Paper Number O85

FRG strengthening systems for masonry building

2014 NZSEE Conference

A. Balsamo & I. Iovinella

Department of Structures for Engineering and Architecture University of Naples, Italy.

G. Morandini

Structural Strengthening Line - Mapei S.p.A.

ABSTRACT: Up to the middle of the last century the main building material was masonry, that is why in most countries there are many masonry buildings and many of these have great historical or social value. These buildings, located in seismic areas, due to their age are deteriorated by environmental and human factors. This combination of factors causes a very alarming situation that causes a growing interest toward a new strengthening system. Researchers are orientated toward a less invasive and reversible system possibly avoiding resin and organic materials. Thanks to recent applications, especially on historical buildings, it was possible to realize a strengthening system based on application of FRP grid and mortar matrix (FRG). Much experimentation was conducted to test the effectiveness of this technology on different kinds of masonry. The present paper resumes the latest tests carried out by the University of Naples on different masonry panels tested under diagonal compression. Experimental campaign investigated preliminarily on material properties of based component from bricks to reinforcing mortar. The experimental results confirmed the effectiveness of the investigated strengthening technique to increase the panels shear strength and validated the effectiveness of this reinforcing system on different kinds of masonry.

1 INTRODUCTION

A large number of existing masonry structures shows damages due to a wide range of events (i.e. environmental deterioration, inadequate construction techniques and materials, design for gravity loads only) or, for the same reason, are subject to an high rise. Several strengthening technique are available to reduce the seismic vulnerability of these buildings; however, some of these techniques may be too invasive or expensive. Techniques based on the use of technologies and materials compatible with physical and mechanical properties of masonry are required to enhance performance of such buildings. Among new strengthening strategies, the use of Fiber-Reinforced Polymer (FRP) or Fiber-Reinforced Grouting (FRG) strengthening technique offers a series of advantages as the high strength-to-weight ratios, low influence on global structural mass, corrosion and fatigue resistance, easy handling and installation, and negligible architectural impact. Effectiveness of this technique was evaluated by means of a different experimental campaign on ma-sonry panels tested under diagonal compression.

The present paper resumes the main experimental results in terms of shear strength, diagonal strains, shear deformation as well as elastic parameters (i.e. modulus of rigidity, G, and Poisson ratio, ν) and ductility are herein presented and discussed with reference to five tuff panels tested under diagonal compression.

2

2 EXPERIMENTAL CAMPAIGN

During the 3 experimental campaigns, 22 diagonal compression tests are carried out on square masonry panels as summarized in Table 1:

Table 1. Tests Matrix Cod. Stone Sample n° Matrix Grid TN1 Neapolitan Tuff 8 Lime + Cement. Glass + Basalt TN2 Neapolitan Tuff 9 Lime Glass CA Limestone 5 Lime Glass + Basalt

2.1 Material Properties

Mechanical properties of tuff units and mortar as well as matrix to bond FRP reinforcement were determined by means of experimental tests.

According to UNI EN 772-2002 tests were performed on cubic stone units, prisms of 360mm×60mm×90mm were tested in flexure with three point bending in order to evaluate the flexural strength according to UNI EN 14580- 2005.

Prismatic sample of 40x40x1600 mm were tested in flexure and compression tests was been carried out on two resulting half according to UNI EN 1015-11 and UNI EN 998-2.

The mechanical properties of the matrix after 28 days of curing were computed in the same ways; cementitious based mortar has: flexural strength of 14.5 MPa; compressive strength of 30.2 MPa; lime based mortar has: flexural strength of 8.0 MPa; compressive strength of 16.1 MPa. The mechanical properties of GFRP (see Figure 1a) and BFRP (Figure 1b) grids were provided by the manufacturer: tensile 45 kN/m, Young's modulus of 72.0 GPa, and ultimate axial strain of 2.0% for glass grid; and tensile 60 kN/m, Young's modulus of 91.0 GPa, and ultimate axial strain of 2.0% for basalt grid.

Ties are realized with steel fabric (Figure 1c) with tensile strength of 2086 MPa and elastic modulus of 210000 MPa, glued by epoxy putty directly on FRP grid.

(a) (b) (c)

Figure 1. Glass (a) and Basalt (b) grid ; scheme of steel tie (c)

2.2 Reinforcement Scheme

The strengthening system installation procedure involved the following steps: a first layer of mortar (thickness of about 6mm) was applied; FRP grid was applied above this and the second layer of mortar (thickness of about 6 mm) was applied covering the grid. Details about installation process are depicted in Figure 2.

3

The construction procedure of the IMG strengthening system with SFRP ties was approximately the same. Five squared pockets were left in the second matrix layer and 200 mm deep and 20 mm large holes were drilled to install SFRP ties of 500mm length, which were previously impregnated with epoxy resin. Inside the hole was cleaned and prepared with epoxy primer, half of each tie was inserted in the hole and another side was spread with in the squared pocket over the grid. Another layer of FRP grid was applied above tie spread and finally, squared pockets were filled with mortar. The number and size of SFRP ties were selected to strengthen both the central and peripheral parts of the specimen.

Figure 2. Strengthening system installation procedure

2.3 Test Setup

The panels were tested in a four column testing frame, characterized by a 1000 mm wide and 4000 mm long steel base floor, capable of testing specimens which are more than 4 m high. Its load capacity is 3000 kN both in tension and in compression with a total stroke equal to 150 mm.

The tests were carried out under displacement control, in order to record the panels post-peak response; the displacement rate was 0.015 mm/sec for all tests. Tests were stopped when the reduction in strength with respect to the peak value was of about 50%. Two steel loading shoes placed on two diagonally opposite corners of the panels were used in order to apply the compression load; the test layout conforms with ASTM E 519-81. Some changes were introduced in order to adapt the ASTM requirements to the masonry blocks dimensions. To avoid a premature splitting failure of panel edges, the spaces between the specimen and steel plates were filled with fast setting and shrinkage free mortar.

Five linear displacement variable transducers over a gauge length of 400 mm were used to monitor in-plane and out-of-plane displacements: two LVDTs were placed on each panel side along the diagonals to record the shortening and elongation of vertical and horizontal diagonals, respectively; one more LVDT was installed perpendicularly to the panel surface to measure out-of-plane displacements..

3 RESULTS ANALYSIS

The shear strength, τ, is computed on the net section area An of the uncracked section of the panels , according to ASTM E519-81 standard test method; in general the shear stress can be obtained as

0.707n

V

Aτ = , where V is the current experimental load. The average strains, εv and εh, have been

computed as the average displacement on the two sides over the gauge length (400 mm) along the compressive and tensile diagonals, respectively. As-built specimens suffered stair-stepped cracking involving both bed and head joints along their compressed diagonal. First cracking is shown generally along mortar – bricks interface due to low strength of mortar adhesion bond. Crack pattern is characterized by few and wide cracks while no cracks are visible along load direction or on the edge of the panel near load application. No significant out-of-plane deformations were observed on them.

4

FRG reinforcement changes cracking propagation toward a more dissipative scheme. In fact strength-ened specimens were characterized, independently by the type of grid adopted as reinforcement, by a more uniform crack pattern; several cracks less wide than those achieved on control specimens were attained on the mortar reinforced by FRP grids.

Significant out of plane deformations were observed on one side reinforced panels due to the different deformation of the two sides. However this deformation takes a significant value after post peak load, so at load value is never reached by non-reinforced panel.

3.1 One Head Neapolitan Tuff

The experimental program consists in 9 diagonal compression tests on masonry panels. Masonry was made of yellow Neapolitan tuff bricks (360mm x 250mm x 115 mm) and a pozzolan (local volcanic ash) based mortar (thickness 10 mm). Each panel was eight courses high and one tuff block wide; with mortar joints of 10-15 mm thick the resulting dimensions were 1000×1000×250mm.

Tuff bricks have a compression strength of 4.0 MPa and flexural strength of 2.6 MPa, mortar joint has a compression strength of 3.72 MPa and flexural strength of 1.3 MPa. Tests layout is two as built and 6 reinforced panel. Three of them are reinforced with cementitious matrix and glass grid, three with lime matrix and glass grid and one with lime matrix and basalt grid.

The maximum shear stress (τmax) (see Table 2), recorded on the reinforced panels was significantly higher than that achieved on control panels with an increase that can reach 300%. Increase is also clear in ultimate shear strain (τulti) that is post peak strain at 80% of ultimate load. Reinforcement in post peak phase always produces a great increase of shear strain (γu).

Table 2. Tests Results

Sample ID Grid Reinforcement MatrixVmax

[kN]

τmax

[MPa]

τu

[MPa]

γmax

[%]

γu

[%]

ΔVmax

[%]

P1 - - - 49.98 0.14 0.11 0.10 0.53 P2 - - - 60.00 0.17 0.14 0.27 0.46

PGG1

Glass 2 Sides Lime

157.31 0.44 0.36 0.23 1.12

233 PGG2 187.55 0.53 0.42 0.53 1.91

PGG3 204.60 0.58 0.46 0.28 1.64

PGG1C

Glass 2 Sides Cement.

228.81 0.65 0.52 0.18 0.50

307 PGG2C 231.62 0.66 0.52 0.12 0.81

PGG2C 210.53 0.60 0.48 0.19 1.00

PBB1 Basalt 2 Sides Lime 138.78 0.39 0.31 1.23 2.98 152

Crack patterns, according to the test results, show few, wide and scaling cracks for non-reinforced panel and, many little and vertical cracks for reinforced panel. Figure 3 shows a moderate increase in stiffness for a strengthened specimens only in correspondence to a load value greater than peak value of non-reinforced panel. More details can be found on Balsamo et al.2010.

3.2 Tw

The 9 tumasonrymm. Dispecimenstrengthe

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Tabl

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ment performe of the tieow the use o

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d and were 350×300 mm

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ax

N]

τmax

[MPa]48 0.2140 0.1932 0.2740 0.4562 0.4117 0.5372 0.5234 0.7145 0.68

ed on both fes improves of connectors

(c) Cracks Patter

310×1200×12m and mortar jt specimens ng with tie

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rength from h shear stren

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[MPa] 0.17 0.15 0.22 0.36 0.32 0.43 0.42 0.57 0.55

facings is mothe effectiv

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rn (b)

200 mm in sjoints of thicand 6 stre

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Pa, tensile stean unit weig:6.25 water/sof 2.50 MP

single to dongth increasi

γmax

[%]

γu

[%] 0.04 0.06 0.11 0.41 0.20 0.37 0.07 0.17 0.10 0.31 0.05 0.68 0.18 1.18 0.85 1.09 0.20 1.12

ore effectiveveness of thwing effectiv

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ΔVmax

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170

206

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6

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Tabl

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Sides Lim

Sides Lim

w shear strengties of the mshear strengtlt and 146%strain of the

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7

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[kN] 53.79

me 141.18183.86

me 138.29126.88

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% in the case non-reinforc

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ansverse andry buildings

resistance ompressive strrcement wit

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[MPa] τ

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robably due tever, as showd panel that r

of reinforceced panels b

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[%]-

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0.11 0.37

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An impoor stiffneintervent

Based otechniquobtainedspecializ

REFER

ASTM E ciety f

Balsamo with Ling FR

UNI EN1

UNI EN1sive s

UNI EN9

Parisi F., with iPages

UNI EN 7

NCLUSION

ults obtainedes a sharp im

on which it

fectiveness isd on two side

o demonstraation in partie for the confsorderly mas

ortant considess in the wtions with th

on these resuues of reinfod with this mzation require

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772/1 - 2002 “

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the configura

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d by injury co

does not prongly it is postion of the bu

ered as a vaonsidering thsimplicity ofrvention itse

on (Shear) in M

erimental Behrkshop on Con, 11-13 Augus

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– Part 11: Det

Masonry morta

In-plane behavEngineering

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amples

he application and mason

ation of reinf

increase in on single s

ese have a paontained on t

oduce signifissible to incluilding.

alid alternatihe high mecf implementalf.

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havior of Tuff nservation of Hst 2010, pp. 97

elastic modulu

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ar .

viour of tuff mVolume 45, Is

ve strength”.

on of the FRnry regardle

forcement on

both resistaside, resultinarticular effethe wall surf

icant changelude among

ive to the trchanical perfation and low

emblages. Ame

f Masonry StrHeritage Struc7-107.

us.

f flexural and

masonry strengssue 1, Februa

RP system ss of the

n a single

ances and ng in less ectiveness face.

s of mass the local

raditional formance

w level of

erican So-

engthened ctures Us-

d compres-

gthened ary 2013,