Influence of water/lime ratio on the characteristics of lime mortars for use as substitution renders in ancient buildings

Transcription

1 Influence of water/lime ratio on the characteristics of lime mortars for use as substitution renders in ancient buildings Maria Paula Raposo Pacheco Algarvio Extended Abstrat Orientador: Prof. Doutor Augusto Martins Gomes Co-Orientador: Prof.ª Doutora Ana Paula Teixeira Ferreira Pinto França de Santana Outubro 2010

2 ABSTRACT Renders are the fundamental elements of a built structure, because they play an important role in protecting masonry constructions. Renders degrade through time due to numerous factors which they are particularly exposed to. Walls renders significance is the focus of analysis and discussion whenever the preservation and restoration of old buildings is concerned. This work aims at a contribution to the increment of scientific knowledge in the preservation of renders in old buildings. It will emphasize the evaluation of the influence of the amount of kneading water in aerial lime mortars characteristics, taking into account their physical and mechanical behaviour through the time. It will also analyse the consequences of the composition variation in the fresh mortars characteristics and the consequent influence in the technical use in buildings under construction. In order to achieve the above objectives, we produced five mortars formulation of aerial lime (hydrated lime powder) with the same volumetric proportion (1:3), but with different water/lime ratio. The characterization of the fresh mortars included the determination of the consistency, the bulk density and the water retentivy. The characterization on the hardened state of the mortars included the assessment of physical and mechanical properties of the five formulations, after 60 days of curing time, conducted on prismatic specimens. Therefore, this thesis also focused on the analysis of the five formulations response concerning the salt crystallization. That is why we made crystallization tests using a solution of sodium chloride. In general the obtained results show that it is possible to verify the suitability of the studied materials in order to repair or substitute the existing mortars of ancient buildings. KEY-WORDS: Mortars; Aerial lime; Plasters and renders; Old buildings; Kneading water; Soluble salt crystallization. I

3 1. Introduction Renders of the old buildings are the key elements of a built structure, because they play an important role in protecting the masonry. As being particularly vulnerable to degradation, these coatings have to give protection against mechanical actions (shock), chemical actions (pollution and soluble salts) and climatic actions (rain and wind). Besides, the buildings aesthetic depend on the renders and, therefore, they need to be subjected to conservation actions concerning that issue. [18] [20]. Wall coatings mostly used on old buildings are directly related to the materials used in their construction, in order to obtain the right compatibility between them. These coatings consist of frail sand and aerial lime mortars, with the adequate characteristics for the underlying masonry [12] [13]. In old buildings, mainly monuments of a significant historic and aesthetic value, the restoration of the pre-existent coatings is the correct option to take, instead of their replacement, thus, fulfilling the conservation ethics view requested [19]. [20]. This work aims at a contribution to the increment of scientific knowledge in the preservation of renders in old buildings. It will emphasize the evaluation of the influence of the amount of kneading water in aerial lime mortars characteristics, taking into account their physical and mechanical reaction through time. It will also analyse the consequences of that element s variation in the fresh mortars characteristics and the consequent influence in the technical use in buildings under construction. 1

4 2. EXPERIMENTAL WORK 2.1 MATERIALS For the mortars preparation under study, it was used an aerial powdered hydrated lime type CL90,, supplied in a 22 kg bag of the Calcidrata brand. It was used a mixture of equal proportions of river sand and yellow sand. The main aggregate characteristics are presented in Table 2.1 (fineness modulus, maximum and minimum particle size). The two types of sand are very similar, saving the river sand that is slightly finer. Table 2.1 Aggregate characterization. Yellow sand River sand Max. Particle size D max [mm] Min. Particle size D min [mm] 2,38 2,38 0,149 0,149 Fineness modulus 3,1 2,7 2.2 MORTAR COMPOSITIONS In order to study the influence of water/lime ratio on the characteristics of aerial lime mortars a total of five mortars was produced. For that, it was taken as reference a lime mortar, with a binder/aggregate (B/Ag) ratio of 1:3 by volume and a consistency of 65%. This mortars ratio by volume was converted into a 1:8 by mass ratio in order to facilitate and minimize the measurement errors of the mortars constituents amouts. The water/lime proportion was not defined from the beginning. It varied according to the intended flow value for each of the five formulations. It is presented in Table 2.2. Table 2.2 Composition of the mortars. Mortar Binder Aggregate Ratio by volume Ratio by mass Flow value CA1 85 ± 2,0 % CA2 75 ± 2,0 % 50% Yellow sand Hydrated CA3 + 1:3 1:8 65 ± 2,0 % lime 50% River sand CA4 55 ± 2,0 % CA5 45 ± 2,0 % 2

5 3. TESTING PROGRAM AND RESULTS Fresh mortars characterization tests were based on the determination of: consistency by flow table and plunger penetration, bulk density and water retentivy. Hardened mortars characterization was made through: Mechanical characteristics: ultrasound velocity, flexural and compression strength. Physical characteristics: water absorption due to capillarity action, porosity accessible to water, drying kinetic, water absorption by immersion at 48 h and salts crystallization. For each mortars formulation, nine prismatic specimens were molded at 60 days. 3.1 FRESH MORTAR CHARACTERIZATION To establish the value of the mortars consistency, adapted procedures were taken according to the standards EN :1999 [3] and EN :1999 [4]. The results of the two consistency measurement methods are presented in Table 3.1. Table 3.1 Flow value and penetration. Mortar Flow value [mm] Penetration [mm] Bulk density [kg/m 3 ] Average Average Average water retention [%] CA1 1, ,1 CA2 1, ,8 CA3 1, ,9 CA4 1, ,6 CA5 1, ,2 The two methods of measuring the consistency, point to the increase of the mortars fluidity with the raise of the water content and we obtained a linear regression with correlation coefficients which are considered very good. Mortar CA1 showed the highest fluidity and mortar CA5 the driest consistently (Figure 3.1). 3

6 Bulk density [Kg/m 3 ] Water retention [%] Flow value [mm] Penetration [mm] R² = 0, R² = 0, Figure 3.1 Consistency methods versus water/lime ratio. The analysis of Figure 3.2 shows that mortars CA2 and CA3 present the lowest capacity to retain water and the driest mortar (CA5), presents a better performance in this matter R² = Figure 3.2 Water retention versus water/lime ratio. In Figure 3.3 it can be verified that mortars show similar values, but mortars CA3 and CA4, with a water/lime ratio of 1,53 and 1,60, respectively, present slightly lower values Figure 3.3 Bulk density versus water/lime ratio. 4

7 Ultrasound velocity [m/s] Ultrasound velocity [m/s] It can be observed that there is not a correlation between the variation in bulk density and the water/lime ratio. 3.2 HARDENED MORTAR CHARACTERIZATION MECHANICAL CHARACTERISTICS Table 3.2 shows the average ultrasound velocity values and flexural and compression strength results, obtained from the different mortars. Table 3.2 Ultrasound velocity, flexural and compression strength. Mortar R f [Mpa] R c [Mpa] Ultrasound velocity [m/s] CA1 1,68 0,26 0, CA2 1,65 0,27 0, CA3 1,60 0,28 0, CA4 1,53 0,30 0, CA5 1,48 0,33 0, Ultrasound velocity tends to increase when a raise of the compression strength occurs and when the water/lime ratio decreases. We registered the maximum velocity value when there is a water/lime ratio of 1,6 (Figure 3.4) Compressive strength [Mpa] Figure 3.4 Ultrasound velocity versus compressive strength and water/lime ratio. 5

8 Flexural strength [Mpa] Compressive strength [Mpa] However, this trend is not clear for the mortars CA4 and CA5, surprisingly, because of their higher mechanical strength, but lower ultrasonic velocity and the differences of the obtained values are not significant. This behaviour may be related to the existence of cracks in the internal structure of the mortars. The compressive and flexural strength tests were performed following procedures adapted from EN :1999 [1] and according to LNEC E29 specification [7]. According to the results presented in Table 3.2 and in Figure 3.5 there is a trend of a decrease in flexural strength, whenever there is an increase of the water/lime ratio. The relation between compressive strength and water/lime ratio showed in Figure 3.5 is similar to that described for the flexural strength, i.e. the compressive strength decreases linearly with the water/lime ratio. The correlation coefficients obtained were quite significant. 0.4 R² = R² = Figure 3.5 Flexural and compressive strength versus water/lime ratio. 3.3 PHYSICAL CHARACTERISTICS WATER ABSORPTION DUE TO CAPILLARITY ACTION The water absorption through capillarity rise test was based on EN :2002 [2] and on LNEC E393 [8]. Table 3.3 presents the mean values of capillarity water absorption coefficients (C.C.) along the initial 30 minutes of testing period. It also shows the asymptotic absorption values. Table 3.3 Capillarity water absorption coefficient and asymptotic value. Mortar C.C min [kg/m 2.s 0.5 ] Asymptotic absorption value [kg/m 2 ] CA1 1,68 0,242 29,4 CA2 1,65 0,270 29,4 CA3 1,60 0,279 29,3 CA4 1,53 0,312 29,1 CA5 1,48 0,276 28,1 6

9 ΔMass/S de 0-30 min [kg/m 2 ] ΔMass/S [kg/m 2 ] The water absorption due to capillarity of the five mortars studied is showedy in Figure 3.6. These curves show a predictable development, with a decreasing slope along the time, when water amount increase CA1 CA2 CA3 CA4 CA Time [ s] Figure 3.6 Capillarity water absorption coefficient. Figure 3.7 specifies the capillarity water absorption coefficient along the initial 30 minutes testing period CA1 CA2 CA3 CA4 CA Time [ s] Figure 3.7 Capillarity water absorption coefficient versus time The results presented in Table 3.3 and in Figure 3.7 demonstrate that mortar CA1 was, among the other mortars the one that showed the slower water absorption rate. Mortar CA4 registered the higher water absorption value. 7

10 Asymptotic value [kg/m 2 ] CoeCoefficient of capillarity absorption 0-30 min [kg/m 2 S 0,5 ] Analysing Figure 3.8 it can be seen that the capillarity water absorption coefficients tend to increase when the water/lime ratio decreases. However, the correlation coefficient value obtained was not significant and it is considered not acceptable (R 2 = 0,441) R² = Figure 3.8 Capillarity water absorption coefficient of versus water/lime ratio. Regarding the total amount of water absorbed, Figure 3.9 points to a generalized trend to a raise of the asymptotic values when water/lime ratio increases (reasonable correlation, R 2 = ) R² = Figure 3.9 Asymptotic value versus water/lime ratio. 8

11 Open porosity [%] OPEN POROSITY The open porosity test was based on the RILEM I.1 [14] guidelines. Table 3.4 shows the open porosity average values tested for each mortar. Table 3.4 Open porosity. Mortar P open [%] CA1 1,68 27,5 CA2 1,65 27,2 CA3 1,60 26,9 CA4 1,53 26,4 CA5 1,48 25,6 There is a high correlation (R 2 = 0,9715) between open porosity and the water/lime ratio in mortars, as verified in Figure 3.10 with a consequent slight raise in porosity, whenever the water/lime ratio increases R² = water/lime Figure 3.10 Open porosity versus water/lime ratio WATER ABSORPTION BY IMMERSION (48 H) Water absorption by immersion test (48 h) was based on LNEC E394 [9] specification. Table 3.5 and Figure 3.11 show the results of the tests made to quantify the amount of water in mortars, after an immersion period of 48 h. 9

12 Water absorption by immersion(48h) [%] Table 3.5 Water absorption by immersion (48 h) value. Mortar water absorption by immersion (48 h) [%] CA1 1,68 10,3 CA2 1,65 10,2 CA3 1,60 10,2 CA4 1,53 9,8 CA5 1,48 9,7 Examining Table 3.5 and Figure 3.11 it can be clearly seen that there is an increase of the water content after 48h, when water/lime increases. There is a high correlation (R 2 = ) R² = water/lime Figure 3.11 Water absorption by immersion (48 h) versus water/lime ratio DRYING KINETIC The drying test was performed following procedures based on the RILEM I.5 [16]. The drying index (I.S.), was determined based on the following drying curves: f(w i ) amount of water inside the sample according to time, expressed in percentage in relation to drying mass; W 0 initial amount of water, expressed in percentage in what concerns the drying mass; t f final time of the test [h]; t 0 initial time of the test [h]. Table 3.6 presents the drying index values obtained in the different mortars. 10

13 water content[%] Table 3.6 Drying kinetic value. Mortar Drying index CA1 1,68 0,25 CA2 1,65 0,21 CA3 1,60 0,30 CA4 1,53 0,34 CA5 1,48 0,27 Analysing Figure 3.12, we verify that drying rates are higher in the early hours of the test and that they diminish over time, until the samples mass stabilizes. We can also see that mortar CA2 shows a higher drying rate, i.e. it dries much easier. We verify that the drying kinetic in mortar CA4 occurs more slowly ((higher drying rate), when compared to the other mortars. Mortars CA1, CA3 and CA5 show intermediate behaviours CA1 CA2 CA3 CA4 CA Time [h] Figure 3.12 Drying kinetic curve CRYSTALLIZATION OF SALTS The salt crystallization test was performed following procedures based on the test described by Charles Selwitz and Eric Dowhne [17]. The weight changes recorded during the test allowed to evaluate the loss or gain arose from the mass action of saline or distilled water. The test results of salt crystallization are presented in Table 3.7, showing the mean values of the masses and the mass variation of the different mortars analyzed. 11

14 Variation of mass [%] Table 3.7 Variation of mass value. 1º Cycle 2º Cycle 3º Cycle Mortar Initial mass Average (m 0 ) [g] Mass after drying [g] Variation of mass value [%] Mass after drying [g] Average Variation of mass value [%] Mass after drying [g] Variation of mass value [%] CA1 463,25 467,85 0,99 465,60 0,51 444,60-4,03 CA2 468,20 471,50 0,71 470,05 0,40 429,95-8,17 CA3 470,55 475,90 1,14 475,00 0,95 457,20-2,84 CA4 466,35 471,50 1,10 467,45 0,24 448,25-3,88 CA5 469,50 473,70 0,89 467,20-0,49 443,90-5,45 Figure 3.13 shows the mass change by action of the chlorides in correlation with the time used in the test CA5 CA4 CA3 CA2-20 CA Time [days] Figure 3.13 Variation of mass by action of the chlorides. Thus, there is a slight gain on the initial mass in all the mortars in the order of 1%, which confirms the uniformity of behavior, on the first cycle test. On the 2nd cycle mortar CA5 had a different behaviour. From this cycle until the end of the test specimens began to have more significant weight loss. Weight losses are very prominent in CA3 mortar (-2.84%) when compared with the mortar CA2 (more than - 8%).The analysis presented leads to a reflection on the internal microstructure of the mortars, including porometry (which was not examined in this paper). 12

15 4. CONCLUSIONS In order to use mortars of aerial lime when replacing mortars in old buildings, it is crucial to know their properties in terms of mechanical behavior and durability. We present the main conclusions obtained from this experimental study next. For the consistency tests we verified that the fluidity of mortars increased when the water content rose too. We also observed a high correlation between the two consistency methods evaluated. It was not possible to establish a clear correlation between water/lime ratio and bulk density. There was a trend to an increase of water retention when the water/lime ratio decreased. The increment of water/lime ratio in mortars studied led to a decrease of mechanical strength and ductility (ratio between flexural and compressive strength). It was verified that there was a trend to a raise in the ultrasound velocity propagation whenever water/lime ratio diminished. The open porosity values were conditioned by water/lime ratio and showed a clear increasing trend due to the water/lime increase. The rates of water absorption due to capillarity action decreased (capillarity coefficient) whenever water/lime ratio increased, causing a raise in the total amount of absorbed water (asymptotic value). With regard to the water content s evaluation after 48h, the conclusions were similar in what concerns the total water absorption capacity of the mortars analysed. There was a decrease in the drying velocity related to the decrease of water/lime ratio. With reference to salt crystallization behavior it was verified that mortars with intermediate water/lime ratio (a/l = 1,6), showed features that point to a better performance when compared to the other mortars. 13

16 REFERENCES [1] EN :1999 Methods of test for mortar for masonry Part 11: Determination of flexural and compressive strength of hardened mortar. European Committee for Standardization, Brussels (August 1999). [2] EN :2002 Methods of test for mortar for masonry Part 18: Determination of water absorption coefficient due to capillary action of hardened mortar. European Committee for Standardization, Brussels (December 2002). [3] EN :1999 Methods of test for mortar for masonry Part 3: Determination of consistence of fresh mortar (by flow table). European Committee for Standardization, Brussels (February 1999). [4] EN :1999 Methods of test for mortar for masonry Part 4: Determination of consistence of fresh mortar (by plunger penetration). European Committee for Standardization, Brussels (October 1998). [5] EN :1998 Methods of test for mortar for masonry Part 6: Determination of bulk density of fresh mortar. European Committee for Standardization, Brussels (October 1998). [6] EN :1999 Methods of test for mortar for masonry Part 8: Determination of water retentivity of fresh mortar. European Committee for Standardization, Brussels (September 1999). [7] Especificação LNEC E 29. (1979). Cimentos Determinação da Resistência Mecânica, Documentação Normativa. Lisboa: LNEC. [8] Especificação LNEC E 393. (1993). Betões Determinação da Absorção de Água por Capilaridade Documentação Normativa.. Lisboa: LNEC. [9] Especificação LNEC E 394; " Betões Determinação da Absorção de Água por Imersão. Ensaio à pressão atmosférica". Documentação Normativa. (1993). Lisboa: LNEC. [10] NP EN :1999 Methods of test for mortar for masonry Part 18: Determination of water absorption coefficient due to capillary action of hardened mortar. [11] NP EN 13139: Agregados para argamassas. [12] PALOMO, A. e. Historic Mortars: Characterization and Durability. New Tendencies for Research. [13] PINHO, F. F. (2000). Paredes de edifícios antigos em Portugal. Lisboa: LNEC. 14

17 [14] RILEM Test No. I.1 - Porosity accessible to water. RILEM 25-PEM - Recommandations provisoires. Essais recommandés pour mesurer l altération des pierres et évaluer l éfficacité des méthods de traitement. Matériaux et Construction, Vol.13, Nº75. (1980). [15] RILEM Test No. I.2 Bulk densities and real densities. RILEM 25-PEM Recommandations provisoires. Essais recommandés pour mesurer l altération des pierres et évaluer l éfficacité des methods de traitement. Matériaux et Construction, Vol.13, Nº75. (1980). [16] RILEM Test No. II.5 - Evaporation Curve. Recommendations provisoires. RILEM TC 25-PEM. (1980). [17] SELWITZ, C., & DOWHNE, E. (2002). The evaluation of crystallization modifiers for controlling salt damage to limestone. Journal of Cultural Heritage Nr. 3 (p ). [18] VEIGA, M. R. (2005). Comportamento de rebocos para edifícios antigos: Exigências gerais e requisitos específicos para edifícios antigos. Seminário Sais solúveis em argamassas de edifícios antigos. Lisboa: LNEC. [19] VEIGA, M. R., AGUIAR, J., SILVA, A. S., & CARVALHO, F. (2004). Argamassas para revestimento de paredes de edifícios antigos. Características e campo de aplicação de algumas formulações correntes. Lisboa: LNEC. [20] VEIGA, M. R., AGUIAR, J., SILVA, A. S., & CARVALHO, F. (2004). Conservação e renovação de revestimentos de paredes de edifícios antigos. Lisboa: LNEC. 15