REINFORCED MASONRY USING TEXTILES -...

11th International Symposium on Ferrocement and Textile Reinforced Concrete 3rd ICTRC

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REINFORCED MASONRY USING TEXTILES

Dorothea Saenger; Wolfgang Brameshuber, Institute of Building Materials Research and Chair of Building Materials, RWTH Aachen University

Abstract: The idea of strengthening masonry constructions with textiles emanates from the same principle as textile reinforced concrete. The low tensile strength of masonry is to be increased by the use of textiles taking advantage of the benefits of this relatively new rein-forcement material compared to steel. Textile reinforcement can be bonded externally in mortar-based systems of new or existing masonry building components. Particularly for strengthening new masonry constructions textiles can be also embedded even in thin-layer mortar bed joints. Masonry strengthening using textiles can only be ensured if the bond performance between textile and mortar as well as between mortar and masonry unit is high enough to transmit the tension forces acting on masonry. This can be achieved by selecting compatible textiles to mortar and masonry unit, which – when applied in/on ma-sonry –lead to an improvement of their mechanical properties. Therefore, the first mile-stone of a recent research project is to find suitable textiles for use in masonry for both application methods. With the help of an adapted uniaxial tensile test method originally developed for textile reinforced concrete, the increase of the tensile strength of textile rein-forced mortar specimens can be examined. This paper presents the test method and first results of ongoing tests.

INTRODUCTION

Masonry is a heterogeneous building material consisting of masonry units bonded with mortar in the horizontal joints and occasionally also in the vertical joints. The unit material can be homogeneous or heterogeneous depending on the type of masonry unit used. Be-sides the compressive and shear strength, one of the essential mechanical properties for the design of masonry building components particularly subjected to flexural loads is the flex-ural tensile strength. Examples of building components subjected to flexural loads are basement walls under earth pressure or outer leaf of cavity walls under wind load. Depend-ing on the leading flexural load case – parallel or perpendicular to the bed joint – as well as the geometry and boundary conditions of the considered masonry building component a differentiation of the flexural tensile strength is made. As concrete, the tensile strength and thereby the flexural tensile strength of masonry is quite low compared to the resistance to pressure. Surpassing the masonry flexural tensile strength a unit and/or a joint failure can occur in both load cases namely parallel or perpendicular to the bed joint [1].

In seismic vulnerable zones, earthquake-induced horizontal accelerations generate high horizontal stresses on masonry constructions. The resistance to the in-plane shear stresses is however low, so that the fail-free recordable horizontal load is limited. To prevent fail-ure and increase the load-bearing capacity of masonry building components subjected to


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out-of-plane or in-plane loads, reinforcement can be added to masonry. One of the tradi-tional methods is to embed reinforcing bars horizontally in the bed joint or collocate rein-forcing bars surrounded from mortar in cavities of masonry units. Another more modern and effective method is to use textiles for masonry strengthening taking into account the same advantages of reinforcing concrete with textiles. Textiles can be embedded in thin-layer mortar not needing such a thick mortar cover like rebars. This enables the use of thin-layer mortar, which is the leading mortar in the German market due to its significantly larger compressive strength and its high bond performance to masonry units compared to general purpose mortar [2]. Textiles can be also applied in mortar-based systems like ren-dering or plaster bonded to the tensile strained side of masonry surfaces enabling the strengthening of masonry externally. For this reason it is one of the most suitable methods for strengthening particularly existing masonry constructions like heritage buildings and therefore nowadays gaining upswing in Mediterranean countries subjected to earthquake. A summary of methods for mortar-based strengthening of masonry structures is given in [3].

The aim to reinforce the bed joints with textiles is to improve the shear strength and the flexural tension strength parallel to the bed joints (see Fig. 1). Apart from this, it is at-tempted to improve the flexural tension strength parallel and perpendicular to the bed joint (see Fig. 2) as well as the shear strength of masonry by applying textile reinforced render-ing on the masonry surface. This reinforcing method may be less effective than the other one, because of the externally application. Combining the two types of textile reinforce-ment a much better three-dimensional strengthening of new masonry buildings could be achieved.

Fig. 1 Textile-reinforcement embedded in the bed joint for strengthening of new masonry constructions


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Fig. 2 Textile reinforced plaster or rendering for externally bonded strengthening of masonry

To ensure strengthening of masonry using textiles a high bond performance between the composite materials is required. On one hand the transfer of tensile stresses acting on ma-sonry building components is determined by the bond properties between reinforced mor-tar and masonry unit surface. On the other hand once the mortar has cracked, the capacity to transfer and absorb higher acting tensile stresses than the mortar tensile strength depends on the bond performance between textile and mortar. This demonstrates the importance of material compatibility and how their mechanical properties can be improved if used in con-junction. Therefore, the first milestone of a recent research project investigating textile reinforced masonry especially for new constructions is to find suitable textiles for the use in bed joints and in rendering/plaster. The bonding behaviour under tensile load of textile reinforced mortar should be examined with the help of uniaxial tensile tests [4]. In this process different geometries and material properties of textiles, mortar as well as rendering or plaster should be taken into account.

TENSILE TESTS

General

With the aim to characterize the stress-strain behaviour of textile reinforced mortar for uniaxial tension loading, uniaxial tensile tests are being conducted according to the rec-ommendations of RILEM TC 232-TDT [4]. These recommendations include information about the specimen shape, the kind of load introduction and the anchorage length of the specimens. The testing method derives from the round robin tensile test for textile rein-forced concrete [5] carried out by seven participants from all over the world.

As mortar is a different material with a significantly lower compressive strength than con-crete, the uniaxial tensile test method may need to be adapted to the specific material prop-


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erties of mortar. The first results of the ongoing tensile tests on textile reinforced mortar specimens are presented below. Besides varying the material combination (2 mortars in combination with 2 textiles), the load introduction type (glued or screwed steel plates) and the test direction of the textiles (warp and weft direction) were varied to optimize/adapt the test method for textile reinforced mortar.

Materials

The process of choosing testing materials will be iterative in order to find the most suitable textile and mortar or rendering for the selected application. Up to now, two AR-glass tex-tiles were used (see Fig. 3). These textiles were chosen primarily to calibrate the test meth-od, without regarding their properties and aptness for the intended specific use in masonry at all. One of the textiles is uncoated (T1) and has been often used as reference for many investigations on textile reinforced concrete in the past. The other textile is coated with polyvinyl acetate (PVAC) (T2) and usually finds application in renderings and thermal insulation composite systems. The characteristics of both textiles are shown in Table 1. With respect to the mortar two representatives of mortar compression classes as well as application thicknesses were chosen so far. The thin-layer mortar (M1) is suitable for the bed joint of thin-layer masonry, while the general purpose mortar (M2) is applied in thick-bed masonry. The material properties of both mortars are given in table 2.

Fig. 3 Structure of the AR-glass textiles T1 (left) and T2 (right)

Table 1: Characterization of the AR-glass textiles

textile type

weave construction coating yarn fineness mesh width

0° 90°

- - [tex] mm

T1 core - 2400 8.4 8.3

T2 turner PVAC 320 6 5


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Table 2: Characterization of the mortars according to DIN EN 1015

mortar type class

grain size

oven-dry density

compression strength

flexural tension strength

[mm] [kg/dm³] [N/mm²] [N/mm²]

M1 thin-layer mortar M10 0 - 1 0.84 10.80 2.50

M2 general purpose mortar M5 0 - 4 1.78 5.40 2.10

Test Specimens

The mortar specimens tested were strips with rectangular shape (l = 500 mm, w = 60 mm and t = 10 mm) and a width ratio of 5:1. These parameters were chosen according to the recommendation in [4]. The specimens constituted of two layers of mortar and one layer of textile located in between. To cast the specimens, a hand-lamination process was used. Within a steel formwork, first a thin mortar layer was applied, then the textile was put over this mortar layer and covered with a second thin mortar layer. Fig. 4 shows the production of the specimens. The specimens were stored in a humid environment, demoulded after 2 days and stored in water for another 12 days. Finally, they were stored in laboratory cli-mate (20 °C/65 % rel. humidity) 1 day before testing. A total of 8 series with each 6 spec-imens were produced.

Fig. 4 Production process of the rectangular specimens

Test Method

The test procedure used to analyse the tensile stress-strain behaviour of textile reinforced mortar specimens is shown in Fig. 5. During the experiments particularly attention was paid in gripping the specimen. On one hand a slipping between clamping and specimen


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should be avoided by gripping the specimen with sufficient and homogeneous distributed compression. On the other hand serious damages on the mortar in the clamping area should be avoided by not exceeding the low mortar compression strength. In the majority of cases, the load introduction was carried out by steel plates which were screwed to the mortar specimen according to [4] (Fig. 5, left). The load was transferred to the specimen by means of friction. To avoid local load concentrations during the test, a rubber interlayer was col-located between the steel plates and the specimen. To exert a sufficient and controlled tightening pressure, the screws were tightened with a specific moment of torque using a torque wrench. From the set moment of torque, the pre-compression on the gripping area can be calculated approximately.

Alternatively to the gripping method with screws, steel plates were glued to the specimen one day before testing. The advantage of this method compared to the other one is, that no mortar damage in the clamping area may occur, because no pressure must be exerted on the specimen for clamping the steel plates. In Fig. 5, right, this testing method is shown. The clamping length of both testing methods was 115 mm. The uniaxial tensile tests were carried out in a displacement controlled way with a rate 0.5 mm/min. The load was meas-ured with standard load cells. The deformations were recorded by compact strain transduc-ers (DD1) attached on both surfaces in the middle of the specimen. The base length ana-lysed was 250 mm for both testing methods.

Fig. 5 Test setup; screwed steel plates (left) and glued steel plates (right)


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Results

During the tests the following problems were observed:

• partial or full pull-out of fibres (Fig. 6) • partial compressive failure of mortar in the clamping area • failure outside the measuring range • no rupture of textile

Similar problems were also reported within the scope of optimizing the test method for textile reinforced concrete [5]. Some recommended solutions for textile reinforced con-crete are however material dependent and must be adapted for textile reinforced mortar.

Since pull-out of fibres may occur under relatively low gripping pressure, the applied pre-compression on the clamping area was increased from originally 2.00 N/mm² to 4.25 N/mm² for new test series (Fig 10, left). Despite tightening the specimens of these new test series with a higher pre-compression, partial pull-out failure of the yarns could not be avoided at all. For this reason and due to the less inner bond between the filaments no complete rupture of the uncoated textile occurred apart from the outer filaments of the yarns as shown in Fig. 6. In addition to the partial pull-out problem, in some tests partial compressive failure of the mortar in the clamping area led to pull-out failure outside of the measuring area.

The stress-strain curves were determined for each tensile test. The measured force was related to the cross-section of the specimen to obtain the tensile stress. To determine the strain of the tensile specimen, the measured deformation of the strain transducers were averaged and then related to the measuring length. In addition, the first crack stress and the type of failure as well as the crack pattern were documented for each single test. The re-sults were evaluated with respect to the main influencing parameters tested: testing meth-od, material combination, applied gripping force and textile test direction (see Fig. 7, 8 and 10).

In the tests only two of the three stages of the concrete multi-layer stress-strain-behaviour [5] were achieved, namely uncracked and multiple cracking. The multiple cracking was defined as diffuse. The third stage, namely complete cracking [5], when the tensile strength of the textile is reached, was not recorded. The comparison between both testing methods for the same material combination shows that the tensile stresses obtained with the glued steel plates were lower than with the screwed steel plates (Fig. 7). This demonstrates that without applying a pre-compression force in the clamping area the anchorage length of the specimen with the glued plates should be increased to improve the bond between textile and mortar and thus to obtain better results.


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0

1

2

3

4

0 2 4 6 8

glued steel platesscrewed steel plates (2.00 MPa)

tensile stress σ [N/mm²] M1-T1

longitudinal strain ε [mm/m]

Fig. 6 Tensile failure of external yarns and pull-out of internal yarns

Fig. 7 Stress-strain curves of specimens under uniaxial tension; weft direction; comparison of specimen gripping

Fig. 8 presents a comparison of the stress-strain curves obtained by testing different mate-rial combinations. The comparison of both textiles applied in combination with mortar M1 generally demonstrates a poor bond of textile T2 with the mortar in opposition to textile T1 (Fig. 8, left). One of the reasons for less bond performance between textile T2 and mortar M1 could be the small mesh size and the smooth surface of this textile due to its coating. The results of the tensile tests on specimens reinforced with textile T1 using two types of mortar (Fig. 8, right) show the influence of the mortar on the curve progression.

Due to the low compressive strength of mortar M2 (5 N/mm²) the first crack stress reached was much smaller than of mortar M1 (10 N/mm²). Fig. 9 shows the failure case (pull-out of fibres) and the crack pattern of these specimens. Both material comparisons (textile and mortar) generally give evidence of the importance of selecting matching materials to im-prove the properties of the composite material.


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0

1

2

3

4

0 2 4 6 8 10

T2T1

tensile stress σ [N/mm²] M1


0

1

2

3

4

0 2 4 6 8

M2M1

tensile stress σ [N/mm²] T1


Fig. 8 Stress-strain curves of specimens under uniaxial tension; weft direction; comparison of both textiles (left) and comparison of both mortars (right)

Fig. 9 Crack pattern of reinforced mortar specimens using M1 (left) and M2 (right), both reinforced with the AR-glass T1

Comparing the two diagrams in Fig. 10, the influence of the pre-compression for specimen gripping (2.0 N/mm² or 4.25 N/mm²) and the influence of the testing direction (weft or warp) can be analysed. All specimens shown in these diagrams are made with the same material combination (M1-T1) and reached, due to the mortar used, approximately the same first crack stress. Both the improvement of the anchoring as well as the change of textile testing direction from weft to warp reached a higher maximal stress before failure.


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0

2

4

6

8

0 2 4 6 8

weft direction (4.25 MPa)weft direction (2.00 MPa)



0

2

4

6

8

0 2 4 6 8

warp direction (4.25 MPa)



Fig. 10 Stress-strain curves of specimens under uniaxial tension for both testing directions

Concluding, textile reinforced mortar shows a similar bonding behaviour under tensile load as textile reinforced concrete. Despite similarities, extensive investigations on textile rein-forced mortar for reinforcing masonry are required, due to differences concerning the spe-cific mortar properties – influenced additionally by contact with the masonry unit – and the planned specific application.

OUTLOOK

In further uniaxial tensile tests the anchorage length of longer specimens should be extend-ed to avoid pull-out failure as far as possible and to determine the minimum anchorage textile length. Apart from the pre-compression and the length of gripping, pull-out of fibres may occur with textiles providing a poor bond/interlocking with the mortar matrix. For this reason besides improving the test method, suitable textiles should be selected based on the gained results and findings for both reinforcing methods – in the bed joint or externally bonded on the masonry surface. In addition to the bond performance between textile and mortar, the bond performance of reinforced mortar/rendering to the masonry unit/surface should be investigated. The overriding goal is to investigate and analyse the load-bearing and deformation behaviour of textile reinforced masonry components for their dimension-ing.

REFERENCES

[1] Schmidt, U.: Modelling of masonry under horizontal loads. Bagneux: RILEM, 2010. In: Proceedings of the International RILEM Conference on Materials Science (MatSci), Vol. II: HetMat Modelling of Het-erogeneous Materials, Aachen, September 6-8, 2010, (Brameshuber, W. (Ed.)). pp. 125-134 ISBN 978-2-35158-108-7

[2] Brameshuber, W.; Saenger, D.; Winkels, B.: Recent developments in masonry construction. In: Mau-erwerk 18 (2014), Nr. 3/4, pp. 151-163 ISSN 1432-3427


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[3] Papanicolaou, C.; Triantafillou, T.; Roca Fabregat, P.; Increase of load-carrying capacity of masonry with textile reinforced rendering, Mauerwerk (Masonry), Vol. 19, No 1, 02.2015, pp. 40-51, ISSN 1432-3427

[4] RILEM TC 232-TDT: Recommendation of RILEM ´Uniaxial tensile test - Test method to determine the load bearing behaviour of tensile specimens made of textile reinforced concrete´, Materials and Struc-tures, 2015, to be published

[5] Brameshuber, W. et al.: Round Robin Tensile Test for Textile Reinforced Concrete, Materials and Structures, 2015, to be published


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