David Marques, João Veludo, Pedro Santos The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles
12th International Workshop on Micropiles, Kraków, Poland, June 2014 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 1/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 2/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles 1. SCOPE Grouted connections One of the methods currently used in foundations strengthened with micropiles, consists in installing Pre-drilled Grout Hole micropiles through existing RC footings which are then sealed with cement grout. Micropile In this case the efficiency of the Existingfoundation load transfer mechanism depends on the bond strength at steel / grout / concrete interfaces.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 3/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles
• Key parameters on grouted connections:
i. Diameter of the hole;
ii. Embedment length of the insert;
iii. Roughness of the hole surface;
iv. Textured of micropile surface (smooth and textured);
v. Confinement provided for concrete and reinforcement
(Passive / Active)
vi. Strength of concrete and grout 12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 4/48
The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 5/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles 2. AIMS The study herein presented describes the behaviour of grout / concrete interfaces. Two major aims have been defined for this research study: • To quantify the influence of the roughness of the hole surface on the bond strength at grout / concrete interfaces; • To quantify the influence of different confinement levels in the connection behaviour using different levels of passive confinement.
26 compression tests were performed with a reinforcing bar grouted in RC cylinder blocks: 4 different types of hole surface treatment and 5 different confinement levels were used.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 6/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 7/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles 3. Previous Research • Jesús Gómez and Allen Cadden, Schnabel Engineering (2005)
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 8/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles 3. Previous Research • Jesús Gómez and Allen Cadden Schnabel Engineering (2005)
Textured micropiles
100 psi=0.69 MPa; 1 in.=25.4 mm
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 9/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles 3. Previous Research • Jesús Gómez and Allen Cadden Schnabel Engineering (2005)
“For textured micropiles, the connection capacity is mostly controlled by frictional effects due to the large dilation that takes place during relative movement at the grout-steel interface. The splitting action induced by shear rings will typically induce yielding of the reinforcement steel of the footing. It was found that, for cased micropiles with shear rings, the frictional effect provided by the footing reinforcing steel may be estimated using the total yield capacity of the reinforcing steel multiplied by a friction factor.”
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 10/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • João Veludo, PhD Thesis (2013) 99 compression and pull-out tests were performed with smooth and textured micropiles inserts grouted in RC blocks and in PVC/steel tubes
Steel plate Parameters evaluated: (150×150×20) Compression Tests D t=60
150 e t=6 i. Diameter of the hole (Dh); Hole Grout Tests with smooth Dywidag lb bar (φ16) D 500 ii. Embedment length of the insert (lb); Tube h micropile inserts Styrofoam insert iii. Roughness of the hole surface;
Steel plates (100×100×10) D t=60 iv. Textured of micropile surface e t=6 Pe Pe 50 lb (smooth and textured micropiles); 150 Pe Pe Tests with textured
Dh micropile inserts Dywidag v. Lateral active confinement (Pe). bar 450×450 (units in mm)
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 11/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles .Materials Properties Concrete Grout Steel
Indented surface Young Modulus Shear Rings 60 tr=5 mm; tr fcg,28 (MPa) 32.8 W/C ratio 0.40 br br=5.5 mm; (mm) 71 sr=75 mm E (GPa) 35.2 f (MPa) 53.0 g,28 cg,28 Reinforcing Bar Dywidag bar (φ=16mm), Slump (cm) 13.0 E (GPa) 14.2 g,28 Grade 500/550
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 12/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles .Test set-up and Procedures
Load Cell (100 tf) LVDT LVDT (25 mm) (25 mm)
Load cells Styrofoam (10 tf) insert
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 13/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Failure Modes in Tests Performed with Textured Micropiles
Confinement
Level 1
Level 2
(1) Monolithic failure (2) Failure at (3) Mixed Bond Failure Level 3 grout/concrete interface 12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 14/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles .Results of Compression Tests with Textured Inserts
Hole D L P P f δ K δy Test h e e u b u 0 δu/δy surface (mm) (mm) (kN) (kN) (MPa) (mm) (kN/mm) (mm)
C 1,2 Wire brushed 102 200 120 (Level 1*) 301.8 4.71 13.55 275 1.09 12.4
C 3,4 Wire brushed 102 275 240 (Level 1) 486.3 5.52 8.38 775 0.66 12.6
C 5,6 Wire brushed 102 350 240 (Level 1) 802.6 7.16 1.33 1482 0.55 2.4
C 7,8 Wire brushed 92 350 240 (Level 1) 864.6 8.55 2.54 1548 0.56 4.5
C 9,10 Wire brushed 122 350 240 (Level 1) 638.4 4.76 4.61 1130 0.57 8.1
C 11,12 Indented 102 350 0 (Level 0) 842.1 10.79 1.15 1075 0.78 1.5
C 13,14 Indented 102 350 240 (Level 1) 980.5 12.56 1.58 1529 0.62 2.6
C 15,16 Indented 102 350 360 (Level 2) 1040.4 13.33 4.54 1544 0.67 6.8
C 17,18 Indented 102 350 480 (Level 3) 1097.6 14.06 5.66 1611 0.68 8.3
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 15/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles .Load / Displacement Curves
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 16/48
The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Bond at Grout / Concrete Interface 1200 • The roughness of the hole surface Micropile failure: 1080 kN
1000 980.5 Micropile yield: 925 kN influence the behaviour of the
802.6 800 14 connection;
600 • For the same situations considered, it
Load (kN) can be seen that the use of indented 400 5,6 surfaces guarantee an increase in the 200 connection capacity and prevents 0 0 5 10 15 failure at grout / concrete interface in Displacement (mm) BC-TT-5,6-wire-brushed surface BC-TT-13,14-Indented surface the confining region.
Pd≤τπ gd ⋅⋅ Dnh h ⋅ d ⋅ d + f bd,, sr ⋅⋅π D h ⋅(ln b − d ⋅hP d ) +⋅µe total number of indentations in the hole surface
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 17/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Lateral Active Confinement
1200 . In specimens with indented Micropile Failure: Pu,m=1080 kN hole surface and with 1000 17,18
Py,m=925 kN 15,16 confinement a ductile 800 14 response was observed.
600 The connection capacity Load (kN) and the ductility increases 400 with the confinement level. 11,12 200 . Contrarily in specimens
0 without confinement a 0 5 10 15 Displacement (mm) brittle failure were BC-TT-11,12 (No confinement) BC-TT-14 (Pe,total=240 kN) BC-TT-15,16 (Pe,total=360 kN) BC-TT-17,18 (Pe,total=480 kN) observed.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 18/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles .Variation in the Tension Forces .Idealized Tensile in Dywidag Bars Stress Field 1200 100 Micropile failure: P =1080 kN radial u,m 90 Pe Pe stresses 1059.7 1000 Micropile Yeld: Py,m=925 kN 80 1 70 Pe tensile stress Pe 800 3 2 field 4 60
600 50
Load (kN) 40 400 30 Pe Pe
20 Dywidag (kN) Bars in Force 200 10
0 0 radial 0 5 10 15 Pe Pe splitting Displacement (mm) cracks BC-TT-15 Bar 1 Bar 2 Bar 3 Bar 4 The tension force, initially applied to the Dywidag bars, starts to increase immediately after the failure of the connection, according to the splitting patterns, until a constant value is reached.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 19/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Bond strength vs. Lateral Active Confinement . The results of the tests with different confinement levels shows that the bond strength increases linearly with the confinement level. . The increase in the connection capacity seems to be proportional to the friction developed in the steel / grout interface.
∆=⋅PPuµ e, total
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 20/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles
. Design Recommendations: Smooth and Textured Micropiles
Grout Dh Pe dt 1. Bond at Steel / Grout Interface
hc fbu Pe 0.30+ 0.0101Kfr ≤≤ bu 1.75 + 0.0101Kr fbu lb Pe 4.14−×≤≤−× 0.054ea f bu 4.83 0.054ea ( Gómez et al .,2005) 2. Bond at Grout / Concrete Interface Tube Pd Pd P≤τπ ⋅⋅ Dnh ⋅ ⋅ + f ⋅⋅π D ⋅( l −nP ⋅h ) +⋅µ 1) Bond at Steel/Grout 2) Bond at Grout/Concrete d gd h d d bd,, sr h b d d e total Interface Interface → number of indentations in the hole surfa c e
Grout Dh 3. Local Crushing in Grout dr dr b P≤⋅ nF →number of shear rings d Punching d r Rdu, g sr br shear surface tr hd FRdu, g=⋅⋅⋅νν fAAA cd ,0 g 10/ ≤⋅⋅⋅ 3.0 fAcd,0 g d dt 4. Punching Shear
Pd Pd 3) Local Crushing 4) Punching Shear 5. Structural Capacity of Micropile in Grout
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 21/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles
. Most Relevant Conclusions: Textured Micropiles
1. The behaviour of a grouted connection with textured micropiles depends on: the hole diameter, the hole surface treatment, number of shear rings, confinement level, grout and concrete strengths;
2. Textured micropiles provide a higher connection capacity than smooth micropiles. The bond mechanism is controlled essentially by bearing and friction;
3. The roughness of the hole surface influence the behaviour of the connection;
4. Active confinement increase the connection capacity and the ductility and spread cracking in concrete blocks;
5. The bond strength decreases with the increase of the hole diameter and increases with the embedment length;
6. Textured micropiles reached yield at embedment lengths substantially less than predicted by design codes for RC structures (6 to 7.5 bar diameter).
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 22/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 23/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Experimental Programme 26 compression tests were performed with a reinforcing bar grouted in RC cylinder blocks • 4 different types of hole surface treatment and 5 different confinement levels were used. Concrete Hole Concrete Hole
Steel Steel PVC PVC bar Ø25 tube tube bar Ø6 Circular 300
300 hoops Ø6 Grout Grout (in mm) 114 75 114 114 75 114 (in mm) 6 303 6 6 303 6 315 315 a) Specimens with different b) Specimens with different hole surface treatment confinement levels
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 24/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Materials Grout : W/C = 0.36; Flow time =15 s; Air content = 1.1 %
fc,g= 64.6 MPa; Eg=16.9 GPa
Concrete: Class C20/25; Consistence S3
Steel (S400NR): Reinforcing bar 25 mm Transverse reinforcement:6 mm
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 25/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Geometry of the specimens . Hole surface treatment (Specimens without transverse reinforcement)
Concrete Hole
PVC Steel tube bar Ø25 300 300 300 300
Grout
(in mm)
6 114 75 114 6 6 114 75 114 6 6 114 75 114 6 6 114 75 114 6 303 303 303 303 315 315 315 315 1) Left as drilled 2) Sandblasted 3) Helical 4) Indented
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 26/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Geometry of the specimens . Level of Confinement (Specimens with transverse reinforcement)
Concrete PVC 20 20 20 20 tube 52 65 87
bar Ø6 130 52 65 87 Circular 52 300
hoops Ø6 65 52 130 87
Grout 65 52 20 20 20 20 (in mm) 6 114 75 114 6 6 114 75 114 6 6 114 75 114 6 6 114 75 114 6 303 303 303 303 315 315 315 315 1) Level 1 2) Level 2 3) Level 3 4) Level 4
Tests were performed in specimens with a indented hole surface and different levels
of confinement, considering four different levels :
• (1) 3 stirrups; (2) 4 stirrups; (3) 5 stirrups; and (4) 6 stirrups.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 27/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles . Test Programme
Specimen Hole Surface Treatment Transverse Reinforcement L1 - L3 Left as drilled No confinement
R1 - R3 Sandblasted No confinement
H1 - H3 Helical No confinement
A1 - A3 Indented No confinement
C3.1 - C3.3 Indented 3 stirrups (s=130 mm)
C4.1 – C4.3 Indented 4 stirrups (s=87 mm)
C5.1 – C5.3 Indented 5 stirrups (s=65 mm)
C6.1 – C6.3 Indented 6 stirrups (s=52 mm)
T1-T2 Indented 6 stirrups (s=52 mm)
s = spacing of the transverse reinforcement
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 28/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Preparation of the Test Specimens The hole surface treatment was prepared on the internal formwork tubes (75 mm diameter), prior to the concrete cast.
left as drilled sandblasted surface helical surface indented surface 1. The left as drilled surface condition was obtained by cast against the PVC formwork without further treatment. 2. The sandblasted surface condition was simulated by glued sand on the formwork. 3. The indented surfaces were obtained by creating an irregular shape in the hole surface. The grooves for the indented and helical hole surfaces were 12,5 mm high and 10.0 mm deep.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 29/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Preparation of the Test Specimens
a) Formwork b) Smooth surface c) Rough surface d) Helical surface
e) Indented surface f) Reinforcement g) Cast h) Grouting
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 30/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Instrumentation Details
Strain Transverse gauges reinforcement
13 14
11 12 9 10 1 2 7 8 5 6 3 4
• The axial strain gauges were used to evaluate the load transfer between the grout and the reinforcing steel bar and the horizontal ones to evaluate the tensile stress of the stirrups and the level of passive confinement. • Two TML PFL-10-11 (120Ω) strain gauges were bonded in the surface of each stirrup in horizontal direction and two were mounted in vertical direction.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 31/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Tests set-up and Procedures
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 32/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Instrumentation Load / Displacement Curve Load / Axial Strain 600 600 Propagation of Theoretiical Composite Axial Test C6-3 Level 4 Pmáx=530.4 kN longitudinal cracking Stiffness EA=157900 kN P=462 kN 500 500
Cracking Load 400 400
300 300 Load (kN) Load (kN)
200 200
100 100
Strain Gauges 1,2 0 0 0 5 10 15 20 -2800 -2400 -2000 -1600 -1200 -800 -400 0 Axial Displacement (mm) Axial Strain (x10-6) • The load / displacement curve shows a The load supported by the bar is about ductile behaviour of the connection. 60% of the total load. • The response is almost linear until the Composite value of 400 kN is reached and then EA= Ess A + E g Ag becomes nonlinear. axial stiffness
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 33/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Tests set-up and Procedures
Load / Tangential strain variation left side Load / Tangential strain variation right side
600 600 Yield strain Longitudinal Longitudinal Yield strain (1.74%E-3) cracking cracking (1.74%E-3) 500 500
Cracking Load Cracking Load 400 400
300 300 Starin Gauge 14 Strain Gauge 13 Strain Gauge 11 Axial Load (kN) Axial Axial Load (kN) Axial 200 Strain Gages 12 200 Strain Gauge 9 Strain Gauge 10 strain Gauge 7 100 Strain Gauge 8 100 Starin Gauge 5 Starin Gauge 6 Starin Gauge 3 0 0 -400 0 400 800 1200 1600 2000 2400 2800 3200 -400 0 400 800 1200 1600 2000 2400 2800 3200 Tangencial Strain (x10-6) Tangencial Strain (x10-6)
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 34/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Failure Modes
• In all the specimens splitting radial cracks are visible at the surface of the concrete, three or four cracks in each specimen. • The cracks propagate along the entire length of the concrete cylinder. • For the specimens with indented hole surface the failure occurs by shearing in the grout in the grooves and shear in concrete.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 35/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Summary of results Hole surface Transverse L (1) P δ P f K K δ Test g máx Pmáx r bu 0 Pmax y μ treatment Reinforcement. (mm) (kN) (mm) (kN) (MPa) (kN/mm) (kN/mm) (mm) g L1 Left as cast No 290 237.2 0.731 3.47 380.2 324.5 0.624 1.17 L2 Left as cast No 293 241.3 0.697 62.6 3.50 382.3 346.2 0.631 1.10 L3 Left as cast No 295 178.3 1.217 52.5 2.57 163.4 146.5 1.091 1.12 R1 Sandblasted No 296 192.8 0.935 72.8 2.77 246.1 206.2 0.783 1.19 R2 Sandblasted No 293 221.0 0.670 73.1 3.20 383.9 329.9 0.576 1.16 R3 Sandblasted No 294 189.7 0.754 68.3 2.74 317.0 251.6 0.598 1.26 H1 Spiral No 292 269.2 0.887 109.4 3.91 367.5 303.5 0.733 1.21 H2 Spiral No 292 305.5 0.967 135.5 4.44 385.1 315.9 0.793 1.22 H3 Spiral No 297 303.8 0.945 129.0 4.34 356.7 321.5 0.852 1.11 A1 Indented No 292 242.3 0.953 87.1 3.52 301.7 254.2 0.803 1.19 A2 Indented No 295 305.2 0.947 87.2 4.39 379.4 322.3 0.804 1.18 A3 Indented No 292 338.5 1.067 94.9 4.92 378.7 317.2 0.894 1.19 C3.1 Indented 3 Stirrups 294 285.5 0.921 225.3 4.12 334.1 310.0 0.855 1.08 C3.2 Indented 3 Stirrups 296 311.3 1.055 278.7 4.47 335.5 295.1 0.928 1.14 C3.3 Indented 3 Stirrups 295 305.0 0.963 262.3 4.39 401.2 316.7 0.760 1.27 C4.1 Indented 4 Stirrups 294 403.7 3.031 323 5.83 381.9 133.2 1.057 2.87 C4.2 Indented 4 Stirrups 294 325.3 2.135 332.5 4.70 333.4 152.4 0.976 2.19 C4.3 Indented 4 Stirrups 296 285.5 1.851 291.1 4.10 301.0 154.2 0.949 1.95 C5.1 Indented 5 Stirrups 296 351.0 1.205 277.7 5.04 374.8 291.3 0.936 1.29 C5.2 Indented 5 Stirrups 295 379.4 2.458 333.1 5.46 395.3 154.4 0.960 2.56 C6-1 Indented 6 Stirrups 293 387.9 2.347 346.7 5.62 393.4 165.3 0.986 2.38 C6-2 Indented 6 Stirrups 296 364.4 2.035 370.0 5.23 353.1 179.1 1.032 1.97 C6.3 Indented 6 Stirrups 296 382.7 2.339 354.6 5.49 361.5 163.6 1.059 2.21
Lg - Embedment length of the grout pocket; Pu - Peak load; fbu - bond strength;δpmáx - total displacement of grout pocket at peak load; K0 - initial stiffness; Kpmáx ~ final stiffness; δy - displacement at yield; δu/δy –ductility
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 36/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Load / Displacement curves Specimens without transverse reinforcement and different hole surface treatments
450 L-1 left as drilled L-2 left as drilled • A brittle failure was observed in 400 L-3 left as drilled all the tests. R-1 sandblasted 350 R-2 sandblasted • First, a linear elastic branch is R-3 sandblasted 300 H-1 helical identified, presenting similar A-3 H-2 helical 250 H-3 helical values of tangent stiffness, until A-1 Indented the failure of the connection. 200 A-2 Indented
Load (kN) A-2 H-3 A-3 Indented • After the failure of the 150 H-2 L-2 connection, a sudden loss of the H-1 100 A-1 connection capacity is R-1 50 L-3 observed. R-3 R-2 • Residual value is about 30 to 0 0 5 10 15 40% of the maximum load. Axial Displacement (mm)
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 37/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Load / Displacement curves Specimens with transverse reinforcement and an indented surface
450 • A ductile failure was observed in all
400 the tests. C4-1 C6-2 C6-1 • First, a linear elastic branch is 350 C6-3 C5-2 identified, presenting similar values 300 C3-2 C5-1 of tangent stiffness, until the 250 C3-3 C3-1 appearance of the first splitting C4-2 200 C4-3 C3-1: Level 1 cracks in concrete top surface. Load (kN) C3-2: Level 1 C3-3: Level 1 150 C4-1: Level 2 • The response becomes non-linear C4-2: Level 2 C4-3: Level 2 at 80-90% of the peak load 100 C5-1: level 3 C5-2: Level 3 • After the peak load is reached, the 50 C6-1: Level 4 C6-2: Level 4 confinement provided by transverse C6-3: Level 4 0 0 5 10 15 reinforcement guarantees a high Axial Displacement (mm) capacity and ductility of the connection.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 38/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Bond strength vs. hole surface treatment
5.0 • Specimens with helical and
4.18 4.14 indented hole surface lead to 4.0 an increase in the bond strength of approximately (MPa) 3.10
bu 2.85 3.0 35%. • The bond strength obtained for 2.0 specimens prepared by
Bond Strength, f sandblasting is lower than the 1.0 initially expected value due to an excessive use of release 0.0 Left as Drilled Sandblasted Helical Indented agents Hole Surface Treatment
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 39/48 bond strength increase with the confining pressure provided by passive reinforcement. A high correlation coefficient is observed (
The volumetric ratio of transverse reinforcement (ρω=ωTheω.fcd influence/fyd), in percentage, of hole surface for each treatment specimen and obtained confinement is: 0.34 forlevel Level in the 1; 0 connection capacity between RC footings and strengthening micropiles • Bond strength vs. confining pressure 6.00 Level 4:C6 • Bond strength increase with the Level 3:C5 (1.47;5.41) (1.18;5.17) 5.50 confining pressure provided by Level 2:C4 passive reinforcement. 5.00 (0.88;4.88) Level 1:C3 • A high correlation coefficient is (MPa) (0.59;,4.25) bu 4.50 observed (0.94). • These results are consistent with 4.00 those obtained by Veludo (2013) in
3.50 tests performed with textured Bond Strength, f micropiles grouted in predrilled 3.00 holes in RC footings with active fbu = 1.2718σ2 + 3.6138; R² = 0.94 2.50 confinement. 0.40 0.60 0.80 1.00 1.20 1.40 1.60
Confining Pressure, σ2 (MPa)
• The volumetric ratio of transverse reinforcement (ρw=ww.fcd/fyd), in percentage, for each specimen obtained is: 0.34 for Level 1; 0.51 for Level 2; 0.68 for Level 3; and 0.85 for Level 4. The latter provides an increase in the bond strength of 27%.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 40/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Most Relevant Conclusions 1. From previous studies it can be stated that confinement is the most relevant parameter in grouted connections. Active and passive confinement has a notable increasing effect in ductility and in the load capacity of the connection.
2. From the research study herein described, it was concluded that: i. specimens without transverse reinforcement show a brittle failure and in specimens with transverse reinforcement a ductile behaviour was observed; ii. the use of an indented hole surface leads to the highest bond strength at the grout / concrete interface; iii. the use of an indented hole surface in grouted connections provides an adequate load capacity and ductility of the connection if the concrete specimen has sufficient reinforcement. In this situation, by increasing the volumetric ratio of transverse reinforcement the bond strength at grout / concrete interface increases.
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 41/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles Outline
1. Scope
2. Aims
3. Previous Research
4. Present Research
5. Future Research
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 42/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models João Almeida; Miguel Lourenço • To develop Strut and Tie Models to design foundations strengthening with micropiles according to: i. Minimum embedment length; ii. Variation in geometric characteristics and type of micropile; iii. Applied force (concentric / eccentric) and Pd location of active prestressing and the amount Pe Pe of existing reinforcement; iv. Influence of combined loading, relative shear Pe Pe ring and grooves location, and grout strength; v. Grout constitutive relationships.
12 th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 43/48
The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models
• Modeling grout connections
Pg active σ t,g τg prestressing τg grout compression Pe σ t,g strut Pf
Pf Pe strut Pg ea
P f - force carried by compression strut due to friction P g - force carried by compression strut due to shear ring bearing; P e- active prestressing force e a - annular width; σ t,g - tensile stress of grout; τ g - shear stress of grout
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 44/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models • Concentric Load • The value (and location) of the resulting of tensile forces as well as the possibility of using the direct load transfer depend on the embedment length. • This may lead to the need to increase the thickness of the existing foundation. • How to balance tensile forces?
João Almeida Miguel Lourenço
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 45/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models • Concentric Load • Model for grouted connections with lateral confining bars (prestressed), active or passive. • The anchorage of tensile force introduces compression in the connection zone. • The solution with prestressed bars also improves the state of stress on the anchorage zone of the micropiles. • If the prestressing force introduced is greater than necessary to balance ∆N, it is possible to have load transfer between the soil and the micropiles. João Almeida; Miguel Lourenço
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 46/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models • Eccentric Load The value (and location) of the resultant of tensile forces, acting in the upper and lower faces of the foundation, as well as the ability to use the direct transfer of loads, depends: • on the embedment length of the micropile (which may act in compressive and tension); • on the reinforcement in tension
João Almeida; Miguel Lourenço in column section. 12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 47/48 The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles • Strut and Tie Models • Enlarge Existing Spread Footing and Overlay
João Almeida; Miguel Lourenço Shear at the interface between concrete cast at different times Coesão Atrito Dowel action
vRd, i=⋅ c f ctd +⋅µσ n +⋅ ρf yd ⋅( µ ⋅ sen α +cos α) ≤ 0.5 ⋅⋅ ν fcd
12th International Workshop on Micropiles, Kraków, Poland, 12-14 June 2014 48/48 David Marques, João Veludo, Pedro Santos The influence of hole surface treatment and confinement level in the connection capacity between RC footings and strengthening micropiles
12th International Workshop on Micropiles, Kraków, Poland, June 2014