RETROFITTING CONCRETE AND MANSONRY BUILDING: FRCM (FIBER REINFORCED CEMENTITIOUS MATRIX) A NEW EMERGING TECHNOLOGY

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Aileen Willis
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1 Giovanni MANTEGAZZA RETROFITTING CONCRETE AND MANSONRY BUILDING: FRCM (FIBER REINFORCED CEMENTITIOUS MATRIX) A NEW EMERGING TECHNOLOGY Abstract Nowadays, the existing buildings need strengthening design due to different loading bearing capacity, serviceability and durability requirements. These aspects are more evident in historical building constituted of masonry structural elements, which can only bear small tensile stresses. Strengthening design could be more important for such buildings when the designer has to consider earthquake effects. In Italy, seismic areas are numerous and the main historical monuments are placed in these areas. Strengthening and rehabilitation of existing buildings, in particular historical building, is in daily practice, especially when the umbro-marchigiano earthquake provoked the collapse of masonry ribbed vaults of St. Francis Church, in Assisi (Italy). Starting from this strengthening application the employment of composite materials started to be more widespread between designers. Usually composite material means the combination of fibres of different type (glass, aramid and carbon) and epoxy resin FRP( fibre reinforced polymers). This composite material offers a high ratio strength/density and the increment of mass due to strengthening application is insignificant compared to other methodology (steel or concrete elements installation). Nevertheless, FRP (fibre reinforced polymers) material could lose the efficiency due to fire event and present defects due to moisture movement within the structural members. A new advanced composite material is described in this paper. A carbon fibre net is embedded in cementitious matrix; the strengthening system was called FRCM (fibre reinforced cementitious matrix). The matrix was differed for masonry and for concrete substrate. The carbon fibers carry the tensile stress whereas the cementitious matrix is more compatible to the substrate in terms of bond, moisture permeability and fire loading. The present paper deals with masonry shear FRCM strengthening.actually the system could be applied easily for strengthening structural elements loaded in plane or out of plane. This system modifies the failure mechanism, increasing maximum applied force and panel stiffness and can be applicable easily by every bricklayer. Besides the paper shows the results of flexural strengthening effect of adding longitudinal FRCM reinforcement to the tension face of a concrete beam compared with traditional FRP.

314 1. Description of FRCM FRCM is a strengthening system which was patent by Ruredil (2001).

2 Description of FRCM FRCM is a strengthening system which was patent by Ruredil (2001). It is composed of: X MESH C10: a carbon fibre net in which the bundles of filaments have 0 /90 orientation and spaced 10 mm. X MESH M25: inorganic cementitious matrix which has to be mixed with water to become like mortar for carbon fibre net application on substrate. The mechanical and geometrical characteristics of FRCM are reported in table 1. X MESH C10 Weight of fibre net (g/m2) Thickness for design (mm) Fibre tensile strength (N/mm2) Tensile elastic modulus (N/mm2) Fibre max. deformation (%) X MESH M (N/mm ) t c (N/mm ) 7.09 (15 days) (15 days) 7.52 (28 days) (28 days) The application procedure is described in figure Figure 1. Application procedure of FRCM on masonry surface Firstly the substrate surface has to be cleaned by sandblasting, grinding or similar abrasion process in order to remove superficial refinement or weak surfaces. The dust has to be removed mechanically or manually and the final surface has to be wet with water to prevent shrinkage phenomena on cementitious matrix. After surface preparation, a thin layer of matrix is applied to the substrate surface. Its application is similar as plaster application on masonry or concrete walls. Then the carbon fibre net is applied on the matrix surface and it is pressed into matrix by light pressure following the fibre orientation. The procedure could be repeated until the required strength and stiffness are reached. Finally a covering thin layer of cementitious matrix has to be applied for embedding the fibre net and refine the strengthened element surface. 2. Evaluation of FRCM efficiency The FRCM efficiency was evaluated for masonry strengthening carrying out an experimental investigation on 11 masonry panels, made of clay bricks and cement mortar, loaded in diagonal compression. The panels had reduced dimensions, compared to ASTM

3 prescriptions, because of the will to investigate different configuration of reinforcement and during a short time Experimental Set up 315 The specimen dimensions were mm 3 and they were made of clay brick of mm 3. The horizontal mortar joint had a mean thickness of 10 mm. During specimens construction, the bricks were wet for preventing pre-cracks in masonry panels due to mortar shrinkage. The masonry was characterised by compression test on 3 prisms, which dimensions were mm 3. The average elastic modulus of masonry was 5418 N/mm 2, and the mean compression strength was equal to N/mm 2. The mortar used for panels construction was characterised following the Italian prescription ; the flexural strength was 5.52 N/mm 2, whereas the compression strength was N/mm 2. The diagonal compression test was carried out on 11 specimens, two of them (PM1 and PM2) were tested without strengthening system for evaluating the maximum load and failure mechanism. The others 9 specimens were strengthened with the following configuration: A. application of cementitious matrix on one panel side (PM3 and PM4); B. application of one FRCM layer on one panel side with carbon fibre net orientation 0 /90, as mortar joints path (PM5 and PM6); C. application of two FRCM layers on one panel side with carbon fibre orientation 0 /90 and ±45 (PM7 and PM8); D. application of two FRCM layers on both panel sides with carbon fibre orientation 0 /90 and ±45 (PM9, PM10 and PM11). The specimens were tested after 28 days of curing in controlled environment. Two displacement transducers were applied on both sides of panels, along the trust and the tie that could be defined in masonry panel loaded in diagonal compression. The vertical transducers measured the positive displacement due to compression force, whereas the horizontal transducers measured the negative displacement due to tensile force Experimental details The FRCM thickness was reduced close to panel boundaries so that the compression loading was applied only on masonry. In this way, the compression force loads the masonry structure up to failure.when the stresses due to applied force induces cracks in masonry, it should not be completely damaged because tensile stresses are carried on by FRCM. This phenomena depends on shear (bond) stresses between substrate and matrix as between matrix and fibres. The application of FRCM allows to cover a large surface of strengthened members so that the bond stresses are spread on a bigger area, reducing the bond stresses between substrate and matrix. The only gap could be at the interface between matrix and fibre. In fact, a particular matrix was formulated that could guarantee a perfect bond between matrix and fibres Experimental results The tests carried out allowed to evaluate the maximum diagonal compression force and corresponding displacement, as the post failure behaviour. Two different failure mechanisms were observed during the tests. Panel PM1 showed a diagonal crack that involved mortar joints and blocks at 67.2 kn, whereas panel PM2 showed a crack path between mortar and block interface and sliding of the upper panel part respect to the lower one at 35 kn. The different failure mechanism was due to different loading gradient applied during tests: it was higher in panel PM1 than in

316 panel PM2. The loading rate of panel PM2 was applied for testing the others panels (strengthened panels).

the failure mechanism changed soon as the strengthening system was applied. A thin layer of cementitious matrix was applied at the surface of panels PM3 and PM4, at one side.

Figure 2. Failure mechanism in panel PM3 Figure 3. Failure mechanism in panel PM5 The carbon fibre net was applied in the successive panels with different configurations.

The maximum compression force increased up to 46.4 kn in panel PM5 and 61.9 kn in panel PM6.

4 316 panel PM2. The loading rate of panel PM2 was applied for testing the others panels (strengthened panels). The panel PM2 was assumed as reference specimen and its structural behaviour described the failure mechanism of un-strengthened panels.the failure mechanism changed soon as the strengthening system was applied. A thin layer of cementitious matrix was applied at the surface of panels PM3 and PM4, at one side. The observed failure mechanism was characterised by a single crack along the diagonal trust (Fig. 2). The maximum compression force increased up to 54.2 kn in panel PM3 and 43.2 kn in panel PM4. Figure 2. Failure mechanism in panel PM3 Figure 3. Failure mechanism in panel PM5 The carbon fibre net was applied in the successive panels with different configurations. The crack path showed by panels PM5 and PM6 was different: secondary small cracks or a second single crack were observed close to the main vertical diagonal crack (Fig. 3). The maximum compression force increased up to 46.4 kn in panel PM5 and 61.9 kn in panel PM6. The carbon fibre net had not the best orientation because the fibres were inclined about 45 respect the tensile direction.the failure mechanism of panels PM7 and PM8 was analogous to that was observed in panels strengthened with one layer of FRCM disposed at 0 /90. However the maximum compression force was increased up to kn in panel PM7 and 88.2 kn in panel PM8.The best strengthening configuration was reached in the last three panels (PM9, PM10, PM11). During the tests no cracks were observed on the strengthening surfaces. The structural behaviour of these panels looked to be linear elastic up to failure mechanism was reached. It started with cracks development between strengthening system and masonry substrate (Fig. 4). The cracks path carried on up to debonding of FRCM from masonry surface. At this point the tests were stopped. The maximum compression force was kn in panel PM9, kn in panel PM10 and kn in panel PM11. Observing the interface between strengthening system and masonry panel (Fig. 5), a diffused crack net could be observed on masonry surface. That means the FRCM involved the whole masonry to carry the applied loading. The tensile strength was supplied by FRCM system up to interface failure on masonry side (Fig. 5). Figure 4. Debonding in panel PM11 Figure 5. Interface in panel PM10

5 Definition of chracteristic shear strength The failure mechanism, observed in panels tested under diagonal compression force, is the starting point for define the characteristic shear strength of this masonry type τ m τ m τ m τ m 2.38 τm 0.73 τ m τ m τ m τ m 0.73 τ m 2.38 τm τ m Figure 6. Stresses in an infinitesimal element, in the middle of panel Figure 7. Principal stresses in an infinitesimal element, in the middle of panel The stresses in the middle of masonry panel, considering an infinitesimal element, could be described in term of compression stresses and shear stresses, which could be translated in term of principal stresses, as it is showed in figures 6 and 7. The stresses at failure are defined in function of m, mean shear stress, evaluated in (1) as: 2 1 τ m = P (1) 2 h t where P is maximum diagonal compression force; h is side of square panel (467.5 mm); t is the thickness of panel (105 mm). The principal tensile stress f tm is the parameter which define the maximum load that could be carried by masonry panel. It is defined by (2) as: f tm = τ (2) m replacing m with expression (1), the maximum tensile stress carried by masonry could be defined. In fact, its value is evaluated introducing the geometrical data of masonry panel and the maximum compression force recorded experimentally in equation (3): f tm 2 1 = 0.73 P (3) 2 h t Finally the characteristic shear stress is defined by (4): τ = b (4) k f tm / where b is a geometric coefficient which depends on panel geometry; b = 1.1 for square panel; b = 1.5 for rectangular panel).the experimental results were elaborated following these consideration and the final findings are reported in table 2.

6 318 P max Specimen [N] [N/mm 2 ] [N/mm 2 ] k0 / ki PM2 35x PM3 (UM) 54.2x PM4 (UM) 43.2 x PM5 (US) 46.4 x PM6 (US) 61.9 x PM7 (UD) x PM8 (UD) 88.2 x PM9 (BD) x PM10 (BD) x PM11 (BD) x k0 shear stress in control panel; ki shear stress in strengthened panel evaluated by (4) with b=1.1; U unilateral FRCM system; B bilateral FRCM system; M only cementitious matrix; S single layer of FRCM (0 /90 ); D double layer of FRCM (0 /90 and ±45 ). f tm 4. Flexural strengthening of concrete The performance of FRCM reinforced specimens RUREDIL XMesh C10/M50 is compared with the performance of specimens without reinforcement and CFRP reinforced specimens (Ruredil X Wrap 310) Experimental Programme 4.2. Description of specimens Tests were conducted with 8 reinforced concrete beams. Of these 8 beams: - two are not reinforced ; - two are reinforced with two layers of FRCM on the intrados; - two are reinforced with a layer of FRCM laid on the intrados and another laid on the intrados and folded over onto the side surfaces - two are reinforced with two layers of FRCM on the intrados and U-shaped strips at the ends - two are reinforced with a unidirectional layer of CFRP and U-shaped strips at the ends Flex tests were conducted on all specimens, at four points, with acquisition of the load and the camber on the mid-line. The beams were released and reloaded upon reaching loads of 30 kn and 70 kn. 5. Results of the experiments 5.1. Load-camber diagrams The experimental load camber diagrams are shown in figures 8. The figure shows the average for each pair of identical specimens.the load-camber diagrams of the specimens without reinforcement display an uncracked phase (first segment), a cracked phase when the steel is still in the elastic phase (up to the bend point, corresponding to the yield point of steel under tension) and a phase with yielded steel in which load is increased (moderately) as a result of the work hardening of the steel and the increase in the arm of the internal torque.the reinforced specimen diagrams are the same as those of the specimens without k

7 319 reinforcement, up to the yield point of the steel; after this point, load can still be increased considerably thanks to the linear elastic behaviour of the fibrous reinforcement up to the breakage point, during which tensile stress continues to increase with curvature.as compared to the specimens without reinforcement, all the reinforced specimens revealed an increase in the yield point of the steel (due to the contribution of the reinforcement, which absorbs a part of the tensile stress) and an increase in the collapse point; the type of crisis was more ductile in the cases of the cement matrix (which clearly permitted gradual creeping of fibers and therefore their gradual release as they approach collapse) as compared to the epoxy matrix, in which the increase in collapse point was greater. In all cases, when the effect of reinforcement is annulled, the load-shift diagrams turn out to be the same as those of the specimens with no reinforcement. In all the reinforced specimens the cracks were closer together and less open than in the specimens with no reinforcement, confirming that tangential tensions interfacing between the reinforcement and the support were transferred. 200 F (kn) E C D B A A D 50 F/2 F/2 η B C E η (mm) Figure 8. Average load-camber diagrams for each pair of identical specimens Table 3. Results of experiments. F u : maximum load SPEC. DESCRIPTION F u (kn) A Two FRCM layers A B No reinforcement B C1 Two FRCM layers, one of which C2 is folded over D1 Two FRCM layers and U-shaped D2 strips E1 One CFRP layer and U-shaped E2 strips AVE. (kn) INCR. (%) Conclusions The failure mechanism in strengthened panels was due to masonry cracking and FRCM ripping. The masonry portion involved in load bearing capacity is bigger in FRCM strengthened

8 320 specimens than that involved in un-strengthened specimens. The strengthening system modifies the failure mechanism, increasing maximum applied force and panel stiffness. These aspects has to be considered in strengthening design. Besides, this technology is applicable easily by every bricklayer.further research has to verify if anchorage bolts, used for connecting strengthening system to masonry substrate, may stop FRCM ripping increasing panel ductility. The next experimental investigation has to be carried on full scale structural elements to analyse mechanical characteristics and stiffness variation due to strengthening. The point is how to evaluate elastic modulus and shear modulus for strengthened material, as if the characteristic curves T-, defined from Italian code prescription, are still applicable for FRCM strengthening.this report reveals the effectiveness of the flex fatigue of a reinforced concrete beam made with a carbon mesh immersed in a cement matrix (FRCM). In all the tests conducted, flex performance was better than in control specimens with no reinforcement. The increase in the point of collapse was between 9% and 18%, depending on the reinforcement configuration (greater benefits can presumably be achieved by using a greater number of layers). Comparison of the theoretical results based on the hypothesis of conservation of flat sections and the results of the experiment reveal that the hypothesis is substantially applicable up to reinforcement dilations of 8 to 9. With dilation in excess of this, fiber creeps in the matrix and the reinforcement gradually loses its effectiveness. Reinforcement must therefore be scaled taking into consideration perfect adherence between the reinforcement and the support and limiting tension in the reinforcement. The increase in the collapse point obtained with mesh and cement mortar (FRCM) is less than that obtained with a sheet of unidirectional carbon fibers and an epoxy resin (CFRP), which is 29%. This result is attributable to both the greater section of the carbon fibers present (about 30% more) and a greater limit on adherence (detachment took place with a dilation of 12 ). On the other hand, the crisis in the specimens made using epoxy resin was more fragile, in that the reinfor-cement was instantaneously detached from the support, so that its contribution was suddenly eliminated, while, in the case of the cement mortar, crisis was more ductile in that the contribution made by the reinforcement was decreased gradually due to creep of fibers inside the matrix. References G. CROCI, A. Viskovic, (2000), The use of FRP of Aramidic Fibers to strengthen the vaults of the Basilica of St. Francis of Assisi, Proceedings of National Conference: Mechanics of masonry structures strengthened with FRP-materials, 7-8 December, Venice, Italy, pp CNR-DT 106/98, 1998, L impiego di armature nonmetalliche nel calcestruzzo armato, CNR BOLLETTINO UFFICIALE Part IV Technical Document. Fib CEB-FIP Bulletin 14 (Task Group 9.3), 2001, Externally bonded FRP reinforcement for RC Structures, Technical Report on the design and use of externally bonded fibre reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures ARDUINI, DI TOMMASO, NANNI, 1997, Brittle failure in FRP plate and sheet bonded beams, ACI Structural Journal, vol. 4, July August 1997 NANNI, FOCACCI, COBB, 1998, Proposed Procedure for the Design of RC Flexural Members Strengthened with FRP Sheets, ICCI-98 Convention Records, Tucson, AZ, 5 January , Vol. I, pp DI TOMMASO, ARDUINI, FOCACCI, RUSSO, 2002, Le strutture in materiale composito, Chapter XVIII of Ingegneria delle strutture by Elio Giangreco, Vol. III, UTET. UZUPEŁNIANIE ZAPRAWĄ CEMENTOWĄ WZMOCNIONĄ WŁÓKNAMI (FRCM) STOSOWANE W OBIEKTACH BETONOWYCH I CEGLANYCH JAKO NOWA TECHNOLOGIA

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