Abstract. 1 Introduction

Transcription

1 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Creep behaviour of mortars used in restoration M. Karaveziroglou, P. Koulikas, P. Panagiotopoulos, C. Dimitreli, G. Triantafillou Department of Civil Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece Abstract Experimental results on mortar deformations used in Restoration of historical buildings and monuments are given in this paper. The time depending deformations of specimens from mortars with different composition (lime, natural sand, pozzolana, crushed bricks and a varying percentage of cement) were measured in laboratory up to 6 month. The difference in creep behaviour between mortars having varying compositions studied in this research can be used in the evaluation of the mortar influence concerning the deformation of masonry with thick mortar joints (stone or brick masonry of historical structures). Introduction Results of a new research on the creep behaviour of masonry have been published in the recent decades. Obviously the creep coefficients of modern masonry cannot be directly applied in performance evaluations by repairing studies and analysis of masonry in historical buildings. The reason is that this type of masonry differs from a modern one (with known mechanical properties) in building materials as well as in geometrical characteristics (large shaped full bricks or stones, thick mortar joints, thick walls); e.g. Karaveziroglou, Papayianni & Penelis []. The investigation of the time depending deformations of a restored masonry in historical structures requires special knowledge of the creep and shrinkage behaviour of mortar in joints, since its properties are not similar to that of cement mortars used in modern buildings; e.g. Bagda & Lipp [2].

2 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis 2 Experimental Procedure 2. Composition of mortars The constituent materials of mortars used in repairing of masonry in historical buildings are selected according to the analysis results of the original mortar in joints (between bricks or stones). Generally old mortars contain lime, pozzolana, sand and crushed bricks, e.g. Penelis, Papayianni & Karaveziroglou []. The mortars tested in this work consist of hydrated lime, natural pozzolana (Santorin earth with max size mm), river sand (max size 4 mm), crushed bricks (max size 4 mm) and cement. The proportions are given in Table. The mixtures differ in cement and sand or ceramic content in order to examine the influence of these materials on mortar deformations. The amount of water was assessed for each mortar to be that required for afluidity4 cm up to 5 cm. Mortar type LM LM2 Table. Proportions of mortars (by weight) Lime Pozzolana Cement Crushed Sand bricks Water LM LM LM Studies and test results of hardened mortars Samples 40 mm x 40 mm x 60 mm were to be tested for: bending strength and compressive strength dynamic modulus of Elasticity (ultrasonic test) shrinkage creep Mortar samples 70 mm x 70 mm x 70 mm were also prepared for testing the compressive strength and the modulus of Elasticity. Measurements of bending strength, compressive strength and dynamic modulus of Elasticity were taken after 7, 5 and 28 days for all mortar types except LM2 (first measurement after 5 days) because of its slow setting time. The average values (three prismatic samples for each test) are given in Table 2.

3 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis 2 Table 2. Strength and modulus of Elasticity (mortar samples 40x40x60 mm) Mortar Bending Compressive Modulus of Elasticity [N/mitf] [N/mnf] [N/mnf] type days days days days days days days days days LM LM LM LM LM Shrinkage measurements were taken immediately after the removal of samples from the moulds; e. g. DIN [4]. At the same time, creep was measured as the hardened samples (40 x 40 x 60 mm) could be loaded. The deformation at time zero (the moment the full load was applied) is the instantaneous deformation. This time was varying for each mortar (0 days for LM, 5 days for LM2, LM and days for LM4, LM5). The samples were placed in testing devices (two samples in each device) under a sustained load corresponding to 0 % of the compressive strength taken after 28 days from the cubic mortar samples. The load for each type of mortar was N for LM, N for LM2, 0 N for LM, N for LM4 and 70 N for LM5. The testing devices (photo ) were designed and completely constructed in our laboratory patterned upon the spring loaded creep frame of ASTM S test method (photo 2) which was used for measuring the creep of concrete or brick masonry in cylindrical cores; e.g. Karaveziroglou, Papayianni & Penelis []. The devices were installed in a room at constant relative humidity 60 % and temperature 20 C. Measurements were taken every day at the first time (about 0 days). Afterwards, every three days up to the end of the deformation process. According to the test results the shrinkage deformation of mortars increases by the time and after a period of 22 up to 29 days (after mortar mixing) remains unchanged. The values obtained are in the range of 0.9 to. mm/m. The measured values of the long-term deformations under load are given in Fig. (average values of four samples). The difference between shrinkage and creep behaviour of mortar LM2 is shown in Fig. 2 and that of LM in Fig. (shrinkage subtracted from the whole deformation of samples in testing device, no consideration of the elastic deformation).

4 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis Photo : Creep testing device for mortar Photo 2: Creep testing device for concrete or masonry samples

5 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis Figure : Time depending deformations of mortars.

6 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis I I / Shrinkage + Creep, Creep c 2 a Q -, Shrinkage Time [days] Figure 2: Creep and shrinkage of mortar LM2. 4 i i i., Shrinkage + Creep Deformation [mm/m] iw w F. Creep Shrinkage _ n / i l l Time [days] Figure : Creep and shrinkage of mortar LM.

7 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis 27 Discussion The data obtained through this experimental work lead to the following considerations: the slope of the ascending branch of the deformation curves (Fig. ) is much steeper at the first days (development of shrinkage, creep and elastic deformations at the same time). the deformation curves LM, LM2, LM and LM4, LM5 in Fig. show a significantly different behaviour depending on the strength of the hardened mortars. These results should be considered in connection with the amount of cement in these mixtures (Table, Table 2). the deformation of the mortar depends on its strength; i.e. lower strength of the mortar, earlier stopping of the deformation process (0 days for LM, 5 days for LM2 and LM, 5 days for LM4 and 90 days for LM5). the addition of crushed bricks to mortar leads to a significant decrease (about 50 %) of time depending deformations (Fig. ). The results of a previous experimental work on masonry cylindrical specimens constructed with full bricks and lime - pozzolana mortar joints confirm this estimate; e. g. Karaveziroglou, Papayianni & Penelis []. as expected, creep - strain in mortars is higher than shrinkage as shown in Fig. 2 for mortar LM; e. g. Kirtschig & Metje [5]. Mortars with no significant difference in creep behaviour (Fig. ) have in shrinkage almost the same deformation development (Fig. 2, Fig. ). 4 Conclusions The experimental results presented in this paper show that time depending deformations of lime - pozzolana - cement mortars remain unchanged after a period which could be characterized "short" compared to that one of a modern cement mortar. Parameters as the sustained load, the curing conditions, the water/binder ratio as well as the bond between new (repairing) mortar and the natural stone or brick in the masonry of historical buildings must be also investigated in order to have a completed study of creep process as the mortar in joints located between bricks or stones cannot deform freely. Aim of the further research would be the estimation of creep coefficients for masonry with full bricks or stones at thick mortar joints, which is common in historical structures.

8 Transactions on the Built Environment vol 5, 995 WIT Press, ISSN Architectural Studies, Materials & Analysis References. Karaveziroglou, M., Papayianni, J. & Pencils, G. Time dependent deformations on brick masonry, pp. -8, Proceedings of the Int. Tech. Conference on Structural Conservations of Stone Masonry, Athens, Greece, Bagda, E. & Lipp, G. Ausbesserungsmortel, Bautenschutz + Bautensanierung, 984,2, Penelis, G., Papayianni, J. & Karaveziroglou, M. Pozzolanic Mortars for Repair of Masonry Structures, in Structural Repair and Maintenance of Historical Buildings (ed. C. A. Brebbia) pp. 0-69, Proceedings of the Int. Conf on Structural Studies, Florence, Italy, DIN 52450, Testing of inorganic non - metallic building materials; Determination of shrinking and swelling on small test pieces, August Kirtschig, K. & Metje, W. R., Tragverhalten von Mauerwerk mit Leichtmortel, Mauerwerk-Kalender, 98,,66-67.