Textile Reinforced Mortar at High Temperatures

Isabella Colombo1, a, Matteo Colombo1,b, Anna Magri1,c, Giulio Zani1,d and Marco di Prisco1,e

1Department of Structural Engineering, Politecnico di Milano, Milan, Italy acolombo@stru.polimi.it, bmcolombo@stru.polimi.it, cmagri@stru.polimi.it, dzani@stru.polimi.it,

emdiprisc@stru.polimi.it

Keywords: textile reinforced mortar, high temperature, tensile strength Abstract. Textile reinforced mortar (TRM) is a composite made by fine grained matrix and glass fabric reinforcement. The main advantages of such a material are the orientation of the reinforcement in the direction of tensile forces, no concrete cover requirement against corrosion and the capability to produce thin and light weight elements. Special attention was given by researchers to the time dependent loss in strength of AR-glass reinforcement embedded in a cement based matrix. Some research has shown durability models to calculate the amount to the strength loss related to material, humidity and temperature. Nevertheless, the behaviour of TRM when exposed to high temperature requires further investigations. A suitable experimental programme was planned to investigate the behaviour of TRM when exposed to high temperatures. Uniaxial tensile tests were performed after thermal cycle on 400 mm x 70 mm specimens 6 mm thick, reinforced with 2 layer of AR-glass fabric. Several thermal thresholds (20, 200, 400 and 600°C) were considered for the mechanical characterization in fire condition. Thermal cycles were performed in an oven considering a heating rate of 30°C/h up to the maximum temperature and by a cooling branch at 15°C/h after a stabilization phase at maximum temperature.

Introduction

In civil engineering an increasing interest in textile reinforced mortar, mainly made of AR-glass fabrics, is developing. Thanks to its advantages (orientation of the reinforcement in the direction of tensile forces and no concrete cover requirement against corrosion) TRM allows to create thin and lightweight structures with high load-bearing capacity.

Many researchers have developed durability models to quantify the loss in strength of textile reinforced composites due to AR-glass degradation problems and weathering conditions (humidity and temperature) [1,2].

One application that could be considered refers to a layered prefabricated structure (for instance a tunnel lining) where the TRM is located on the tensile side and could be subjected to high temperatures and tensile stresses.

Only few results about TRM under fire loading exist and the behavior of the compound material exposed to high temperature requires further investigations. Some researchers focused on the load bearing capacity of TRM during a fire test under static load underling the behavior dependency on fabric material and on concrete spalling [3,4] and the increment of deflection during temperature exposure [3,4,5]; some others focused on the remaining load bearing capacity after exposure at high temperature [6].

The study presented in this paper analyses the mechanical residual properties of the material: uniaxial tensile tests were performed on 400 mm x 70 mm x 6 mm specimens reinforced with 2 layers of AR-glass fabric after exposing them to thermal cycles up to three different maximum temperature (200 – 400 – 600°C). The test results are compared with the behavior of specimens at room temperature.

Experimental programme

Materials

Matrix. The matrix of the TRM investigated is a high strength mortar characterized by a water

to binder ratio equal to 0.20. The maximum aggregate size was taken equal to 600 µm in order to allow the matrix to flow through the fabric mesh. The mix design is summarized in Table 1. Table 1 – Mix design. Table 2 – Bending and compressive strength. Component Content Specimen fctf [MPa] fcc [Mpa] Cement I 52.5 600 kg/m3 N1 18.15 86.46 79.66 Sand 0-600µm 928 kg/m3 N2 18.29 90.35 84.76 Water 220 l/m3 N3 20.30 88.94 88.01 Superplasticizer 44 kg/m3 N4 20.96 86.99 85.25 Slag 500 kg/m3 N5 19.93 98.99 89.09 Average 19.53 87.85 STD 1.25 4.93 STD% 6.39 5.62 The mechanical properties of the matrix were characterized according to EN 196 Standard for mortar. Table 2 shows the bending tensile strength (fctf) together with the cubic compressive strength (fcc).

Fabric. The fabric used was chosen after performing several investigations in order to optimize

performances in terms of ductility of the composite material, bond between matrix and fabric and internal filaments slip. The variables considered were the fabric geometry (warp and weft spacing and their cross section), fabric weaving and fabric coating. The final purpose was to achieve in the TRM the largest strength and ductility. The geometrical characteristics are summarized in Table 3. Table 3 – Geometrical characteristics of fabric. Material AR-glass Fabrication tecnique Leno weave Warp wire spacing [mm] 5.0 Weft wire spacing [mm] 7.1 Warp [Tex] 640 Weft [Tex] 1200 Tensile load peak [kN]a 5.99 a average of 10 specimens (400 x 70 mm) tested under uniaxial tension Fig.1 – View of the fabric.

Specimens preparation. The specimens used in the experimental investigation were 400 mm long, 70 mm wide and 6 mm thick and were reinforced with 2 layers of the fabric presented before. The reinforcement ratio here considered is equal to 3.30%.

The specimen preparation was made by using a proper formwork with the transparent bottom plate in order to check the penetration of the matrix into the fabric mesh. Some overlapping steel rails (2 mm thick) were designed in order to interpose textile layers. The mortar was spread and smoothed with a roller. After that, the reinforcement was tight and fixed at the edge. Repeating the procedure it was possible to create a multilayer specimen. The layers of textile were divided by a mortar layer 2 mm thick. In all the cases the warp was parallel to the long side of the specimen.

The specimens were cured in wet environment for 28 days until carrying out the thermal treatments. The thermal cycles were performed in an oven considering three different maximum temperature (200, 400 and 600°C). When the maximum temperature was reached (with an heating

warp

weft

70 mm

rate equal to 30°C/h) a 2 hours stabilization phase was imposed and then a cooling branch of 15°C/h was performed. In each thermal cycle, 3 nominally identical specimens were placed into the oven. Fig.2(a) and Fig.2(c) show the thermal cycles for the three maximum temperature considered and the distribution of the specimens into the oven.

Fig.2 – (a) Thermal cycles temperature-time curves; (b) tensile test set-up; (c) specimen distribution in the oven: section (top) and plan (bottom).

After the thermal treatment all the specimens, included those heated at 600°C, did not present

any visible crack.

Test set up. The tensile tests were carried out using an electromechanical machine INSTRON 5867 with a maximum load capacity of 30 kN. The specimens were clamped to the machine and two 70 mm x 55 mm steel plates were glued on each surface of the specimen where clamped in order to minimize the damage related to the local crash of the material when subjected to the clamping pressure. To prevent torsional and bending moments owing to a misalignment of the constraints, spherical joints were placed at the ends. The pressure applied to the pneumatic clamp was 2.7 bar and the tests were displacement-controlled by imposing a constant stroke rate of 0.02 mm/sec. The set-up is shown in Fig.2b.

For each temperature considered, 3 nominally identical tests were carried out. Three specimens not thermally damaged were used to compare the mechanical properties.

(a)

(b)

δ

(c)

Experimental results

The experimental results are reported in the paragraph for all the specimens investigated by means of the nominal stress (σ) vs strain (ε) curves (Fig.3). The stress was obtained by dividing the load by the nominal area of the specimen cross section and the strain was evaluated by the ratio between the elongation (δ) of the specimen and its initial length.

Fig.3 – Load vs displacement curve in uniaxial tension at different temperatures.

A common trend can be observed in the case of 20°C and 200°C: three sections characterize the non linear response. The first cracking was reached when the tensile strength of the matrix was exceeded and then multicracking strain hardening behaviour occurred. A final branch in which no further cracks develop takes place. In the case of 400°C only a multicracking phase up to the failure can be observed. At 600°C just a linear phase followed by a brittle failure is clear.

The comparison of the average stress-strain curves at different temperature is shown in Fig.4. An increment of first cracking strength and a better multicracking behaviour are evident in the case of 200°C compared to 20°C. This phenomenon is probably due to the shrinkage of the mortar at 200°C that leads to an increase of bond between the fabric and the matrix. For higher temperatures (400°C - 600°C) the coating degrades and sliding occurs.

Fig.4 – (a) Comparison between average curves and (b) view of specimens after tests at different temperatures (one specimen for each temperature; from the left: 20 – 200 – 400 – 600°C).

The results after each thermal cycle are reported in Table 4 in terms of peak strength, peak strain, first cracking strength and first cracking strain. The dependence of these parameters on temperature is also shown in Fig.5.

Table 4 – Test results.

T [°C]

Specimen

σp [Mpa]

εp [%]

σI [Mpa]

εI [%] T [°C]

Specimen

σp [Mpa]

εp [%]

σI [Mpa]

εI [%]

N1 21.30 2.31 6.84 0.12 N1 20.60 1.94 8.28 0.17 N2 17.32 1.94 4.54 0.08 N2 18.65 1.59 11.52 0.24 N3 20.65 2.12 5.22 0.10 N3 22.67 2.15 9.78 0.23 Average 19.76 2.12 5.53 0.10 Average 20.64 1.89 9.86 0.21 STD 2.14 0.18 1.18 0.02 STD 2.01 0.28 1.62 0.04

20

STD% 10.81 8.66 21.38 19.44

200

STD% 9.72 15.00 16.42 17.46N1 6.11 1.08 4.71 0.12 N1 3.98 0.10 3.98 0.10 N2 7.16 1.10 3.71 0.11 N2 3.98 0.09 3.98 0.09 N3 7.42 1.20 3.43 0.09 N3 4.05 0.11 4.05 0.11 Average 6.90 1.13 3.95 0.11 Average 4.00 0.10 4.00 0.10 STD 0.70 0.06 0.68 0.01 STD 0.04 0.01 0.04 0.01

400

STD% 10.08 5.64 17.10 11.91

600

STD% 1.04 9.46 1.04 9.46

Fig.5 – Test results: (a) peak strength vs temperature, (c) peak strain vs temperature (b) first cracking strength vs temperature and (d) first cracking strain vs temperature.

(a) (b)

In the case of 20°C specimens the working ratio, computed as the ratio between the strength in terms of load of the TRM composite and the reinforcement only, is equal to 0.78.

Conclusions

The experimental investigation shows that the mechanical residual properties of the TRM exposed to high temperatures depend on the loss in strength of the fabric. In the case of 200°C the fabric keeps its strength, even if the coating starts to deteriorate, showing a golden color: the maximum tensile strength, comparable with the one obtained in the case of 20°C, is reached. The specimens show the best behaviour in terms of first cracking strength and multicraking pattern (a crack occurs at each weft wire, approximately every 7 mm). This interesting phenomenon requires further investigations in order to optimize the characteristics of TRM and its casting and curing procedure.

After the exposure to higher temperatures (400 and 600°C), the coating completely deteriorate and the behaviour becomes brittle. However TRM elements present a tensile strength higher than 3.98 MPa.

Acknowledgement

The authors would like to thank Gavazzi company for its cooperation and supplying the AR glass fabrics. The research was financially supported by European INTERREG IT/CH 2006_2013 project ACCIDENT ID7629770, Measure 2.2.

References

[1] J. Orlowsky, M. Raupach, Durability model for AR-glass fibres in textile reinforced concrete, Material and Structures 41 (2008) 1225-1233.

[2] P. Purnell and J. Beddows, Durability and simulated ageing of new matrix glass fibre reinforced concrete, Cement & Concrete Composites 27 (2005) 875-884.

[3] T. Buttner, J. Orlowsky, M. Rapauch, Fire resistance tests of textile reinforced concrete under static loading- results and future developments, H.W. Reinhardt and A.E. Naaman (Eds), Proceedings of the fifth international RILEM workshop (HPFRCC5 PRO53), RILEM Publications S.A.R.L., Mainz, 2007, pp 361–370.

[4] M. Kruger, H.W. Reinhardt, Fire resistance, in: W. Brameshuber (Eds.), Textile Reinforced Concrete. State-of-the-Art Report of RILEM Technical Committee 201 – TRC: Texile Reinforced Concrete (Report 36), RILEM Publications S.A.R.L., Aachen, 2006, pp. 211- 219.

[5] D. Ehlig, F. Jesse, M. Curbach, High temperature tests on textile reinforced concrete (TRC) strain specimens, in: W. Brameshuber (Eds.), 2nd ICTRC. Textile Reinforced Concrete. Proceedings of the International RILEM Conference on Material Science (MatSci) Volume I (PRO 75), RILEM Publications S.A.R.L., Aachen, 2010, pp. 141- 151.

[6] M. Iakimov, Untersuchungen zum Brandverhalten von textilbewehrten Betonelement (Investigations on fire behaviour of textile reinforced concrete elements), Diplomarbeit, Universität Stuttgart, Institut für Werkstoffe im Bauwesen, 2003.