Designing adhesives to cope with stress
Richard P Goldberg* highlights the importance of selecting adhesives based on deformation, shear bond strength and shear modulus, after evaluating the anticipated movement of the substrate, tile and adhesive.
01 November 2010
With the development of a new generation of polymer-modified cement adhesives, there has been an increasing emphasis on the importance of flexibility and deformation capabilities of these adhesives.
While this characteristic has been one of the key elements in the successful application of this type of construction adhesive, the current international standards initiatives to qualify polymer-modified cement adhesives primarily by deformation characteristics do not fully consider the equally important balancing characteristic of high shear bond strength and resulting shear modulus of the adhesives.
The behaviour of structural and non-structural adhesive connections has been extensively studied and quantified in mechanical and aerospace applications. However, considerable study and development of quantitative methods for engineering adhered composite tile and stone veneer assemblies in building or infrastructure design remain a priority.
Polymer-modified cement adhesives that are formulated to provide a balanced shear modulus (moderate flexibility with high shear strength) have a 50-year proven empirical history of successful application, as well as basis in established adhesive technology engineering theory that is unique to the specialized field of direct adhered tile and natural stone.
Shear modulus of materials
Anyone would feel a certain level of discomfort if they learned that a structural engineer guessed the size of steel beams needed for a bridge, with the design rationale based simply on the fact that steel is a very strong material. Yet for some reason, it seems acceptable to select adhesives to adhere tile to a building façade on qualitative characteristics, such as a range of deformation or shear strength alone, without even determining the anticipated movement in the façade substrate, or worse, without knowing or assessing the physical characteristics of the tile, the substrate, or the tile adhesive used. This “guessing game” can result in significant consequences.
Shear modulus is one of several such quantitative measures of the strength of a material. The shear modulus of a material is essentially a numerical constant that describes its elastic deformation properties, or degree of rigidity, under the application of transverse internal forces. In the construction of adhered, composite tile assemblies, such forces will typically result from differential thermal, moisture or structural movement between an adhered veneer material such as porcelain tile, and a typical substrate such as a concrete slab or wall assembly constructed of concrete masonry units.
Physically, deformation can be characterised by a small cubic volume that is slightly distorted in such a way that two of its faces slide parallel to each other at a small distance, and two other faces change from squares to diamond shapes (Figure 1). The shear modulus is a measure of the ability of a material to resist transverse deformations, but is a valid index of elastic behaviour only for small deformations, after which the material is able to return to its original configuration. More flexible materials with large deformations can transition from an elastic state to a plastic state, also known as the material’s yield point, resulting in permanent deformation, or fracture under high shear stress.
In material science, shear modulus, denoted by the term G, or sometimes S or m, is defined as the ratio of shear stress to the shear strain. Shear modulus is usually measured in GPa (gigapascals) or ksi (thousands of pounds [kips] per square inch):
 |
In order to provide a better understanding of the concept, consider two different polymer-modified cement adhesives, each conforming to current EN 12004/ISO 13007-2 tile industry standards for a highly deformable category S2 adhesive, which requires that the adhesive be capable of a transverse deformation of ≥ 5 mm (0.2 inches) without adhesion failure.
Both adhesives could have the same deformation or strain characteristics (for example 5 mm). However, without knowing the shear stress required to induce such deformation, or the adhesive’s shear modulus value, it is not possible to know whether the adhesives have the shear strength resistance to avoid fatigue from cyclical shear stress, or sudden failure from the stress of unrecoverable deformation once the adhesive reaches its “yield point”.
In other words, both adhesives may comply with the performance standard, but a more flexible adhesive with lower shear modulus and resulting reduced shear strength characteristics may be more susceptible to failure under certain adverse conditions. Therefore, flexibility of polymer-modified cement adhesives alone is not a valid measure of performance when exposed to transverse deformation caused by differential movement between tile and substrates.
Shear modulus of adhesives
In order to provide a reference framework for comparison of various adhesives, it is helpful to understand the range of flexibility performance for various types of adhesives used in the tile and construction industry.
Adhesives formulated with polyurethane polymers are typically considered relatively flexible, and exhibit low shear modulus values in the range of 0.05 to 0.2 Gpa (7.2x103 to 2.9x104 psi). Adhesives formulated with epoxy resins – typically considered more rigid, despite having relatively low shear modulus values compared to typical adherends, such as porcelain tile – exhibit shear modulus values in the range of 0.2 to 3.5 Gpa (2.9x104 to 5x105 psi). Moderately deformable polymer-modified cement adhesives may have a shear modulus in the range of 0.3 Gpa (4x104 psi).
By comparison, the value of the shear modulus for aluminium and glass is about 24 Gpa (3.5x106 psi), and steel under shear stress is more than three times as rigid as aluminium. On the other end of the spectrum, rubber has a shear modulus of 0.006 Gpa. So in relative terms, polymer-modified cement adhesives generally would be considered materials that exhibit relatively lower shear modulus properties compared with stronger, more rigid materials such as porcelain tile or steel.
Adhesive characteristics
There are several established test methods for determining the shear modulus and shear strength of adhesives. ASTM D 4027-04 “Standard Test Method for Measuring Shear Properties of Structural Adhesives by the Modified Rail Test”[1], is a test protocol that determines shear strength values for adhesives with a degree of accuracy, which allows use in engineering and predicting the characteristics of composite adhered tile assemblies bonded with adhesives.
Structural design based on strength of materials principles or the theory of elasticity requires knowledge of the mechanical properties of the adhered materials, including adhesives. By the nature of their use, the most important physical characteristic of an adhesive is shear modulus, and shear modulus determined by both shear strength and shear strain (that is, deformation).
Based on the theory of elasticity, shear modulus of polymer-modified cement adhesives can also be calculated as follows:
 |
Adhesive technology theory
An important aspect to consider in assessing compatibility and selection of an adhesive is the difference between the adhered materials’ shear modulus characteristics. In an adhered tile assembly, the tile (G1) has a much greater shear modulus than a cementitious substrate (G2). Therefore, the tile-adhesive interface is often more susceptible to concentration of shear stress and potential failure. As a result, shear strength becomes the dominant design characteristic, despite the capability of an adhesive to deform. When adhering materials of different compositions and characteristics, research suggests that the shear modulus of an adhesive should be ½ (G1 + G2)[2].
So, it is important to select an adhesive with balanced flexibility or rigidity characteristics that are compatible with the adherends, such as the tile and the type of substrate. Construction industry standards such as ASTM standard C0623-05 “Test Method for Young's Modulus, Shear Modulus, and Poisson's Ratio for Glass and Glass-Ceramics by Resonance” provides a method for determining the rigidity of tile for engineering design purposes.
Similar test protocols and established engineering formulae are available to determine the shear modulus of various tile substrates to assure that substrates have shear strength to resist shear stress that may be induced by adhesion of disparate materials with higher shear modulus characteristics, including the adhesive.
Many studies regarding compatibility of adhesives and adhered materials have been conducted in the construction industry, most notably in brick masonry construction, where assessing compatibility between brick masonry and mortar compressive, shear strength, and flexural strength are important for the proper performance of the composite brick masonry assembly[3].
While tile industry standards and basic engineering theory recognise that more deformable polymer-modified cement adhesives can absorb and isolate differential movements common in high-performance applications such as exterior building façades, there are many situations where a more flexible adhesive could be detrimental, or where shear strength of an adhesive may govern design.
It is a well-known phenomenon that shear stress is uniformly distributed at the adhesive interface with more rigid adhesives, whereas it is concentrated at the perimeter of the adhesive interface with more flexible adhesives. Engineering data also demonstrates that higher shear modulus adhesives exhibit a much more linear shear versus strain behaviour over a large range of stresses, and that lower modulus adhesives exhibit non-linear behaviour, and consequently exhibit greater strains. So while highly-flexible polymer-modified adhesives are better able to absorb differential movement between components of a composite tile assembly, their behaviour is less mathematically predictable.
Leading manufacturers of re-dispersible polymers used in tile adhesive products have conducted numerous studies[4] regarding the capabilities of various polymer-modified tile adhesive formulations. It is generally known that deformable tile adhesives in compliance with EN 12002/ISO 13007 Standard S1 class contain approximately three to five per cent polymer modification as a percentage of dry content weight, while S2 class highly-deformable tile adhesives require more than 9 to 10 per cent polymer modification (Figure 2).
However, as shown in Figure 3, there is a balance between linear, predictable performance of a moderately deformable adhesive, and non-linear performance of highly deformable adhesives. With very flexible adhesives, as polymer-modification increases, deformation may result in unrecoverable fatigue, whereby the adhesive’s shear strength capabilities are diminished and can become more critical.
Therefore, deformation capability alone is not an indication of ultimate performance of a tile adhesive. As a result, testing and determination of shear strength and shear modulus characteristics, together with flexibility characteristics, can enable a more accurate assessment of a polymer-modified cement adhesive’s performance under adverse conditions.
As in structural engineering of a building’s structure, it is also helpful to know the ultimate strength characteristics of a polymer-modified cement adhesive itself, which differs from its shear strength when adhered to another material. This enables the designer to quantify the extent of shear stress the adhesive itself can sustain, and also whether that characteristic would govern the design under certain conditions.
Fatigue: Cyclical loading/stressing of flexible, highly-deformable adhesives may result in unrecoverable fatigue, whereby the adhesives may yield to a plastic state, with accompanying reduction in shear strength capabilities. When subject to cyclic loading, these stresses and strains intensify and accumulate, resulting in potential internal cohesive failure or, failure at the adhesive interface.
Plasticity and elasticity: When shear stress is induced on a flexible, highly-deformable polymer-modified cement adhesive interface, the adhesive behaves initially in an elastic manner. Shear stress is accompanied by a proportional increase in deformation, and when the shear stress load is removed, the adhesive returns to its original shape/size.
However, once the load exceeds a certain threshold (known as yield strength), the deformation increases more rapidly than in the adhesive material’s elastic region. So when the shear stress load is removed, some amount of the deformation remains permanent and is unrecoverable.
Plasticity describes the behaviour of materials that undergo irreversible deformation without failure or damage. However, even highly deformable polymer-modified cement adhesives cannot sustain any significant plastic behaviour before internal fracture or shear failure. It is important to note, though, that predictable elastic deformation depends on time frame and loading speed, and that rapid loading, such as caused by a seismic event or thermal shock, can also result in sudden adhesion failure.
Duration of loading: Duration of shear stress exerted on a polymer-modified cement adhesive is another factor that is of greater concern with a more flexible, deformable adhesive. Studies have shown that more rigid adhesives behave more favorably than flexible adhesives under sustained loading. Flexible adhesives exhibit greater initial creep.
Durability of flexible polymers: The issue of most concern regarding the performance of highly-deformable polymer-modified cement adhesives is their long-term performance under actual temperature and moisture conditions. Recent research indicates that prolonged moisture exposure has a significant effect on deformation characteristics of certain formulations of polymer-modified cement adhesives when compared to laboratory samples of the same type and age. Bond strength degradation appears a less significant issue[6].
When stored outside in normal in-service conditions over 180 days, transverse deformation characteristics were reduced from 15 to 18 per cent in some of the adhesive formulations, compared to 28-day cured laboratory samples. For the transverse deformation, the highest result obtained under external conditions was 3.55 mm, which was 50 per cent lower than the lowest value obtained from all adhesive formulations under laboratory storage conditions (Figure 4).
One school of thought on this issue is that flexibility of tile adhesives is more critical during the initial progress of construction of a building, as the majority of differential movement attributable to shrinkage and creep diminishes with age, and a more stable tile assembly and substrate requires less flexibility as a building ages. Nonetheless, studies indicate that shear strength remains an important and proven attribute of a polymer-modified cement adhesive, as does the need to qualify and quantify the long-term performance claims of certain deformable adhesive formulations.
References
1. ASTM International ASTM D4027-04 “Standard Test Method for Measuring Shear Properties of Structural Adhesives” by the Modified Rail Test, West Conshohocken, PA.
2. Handbook of Adhesives & Sealants, Edward M Petrie, 2006 p-54.
3. Technical Notes on Brick Construction 8B, Brick Industry Association, Reston, VA, 2006.
4. Shape the Future with Modern Tile Adhesives-Vinnapas, Wacker Chemie, Adrian, Michigan, 2007.
5. Behaviour of Construction Adhesives under Long-term Load, USDA, Forest Products Lab, 1981.
6. Bond Strength & Transversal Deformation Aging on Cement-Polymer Adhesive Mortar, Maranhao & Vanderley, University of Sao Paulo, Brazil, 2007.
* Richard P Goldberg (AIA, CSI, NCARB) is an architect at Professional Consultants International and is an ex-technical director of Laticrete International.
- Designing adhesives to cope with stress
- Formtexx gives flight to architect’s fancy
- $45m projects keep alabniah busy
- PanelMaker makes all shapes possible
- UMC rolls out coils from new facility
