ANALYSES OF THE INTERFACE BETWEEN WALL ELEMENTS AND RENDERING LAYERS. Extended Abstract

Transcription

1 INSTITUTO SUPERIOR TÉCNICO Universidade Técnica de Lisboa ANALYSES OF THE INTERFACE BETWEEN WALL ELEMENTS AND RENDERING LAYERS Exended Absrac Sara Maria Garcia Gaspar Ocober, 2011

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3 1 INTRODUCTION Adhesion srengh is one of he mandaory properies of renders, so ha consrucion works in which hey are applied can mee he requiremens of safey in-use and durabiliy, esablished in he Consrucion Producs Direcive (Council Direcive 89/106/EEC). The European sandard EN (CEN, 2003) liss he requiremens and properies for hardened morar, in which figures adhesion o he subsrae. According o he laer norm, he value of he adhesion srengh of morars can be deermined wih pull-off ess described in he European norm EN (CEN, 1996). There have been several experimenal sudies o exend he knowledge of he adhesion mechanism beween a morar and is subsrae, and deermine which facors can aler he render propery. In hese sudies, researchers ofen perform microscopic analysis of he inerface of he sysem subsrae/morar and measure he srengh of he inerfacial bond wih pull-off ess. Adhesion loss is a frequen defec of exernal renders and canno be negleced as he render deachmens can cause hazardous siuaions o passersby. Therefore, in-siu evaluaion of he renders adhesion srengh, in-service condiions, mus be performed periodically. For insance, he American sandard ASTM E2270 (ASTM, 2005), in a periodic deailed building facade inspecion, recommends performing a leas hree pull-off ess per coaed surface. Performing a pull-off es in a rendered facade causes a cerain degree of damage which mus be repaired afer he es. This is a relevan disadvanage of he echnique and is one of he causes for he reduced number of experimenal campaigns in which in-siu pull-off ess are performed. However, here are a few sudies available in which an evaluaion of he adherence of a morar was performed for in-service condiions. For insance, Flores-Colen e al. (2009) performed pull-off ess in building facades wih render adhesion loss in some areas o evaluae he adhesion capaciy of he remaining render o he wall; he auhors were able o idenify some causes for he adhesion deficiency. Quinela (2006) also performed in-siu evaluaion of morar adhesion o sudy he evoluion of his propery hrough weahering cycles, in a wall buil for he purpose of he sudy. The presen disseraion inends o idenify facors ha migh have an impac in he morar-subsrae adhesion and o build a numerical model of morar coaed wall o sudy he influence of hose facors in he modeled sysem inerface (beween he wall and he render). 2 SUBSTRATE/MORTAR ADHESION 2.1 Bond mechanism of a morar o a subsrae The mechanical bond of a morar o is subsrae is due o he peneraion of morar fluids and fines ino he pores and caviies of he subsrae followed by he crysallizaion of binder hydraion producs; he solid crysals provide he mechanical connecion beween morar and subsrae (Figure 1). The subsrae capillary sucion is responsible for he

4 movemen of he morar fluids and fines in he subsrae/morar inerface and in he pores of he firs. When he morar conacs wih he subsrae, is pores are bigger han he subsrae pores, which are unsauraed, hus he capillary sucion by he laer. The subsrae sucion of morar fluids lowers he morar waer/solids raio, which leads o is plasic shrinkage, and, as a consequence, he morar pores size decreases and evenually become smaller hen he subsrae pores; ha is when he occurring sucion sops. Chemical bonding, namely, covalen and Van der Waals bonding, play also a par in he adhesion of he morar, bu wih a minor share. 2.2 Facors ha influence he rendering morar adhesion o he subsrae Several sudies have been performed by differen auhors wih he aim o idenify facors ha influence he adhesion bond beween a morar and is subsrae, in hese sudies several facors are modified in a subsrae/render model, and pull-off ess are performed o evaluae he normal srengh of he bond beween hem. The facors ha influence a morar adhesion have differen naure; some are relaed wih he morar characerisics (cemen proporion, consisency), ohers are relaed wih he subsrae ype (surface roughness, iniial rae absorpion) and ohers are exernal o he subsrae/render sysem (weaher condiions during he morar applicaion, morar applicaion process). Regarding he facors relaed o morar characerisics, i is known ha he increase of cemen proporion in a morar also increases he srengh of is normal and shear bond o he subsrae, and morars wih higher compressive and ensile srengh have more adherence capaciy; morars less viscous and easy o apply or wih a higher capaciy o reain waer perform beer in erms of adhesion (Gaspar, 2011). Figure 1 Mechanical bond beween a morar and is subsrae (Quinela, 2006). The subsrae naure is also significan in he developmen of he adhesive bond wih he subsrae. A rough surface exure helps he peneraion of morar fluids ino he subsrae pores. The iniial rae absorpion can also influence he subsrae/render bond srengh; here is an opimal range of values for his subsrae propery for which he srengh of he menioned bond is maximum. The presence of dus and oil in he surface of he subsrae prevens he peneraion of morar fluids ino is pores and caviies, and he subsrae/morar bond is unable o develop. Mois-curing and he applicaion of render hrough mechanical projecion are exernal facors ha have proven o improve he morar adherence capaciy (Gaspar, 2011).

5 Table 1 Lis of facors ha according o he lieraure can influence he subsrae/morar bond (Gaspar, 2011). Facors relaed o he rendering morar composiion (includes cemen and aggregaes characerisics and proporion) characerisics such as: compressive and ensile srengh, consisency, waer reenion hickness number of rendering sysem layers Facors relaed o he subsrae surface exure iniial rae absorpion porosiy iniial humidiy conen cleaning surface reamens Exernal facors render applicaion process mois-curing weaher condiions during applicaion 2.3 Mehods o evaluae he adhesion of morars The pull-off es is he mos used mehod o measure he adherence capaciy of a morar o he subsrae and is described in he European norm EN (CEN, 2000); his es measures he srengh of he normal bond beween a morar and a subsrae. The RILEM mehods MR14 (RILEM, 1982) and MR 20 (RILEM, 1982) describe wo es mehods o measure he srengh of a subsrae/morar shear bond. 3 MODELING A SUBSTRATE/RENDER SYSTEM IN ANSYS SOFTWARE 3.1 Overview of he ANSYS sofware ANSYS is a general purpose finie elemen modeling package for numerically solving a wide variey of engineering problems. These problems include: saic/dynamic srucural analysis (boh linear and non-linear), hea ransfer and fluid problems, as well as acousic and elecromagneic problems. ANSYS is ofen used by mechanical engineers o deermine in-use sress disribuions of vehicles componens. In he civil engineer area, ANSYS can be a helpful ool in modeling he mos complex srucures if combined wih he CivilFEM package sofware ha runs wihin ANSYS Muliphysics. 3.2 Modeling an inerface wih ANSYS Muliphysics 11.0 ANSYS Muliphysics 11.0 provides, in is finie elemen library, he called inerface elemens o model inerfaces beween srucural elemens. The several inerface elemens available differ in heir node number. The 2D models of a rendered facade (Figure 2) and of he pull-off mechanism ha were buil in ANSYS o sudy he influence of he inerface beween a render and is background wall have a plane geomery; herefore, he finie inerface elemen ype INTER202, which is a plane elemen wih four nodes, was used o model he inerface (Figure 3).

6 Undeformed shape Deformed shape I,J I J Y X Render Inerface Background wall Y X K,L n K n L Capion: I,J,K,L - elemen nodes X,Y - global coordinae sysem n, - elemen coordinae sysem Figure 2 Building facade geomery. Figure 3 Finie elemen INTER202 geomery. In he beginning of he soluion, he inerface elemen doesn have hickness, which means some of is nodes are iniially coinciden; wih he applicaion of forces in he model, he displacemen beween nodes wihin he inerface elemen increase, simulaing an inerfacial displacemen beween he wall and he render surfaces. The inerface elemens behave according o he exponenial cohesive zone model inroduced by Xu and Needleman (1993). The cohesive zone model consiss of a consiuive relaion beween he racion (T) acing on he inerface and he corresponding inerfacial separaion (Δ). The inerface racion and separaion are defined in he normal (n) and angenial () inerface direcion; hese direcions are based on he elemen coordinae sysem which is shown in Figure 4. The exponenial form of he cohesive zone model uses a surface poenial (ϕ) o define he inerface normal and angenial racion, according o Eq. (1) and Eq. (2): T n = ϕ(δ) n (1) T = ϕ(δ) (2) If he work associaed wih he separaion process in he normal and angenial direcion is assumed o be he same, hen he surface poenial is given by Eq. (3): ϕ (δ) = exp (1) σ max δ n Δ n exp ( Δ n ) exp ( Δ 2 δ n δ 2 n δ ) (3) where σ max represens he maximum normal racion a he inerface, δ n is he normal inerfacial separaion where σ max is aained in a paricular case in which here is no angenial separaion (Figure 4, righ) and δ is he shear inerface separaion for which 2 2 δ is he shear separaion corresponding o he maximum shear sress a he inerface (τ max ), when here is no normal inerfacial separaion (Figure 4, lef). The normal and shear

7 inerface racion of he inerface are expressed in Eq. (4) and (5), hese expressions are obained from Eq (1), (2) and (3). T n = exp (1) σ max Δ n δ n exp ( Δ n ) exp ( Δ 2 δ 2 n δ ) (4) T = 2 exp (1) σ δ n max 1 + Δ n exp ( Δ n ) exp ( Δ 2 δ δ δ n δ 2 n δ ) (5) T n σ max T τ max δ n Δ n 2 2 δ Δ Figure 4 Righ, graphical represenaion of Eq. 4, for Δ = 0; lef, graphical represenaion of Eq. 5, for Δ n = 0. The value of he parameers σ max, δ n, and δ mus be specified in ANSYS Muliphysics 11.0 so ha he inerface can be modeled; hey represen he inerface elemen inpus. As for he inerface elemen oupus, hey consis in he normal and shear inerface racion and separaion. 3.3 Modeling a subsrae and a render wih ANSYS Muliphysics 11.0 In he modeling of he subsrae and he render of he facade wih he geomery presened in Figure 3, he srucural, plane elemen wih four nodes, PLANE182 was used. The render and he subsrae maerial behave according o a linear consiuive model wih a maximum Von Mises sress (σ 1 ) ha he maerial can resis, and afer his sress is reached, hey can break. σ σ 1 E ε Figure 5 Render and subsrae behavior consiuive model.

8 The inpus of he PLANE182 elemen are maerial properies such as Poisson raion, Young's modulus (E), densiy and maximum Von Mises sress (σ 1 ). The oupus of elemen are mainly sresses and srains. 4 NUMERICAL MODEL OF THE WALL/MORTAR SISTEM 4.1 Parameric analysis None of he hree inerface parameers (σ max, δ n and δ ) are given in experimenal ess; he resul of he pull-off es is assumed o be close o he value of σ max. δ n and δ represen he disance ha he bonding crysals can be pushed from he background pores before he adhesion sars losing srengh. In hese analyses he wall/render models were subjec o a uniform racion applied on he render surface, besides he acion of heir weigh; he value of he hree inerface parameers were alered and he resuling inerface racions and separaions were compared. The firs referred acion inends o simulae he wind sucion ha affecs exernal render in-use condiions, which is an acion ha can be direcly inpued in he models. By increasing he value of δ n, he value of Δ n increase and he inerface deforms more. A very small value of Δ n implies ha he render and he wall ac as one, like in a model wihou inerface. Wih a large value of Δ n, he sress of he render is no ransmied o he wall; in his case, a render fracure is more likely o occur. As he value of σ max increases, he value of Δ n decreases. As he sudied acion acs in he normal direcion, he normal sress is higher han he shear one. The influence of Δ in he shear ension (T ) is idenical o he influence of Δ n in he normal sress (T n ). 4.2 Modeling of facors ha influence subsrae/render adhesion In secion 2.2, a lis of facors ha can influence he render adhesion o is background was presened. There are wo possible ways in which hese facors can be inroduced in he numerical model. Each facor ha causes a geomerical change in he sysem, like render cracks, can be inroduced by a change in he finie elemens. Oher facors, such as he wall roughness, can be inroduced by alering he inerface parameers. For insance, i is known hrough experimenal ess ha he wall roughness increases he adhesive srengh of he render; so by increasing he value of σ max in he numerical inerface, wall roughness is aken ino accoun. The ype of he rendering morar, is consisence, wall and render cracking, he render applicaion process and he presence of differen render layers were he facors inroduced in he numerical model. A bad render consisence can cause macro-defecs in he inerface wih he wall. The macro-defecs are creaed by deleing finie elemens in he inerface. The presence of a higher concenraion of macro-defecs induces more sress in he inerface, which will deform more. (Figure 6).This behavior is wha was expeced: he numerical model is predicing real behavior.

9 10% of macro-defecs 20% of macro-defecs Deails T n (Pa) 10% of macro-defecs 20% of macro-defecs Figure 6 Normal inerface sress (T n ) due o he presence of macro-defecs. Wall and render cracking will also submi he inerface o more sress. As for he render applicaion, he ergonomics of he applier will cause an heerogeneous inerface wih areas wih a beer bond; he exisence of differen bond srenghs hrough he inerface, will generae more sress in i. 5 CONCLUSION There are many facors ha can have an impac in a morar adhesive srengh for insiu condiions; he mehods used o measure adhesive srengh like he pull-off es have many limiaions for in-siu applicaion. Numerical models can be a useful ool o evaluae he influence of any facor in a wall/render adhesion in service condiions. The numerical models predic he expecable behavior and are capable o incorporae any facor. From he numerical model, we can conclude ha he presence of cracks in he render and in he wall, as well macro-defecs in he inerface, have a negaive impac on he render adhesion. The heerogeneiy of he render or of he inerface also decrease he srengh of he inerfacial render-wall bond.

10 REFERENCES Xu, X. P.; Needleman, A. (1993) - Numerical simulaion of fas crack growh in brile solids. Journal of he Mechanics and Physics of Solids, vol 42: pp Chandra, N.; Li, H.; She, C.; Ghonem, H. (2000) - Some issues in he applicaion of cohesive zone models for meal-ceramic inerfaces. In: Inernaional Journal of solids and Srucures 39 (2002): pp ANSYS (2007) - Release 11.0 Documenaion for ANSYS. ANSYS (2009) - Fluid Analysis soluions brochure, 12.1 release.