress were

calculated using Equations (4.1), (4.2) and (4.3). From the principal stress

evaluated, Von-mises stress ( Von) was calculated from Equation (4.6).

212

22

1Von (4.6)

The principal strains / stresses and direction of principal stress

calculated from the released strain obtained from numerical analysis are given

in Table 4.1. It is seen that the relieved principal stresses for 10mm depth of

cut are -1.45 N/mm2 and -1.91 N/mm2 which is 0.64 and 0.72 times of the

applied stresses. This indicates that partial strain only released at 10mm

depth. The maximum stress release occurred at a depth of 20mm with

-2.33 N/mm2 and -3.29 N/mm2, where as the applied stresses are -2.0 N/mm2

and -3.0 N/mm2. The maximum principal stress obtained was slightly higher

than the applied stress. This may be due to the shear stress that was not

considered in the analysis. For 30mm depth the evaluated stresses are

-2.20 N/mm2 and -3.28 N/mm2. For 40mm and 50mm depths the evaluated

stresses are -2.05 N/mm2, -3.12 N/mm2 and -1.99 N/mm2, -3.05 N/mm2

respectively. The Von-mises stress for the applied stress is 2.65. Von-mises

stress obtained at 10mm depth is 1.73, which is 0.653 times the applied stress.

Von-mises stress ratio is the ratio of evaluated Von-mises stress to the applied

Von-mises stress (ie. Von(Evaluated)/ Von(Applied)). The maximum ratio for

Von-mises stress occurred at the depth of 20mm. Further the stress ratio

obtained for 30mm to 50mm depths were reduced with the increase in depth

though the reductions are small.

63

Table 4.1 Numerically evaluated strain and stress

Depth of cutQuantity 10mm 20mm 30mm 40mm 50mma(micro-strain) 35.5 56.1 51.9 47.9 46.5b(micro-strain) 44.1 73.8 71.9 67.8 66.1c(micro-strain) 52.7 91.5 91.8 87.8 85.71(micro-strain) -35.5 -56.1 -51.9 -47.9 -46.52(micro-strain) -52.7 -91.5 -91.8 -87.8 -85.71(N/mm2) -1.45 -2.33 -2.20 -2.05 -1.992(N/mm2) -1.91 -3.29 -3.28 -3.12 -3.05 (degrees) 0.007 0.004 0.004 0.004 0.004

Von-mises stressVon (N/mm2) 1.73 2.93 2.89 2.75 2.68

Ratio of Von-misesstress* 0.653 1.108 1.093 1.039 1.013

(* - applied Von-mises stress = 2.65 N/mm2)

Note :

1. Positive value of strain / stress indicates tension2. Positive sign of indicates the direction of maximum principal stress

with respect to gage SG1 is anticlockwise

4.4 EXPERIMENTAL STUDIES

Reliability of the concrete core trepanning technique for in-situ stress

evaluation under biaxial stress field was established in the laboratory, by

conducting experimental investigations on concrete specimens with known

stresses/strains. Experimental studies on in-situ stress evaluation under biaxial

stress field using trepanning technique were carried out on ten specimens of

dimension of 500×500×100mm.

4.4.1 Instrumentation and Testing

Experiments were conducted on 500×500×100mm size concrete

specimens. The test specimens were prepared in the same way as explained in

64

Chapter 3.5.1. On each specimen, three element staked electrical resistance

strain gage rosette of 30mm length, 120 ohm resistance was bonded at the

centre as per the configuration described in Figure 4.1. This rosette was used

to measure applied strain/stress and also used to measure the released strain to

evaluate the existing stress by using concrete core trepanning technique. All

the gages were connected with lead wire by means of terminals. The

specimen instrumented with strain gages is shown Figure 4.13. Standard

procedures were followed for bonding the strain gages. Before bonding the

strain gages, the surface of the specimen was cleaned with emery sheets. A

pre-coat of two component epoxy was applied on the surface. The rosette

gage was bonded to the specimen using cynoacyralic based quick setting

cement. After bonding, the rosette was protected with M-coat and wax.

Additionally, silicon rubber coating was also provided as water was used as a

coolant during drilling the core.

Figure 4.13 Strain gage instrumented specimen for testing

A special test set-up was designed and fabricated to apply biaxial stress

to the specimen, by means of pedestals and hydraulic jacks. The schematic

diagram of test setup is shown in Figure 4.14. The pedestals were fixed to the

heavy duty test floor at Structural Testing Laboratory, CSIR- SERC.

65

Figure 4.14 Schematic diagram of experimental setup

Axial compression was applied to the instrumented specimen, by

means of two 300kN capacity hydraulic jacks (Figure 4.15). The load applied

was measured through 300 kN load cells. These load cells were specially

designed and fabricated for this purpose and calibrated prior to testing. A

strain gage data logger and portable strain measuring equipment were used to

measure the strain response. All the strain gages were connected to the strain

gage data logger in a three lead wire quarter bridge configuration. During

PedestalJack

Load cell

Concretespecimen

ELEVATION

PLAN

66

loading, the load and strain responses were measured in order to have the

applied strain to the specimens. After loading, strain response was recorded

from the strain gages. From the measured strain, the applied strain / stress,

principal strain / stresses and direction of principal stress were computed using

material properties.

Figure 4.15 Experimental test setup for concrete core drilling technique

After the application of load, the bridge circuit was initialized.

Concrete core trepanning technique was carried out by drilling the core on the

stressed specimen as shown in Figure 4.15. Now in the stressed specimen, a

circular core of 50 mm diameter was formed by diamond core drilling for

different depth of cutting from 10mm to 50mm with the increments of 10mm.

At each increment, the released strain was measured from the strain gages

(Figure 4.16.).

67

Figure 4.16 Measurement of released strain after drilling of core

Totally ten specimens were tested with different combination of stress

condition. The test specimens were identified as SP1, SP2, SP3, SP4, SP5,

SP6, SP7, SP8, SP9 and SP10. The results of each specimen are given below:

4.4.2 Results on Concrete Core Trepanning Technique

Experimental studies for assessment of stresses under biaxial stress

condition were carried out on ten specimens with different combination of

loads. The results are given below.

4.4.2.1 Test results of Specimen SP1

Specimen SP1 was biaxially stressed by loading 156kN and 105kN in

two orthogonal directions. The strain developed and the calculated principal

strain / stress and Von-mises stresses based on the measured strain in the

Specimen SP1 are given in Table 4.2.

68

Table 4.2 Applied strain and stress for Specimen SP1

Applied strain Principal strain Principal stress

a = -33micro-strain

b = -14micro-strain

c = -3micro-strain

1 = - 2.5micro-strain

2 = - 33.5micro-strain

= 82.53

1 = -0.27N/mm2

2 = -1.11 N/mm2

Von = 1.00 N/mm2

Note :



Trepanning technique was carried out by drilling the core on the

stressed specimen. Incremental drilling was carried out in steps of 10mm up

to 50mm. At each increment, the released strains were measured from the

strain gages and are given in Table 4.3. Released strain vs. depth of cut for

Specimen SP1 is plotted in Figure 4.17.

Table 4.3 Released strain for Specimen SP1

Released strain in micro-strainDepth of cut in mm a b c

10 19 3 220 29 10 330 31 12 240 27 11 250 23 11 1

Note :

1. Positive value of strain indicates tension

69

0

10

20

30

40

50

0 20 40 60 80 100Micro-strain

SG1SG2SG3

Figure 4.17 Released strain vs. depth of cut for Specimen SP1

It is seen that, at 10mm depth of cut, partial strain is only released.

When the depth of cut increases, the strain release also increases. But after

30mm the released strains almost stabilised. From the released strain the

principal strain / stresses and direction of principal stresses were calculated by

using Equations (4.1), (4.2) and (4.3) and given in Table 4.4.

Table 4.4 Evaluated principal strain /stress for Specimen SP1

1 2 1 2 VonVon

ratioDepth of

cut inmm Micro-strain Degree N/mm2

10 0.8 -21.8 69.29 -0.05 -0.75 0.72 0.7220 -1.7 -30.3 77.61 -0.16 -1.06 0.99 0.9930 -1.3 -31.7 81.38 -0.15 -1.11 1.04 1.0440 -1.5 -27.5 82.18 -0.14 -0.96 0.90 0.9050 -1.0 -23.0 87.40 -0.11 -0.81 0.76 0.76

Note :



70

It is seen that the maximum strain release occurred at the depth of

30mm. Von-mises stresses were also calculated for applied and evaluated

stresses. At 30mm, 1.04 times of the applied Von-mises stress was measured

by using trepanning technique. The applied and evaluated stresses are shown

schematically in Figure 4.18.

Figure 4.18 Applied and evaluated stresses for Specimen SP1


Biaxial stress was applied to the Specimen SP2 with the load of 221kN

and 109kN in two orthogonal directions. The strain developed and the

corresponding principal strain / stress and Von-mises stresses calculated are

given in Table 4.5.

2, 2

a

b

c

1, 1

1 = -0.27 N/mm2

2 = -1.11 N/mm2

= 82.53Von = 1.00 N/mm2

Applied

1 = -0.15 N/mm2

2 = -1.11 N/mm2

= 81.38Von = 1.04 N/mm2

Evaluated

71

Table 4.5 Applied strain and stresses for Specimen SP2


a = -86micro-strain

b = 15micro-strain

c = 26micro-strain

1 = 41.8micro-strain

2 = -101.8micro-strain

= 70.61

1 = 0.80 N/mm2

2 = -3.08 N/mm2

Von = 3.55 N/mm2

Note :



Trepanning technique was applied on the stressed specimen by drilling

the core. At each increment of 10mm, the released strain was measured from

the strain gages. Released strain for different depth of cut for Specimen SP2

is given in Table 4.6 and plotted in Figure 4.19. The maximum strain release

occurred at the depth of 30mm.



10 51 -10 -1520 70 -12 -2130 75 -13 -2240 70 -11 -2050 67 -10 -21

Note :


72

0

10

20

30

40

50

-40 -20 0 20 40 60 80Micro-strain

SG1SG2SG3


From the released strain the principal strains / stresses and direction of

principal stress were calculated and are given in Table 4.7.


1 2 1 2 VonVon

ratioDepth of


10 25.3 -61.3 69.84 0.64 -2.01 2.40 0.6720 33.8 -82.8 70.63 0.86 -2.72 3.24 0.9130 36.0 -89.0 70.42 0.91 -2.93 3.48 0.9840 32.6 -82.6 70.67 0.82 -2.72 3.21 0.9050 32.0 -78.0 71.57 0.82 -2.57 3.06 0.86

Note :



It is noted here that the maximum strain release occurred at the depth of

30mm. Von-mises stress was also calculated for applied and evaluated

73

stresses. The applied Von-mises stress is 3.55 N/mm2 and the evaluated stress

is 3.48 N/mm2 for the depth of 30mm. The ratio of Von-mises stresses is 0.98.

Figure 4.20 shows the applied and evaluated stress for the Specimen SP2.



Specimen SP3 was loaded with 202kN and 83kN in two directions to

create biaxial stress. The strain developed and the corresponding principal

strain / stress and Von-mises stresses calculated are given in Table 4.8.

a

2, 2

b

c

1, 1

1 = 0.80 N/mm2

2 = -3.08 N/mm2

= 70.61Von = 3.55 N/mm2

Applied

1 = 0.91 N/mm2

2 = -2.93 N/mm2

= 70.42Von = 3.48 N/mm2

Evaluated

74



a = -117micro-strain

b = -26micro-strain

c = 39micro-strain



= 85.27

1 = 0.65 N/mm2

2 = -3.62 N/mm2

Von = 3.99 N/mm2

Note :






is given in Table 4.9 and plotted in Figure 4.21.



10 59 13 -2220 79 20 -2430 96 22 -2840 95 22 -2550 91 21 -24

Note :


75

0

10

20

30

40

50

-40 -20 0 20 40 60 80 100 120Micro-strain

SG1SG2SG3


From the released strain the principal strain /stresses and direction of

principal stress were calculated and given in Table 4.10.


1 2 1 2 VonVon

ratioDepth of


10 22.4 -59.4 86.13 0.59 -2.00 2.34 0.5920 24.5 -79.5 85.86 0.60 -2.69 3.03 0.7630 29.2 -97.2 84.52 0.70 -3.29 3.69 0.9240 26.4 -96.4 83.89 0.60 -3.27 3.61 0.9050 25.3 -92.3 83.87 0.58 -3.13 3.46 0.87

Note :






76


Figure 4.22 shows the applied and evaluated stresses for the Specimen SP3.



Specimen SP4 was loaded with 231kN and 102kN in two directions.

The strain developed and the corresponding principal strain / stress and Von-

mises stresses calculated are given in Table 4.11.



a = -119micro-strain

b = -92micro-strain

c = 58micro-strain



= -72.60

1 = 1.75 N/mm2

2 = -4.07 N/mm2

Von = 5.18 N/mm2

Note :



a

2, 2

b

c

1, 1

1 = 0.65 N/mm2

2 = -3.62 N/mm2

= 85.27Von = 3.99 N/mm2

Applied

1 = 0.70 N/mm2

2 = -3.29 N/mm2

= 84.52Von = 3.69 N/mm2

Evaluated

77


the core. Released strain for different depth of cut for Specimen SP4 is given

in Table 4.12 and plotted in Figure 4.23.



10 46 30 -3020 98 78 -5030 92 77 -4840 84 73 -4550 83 71 -45

Note :


0

10

20

30

40

50

-100 -50 0 50 100 150Micro-strain

SG1SG2SG3


From the released strain the principal strain / stresses and direction of

principal stress were calculated from Equations (4.1), (4.2), and (4.3) and are

given in Table 4.13.

78


1 2 1 2 VonVon

ratioDepth of


10 35.9 -51.9 -74.97 1.06 -1.67 2.38 0.4620 67.6 -115.6 -71.94 1.91 -3.74 4.97 0.9630 67.0 -111.0 -70.92 1.90 -3.57 4.81 0.9340 64.3 -103.3 -70.16 1.83 -3.31 4.51 0.8750 63.5 -101.5 -70.45 1.81 -3.26 4.44 0.86

Note :



It is seen that the maximum strain release occurred at the depth of 20mm.

Von-mises stresses were also calculated for applied and evaluated stresses.

The applied Von-mises stress is 5.18 N/mm2 and the evaluated stress is

4.97 N/mm2 for the depth of 20mm. The ratio of Von-mises stresses is 0.96.

Figure 4.24 shows the applied and evaluated stress for the Specimen SP4.


a

2, 2b

c

1, 1

1 = 1.75 N/mm2

2 = -4.07 N/mm2

= -72.60Von = 5.18 N/mm2

Applied

1 = 1.91 N/mm2

2 = -3.74 N/mm2

= -71.94Von = 4.97 N/mm2

Evaluated

79


Specimen SP5 was stressed biaxially by loading 342kN and 94kN in

two directions. From the developed strain, the corresponding principal strains

/ stresses and Von-mises stresses were calculated and are given in Table

4.14.



a = -93micro-strain

b = -22micro-strain

c = 18micro-strain



= 82.20

1 = 0.13 N/mm2

2 = -2.99 N/mm2

Von = 3.05 N/mm2

Note :



Trepanning technique was applied on the stressed specimen. The

released strain was measured from the strain gages up to a depth of 50mm in

increments of 10mm. Released strain for different depth of cut for Specimen

SP5 is given in Table 4.15 and plotted in Figure 4.25.



10 52 11 -1020 72 16 -1330 81 19 -1740 74 18 -1550 70 17 -13

Note :


80

0

10

20

30

40

50

-40 -20 0 20 40 60 80 100Micro-strain

SG1SG2SG3


From the released strain, the principal strains / stresses and direction of



1 2 1 2 VonVon

ratioDepth of


10 11.6 -53.6 81.06 0.22 -1.82 1.94 0.6420 15.1 -74.1 81.19 0.28 -2.52 2.67 0.8830 18.7 -82.7 82.65 0.38 -2.81 3.02 0.9940 16.5 -75.5 82.76 0.32 -2.57 2.75 0.9050 14.6 -71.6 82.26 0.27 -2.44 2.59 0.85

Note :



81





The applied and evaluated stress for the Specimen SP5 is shown in

Figure 4.26.



Biaxial load was applied to the Specimen SP6 by loading of 178kN and

130kN in two orthogonal directions. From the strain developed the principal

strain / stress and Von-mises stresses were calculated and are given in

Table 4.17.

1 = 0.13 N/mm2

2 = -2.99 N/mm2

= 82.20Von = 3.05 N/mm2

Applied

1 = 0.38 N/mm2

2 = -2.81 N/mm2

= 82.65Von = 3.02 N/mm2

Evaluated

a

2, 2

b

c

1, 1

82



a = -77micro-strain

b = -77micro-strain

c = -49micro-strain



= -67.50

1 = -1.87 N/mm2

2 = -2.94 N/mm2

Von = 2.57 N/mm2

Note :



By core trepanning technique the released strain was measured from

the strain gages during drilling the core. Released strain for different depth of

cut for Specimen SP6 is given in Table 4.18 and plotted in Figure 4.27. The

maximum strain release occurred at the depth of 20mm.

Table 4. 18 Released strain for Specimen SP6

Released strain in micro-strainDepth of cut in mma b c

10 42 38 2320 66 72 4330 66 68 4340 65 63 4250 59 61 40

Note :

1. Positive value of strain / stress indicates tension

83

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80Micro-strain

SG1SG2SG3





1 2 1 2 VonVon

ratioDepth of


10 -21.5 -43.5 -74.97 -0.90 -1.58 1.37 0.5320 -33.6 -75.4 -61.66 -1.45 -2.70 2.34 0.9130 -36.8 -72.2 -65.21 -1.54 -2.61 2.28 0.8840 -38.6 -68.4 -70.22 -1.58 -2.50 2.19 0.8550 -34.6 -64.4 -64.78 -1.44 -2.34 2.04 0.79

Note :



Von-mises stresses calculated for applied and evaluated stresses are

2.57 N/mm2 and 2.34 N/mm2 respectively for the depth of 20mm. The ratio of

84

Von-mises stresses is 0.91. Figure 4.28 shows the applied and evaluated

stress for the Specimen SP6.



Specimen SP7 was loaded with 164kN and 104kN in two orthogonal

directions. The strain developed and the corresponding principal strain / stress

and Von-mises stresses calculated are given in Table 4.20.



a = -89micro-strain

b = -31micro-strain

c = 32micro-strain



= 88.82

1 = 0.55 N/mm2

2 = -2.72 N/mm2

Von = 3.04 N/mm2

Note :



a

2, 2b

c

1, 1

1 = -1.87 N/mm2

2 = -2.94 N/mm2

= -67.50Von = 2.57 N/mm2

Applied

1 = -1.45 N/mm2

2 = -2.70 N/mm2

= -61.66Von = 2.34 N/mm2

Evaluated

85


the core. The released strain was measured from the strain gages for depth

from 10mm to 50mm in 10mm increments. Released strain for different depth

of cut for Specimen SP7 is given in Table 4.21 and plotted in Figure 4.29.



10 45 13 -520 69 23 -1930 73 24 -2640 75 22 -1650 73 21 -16

Note :


0

10

20

30

40

50

-40 -20 0 20 40 60 80 100Micro-strain

SG1SG2SG3




86


1 2 1 2 VonVon

ratioDepth of


10 6.0 -46.0 82.18 0.05 -1.58 1.61 0.5320 19.0 -69.0 88.70 0.44 -2.35 2.59 0.8530 26.0 -73.0 89.71 0.67 -2.46 2.86 0.9440 16.6 -75.6 85.32 0.33 -2.58 2.76 0.9150 16.6 -73.6 85.22 0.34 -2.51 2.70 0.89

Note :




30mm. The applied and evaluated Von-mises stress is 3.04 N/mm2 and

2.86 N/mm2 respectively for the depth of 30mm. The ratio of Von-mises

stresses is 0.94. The applied and evaluated stress for the Specimen SP7 is

shown Figure 4.30.


a

2, 2

b

c

1, 1

1 = 0.55 N/mm2

2 = -2.72 N/mm2

= 88.72Von = 3.04 N/mm2

Applied

1 = 0.67 N/mm2

2 = -2.46 N/mm2

= 89.71Von = 2.86 N/mm2

Evaluated

87


Specimen SP8 was stressed in both direction by applying load of 89kN

and 193kN in two directions. The strain developed and the corresponding

principal strain / stress and Von-mises stresses calculated are given in

Table 4.23.



a = 58micro-strain

b = -11micro-strain

c = -71micro-strain



= -2.0

1 = 1.50 N/mm2

2 = -2.00 N/mm2

Von = 3.04 N/mm2

Note :




the core. At each incremental depth, the released strain was measured from


is given in Table 4.24 and plotted in Figure 4.31.

88



10 -36 6 4620 -46 8 5730 -49 9 6240 -45 8 5850 -40 8 55

Note :


0

10

20

30

40

50

-60 -40 -20 0 20 40 60 80Micro-strain

SG1SG2SG3




89


1 2 1 2 VonVon

ratioDepth of


10 36.0 -46.0 -0.70 1.11 -1.49 2.25 0.7420 46.1 -57.1 -1.39 1.42 -1.84 2.83 0.9330 49.1 -62.1 -1.29 1.51 -2.00 3.05 1.0140 45.0 -58.0 -0.83 1.38 -1.88 2.83 0.9350 40.0 -55.0 -0.30 1.22 -1.79 2.62 0.86

Note :



It is seen here that the maximum strain release occurred at the depth of


stresses. The applied and evaluated Von-mises stress is 3.04 N/mm2 and

3.05 N/mm2 respectively for the depth of 30mm. The ratio of Von-mises stress

is 1.01. The applied and evaluated stress for the Specimen SP8 is shown

Figure 4.32.


a

1, 1

b

c

2, 21 = 1.50 N/mm2

2 = -2.00 N/mm2

= -2.0Von = 3.04 N/mm2

Applied

1 = 1.51 N/mm2

2 = -2.00 N/mm2

= -1.29Von = 3.05 N/mm2

Evaluated

90


Specimen SP9 was loaded with 101kN and 237kN in two directions.

The strain developed and the corresponding principal strain / stress and Von-

mises stresses are calculated and given in Table 4.26.



a = 55micro-strain

b = -27micro-strain

c = -114micro-strain



= -0.85

1 = 1.16 N/mm2

2 = -3.41 N/mm2

Von = 4.11 N/mm2

Note :




the core. The released strain was measured from the strain gages for different

depth of cut. Released strain for different depth of cut for Specimen SP9 is

given in Table 4.27 and plotted in Figure 4.33.



10 -38 9 6020 -41 26 7930 -43 19 9540 -42 11 9050 -40 12 85

Note :


91

0

10

20

30

40

50

-60 -40 -20 0 20 40 60 80 100 120Micro-strain

SG1SG2SG3





1 2 1 2 VonVon

ratioDepth of


10 38.0 -60.0 1.17 1.13 -1.97 2.72 0.6620 41.4 -79.4 -3.33 1.18 -2.63 3.38 0.8230 43.4 -95.4 2.90 1.20 -3.18 3.92 0.9540 43.3 -91.3 5.57 1.21 -3.04 3.79 0.9250 40.9 -85.9 4.77 1.14 -2.86 3.57 0.87

Note :




30mm. The applied and evaluated Von-mises stress is 4.11 N/mm2 and

92

3.92 N/mm2 respectively for the depth of 30mm where the maximum stress

released. The ratio of Von-mises stresses is 0.95. The applied and evaluated

stress for the Specimen SP9 is plotted in Figure 4.34.



Specimen SP10 was stressed to the load of 78kN and 200kN in two

directions. The strain developed and the corresponding principal strain / stress

and Von-mises stresses calculated are given in Table 4.29.

a

1, 1

b

c

2, 21 = 1.16 N/mm2

2 = -3.41 N/mm2

= -0.85Von = 4.11 N/mm2

Applied

1 = 1.20 N/mm2

2 = -3.18 N/mm2

= 2.90Von = 3.92 N/mm2

Evaluated

93



a = 60micro-strain

b = -12micro-strain

c = -202micro-strain



= 12.12

1 = 1.18 N/mm2

2 = -6.59 N/mm2

Von = 7.25 N/mm2

Note :






is given in Table 4.30 and plotted in Figure 4.35. The maximum strain release

occurred at the depth of 20mm.



10 -33 8 10520 -43 11 15530 -50 10 15040 -47 10 14650 -44 10 141

Note :


94

0

10

20

30

40

50

-100 -50 0 50 100 150 200Micro-strain

SG1SG2SG3





1 2 1 2 VonVon

ratioDepth of


10 38.5 -110.5 11.04 0.96 -3.70 4.26 0.5920 52.7 -164.7 12.22 1.26 -5.53 6.26 0.8630 57.7 -157.7 10.90 1.47 -5.28 6.14 0.8540 54.8 -153.8 11.17 1.38 -5.15 5.96 0.8250 51.9 -148.9 11.42 1.29 -4.99 5.74 0.79

Note :



95

It is seen here that the maximum strain release occurred at the depth of




The applied and evaluated stress for the Specimen SP10 is shown Figure 4.36.


4.5 DISCUSSIONS

Ten specimens were tested with different combination of loads.

Table 4.32 gives the applied principal and Von-mises stresses based on the

measured strains.

a

1, 1b

c

2, 2

1 = 1.18 N/mm2

2 = -6.59 N/mm2

= 12.12Von = 7.25 N/mm2

Applied

1 = 1.26 N/mm2

2 = -5.53 N/mm2

= 12.22Von = 6.26 N/mm2

Evaluated

96

Table 4.32 Applied principal strain / stresses for tested specimens

1 2 1 2 VonSP

Id. Micro-strain Degree N/mm2

SP1 -2.5 -33.5 82.53 -0.27 -1.11 1

SP2 41.8 -101.8 70.61 0.8 -3.08 3.55

SP3 40.1 -118.1 85.27 0.65 -3.62 3.99

SP4 77.3 -138.3 -72.60 1.75 -4.07 5.18

SP5 20.1 -95.1 82.20 0.13 -2.99 3.05

SP6 -43.2 -82.8 -67.50 -1.87 -2.94 2.57

SP7 32.1 -89.1 88.82 0.55 -2.72 3.04

SP8 58.2 -71.2 -2.00 1.5 -2 3.04

SP9 55.0 -114.0 -0.85 1.16 -3.41 4.11

SP10 72.7 -214.7 12.12 1.18 -6.59 7.25

Note :



Trepanning technique was carried out by drilling the core on the stressed

specimen. Incremental drilling was carried out in steps of 10mm upto 50mm.

At each increment, the released strain was measured from the strain gages.

From the released strain the principal stresses and direction of principal stress

were calculated from the Equations (4.1), (4.2) and (4.3). From the

measurement of released strain, in-situ stress was evaluated and is given in

Table 4.33.

97

Table 4.33 Evaluated principal strain / stresses for tested specimens

1 2 1 2 Von

SPId. Micro-strain degree N/mm2Von

ratio

Max.releasedepth

SP1 -1.3 -31.7 81.38 -0.15 -1.11 1.04 1.040 30mm

SP2 36.1 -89.1 70.42 0.91 -2.93 3.48 0.980 30mm

SP3 29.2 -97.2 84.52 0.7 -3.29 3.69 0.925 30mm

SP4 67.6 -115.6 -71.94 1.91 -3.74 4.97 0.959 20mm

SP5 18.7 -82.7 82.65 0.38 -2.81 3.02 0.990 30mm

SP6 -33.6 -75.4 -61.66 -1.45 -2.7 2.34 0.911 20mm

SP7 26.0 -73.0 89.45 0.67 -2.46 2.86 0.941 30mm

SP8 49.1 -62.1 -1.29 1.51 -2 3.05 1.003 30mm

SP9 43.5 -95.4 2.90 1.2 -3.18 3.92 0.954 30mm

SP10 52.8 -164.8 12.22 1.26 -5.53 6.26 0.863 20mm

Note :



It is observed from the plots of released strain vs. depth of cut, at 10mm

depth of cut, partial strains are only released. When the depth of cut increases,

the strain release also increases. But after 30mm / 20mm, the released strains

stabilises. It is seen that the maximum strain release occurred at the depth of

30mm for seven specimens out of ten specimens. In three specimens

maximum strain release occurred at the depth of 20mm. Von-mises stresses

were also calculated for applied and evaluated stresses. The pattern of the

strain release is similar for all specimens. Comparison of applied and

evaluated stresses by concrete core trepanning technique was made and is

given in Table 4.34. The applied stresses and evaluated stresses are matching

closely. From the experiments the average of ratios of Von-mises

98

stresses is 0.957. This shows that almost 95% of the applied stress was

measured by using trepanning technique. The behavior of strain release is

similar in both experimental and numerical analysis. The pattern of strain

release shows that the results of previous trepanning investigation for uniaxial

stress state (Parivallal et. al. (2001) and Kesavan et. al. (2005)) and the current

trepanning for biaxial stress state are similar. The existing stress can be

evaluated from the measured strain by using a coefficient of 1.05. The

reliability of this technique was established from the laboratory experiments.

Table 4.34 Comparison of applied and evaluated stresses

Applied Evaluated

1 2 Von 1 2 VonSP

Id.N/mm2 N/mm2

Von

ratio

SP1 -0.3 -1.1 1.0 82.5 -0.2 -1.1 1.0 81.4 1.04

SP2 0.8 -3.1 3.6 70.6 0.9 -2.9 3.5 70.4 0.98

SP3 0.7 -3.6 4.0 85.3 0.7 -3.3 3.7 84.5 0.92

SP4 1.8 -4.1 5.2 -72.6 1.9 -3.7 5.0 -71.9 0.96

SP5 0.1 -3.0 3.1 82.2 0.4 -2.8 3.0 82.7 0.99

SP6 -1.9 -2.9 2.6 -67.5 -1.5 -2.7 2.3 -61.7 0.91

SP7 0.6 -2.7 3.0 88.8 0.7 -2.5 2.9 89.5 0.94

SP8 1.5 -2.0 3.0 -2.0 1.5 -2.0 3.1 -1.3 1.00

SP9 1.2 -3.4 4.1 0.9 1.2 -3.2 3.9 2.9 0.95

SP10 1.2 -6.6 7.3 12.1 1.3 -5.5 6.3 12.2 0.86

From the numerical analysis it is observed that for small depth of cut

the strain release is also less. Maximum strain released occurred at a depth of

20mm. Beyond that the strain release is less though the reduction is small.

Comparison was made for the values obtained from the experimental and

numerical studies. In numerical analysis, the maximum strain release

99

occurred at 20mm depth. In experimental testing, for seven out of ten

specimens, maximum strain release occurred at 30mm depth and for the

remaining three specimens the maximum strain release occurred at a depth of

20mm. The behavior of strain release is similar in both the cases namely,

experimental and numerical analysis. The numerical studies and laboratory

experiments have clearly indicated that the proposed technique is efficient in

assessing the existing stresses. These comparisons show that the proposed

trepanning technique can be used for in-situ stress evaluation under bi-axial

stress conditions. The accuracy of the technique depends on the correctness of

the material constant used, since the elastic modulus (E) and Poisson’s

ratio ( ) of concrete are required to calculate the existing stresses from the

measurement of strain.

4.6 SUMMARY

The evaluation of in-situ stresses under bi-axial stress state by using

concrete core trepanning technique was developed. This technique employs a

three element strain gage rosette configuration of 0 /45 /90 orientation to

measure the strain release due to core drilling. The reliability of this technique

was established in the laboratory, by conducting experimental investigations

on ten concrete specimens with known stress/strain field. Numerical analysis

carried out using finite element method for evaluating the validity of the

method was also presented. The comparison of experimental and numerical

studies was carried out. The numerical studies and laboratory experiments

have clearly indicated that the proposed technique is efficient in assessing the

existing stresses. Hence, the concrete core trepanning technique can be used

for in-situ stress evaluation under bi-axial stress conditions.

Documents

CHAPTER 4 CONCRETE CORE TREPANNING TECHNIQUE 4.1 …shodhganga.inflibnet.ac.in/bitstream/10603/15821/9/09... · 2015-12-04 · core drilling machine, with diamond tipped cutting tool