15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

THE NEW ITALIAN GUIDE LINES FOR FRP STRENGTHENING OF MASONRY AND TIMBER STRUCTURES

Benedetti, Andrea1; Sacco, Elio2; 1 PhD, Professor, University of Bologna, DICAM Department, andrea.benedetti@unibo.it

2 PhD, Professor, University of Cassino, DIMSAT Department, sacco@unicas.it

In year 2004 the National Research Council of Italy published the first English version of the well-known DT 200 guidelines “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures”. The document was based on the collaboration of more than twenty Italian professors working on monuments, structural strengthening, seismic protection, new materials and theoretical mechanics. Seven years later, the basic ideas of the scientific community on the mechanics of FRP debonding are well diffused and settled, but many new application systems and techniques have displaced the boundary of the real world far from our basic knowledge. In the meanwhile, a lot of experiments lead us to gather a tremendous set of data which are of very difficult comparison due to the variety of the construction materials and testing procedures even in Europe only. Although the code calibration based over a consistent database of such experiments is far to come, the actual picture shows that the most part of the European guidelines (Bulletin 14 of Fib among the others), and the Italian ones in particular, gave rise to robust applications, with sensible safety margins. In the paper, some proposed modification to the DT 200-2004 document and the underlying experimental database used for revision of masonry clauses are presented. Finally some comments are added in order to present the interaction of technical documents with a complex chaotic reality such the one of L’Aquila city after the 2009, April 6th earthquake.

Keywords: masonry, strengthening, FRP, guidelines, seismic mitigation INTRODUCTION Italy in recent times has been one of the propulsive actors in the development of new strengthening techniques for masonry structures. Unfortunately, the pushing force of this investigation must be identified in a number of earthquakes causing severe damages in all the country. The academic community thus promoted the transfer of a huge amount of experimental and theoretical data into a guide line document, helpful in establishing a common knowledge base for designers. Through this commitment the CNR DT 200 came to reality in 2004. Although the first version of the CNR DT 200 document is not so different from the last prepared revision, in the meanwhile the accumulated data have grown largely, and therefore the revised version includes formulas and parameters which have been calibrated once more. Among the important updates we can cite:

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

- The fracture energy formula has revised coefficients, - The effective length of externally bonded reinforcements has been updated, - A formula for shear design of panels with cross diagonal reinforcement was added.

PREDICTION OF FRP DEBONDING IN MASONRY STRCTURES The fracture mechanics of FRP to concrete interface is today a well understood phenomenon although some shadows still remain, but the case of masonry has several drawbacks which render the problem less solvable. The main concern is consequence of the very large combination of blocks and mortars which are employed in the masonry structure erection. This large variability hinders the need for a general formulation and increases the statistical variability in calibration studies so that obtaining the right design value means the adoption of large safety factors. Moreover masonry is a very irregular solid so that in most cases a regularization layer is needed. By this way the reinforcement is bonded over a material stronger than bricks, while this thicker layer is in turn bonded to bricks. This gives rise to a composite effect, which rarely is taken into account in the design of the anchorage force. In the following we shall consider a simple externally bonded unidirectional FRP strip struck to a brick texture interleaved with mortar (fig. 1). As usual, f stands for “fibre”, b for “bricks”, m for “masonry” and d for “design”; E is the elastic modulus, f the strength, b the breadth, t the thickness. According to the fracture mechanics approach [for a presentation see Brosens, Talijsten, Fib…], we define the debonding stress as the tension in the fibres leading to support detachment in the external reinforcement, and effective length as the length over which the anchoring force of the FRP reinforcement does not increase further.

lb ≥ l

e

b

bft

f

Fmax

Figure 1: Geometry of the externally bonded reinforcement

• 2

f fed u

Fd

max ,150mm8

E tl s

πΓ

⋅ ⋅ = ⋅ (1)

The specific fracture energy of the surface can be calculated by multiplying the cohesion of the supporting material times the ultimate slip and the stress intensification factor:

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

• b GFd bm btm

k kf f

FCΓ ⋅= ⋅ ⋅ (2)

Where FC is the confidence factor (>1) stated for the structure to be reinforced, fbm and fbtm are the mean compressive and tensile strength of the support material, and kG is the slip factor fixed as:

- 0,031 mm for clay bricks, - 0,032 mm for tuff stones, - 0,012 mm for calcarenite stones.

The geometric factor kb linked to the stress spreading in the support material and can be evaluated as:

• fb

f

3 /

1 /

b bk

b b

−=+

(3)

Where the involved widths are represented in figure 2.

a+c=bd

bf a

c

bf a c

a+c=bd

bf

c

a

Figure 2: Geometric stress diffusion in regular and irregular masonry textures

The bond strength of the fibre reinforcement is finally evaluated by solving the complementary energy equation. For distances from the end less than the effective one, a parabolic interpolation is used. The FRP resisting force in the internal zones far from the ends more than 3 le can be increased up to twice the end anchorage force:

• f Fdfdd

f,d f

21( ) ( )

Ef x x

t

Γ λγ

⋅ ⋅= ⋅ ⋅ (4.a)

•

ed ed

( ) 2 if

1 ( ) 2 if

( ) 2 if 3

ed

ed

ed

x xx x l

l l

x x l

x x l

λ

λ

λ

= ⋅ − ≤

≤ ≤ ≥

= ≥

(4.b)

γf,d is the safety factor which in case of end mode II debonding is assumed equal to 1,25. A very important added clause in the new DT 200 version concerns the verification of FRP reinforcement bonded to primer regularization layers. In this case the verification for the

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

primer detachment from the brick surfaces can be worked out with the presented formulas, but considering an equivalent composite material, which is characterised by the homogenised properties derived by mixing fibres and primer:

• h f rt t t= + , f f r rh

h

E t E tE

t

⋅ + ⋅= (5)

It is to highlight that in this type of verification the specific fracture energy of resin must be used in the fibre debonding analysis, while the brick fracture energy in the homogenised composite debonding analysis.

SHEAR RESISTANCE OF DIAGONALLY STRENGTHENED WALLS The shear strengthening of squat walls can be effectively carried out by using strip crosses laying along the diagonals of the panel. In this case a combination formula is necessary in order to combine the shear capacity of the masonry with the increase due to the FRP strips.

Figure 3: Geometry of the diagonally reinforced wall

The shear capacity of the wall is obtained by superposition of the two components due to the masonry resistance and the FRP resistance. The cited guide lines however, take into account the limited ductility of the system, so that if a limit distortion is defined for the wall, even the stretch of the FRP strip has to be limited according to this value. If B, H, and t are base length, height and thickness of the wall, Af is the total reinforcement transversal area, we have the following formulas:

• { }Rd Rd,m Rd,f Rd,maxmin ,V V V V= + (6)

• Rd,m vdRd

1V d t f

γ= ⋅ ⋅ ⋅ (7)

• vd 0 0

0

0,4 1 mvk m vk

vk

f f ff

σ= + σ ≈ + (8)

α

δ

D∆

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

• { }Rd fddRd,1 Rd,2

f

1min , min 0.005,

sin cos

f

H H E

δ δ δα α

= = ⋅

(9)

• Rd,m 2RdRd f fsin cos

0.005

VV E A

H

δ α α = + ⋅ ⋅

(10)

It is to consider that this procedure is almost ever on the safe side; in fact a upper limit to the FRP contribution can be defined as:

• Rd Rd,m fdd fcosV V f Aα= + ⋅ ⋅ (11)

A very important question arises about the consideration of the compressed strips. In general compression strips must be disregarded in the design stage. It is however to consider that if the strips are restrained against buckling by using chord connectors or nailing, the stiffness and the strength can be doubled by the strips acting in compression. As we shall see interpreting experiments, the compressed strips are acting up to the collapse in the tested panels, even in cycles of reversed loading. EXPERIMENTAL EVIDENCE OF THE BOND INTERPRETATION The calibration phase of the new DT 200 version was supported by a large number of new experimental debonding tests carried out by several research teams linked in European projects. A subset of the used data can be found in the following papers: Accardi (2007), Aiello (2006), Briccoli Bati (2009), Capozzucca (2010), Casareto (2006), Garbin (2010), Basilio (2007), Olivito (2007), Pfeiffer (2009), Sacco (2011).

Figure 3: Experimental testing of solid (Garbin) and hollow (Pfeiffer) bricks

The following figures point out the result of the calibration process.

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

Figure 4: Calibration process for the experimental investigation of Briccoli Bati (2010)

Figure 5: Calibration process for all the data used in the analysis

Figure 6: Comparison of experimental and predicted debonding forces for the data set

The value of the slip coefficient leading to the specific fracture energy evaluation does not change so much. It is however to point out that this final result is due to an increase of the mean kG value with a simultaneous increase of its standard deviation. The consideration of a number of new data sets introduces a large variety of different experimental set up, brick types and reinforcement systems, so that even the guide lines increase their soundness, the design resistances do not increase accordingly.

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0 50 100 150 200 250

bf = 10 mm bf = 20 mm

bf = 40 mm bf = 60 mm

bf = 80 mm media RMS

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,00 5,00 10,00 15,00 20,00

Briccoli Capozzucca Casareto Garbin Rilem GFRP Sacco Olivito Lourenco Pfeiffer RILEM CFRP

0

10

20

30

40

50

0 10 20 30 40 50

Perfect

Best Fit kG = 0,065

Design kG = 0,031

kG [mm]

Le [mm]

kG [mm]

(fc ft)0,5 [MPa]

Ffu,exp [kN]

Ffu,th [kN]

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

It is to mention that in the analysis several uncertainties contrast to a better knowledge of the phenomenon; in particular, not all the research report correctly the data of the mechanical parameters of the support masonry, and the fibre pulling schemes are not so similar. Finally, the data set includes both solid and hollow bricks which perform in a different way if the compression force in the support is sufficiently large.

EXPERIMENTAL EVIDENCE OF THE WALL DIAGONAL STRENGTHENING The problem of wall strengthening in in-plane shear is very important for seismic mitigation of ancient structures; it is however of relevant interest even in the analysis of masonry infilled frames, in which the masonry wall is acting as a bracing system. The results of the comparison among some literature experimental investigations is very interesting; the procedure shows a good performance even if the examined walls are of very different materials.

Table 1: Data of the examined walls Author Type B H t N fbc fbt fmc fmt fkc tf bf Ef Vurm Vexp

mm mm mm kN MPa MPa MPa MPa MPa mm mm GPa kN kN Zhao C200-1 Solid Clay 1400 1000 240 403 11,6 1,2 16,9 2,5 11,0 0,104 200 280 224 332 Alcaino C100-1 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,130 100 230 140 185 Alcaino C100-2 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,260 100 230 140 197 Alcaino C200-1 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,130 200 230 140 215 Alcaino C200-2 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,260 200 230 140 251 Alcaino C300-1 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,130 300 230 140 256 Alcaino C400-1 Hollow clay 2000 1975 140 98 28,0 2,0 23,4 2,5 11,3 0,130 400 230 140 268 Marcari C200-1 Tuff 1480 1570 530 400 3,7 0,8 2,0 0,5 1,7 0,167 200 230 132 173 Marcari G200-1 Tuff 1480 1570 530 400 3,7 0,8 2,0 0,5 1,7 0,111 200 74 132 156 Marcari C200-2 Tuff 1480 1570 530 400 3,7 0,8 2,0 0,5 1,7 0,333 200 230 132 204 Marcari G200-2 Tuff 1480 1570 530 400 3,7 0,8 2,0 0,5 1,7 0,222 200 74 132 163

In table 2 the comparison results are summarised; we note that the shear stress at failure obtained from un reinforced walls is very similar to the two formulations (Coulomb law or Turnsek-Cacovic formula) adopted by Italian guide lines. The proposed design formula in the DT 200 revision has however a strong limitation in the reinforcement maximum strain compliant with the maximum drift of the wall. If we limit to 0,5% the maximum drift, the debonding strain of the fibres cannot be activated in most cases, so that a cut in the fibre contribution is introduced. As is evident in the table 2, this reduction is somewhat compensated by the evaluation of the masonry contribution. By comparing the error 1 (related to the full straining of the FRP reinforcements), with the error B (related to the limited straining introduced by the guide lines), we see that these two columns do not provide large differences. This is however due to a sort of compensation occurring since the FRP contribution is limited by the drift limitation rule, but the un-reinforced masonry contribution is larger than the experimental one for those panels having a large strain reduction. The final outcome is a smoothing of the errors. Two comments are due: first of all the presented procedures function in the comparison only if we consider both the compressed and tensioned reinforcements, although in cyclic cases the guide lines suggest to discard the compressed FRP contribution. Secondly, the computed

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

values are all on the safe side, resulting the theoretical values lower than the experimental ones unless the fibres are very stiff. It is however to mention that some of the experiments have been carried out with cyclic imposed increasing displacement, but the FRP strips were stabilised by using transversal rope connectors spread over the uniaxial strips.

Figure 7: Tuff wall reinforced with CFRP and GFRP tested by Marcari

Table 2: Results of the examined walls fvk,exp fvk,1 fvk,2 ffdd Nfd Vfmax Vu+4 Vfmax Err. 1 εεεεf,max γγγγmax Vurm Err. A VRdm Err. B MPa MPa MPa MPa kN kN kN % kN % kN %

0,67 0,78 0,67 1005 20,8 16,9 291,7 12% 0,0076 0,0050 225,4 -1% 268,6 19% 0,50 0,44 0,44 1151 15,0 10,6 182,6 1% 0,0100 0,0050 123,6 12% 161,3 13% 0,50 0,44 0,44 814 21,2 15,1 200,2 -2% 0,0071 0,0050 123,6 12% 182,5 7% 0,50 0,44 0,44 1151 29,9 21,3 225,1 -5% 0,0100 0,0050 123,6 12% 182,5 15% 0,50 0,44 0,44 814 42,3 30,1 260,4 -4% 0,0071 0,0050 123,6 12% 225,1 10% 0,50 0,44 0,44 1151 44,9 31,9 267,7 -5% 0,0100 0,0050 123,6 12% 203,8 20% 0,50 0,44 0,44 1151 59,8 42,6 310,3 -16% 0,0100 0,0050 123,6 12% 225,1 16% 0,17 0,35 0,31 487 16,3 11,2 176,4 -2% 0,0042 0,0042 246,8 -87% 156,5 9% 0,17 0,35 0,31 339 7,5 5,2 152,4 2% 0,0092 0,0050 246,8 -87% 143,0 8% 0,17 0,35 0,31 345 23,0 15,8 194,8 4% 0,0030 0,0030 246,8 -87% 142,2 30% 0,17 0,35 0,31 240 10,6 7,3 161,0 1% 0,0065 0,0050 246,8 -87% 154,3 6%

EXECUTION OF STRENGTHENING WORKS OF DAMAGED BUIDINGS The strengthening of damaged buildings requires a careful execution not only of the design phase but also of the execution of the repair intervention. The prepared guidelines contain the basis for planning and designing reinforcement nets and correlated works. It is however to say that the support preparation in terms of consolidation of the masonry surface and volume, the substitution of deteriorated mortar and the cleaning of the bonding area are of paramount importance in the execution of an effective strengthening work. Among the other problems, checking the capacity of the planned reinforcement system in real conditions is a source of reliability of the design under execution.

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

The DT 200 document presents several non-destructive testing procedures able to help the designer in dimensioning the strength of the FRP. Among the others, the pull off test and the belt shear test are easily worked out and give sensible information on the system performance.

Figure 8: View of the specimen prepared for the pull off and the shear tests

The presented tests can assess the ability of the bonded interface to resist peel and shear stresses. The results can be classified as positive if the values obtained by the tests are larger than the analytical ones computed by assuming all the safety factors as unity. It is however to cite that in obtaining real data about the seismic resistance of consolidated masonry structures, the use of medium size specimen is preferred. In this case, the unusable condition of the building is a factor promoting a real study of the behaviour in diagonal compression of a specific masonry material in order to evaluate the strength increase produced by every suitable strengthening system. This is the only advantage of a design activity in the arena of severely damaged environment such as L’Aquila city. CONCLUSIONS The paper presents an overview of the Italian guide lines on FRP strengthening of existing structures. The document was produced by an open group of Italian academics with the help of many companies dealing with innovative strengthening systems, and finally adopted by the Italian Research Council as a reference document for interventions in seismic damaged buildings as in the case of L’Aquila city. The theoretical basis of the document is widely supported by a huge number of experimental investigations carried out all around the world, but for the case of ancient masonry structures, the Italian contribution is certainly dominant in the panorama of active research centres. The update of the document is still on going, and at present time, the database of FRP shear tests used for code calibration contains more than 500 results. Supports as brick, stone, tuff, timber were considered and explored. Externally bonded strips and near surface mounted bars are at present widely used in the reconstruction of L’Aquila city; the design of these innovative strengthening systems is carried out with the help of the cited guide lines. This decision makes comparable the strengthening techniques copy righted by the different companies involved in the reconstruction in terms of forecast strength increase produced by a reinforcement net. It seems that this choral contribution of the academic world is one of the best results in terms of support to the country development ever given in Italy.

15th International Brick and Block Masonry Conference

Florianópolis – Brazil – 2012

REFERENCES Accardi M. “Strengthening of masonry structural elements subjected to Out-of-Plate loads

using CFRP reinforcement”, Ph. D. Thesis, University of Palermo, Italy, 2006.

Aiello M.A., Sciolti S.M. (2006), “Bond analysis of masonry structures strengthened with CFRP sheets”, Construction and Building Materials, Elsevier, 20 1-2, 2006, pp. 90–100.

Basilio Sanchez I., “Strengthening of arched masonry structures with composite materials”, Ph. D. Thesis, University of Minho, Portugal, 2007.

Briccoli Bati S., Fagone M., “Lunghezza ottimale di ancoraggio per rinforzi in FRP su elementi in laterizio”, Compositi, 4, 2009, pp.27–31.

Camli U., Binci B. Strength of carbon fiber reinforced polymers bonded to concrete and masonry. J Construction & Building Material, 21, 2007, pp. 1431–1446.

Capozzucca R., “Experimental FRP/SRP–historic masonry delamination”, Composite Structures, 92, 2010, pp. 891–903.

Casareto M., Olivieri A., Romelli A., Lagomarsino A. “A Bond behaviour of FRP laminates adherent to masonry”, Int. Conference Advancing with Composites, Milano, 2003.

CNR DT 200, “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures”, 2nd revised version, Rome, 2011.

FIB, “Externally Bonded FRP Reinforcement for RC Structures”, Bulletin 14, Fédération Internationale du Béton, Lausanne, 2001.

Garbin E., Panizza M., Valluzzi M. R., “Experimental Assessment of Bond Behaviour of Fibre-Reinforced Polymers on Brick Masonry”, Structural Engineering International, 4, 2010, pp. 392–399.

Grande E., Imbimbo M., Sacco E., “Bond behaviour of CFRP laminates glued on clay bricks: Experimental and numerical study”, Composites Part B: Engineering, 42-2, March 2011, pp. 330–340.

Olivito R.S., Zuccarello F.A. (2007), “Prove di delaminazione su laterizi rinforzati con materiali compositi”, XXXVI Convegno Italiano AIAS, 4-8 Settembre 2007, Ischia, Italy.

Pfeiffer U., “Experimentelle und theoretische Untersuchungen zum Klebeverbund zwischen Mauerwerk und Faserverbundwerkstoffen”, Kassel University Press, Germany, 2009.

Pfeiffer U., Seim W., “Bonding of FRPs on masonry surfaces”, Proceedings of the 8th IMC, Dresden, Germany, 2010, pp. 1347–1354.

Willis C. R., Yanga Q., Seracino R., Griffith M. C., “Bond behaviour of FRP-to-clay brick masonry joints”, Engineering Structures, 31, 2009, 2580–2587.