Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

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DURABILITY EVALUATION OF EBR CFRP STRENGTHENED MASONRY STRUCTURES

P. Desiderio 1 and L. Feo2 1 D.IN.E. Department of Building Engineering,

University of Naples “Federico II” IT. Email: pdesider@unina.it 2 DICIV Department of Civil Engineering,

University of Salerno IT. Email: l.feo@unisa.it ABSTRACT This work presents the results of an experimental program focusing on the durability of externally bonded FRP reinforcements (FRP EBR) to tuff masonry structures in the case of outside application (wrapping, planting of masonry walls, vaults and arches) and exposition to climatic agents, particularly to temperature cycling, moisture, and UV light exposure. Studying the performance decline of the entire masonry-interface-FRP-plaster system, to finally come to performance-time diagrams for each system component. In this method the accelerated artificial aging tests are used only to discover the incidence percentage of each factor on the system’s durability, while the re-scaling problem is solved elaborating statistical data on the field (real application durability). KEYWORDS Externally bonded reinforcement (EBR), carbon fiber reinforced polymers (CFRP), durability, masonry structures. INTRODUCTION The results presented in this paper are part of an ongoing research program finalized to study the durability problem of externally bonded FRP reinforcements (FRP EBR) of masonry structures. The research is carried out adopting the evaluation of a durability method named “NIC” (UNI norm draft, working group GL 15). This method estimates the service life (ISO 15686-1 2000) of a building component as a deviation from a standard value, named "mid-normal". The mid-normal value of the "duration", that is the reliability of the considered building component in special conditions assumed as mid-normal, is obtained elaborating the statistical data of the duration of real applications. In this research some FRP EBR applications (20 applications) carried out in Italy have been used in order to extrapolate the mid-normal value. Models were chosen privileging the following criteria:

- quality and amount of the available information: application year, materials used, work execution, possible anomalies found;

- possibility to execute inspections: visual controls, ultra-sonic, acoustic sounding (hammer tap), thermographic tests, mechanical tests;

- possibility to compare materials used, serviceability of the support, geometric conformation of the application surface, etc.;

- homogeneity of the influence agents. -

This still running monitoring started about five years ago. As a result of the monitoring of these applications, some performance/time diagrams and the real duration of such strengthening systems will be extrapolated.

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Once the mid-normal value has been determined, the service life of any FRP system will be evaluated through the following expression:

∑ ⋅×=

100FλDD ii

mnpp , (1)

where DPP is the most probable service life, Dmn is the mid-normal duration, Fi is the modifying factor and λi denotes the weight of the i-th factor. In particular, Fi is the modifying factor associated to the i-th agent that influences the service life of the considered building component. The weight (λi) in eq. (1) defines the influence of the agent on the degradation of the considered building element (Nicolella 2000). In this way the mid-normal value is the starting point to determine the durability of the same EBR FRP system applied to any other context, if adjusted by the modifying factors, which are thought to be a function of the specific case. The considered influencing agents for FRP strengthening systems are those summarized in table 1.

Table 1. Influencing agents

n. Group of agents Influencing agents 1 Climatic agents main temperature, UV exposure, humidity and

moisture, freeze-thaw cycles 2 Environmental Agents chemical agents, exposure to salts,

sustained loading 3 Configuration shape/lying, extension,

presence of discontinuity/chines 4 Technological characteristic application surface state,

protection The definition of the influencing agents and the corresponding conditions of variation is a crucial point of the method. These conditions of variation, where possible, have been derived from definition and classification criteria suggested in some technical codes (for example the DPR 412/93 for the temperature), from subdivisions connected to intensity and/or homogenous behaviors of the agents (for example, subdivision relating daily ∆T temperature or exposure) and from limit-values often calculated through statistical treatment of field-collected data. The next step is the attribution of a weight (λi) to each agent. This weight defines the influence of the agent on the degradation of the considered building element. Once the factors have been defined and the weights for every agent have been chosen, based on the estimation methods and the information available, the application of the method consists in the choice of the factors that correspond to the conditions of the investigated EBR FRP system and in the successive weighted arithmetical mean of these coefficients. The result of this mean is multiplied by the reference value of duration, the mid-normal value, in order to obtain the most probable duration of the considered EBR FRP strengthening system in the analyzed case. In order to translate in numerical value the difference of behavior of a building component in the various conditions of variation of the influencing agents, in respect of the mid-normal case, the method suggest to use field-collected data, results of researches, indications and information by designers, producers, constructors, technicians or laboratory tests. In particular, the experimental determination through laboratory tests concurs to estimate the relativity of the several conditions of variation of the influence agents regarding the case assumed as reference (mid-normal values), that is obtained statistically by field collected data. This paper focuses on the effects of two of the climatic agents: UV exposure and freeze-thaw cycles for CFRP externally bonded to masonry structures. Specifically, degradation of the CFRP laminates due to coupled freeze-thaw cycling and UV exposure is experimentally investigated in order to dimensioning the modifying factor associated to these last agents.

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LABORATORY TESTS

Used facilities and specimen preparation After a series of cyclic environmental preconditioning and mechanical loading, the degradation and variable load rate dependency of CFRP lamina are evaluated from these aspects: adhesion to the substrate degradation of due to freeze-thaw cycling and UV exposure, failure mode analysis including FRP failures and substrate failures. For the conditioning of the samples, it has been used an environmental chamber manufactured by Angelantoni (Massa Martana, Perugia, Italy). The specimens dimensions of about 120×20 mm in section and about 500 mm long, have been conditioned by climatic chamber internal dimensions (600×535×700 mm). They are made of Naples yellow tuff, mortar, pre-mixed plaster and siloxane paint (Fig. 1, 2). A tuff stone was split along the longest side in 4 parts, obtaining 4 blocks. Two blocks were used to make up the specimen. Each specimen is made of two tuff blocks of 120×120×240 mm planting with a couple of CFRP laminate with a section of 65×1,5 mm in vinylester matrix with a 35% of fibers in volume (with the characteristics summarized in tables 2), with pre-mixed plaster and siloxane paint. A special steel device allows the system to be placed under traction in order to mobilize the adhesion stresses at interface between laminate and substrate.

Figures 1, 2. Specimens

The interface is made of a bi-component epoxy putty with the characteristics summarized in table 2.

Table 2. Technical characteristics of bi-component epoxy putty Propriety Value Density 1,77 kg/l (A+B)

Tensile Modulus 12.800 N/mm 2 Bond shear stress > 4 N/mm2

Shear strength > 15 N/mm2 Coefficient of thermal expansion 9 E -5 per °C (from -10°C to +40°C)

Density 1,77 kg/l (A+B)

Table 3. Technical characteristics of CFRP laminate Propriety Value

Laminate thickness 1,5 mm Width 65 mm

Laminate area 97,5 mm2 Volume fraction of fibers 35 %

Nominal area of fiber sheet 34,15 mm2 Tensile strength 4.500 N/mm2

Modulus of elasticity 234.000 N/mm2 Ultimate elongation 1,9 %

Preliminary tests Each specimen was weighted, measured and subjected to visual inspection in order to establish changes in color, debonding, peeling, blistering, cracking, crazing, deflections, indications of eventually present cracks in the plaster and other anomalies after conditioning (ACI 440 2001).

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Specimens were also subjected to thermographic tests (Fig. 3, 4). These tests have been finalized to evaluate for delaminations, debondings or air voids between multiple plies or between the FRP system and the masonry prior to testing, after reconditioning, and at any intermediate stage as prescribed in the test procedure.

Figures 3, 4. Execution test

Figure 5 shows some thermographic images of the investigated specimens.

Panel 1A – 1B T main = 40,42 °C

(on the 99,7 % of the surface) Panel 1C – 1D T main = 43,26 °C

(on the 99,7 % of the surface) Figure 5. Thermographic test results

Freeze-Thaw Cycling In a FRP-interface-masonry composite system, self-equilibrating stresses develop in two cases: differential thermal expansion and contraction of the FRP, interface and masonry and when the distribution of temperature over the cross-section of the FRP is non-linear. In the longitudinal direction, CFRP laminates have a thermal expansion coefficient less then that of the substrate, even negative (Fib Bulletin No. 14 2001). In regions of drastic temperature changes, this can negatively affect the bond characteristics and lead to the failure of the lamina. A lot of time was spent analyzing the freeze-thaw cycling capabilities of D.IN.E. laboratory facilities. After extensive research on previous experiments on temperature effects of FRP reinforcement, the conclusion was that there seemed to be a lack of data for CFRP externally bonded to masonry structures for low temperatures. Thus each specimen requiring pre-conditioning (according to D 618 - Standard Practice for Conditioning Plastics for Testing) was subjected to low temperature thermal cycling between 30°C and –18°C excursions with a 1-hour-hold at –18°C and 1-hour-hold at 30°C, achieving a 6 cycles per day rate, according to ASTM C 666 - Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. Figure 6 shows the cycling procedure for a 4-hour period.

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.

Freeze - thaw cycling

-30

-20

-10

0

10

20

30

40

0.00 0.15 0.30 0.45 1.00 1.15 1.30 1.45 2.00 2.15 2.30 2.45 3.00 3.15 3.30 3.45 4.00

time (hh.mm)

tem

pera

ture

(蚓)

Figure 6. Temperature – time diagram in the freeze thaw cycling

Specimens were exposed to 50 and 105 freeze-thaw cycles corresponding to 200 and 420 hours of exposure (fig. 7,8). Specimens that were not subjected to freeze-thaw cycling were stored in the structures testing laboratory at room temperature and standard relative humidity.

Figures 7, 8. Specimens in the climatic chamber subjected to Freeze-Thaw Cycling

After conditioning, specimens have shown the following visual results:

• debonding of large areas of the plaster, while the lamina was still bonded; • large cracks appeared on the residual plaster along CFRP lamina perimeter.

UV exposure Each specimen, requiring pre-conditioning (according to D 618 - Standard Practice for Conditioning Plastics for Testing), was subjected to Xenon-Arc Exposure in laboratory environment (according to D 2565-99 Xenon –Arc Exposure of Plastics Intended for Outdoor Applications) for 50 cycles with intensity of 0,35±0,02W/mq and temperature of 63°C (Table 4, Fig. 9) and has shown the following visual results:

• all areas are still bonded (evaluated with thermographic tests); • small cracks appeared on the plaster along CFRP lamina perimeter.

Table 4. Test cycles commonly used for Xenon-Arc Exposure Testing of Plastics (ASTM D 2565-99)

Cycle Description Temperature Typical Irradiance 18 h, consisting of alternating intervals of 102 min light only followed by 18 min of

light with water spray

63 ± 2 °C 0,35 ± 0,02 W/m2 at 340 nm

41,5 ± 2,5 W/m2 from 300 to 400 nm

6 h dark, at 95 ±4 %RH with no water spray

38 ± 2 °C -

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Figure 9. Specimens in the climatic chamber subjected to UV exposure RESULTS AND DISCUSSION After conditioning, specimens were tested to failure under monotonically increasing concentric tension. The experimental value of the first results are summarized in table 5.

Table 5. Tests results

Specimen n.

Conditioning type n. cycle τad,m (MPa)

% decay Failure modality

1 no conditioning - 0,43 0 tuff failure 2 freeze-thaw cycles 50 0,39 9 lamina – substrate failure 3 UV exposure cycles 50 0,24 44 lamina – substrate failure

The value of the bond stress was established by the following expression:

τad,m= F/(2lad × b), (2) where τad,m is the mean bond stress, F is the applied tension force, lad is the anchorage length and b is FRP sheets width. The analysis of the data contained in the table shows the decrease of bond capacity in case of exposure to freeze – thaw cycles or UV. This degradation is also shown in terms of percentage decay of bond capacity with respect to control samples. It is important to note that in non-conditioned samples the failure is always on the tuff, while in the conditioned ones the failure is at the lamina – substrate interface on the epoxy adhesive layer (Fig.10,11).

Figures 10, 11. Failure mode of the specimen n.1

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CONCLUSIONS The conclusions of the first laboratory tests are summarized below:

• The first results have shown a decrease of the ultimate load and naturally of the bond stresses in the conditioned samples;

• The decay is higher for the UV exposure of the samples; • The failure is at lamina – substrate interface for the conditioned samples, while on the tuff masonry for

no-conditioned ones. At the moment the conditioning of other samples is in progress, with an increase in exposition cycles, in order to confirm the service decay trend which was already registered after the first test. Additionally, after noticing that on non conditioned samples the main reason of the “crisis” is the capacity limit of the shear resistance in the tuff support, other tests will be carried out in order to verify if a superficial pre-strengthening of the tuff support (and additionally anchorage) can determine an increase of the transfer capacity of the agent loads from the tuff supports to the reinforcement. Once performance decays, varying the number of freeze-thaw cycling and of UV exposure, have been evaluated, with reference to non conditioned samples, the dimensioning of modifying factors related to the analyzed influenced agent of “NIC” method will be possible. ACKNOWLEDGMENTS The authors wish to express their gratitude to Eng. Giona Maddaluno of the ATP (Angri , SA), Eng. Massimo Prencipe (COPROMA SUD, Casoria, NA) and B.s. Lucio Iadicicco who made it possible to carry out the experimental program and the tests on the samples. REFERENCES ACI 440 (2001), “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening

Concrete Structures,” ACI Committee 440, Detroit Michigan. Fib Bulletin No. 14, Externally bonded FRP reinforcement for RC structures, Federation International du Beton,

Bruxelles, 2001. ISO 15686-1, “Buildings – Service life planning – part1: general principles.” International Organization for

Standardization, 2000. ASTM D 618, Standard Practice for Conditioning Plastics for Testing;. ASTM. D 2565-99, Xenon –Arc Exposure of Plastics Intended for Outdoor Applications. ASTM D 756-99, Determination of weight and shape changes of plastics under accelerated service conditions. ASTM C 666/C 666M – 03, Standard test method for resistance of concrete to rapid freezing and thawing. Nicolella M., Affidabilità e durabilità degli elementi costruttivi in edilizia – Un’ipotesi metodologica per il

calcolo, CUEN, Napoli. 2000.

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