CREEP TESTS OF DRY AND SATURED CLAY BLOCK PRISMS

Transcription

1 CREEP TESTS OF DRY AND SATURED CLAY BLOCK MASONRY PRISMS Carvalho, Jenner M. 1 ; Ramos, Luís F. 2 ; Lourenço, Paulo B. 3 ; Roman, Humberto R. 4 1 Dr, Professor, Federal Institute Education of Bahia, Applied Sciences Department, jemicarvalho@gmail.com 2 PhD, Assistant Professor, University of Minho, Civil Engineering Department, lramos@civil.uminho.pt 3 PhD, Professor, University of Minho, Civil Engineering Department, pbl@civil.uminho.pt 4 PhD, Professor, Federal University of Santa Catarina, Civil Engineering Department, humberto@ecv.ufsc.br This work has as main objective to provide the creep coefficient and modulus of elasticity of clay block masonry prisms. For this purpose tests were conducted in specimens in dry and saturated conditions under compression. The first stage was the physical characterization of materials, followed by the preparation of the specimens and subsequent testing the compressive strength of blocks, mortar and prisms. Tests on prisms showed low values of strength and a brittle rupture. This happened close to the maximum strength, when the webs of the upper and lower blocks of the prism started to collapse and soon after the mortar began cracking, leading to an explosive collapse. Brittleness was more pronounced in dry prisms. From the test results it was possible to obtain values for the final creep coefficient and to calculate the modulus of elasticity in the long term. The results indicated that the ultimate creep coefficient for the saturated condition remained in the range recommended by Eurocode 6, although the value for the dry condition was a little above. Regarding the elastic modulus, it was noticed that there is a decrease of the saturated prisms when compared with the dry prisms. Keywords: Testing; creep; uniaxial compression; clay block masonry. 1 INTRODUCTION In the structural analysis of a building, the designer should consider all actions on the structure, which includes its weight, imposed loads for buildings and other live loads. These actions can lead to permanent displacements on the building components over time. Progressive deformation of building elements under constant or almost constant loading, as a function of time is called creep or plastic deformation. The magnitude of creep in masonry or concrete structures depends on the stress level, age of the material, duration of load, material quality and environmental conditions, such as the variation of temperature and relative humidity, or wetting and drying. The lack of information on creep in masonry has prevented its introduction into structural analysis, in opposition to the recommendations for concrete and standardss on concrete creep. In Brazil there are thousands of buildings constructed in structural masonry in the last decades, in some cases having the foundation walls subjected to wetting and drying cycles, which might compromising their durability. The prism tests carried out aimed at checking the mechanical behavior of the foundation built in clay block masonry over time. The objective was to analyze the effect of creep on the masonry under saturated and dry conditions. For this purpose, tests of monotonic load and

2 accelerated creep were carried out. The results showed that there is a need to consider the effect of creep when low quality and weakly burnt masonry is subjected to both dry and saturated conditions. 2 - MATERIALS AND METHODS Characteristics of the clay block The clay blocks used in the experimental work are shown in Figure 1, and they were acquired in Recife, Brazil. A sample with 30 blocks was sent to University of Minho in Portugal, where tests were performed. The sample did not present systematic defects such as broken tiles, uneven surfaces or excessive deformation, significant variation in color or cracking. Compressive strength tests carried out in 6 blocks showed values of 1.51 MPa for dry conditions and 0.97 saturated conditions. Width = 90 Height = 190 Length = 190 Figure 1 - Characteristic and size (mm) block Characterization of materials used in the mortar The mortar used for construction of the prisms was based on cement, hydrated lime and sand mix (1:1:6 by volume), recommended by BS 5628 (1992). The cement used was type CEM II / BL 32.5 N. To assess the contents of powdery material and to determine the contents of organic impurities in the sand, norms NBR NM 46 (2003) and NBR NM 49 (2001) were used. The sand particle size was determined by the NBR NM 248 (2003) and the test results are presented in Table 1. The unitary masses of cement, lime and sand were also determined according to NBR NM 45 (2006), and are shown in Table 2. Table 1 - Composition of the sand particle size Sieve (mm) Percentage cumulative retained Percentage cumulative passed Table 2 - Unit mass of material Material unit mass (kg/dm³) Cement 1.08 Hydrated lime 0.76 Sand 1.45

3 The mechanical characterization of mortar indicated a tensile strength in bending, determined by the NBR (2005), equal to 2.30 MPa for dry conditions and 1.17 MPa for saturated conditions. The compressive strength values according to the same norm were 6.29 MPa for dry conditions and 3.86 MPa for saturated conditions. 2.3 Test setup for monotonic loading After the characterization of materials, six prism specimens were prepared. Each sample consisted of two blocks positioned horizontally in the larger dimension as observed in the inspection of the foundation of the building. Furthermore, a thickness of the horizontal joint equal to 25 mm was considered, which was the average value found in the foundation walls. The six specimens were left 28 days in the laboratory for curing. At the end of this period three of the specimens were tested until collapse, while the other three were immersed in a water tank for 28 days before testing. The test loading procedure was performed according to EN (1999). To obtain the data four linear variable differential transformers (LVDT) were positioned, one on each side of the prism and fixed on the top plate and the bottom plate, see Figure 2. Figure 2 - Position of LVDT 2.4 Test setup for accelerated creep test Similar procedures to the setup for monotonic loading test were used for creep, again with six similar test specimens under dry and saturated conditions. To obtain the data four LVDTs were placed, one on each side of the prism and fixed on the top and the bottom plates, see Figures 3a and 3b. It is important to note that the saturated prisms were wrapped in thin foil to prevent the loss of water. According to Eurocode 6 (2005) there is no specific standard for a creep test. However, Binda et al. (2008) and Lourenço and Pina-Henrique (2008) can serve as guidelines for developing the experiment. In the studies conducted by these authors, historic stone or clay brick masonry was used. Therefore, it was necessary to adjust the test for materials with different mechanical and physical properties. The cited researchers pointed out the need to measure the temperature and humidity variation in the laboratory. Measurements were taken every two hours except at night when the interval was a longer. The results of the measurements of temperature and humidity, a total of 72 records, did not indicate significant variations, with values of about 24 C and 60% for

4 temperature and humidity, respectively. From the results of the monotonic loading test, 40% of ultimate stress was taken as initial load and 10% of the remaining load was applied subsequently for each level. Load intervals were about 5 hours during the day and 8 hours during the night. The initial load was applied through the pump shown in Figure 3a, with a compressed air reservoir to keep the load constant during the loading stage. (3a) (3b) Figure 3 - Equipment used in the accelerated creep test 3 - ANALYSIS OF RESULTS Monotonic load test After testing the prisms, the stress-strain diagrams in Figures 4 and 5 were compiled. Figure 4 shows that the dry compressive strength ( ) ranged from 1.17 MPa to 1.50 MPa, with an average of 1.27 MPa for the three prisms. The elasticity modulus ( ) ranged from 378 MPa to 913 MPa, with an average of 685 MPa. Figure 4 - Stress-strain diagrams for the dry prisms Figure 5 depicts the results of tests performed on saturated prisms. The compressive strength ( ) ranged from 0.62 MPa to 1.07 MPa, getting an average of 0.85 MPa. The modulus of elasticity ( ) ranged from 188 MPa to 356 MPa, with an average of 272 MPa.

5 Figure 5 - Stress-strain diagrams for the saturated prisms Accelerated creep test This test was performed for dry prisms and saturated prisms, with the results shown in Figures 6 to 9. Figure 6 provides the stress-time diagrams for the dry prisms, with an ultimate stress ranging from 0.98 MPa to 1.10 MPa, with an average of 1.03 MPa. For the saturated prisms, see Figure 7, the values of the ultimate stress were in the range from 0.85 MPa to 0.95 MPa with an average of 0.90 MPa. It is observed that the approximate time for collapse was 1.5 days and 2.5 days for dry and saturated prisms, respectively. Figure 6 - Stress-time diagrams for the dry prisms The results of the strain-time diagrams for dry prisms are shown in Figure 8. Excluding the effect of the initial load on the final strain of creep in masonry ( ), the creep rate was between 2.1 mm/m and 2.3 mm/m, with an average value found of 2.17 mm/m. Figure 9 shows the strain-time diagrams for saturated prisms. The creep rate ranged between 1.8 mm/m at 2.4 mm/m, with the same average value of 2.17 mm/m.

6 Figure 7 - Stress-time diagrams for the saturated prisms Figure 8 - Strain-time diagrams for the dry prisms Figure 9 - Strain-time diagrams for the saturated prisms

7 4 - DISCUSSION Table 3 was compiled from the information obtained in the previous sections and based on equations proposed by Eurocode 6. The elastic strain of the masonry ( ) equals the average compressive strength of the prisms ( ) divided by the average modulus of elasticity ( ). The final creep strain in masonry ( ) was considered after application of the initial load. For the final creep coefficient ( ), presented in Table 3, the Eurocode 6 recommends to use the relationship between the final creep strain ( ) and the elastic strain of the masonry ( ). The same code provides the modulus of elasticity in the long term ) as: (1) Prism Table 3 - Parameters of the masonry dry and saturated (MPa) (MPa) (MPa) Dry Satured In the monotonic loading test, shown in Table 3 by the parameters and, the ultimate compressive strength decreased by 33%, when changing from dry to saturated conditions, while the modulus decreased by approximately 60%. For the elastic strain of the masonry ( ), there was an increase of 69% when changing from dry to saturated conditions. It is noticed that the final strain of creep in masonry ( ) was the same for both conditions. However, the final creep coefficient ( ) for dry masonry is approximately twice the saturated masonry. It should be noted that the Eurocode 6 (2005) prescribes for values in the range of 0.5 to 1.5 for the clay unit, but does not specify whether the masonry is dry or saturated. Table 3 also shows the modulus of elasticity in the long term ) for dry and saturated masonry, which decreased by approximately 50% when changing from dry to saturated conditions. Analyzing the modulus of elasticity at the end of time, the value decreases by about 76% when changing from dry to saturated conditions. Another aspect that was observed during the monotonic load test and accelerated creep test was the collapse mechanism of prisms, shown in Figures 10. Figure 10a shows a test in dry prisms while Figure 10b shows a test in saturated conditions. The collapse mechanism was similar for both conditions and occurred near the final strength. Firstly, the external webs of upper and lower blocks collapsed, then cracking in the mortar around the holes began to crack leading to the explosively collapse of the whole. This phenomenon was more pronounced in dry prisms, when compared to saturated prisms. 5 - CONCLUSIONS AND RECOMMENDATIONS It can be concluded from the results that there was a decrease in the strength of masonry as it changed from dry to saturated conditions. The value of the final creep coefficient of the dry and the saturated masonry were within the range established by Eurocode, being lower in

8 saturated conditions. In the accelerated creep test a decrease in masonry stiffness over time it was found for both conditions. The results showed also that there is need to consider the effect of creep in heavily loaded masonry structures. As these are first results for materials commonly used in Brazil, more tests are needed to establish definitive conclusions on the values encountered. (10a) (10b) Figure 10 - Mode rupture of prisms: dry (10a) and saturated (10b) ACKNOWLEDGMENTS The first author would like to thank University of Minho UM for hosting the works shown, Federal University of Santa Catarina UFSC where he enrolled as PhD student, Coordenação de Aperfeiçoamento do Pessoal CAPES and Fundação de Amparo à Pesquisa do Estado da Bahia FAPESB for the PhD grant. REFERENCES Associação Brasileira de Normas Técnicas. NBR NM 45: Aggregates Determination of the unit weight and air-void contents. Rio de Janeiro, NBR NM 46: Aggregates Determination of material finer than 75 um sieve by washing. Rio de Janeiro, NBR NM 49: Fine aggregate - Determination of the organic impurities. Rio de Janeiro, NBR NM 248: Aggregates Sieve analysis of fine and coarse aggregates. Rio de Janeiro, Binda, L.; Anzani, A.; Saisi, A. Learning from failure: Long-term behavior of heavy masonry structures. 1ª ed. Boston: WITpress, 2008, v. 23, p ISBN: ISSN: BS 5628 Part 1. structural use of unreiforced masonry. London, Eurocode 6 Part 1. Design of masonry structures: General rules for reinforced and unreinforced masonry structures. Brussels, EN 1052 Part 1. Methods of test for masonry: Determination of compression strength. Brussels, Lourenço, P. B.; Pina-Henriques J. Learning from failure: Long-term behavior of heavy masonry structures. 1ª ed. Boston: WITpress, 2008, v. 23, p ISBN: ISSN: