INFLUENCE OF MATERIAL PROPERTIES AND VERTICAL PRE-COMPRESSION ON THE SHEAR BEHAVIOUR OF MORTAR JOINTS. Miklós Molnár 1

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1 INFLUENCE OF MATERIAL PROPERTIES AND VERTICAL PRE-COMPRESSION ON THE SHEAR BEHAVIOUR OF MORTAR JOINTS ABSTRACT Mikló Molnár 1 The influence of mortar type on the hear behaviour of bed joint ubjected to imultaneou hear and normal pre-compreion wa invetigated. The tet pecimen were manufactured of olid clay brick combined with a typically trong, cement rich repectively a typically weak, lime rich mortar. The tet were carried out under deformation control, which enabled both the elatic and oftening platic propertie of the joint to be oberved. The elatic hear deformation of the joint with the weak mortar wa 2-4 time larger than in the cae with the trong mortar. Joint with the weak mortar exhibited le pronounced hear oftening than the joint with the trong mortar. Reult of the invetigation ugget that a hift from the trong to the weak mortar might improve the reitance to cracking of a horizontally retrained wall by %. Key word: Maonry, mortar, cement, lime, hear, trength, deformation, dilatancy. 1 Diviion of Structural Engineering, Lund Univerity, P.O. Box 118, Lund, Sweden, miklo.molnar@ktr.lth.e

2 INTRODUCTION Deign of maonry ha mainly been concerned with proviion of ufficient reitance againt load threatening the tructural integrity of building. Thi ha created a tendency to maximize the trength of both unit and mortar, which often render maonry with low ductility. In uch maonry, there i an increaed demand for movement joint to accommodate for differential thermal movement and hrinkage. A the undertanding of the phyical phenomena influencing cracking of maonry i generally poor among practitioner, movement joint are often ued in exce. Movement joint need maintenance and lead to a fragmentation of maonry tructure, which i conidered unfavorable from architectural point of view. Development of micro-level modeling trategie in the lat decade ha led to a better undertanding of the cracking of maonry ubjected to in-plane tenion load (CUR (1997), Lourenço (1996), Gottfreden (1997), van Zijl(1999), Molnár (2000)). The hear propertie of mortar joint have, among other, been identified a key-parameter. A a complement to the modeling, uitable tet et-up, able to deliver elatic oftening-platic hear parameter of mortar joint, have been developed by van der Pluijm (1993) and Gottfreden (1997). A the main objective of the hear tet wa model development and verification, generally few material combination were teted. In Hanen et al. (1998), different unit/mortar combination were teted regarding in-plane horizontal ductility. Weak mortar were reported to produce more ductile hear behaviour compared to trong mortar. However, generalization baed on the reult in Hanen et al. (1998) are difficult to make due to the inufficient number of replicate in the tet. The purpoe of the preent reearch i to provide better inight in the hear behaviour of joint built with maive clay brick and typically trong repectively weak mortar. The tet reult can be ued to evaluate the influence of mortar quality and vertical pre-compreion on the tructural repone of maonry ubjected to hear load and horizontal tenion in it own plane. Thi contitute an eential tep toward more rational deign rule concerning deign of movement joint. The paper preent the reult of 31 deformation controlled hear tet on mortar joint ubjected to imultaneou vertical pre-compreion and hear. MATERIALS AND METHODS The hear tet in the preent work were carried out on pecimen coniting of two brick and one mortar joint. The tet were performed at three different level of normal pre-

3 compreion. The teting arrangement wa imilar to thoe ued by van der Pluijm (1993), Gottfreden (1997) and Hanen et al. (1998). Material One type of olid clay brick and two type of mortar were ued. The brick were of type Haga red, manufactured by extruion. The tenile trength and the tatic modulu of elaticity were determined from direct tenion tet on full ize brick. The dynamic modulu of elaticity wa determined by meaurement of the natural reonance frequencie in the flexural mode, prior to the tenion tet. The mechanical propertie of the brick are preented in Table 1. Table 1. Mechanical propertie of maive clay brick (Molnár 2000). Parameter Value Dimenion Number of tet Coefficient of variation [%] Dimenion 218x119x60 mm³ - - Denity 1870 kg/m³ - - Initial rate of uction 2.55 kg/m²/min - - Compreive trength 35* N/mm² - - Tenile trength 2.5 N/mm² Static modulu of elaticity N/mm² Dynamic modulu of elaticity N/mm² * Compreive trength according to the manufacturer. Two mortar, here decribed a B and D repectively, were ued in the experiment. Mortar B i the mot frequently ued pre-mixed mortar in Sweden. The binder in mortar B i maonry cement, compoed of approximately 75% Portland cement, 25% filler of grounded limetone and air-entraining additive. The filler i ued to reduce the cement content wherea the airentraining additive i ued to improve workability. Mortar D i a pre-mixed lime (hydraulic) cement mortar, introduced to the Swedih market in the The flexural and compreive trength of the mortar were determined on prim 40*40*160 mm according to procedure decribed in RILEM (1994). Mortar compoition and mechanical propertie are hown in Table 2. Table 2. Compoition and tandard propertie of mortar (Molnár 2000). Mortar B D Compoition Part by weight maonry cement/and 100/600 Portland cement/lime/and 25/75/750 Compreive trength [N/mm²] Flexural trength [N/mm²]

4 Specimen The pecimen ued in the preent work were o called couplet, coniting of two brick and one mortar joint, ee Fig. 1. The preparation of the brick before manufacturing of pecimen involved: - grinding of one bed face, in order to facilitate the gluing to the teel plate; - aw-cutting to the deired length of 180 mm; - repeated cleaning of the urface by high-preure water, in order to eliminate the dut originating from awing; - drying at a temperature of C and a relative humidity (RH) of % during 3 week. Two type of pecimen, with mortar B and D repectively, were manufactured. The pecimen were cured for 2 week at C and RH 90 %. Until teting, i.e. farther 6-9 week, the pecimen were tored at C and RH %. Tet et-up Figure 1. Shear pecimen The hear tet et-up wa imilar to thoe ued by van der Pluijm (1993), Gottfreden (1997) and Hanen et al. (1998), ee Fig. 2. An approximately uniform hear ditribution in the mortar joint i obtained by introducing an axial force through two L-haped teel frame. The deired normal pre-compreion i generated by pring attached to the L-haped frame.

5 Figure 2. Shear tet et-up adopted from v. d. Pluijm (1993) and Gottfreden (1997). Before teting, two teel plate were glued to the pecimen by mean of an epoxy -baed glue. Next, the pecimen were aembled in the L-haped teel frame and fatened by crew. The pre-compreion force wa generated by mean of a teel frame and two piral pring. The tiffne of the pring wa 500 N/mm, which wa choen o that no larger deviation than 6 % from the target pre-compreion value hould occur due to the normal expanion of the mortar joint during hear oftening. The whole arrangement wa fixed in a teting machine of MTS type with hinged connection. The tet were conducted under contant deformation rate, with the mean tangential deformation of the mortar joint a a guiding input. The choen deformation rate wa mm/min. Six LVDT a hown in Fig. 3 meaured the tangential and normal deformation of the mortar joint. The upport of the LVDT were glued to the bed face of the brick, which enabled the hear deformation of the joint to be recorded directly. The teting machine recorded the axial force. Figure 3. Meaurement of the tangential (LVDT 1&2) and normal (LVDT 3) diplacement. LVDT 4-6 on the oppoite ide not hown.

6 The mean value of the tangential (hear) diplacement δ i calculated a δ ( δ δ ) + ( δ δ ) = (1), 4 where the δ n value are reading from the diagonally placed LVDT in Fig. 3. By placing the LVDT diagonally, the reading ytematically underetimate the tangential diplacement by 2.3 %. The normal diplacement i calculated a the mean of the reading from LVDT 3 and 6. Tet program Altogether 31 tet were carried out, coniting of two material combination, at three different level of normal tre, with five or ex replicate for each material and normal tre combination, ee Table 3. At the time of teting, the pecimen were 8-11 week old. In the following ection, pecimen with mortar type B and D are referred to a erie B repectively erie D. Table 3. Tet program - hear tet on bed joint Number of tet Normal tre level [N/mm²] Serie B Serie D RESULTS AND DISCUSSION Repreentative load-hear diplacement curve from the deformation controlled hear tet are preented in Fig. 4. In mot of the tet, the behaviour wa linear elatic up to 30 % of the peak load. Thereafter, the joint behaved more or le non-linearly. In all tet, failure occurred in the brick mortar interface, with no dicernible crack in the brick. In three cae, all in erie B, untable behaviour wa oberved in the pot-peak tage. Serie B exhibited a teeper decent of the oftening branch of the tre-hear diplacement curve at every precompreion level than erie D. Along the oftening branch of the load deformation curve, udden load drop of vibration type could be oberved. The phenomenon i known a tick-lip motion, ee Oden & Martin (1985) and Giannakopoulo (1989) for more detail.

7 1,60 1,20 hear tre [MPa] 0,80 0,40 n = MPa n = MPa n = MPa 0, hear diplacement [mm/1000] Figure 4. Load-diplacement curve from the hear tet at different level of normal precompreion. Serie B - continuou line; erie D - dotted line. Fig. 5 preent normal veru hear diplacement curve from the hear tet. The normal uplift generally tarted at hear diplacement correponding to the peak load and did not reach it maximum when the hear tet were terminated. The normal uplift for the joint in erie B wa twice a high a in erie D. High normal pre-compreion reulted in low normal uplift and vice vera, regardle mortar type. 600 normal diplacement [mm/1000] erie B erie B erie B erie D erie D erie D σ n σ n σ n σ n σ n σ n =-0.22 MPa =-0.56 MPa =-0.84 MPa =-0.22 MPa =-0.56 MPa =-0.84 MPa hear diplacement [mm/1000] Figure 5. Typical normal veru hear diplacement curve.

8 The tet reult are analyzed a follow in term of propertie determined, either directly or derived, from the load deformation curve. Fig. 6 how a load tangential deformation curve and the definition of the propertie to be analyzed. Figure 6. Typical load diplacement curve from deformation controlled hear tet. δ m - tangential diplacement at peak load; τ m - maximum hear load carried by the joint; σ n - normal tre; µ - coefficient of dry friction; τ dry - hear load at contant dry friction; k - Pre-peak parameter ecant hear tiffne of the joint; II G f - mode II fracture energy. Up to load level of approximately 30 % of the peak load, the joint exhibited a nearly linear elatic behaviour. However, from level correponding to 70 % of the peak load and upward, the behaviour of the joint wa non-linear. From the load deformation curve obtained in the tet, the hear tiffne of the joint wa determined. Chooing ecant tiffne eemed convenient, a it give a realitic etimate of the hear deformation at the peak load. The ecant tiffne k i calculated a k F max = (2), Ajδm where F max are the peak load, A j the joint area and δ m the tangential diplacement at peak load. The obtained hear tiffne value are preented in Table 4.

9 Table 4. Shear tiffne value of the joint determined from the hear tet. Normal pre-compreion Serie B k c.v. Serie D k [N/mm²] [N/mm³] [%] [N/mm³] [%] It can be oberved, that the joint in erie B are 4-5 time tiffer in hear compared to the joint in erie D. In both cae, the variability in k i highet at low pre-compreion level and decreae with increaing pre-compreion. In erie D the joint tiffne k decreae with increaing normal pre-compreion. Thi may be the reult of an early onet of platic deformation in the mortar. The value of hear diplacement at peak load δ m indicate the tangential deformation capacity of a joint before cracking. From Fig. 7 it can be oberved, that at pre-compreion level of 0.22 MPa joint in erie D can deform twice a much than joint in erie B before cracking. At pre-compreion level of 0.56 MPa and 0.84 MPa, thi difference in elatic deformation capacity increae to a factor 3 and 4 repectively. The reult of the tet ugget that the choice of mortar type ignificantly influence the behaviour of horizontally contrained wall ubjected to impoed deformation. If the proportion of mortar joint in maonry i roughly etimated to 15 %, a hift from the trong to the weak mortar in the preent tudy enhance the elatic deformation capacity of a wall in the horizontal direction by %. Thi mean, that the capacity of the retrained wall to reit hrinkage or temperature change i enhanced to the ame extent. 250 c.v. hear dipl. at peak load d m [mm/1000] erie D erie B ,2 0,4 0,6 0,8 1 normal pre-compreion n [MPa] Figure 7. Shear diplacement at peak load δ m veru normal pre-compreion.

10 Strength parameter The hear trength of the joint τ m i calculated from the maximum axial force area of the mortar joint A j a F max and the F max τ m = (3). Aj In Fig. 8 the hear trength value are plotted againt the applied normal pre-compreion. By performing a linear regreion, the parameter of the Coulomb failure criterion, the coheion c and the coefficient of internal friction tanϕ, are determined. The reult are preented in Table 5. Figure 8. Shear trength of the joint a a function of the normal pre-compreion. 1,60 1,20 hear trength [MPa] 0,80 0,40 Serie B Serie D 0,00-1,00-0,75-0,50-0,25 0,00 Table 6. Shear trength parameter for the joint determined by linear regreion. Coheion c normal pre-compreion [MPa] [N/mm²] [-] Serie B Serie D Coefficient of internal friction tanϕ The linear regreion indicate an eential difference between the two mortar regarding coheion, with value four time higher in erie B than in erie D. Compared to imilar mortar in Gottfreden (1997), both mortar in the preent work exhibit low coheion.

11 Pot-peak parameter After the peak, the hear load carried by the joint pecimen gradually diminihe to a (theoretically) contant level τ, known a dry friction, ee Fig. 6. In the preent work, the dry plateau reported by van der Pluijm (1993) and Gottfreden (1997) in the pot-peak tage of the load diplacement curve could not be oberved. Epecially at pre-compreion level of 0.56 and 0.84 MPa, the load diplacement curve exhibited a loping tendency when the tet were terminated. Thi might be caued by the fact that coheion oftening of the mortar joint wa not completed when teting wa topped. The coefficient of dry friction µ, calculated at tangential diplacement of 0.6 mm, varied between 0.84 and dry 0,12 0,09 erie B fracture energi [N/mm] 0,06 0,03 erie D Figure 9. Mode II fracture energy a a function of the normal pre-compreion. The coheion oftening of mortar joint i often characterized by the mode II fracture energy G. Thi i defined a the area encloed by the load diplacement curve and the level of dry II f friction τ dry. Fig. 9 preent the mode II fracture energie plotted againt the normal precompreion. For erie B a linear dependence wa found, wherea erie D howed no dependence on the normal pre-compreion. The oftening behaviour of the joint wa mot pronounced in erie B, at low level of pre-compreion. Leat oftening wa oberved in erie D at high level of pre-compreion. Generally, the hear behaviour of the joint in erie D howed imilaritie with an elato-platic material. Normal uplift - dilatancy 0 0,1 0,3 0,5 0,7 0,9 normal pre-compreion [N/mm 2 ] Granular material like oil, concrete and mortar ubjected to hearing beyond the elatic tage exhibit a normal uplift along with the hear diplacement. The normal uplift i explained by the dilocation taking place in the granular material at the onet of cracking and during the

12 relative tangential movement of the two urface in the crack. The decribed normal uplift i known a dilatancy/dilatation in the literature. The magnitude of normal pre-compreion ha a ignificant influence on the normal uplift of joint in hear. At the ame amo unt of hear diplacement, high level of normal precompreion correpond to low value of normal uplift and vice vera. Another parameter influencing the normal uplift of joint during hear deformation i the mortar type. Serie B exhibited twice a large normal uplift at every pre-compreion level compared to erie D. Thi fact can be explained by difference in coheion and grading of the and in the two mortar. Van Zijl & Rot (1998) analyzed the influence of normal confinement on the hear behaviour of mortar joint. In a erie of FE analye, cae with total confinement and no confinement at all were imulated with different level of dilatancy. At total confinement combined with nonzero dilatancy, no upper limit for the maximum load could be obtained. The effect of dilatancy at zero confinement wa negligible. However, the effect of dilatancy wa not verified on experimental data with documented normal tiffne of the teting equipment. In the preent work the normal pre-compreion in the hear tet wa generated by two piral pring mounted in parallel. The total tiffne of the pring wa 500 N/mm, which reulted in an increae of the normal pre-compreion by 1 and 6 % at pre-compreion level of 0.84 MPa and 0.22 MPa repectively. It can be concluded that the confining effect of the pring wa not ignificant and thu the tet can practically be conidered a unconfined. CONCLUSIONS Deformation controlled hear tet with three different level of normal pre-compreion were carried out on mortar joint built with typically trong (cement rich) rep. weak (lime rich) mortar. The tet howed that: - The trength of the mortar joint could be decribed by the Coulomb failure criterion. The coheion c wa four time higher (0.40 v MPa) in the joint with trong mortar compared to joint with weak mortar. - The hear diplacement at peak load wa 2-4 time larger in the cae of joint with weak mortar compared to joint with trong mortar. Increaing level of normal precompreion reulted in higher value of the hear diplacement at peak load. - The ecant hear tiffne k of the joint with weak mortar decreaed with increaing normal pre-compreion. - Joint with trong mortar exhibited more hear oftening compared to joint with weak

13 mortar. The pot-peak behaviour of the joint with weak mortar howed imilaritie with an elato-platic material. - Concerning dilatancy, joint with trong mortar howed twice a high value a joint with weak mortar. REFERENCES CUR (1997) Structural maonry: an experimental/numerical bai for practical deign rule. Edited by Rot, J.G. A. A. Balkema, Rotterdam, The Netherland. Giannakopoulo, A.E. (1989) The return mapping method for the integration of friction contitutive relation. Computer and Structure, Vol. 32, pp Gottfreden, F.R. (1997) Laterally loaded maonry propertie and behaviour. Ph.D. thei, SBI report 289, Danih Building Reearch Intitute, Hoerholm, Denmark. Hanen, K.F., Nykänen, E. and Gottfreden, F.R. (1998) - "Shear behaviour of bed joint at different level of precompreion". Maonry International, The Britih Maonry Society, Stoke-on-Trent, UK, Vol. 12, No. 2, pp Lourenço, P.B. (1996) Computational trategie for maonry tructure. Diertation, Delft Univerity of Technology, Delft, The Netherland. Molnár, M. (2000) - "In-plane tenion behaviour of horizontally loaded maonry". Licentiate thei, Report TVBK-1018, Lund Univerity, Lund, Sweden. RILEM (1994) Technical recommendation for teting and ue of contruction material. E&FN Spon, London, UK. Van der Pluijm, R. (1993) Shear behaviour of bed joint. Proceeding of the 6 th North American Maonry Conference, Philadelphia, pp Van Zijl, G.P.A.G. & Rot, J.G. (1998) Toward numerical prediction of cracking in maonry wall. Computer method in tructural maonry - 4, ed. by Pande, G.N., Middleton, J., Kralj, B., pp E & FN Spon, London, UK