Flooring products have a history of debonding from cement regardless, it seems, of any and all precautions taken to prevent it. Water and how it reacts with other materials is the driving force behind the failure of water insoluble adhesives to bond permanently to the concrete. The natural physical processes which cause this will be shown.
The problem can be put into the context of process and opportunity. Capillary action is the well known physical process by which water moves through host materials (concrete) and eventually to the adhesive/concrete interface and is believed to be the main cause for debonding. Opportunity for the process is made possible by the physical characteristics of the concrete. In particular, the porosity (void space), permeability (interconnections of the void space) and media discontinues (fractures, microfractures, imperfectly sealed joints), are the opportunities for capillary action to work. and media discontinuities (fractures, microfractures, imperfectly sealed joints) are the opportunities for capillary action to work. If the initial concrete surface is “water wet” because capillary action is bringing water to the surface, proper adhesive bonding will not take place. If the adhesive is sensitive to the chemically reactive water found in the concrete then the bonding problem is compounded. Additionally, if the adhesive forces of the water to the concrete is greater than the adhesive forces between the adhesive and the concrete, debonding will occur. Should any or all of the above happen, other mechanical stresses can further the debonding process. The effect of capillary is that that it produces an environment which prevents a perfect bond between the adhesive and the concrete.
CAPILLARY PRESSURE AND ACTION
To explain capillary, it is necessary to understand some of the physical properties of fluids. Isaac Asimov’s clearly written 1996 text UNDERSTANDING PHYSICS was used to illustrate these concepts. Cohesion is the ability of a material, solid or liquid, to bind itself. Obviously, solids have greater cohesion than liquids, i.e., you can lift a stick from the one but water “breaks” at the droplet level. However, the cohesive forces of liquids are not zero, but are small are best described and demonstrated on liquid surfaces. If there was no gravity, all liquids would form perfect spheres if not in contact with any other objects. Molecules below the surface of the liquid sphere are bound together equally in all directions. At the surface of the sphere, the attractive forces are limited in direction by the radius of the sphere. On a small sphere, a molecule on the surface will be attracted by molecules over an angle of less than 180 degrees. Regardless of the sphere’s size, lateral forces cancel. The net forces are directed perpendicular to the surface, which in zero gravity will form a sphere of any liquid of with any magnitude cohesive forces (FIGURE 1).Similarly, gravity, a fundamental and separate force of nature, caused the planets to be spherical and therefore conservation of energy maintained. In both cases, the sphere represents a minimum energy configuration. The liquid’s surface, therefore, represents a minimum energy position similar to that of how gravity seeks to have all objects at a lower position, for example, when a ball rolls off a table to a lower potential energy floor elevation. The center of the earth has zero potential energy. This intermolecular attraction is called van der Waals force – another fundamental force of nature – which is strong only when molecules are nearly touching as they are in liquid or solid. It is virtually absent in gases under normal conditions because the molecules are too far apart. Mercury and water float identically in zero gravity space as spheres. If a floating sphere of each were struck with a rod, water would be more apt to divide into many more spheres than mercury. It is obvious that mercury’s cohesive forces are much greater than that of water.
If gravity were put into our experiment and given a drop of water on a non-sticking Teflon surface, the forces of gravity are greater than the cohesive (van der Walls) forces which is reflected by the flattening of the sphere to an elliptical spheroid (FIGURE 1). Mercury, however, will be more spherical because its cohesive forces are stronger. As the quantity of mercury increases, gravity will eventually counteract the cohesion and flatten out the mercury as well. Taken to an extreme, water will totally lose its sphericity when a glass container is half filled with water where the shape of the water mimics the vessel and comes to rest at some minimum level. Gravitational forces establish this minimum level just as sea level is established. Cohesive forces are subordinate and control the shape only locally on the molecular level at thewater/air interface (FIGURE1). All molecules below the surface of the liquid are, again, bound together equally in all directions just as in the zero-gravity sphere described above. At the surface of liquid, however, there is an imbalance of forces caused by the “free” surface represented by the air/liquidinterface (FIGURE 1). For any particular molecule on the surface, all the forces are directed in a convex down hemisphere just in the sphere described above. There are no forces from the above the air/liquid interface and all lateral forces cancel. Therefore, the net forces at the molecular level are directed downward perpendicular to the actual surface just as in the sphere. This surface can be thought of as a thin rubber film acting as an elastic “skin” under tension over the mass of water and thus the origin of the term surface tension. This surface tension is especially notable while observing dragonflies resting on water. Each step that the dragonflfy takes stretches the “film” but not enough weight is present to break the intermolecular can der Walls bonds. The weight of the paper clip, however, will provide the force needed to break the bonds and will subsequently enter the mass of water and sink. Thus, when as object is dropped in the water, the surface (film) deforms but will rebound almost immediately back to a flat surface as it seeks the minimum surface area and thereby the minimum energy configuration (conservation of energy, again). Keeping the surface coherent requires work which implies a form of energy called surface energy and is expressed in terms of work per unit area or dynes/cm2 which is the measure of the liquid’s cohesion (or the strength of the film). How much stress (energy) does it take to break the surface? How well does the surface of the liquid stick together? The surface tension strength for water is 72 dynes/cm2 and for mercury it is 435 dynes/cm2.
The same forces acting between liquid molecules also act between solid and liquid molecules, for instance, on the side of a beaker half filled with water. (FIGURE 1,2,3). The solid liquid forces are referred to as adhesion. Adhesive forces between liquid and solid can be stronger or weaker than liquid/liquid cohesive forces. Water adheres more strongly to glass than it coheres to itself shown by water droplets remaining in a beaker after the water is poured out. It is also seen at the intersection of water and glass where the water’s surface (film) is curved up (FIGURE 3). Gravity prevents water from climbing up the beaker and totally covering it. If the container is very narrow as ina tube, the flat surface between opposite sides becomes a concave up surface which is called meniscus (remember high school chemistry and measuring liquid quantities is graduated cylinders?). So, as you can see the driving force in shaping the meniscus is the greater attraction of the water by the glass rather than to water. If a tube is very thin and put in water, water will rise until the gravitational forces acting on the column of water equals the adhesive forces between the glass and water. It seems to “defy” gravity in that way. If you notice a paper towel getting wet, the whole towel is a network of small capillary openings where gravity cannot prevent capillary forces from wetting the whole towel. In an opposite sense, a tube submersed in mercury will have a depressed level as gas actually repels the mercury to a lower level (FIGURE 3). The water/glass, or water/concrete, interface strives to increase the surface exposure of water on the glass, where as in mercury it is reduced (FIGURE 2, 3). If water is spilled on a surface it will make a thin film and increasing the surface exposure (wetting) whereas mercury will bead up which decreases the exposure (non-wetting). In most cases, the effect is to reduce the total energy of each respective system. Taken to an extreme, if the tube has a microscopic diameter, the adhesive forces between the glass and water totally overwhelms gravity where the tube will fill instantly regardless of the height. Note the contrast of forces relative to the size of the tube.
The total upward force, Pc (capillary pressure), by the adhesive forces, now called interfacial tension (IT), is 2(IT)(P)r where r is the radius of the tube. This is counteracted by the force of gravity on the column of water in the tube or r2hdg where h is the height of the water, d is the density, and g is the force of gravity. Thus, there is (pi)r2hdg = 2(P)r (IT) or hdg = 2(IT)/r = Pc. Since d, g and P are all constants, the smaller the radius the taller the water will be in the tube, h=2(IT)/rdg. This is capillary action!Thus, either natural or artificial “tubes” of microscopic radii will immediately “suck” liquid into the space. The smaller the space, the stronger the force. It is this force that draws water through a paper towel (BOUNTY paper towels have more and smaller openings) and helps draw water up the stems of plants. It is this force that draws water through the cracks and porosity in concrete. This is the most powerful force acting on the concrete system.
Hydrostatic pressure is simply the force created by a column of water, commonly termed hydrostatic head or just head (FIGURE4). It is this force which drives artesian wells were the well is connected underground to a source of water higher than itself and therefore it flows naturally. Hydrostatic pressure is directly proportional to the height of the water column. It can be described by Ph=hdg, where h is the height, d is the density of water, and g is the gravitational constant. Since g and d are constants you can see that only needs the height to calculate Ph. It is approximately 0.43 psi per foot of height. If you have a ten-foot column of water then you have buildup 4.3 psi. As you can see, hydrostatic pressure builds very slowly and is virtually a non-factor in a concrete that is perfectly sealed. However, if there are cracks in the concrete then hydrostatic pressure has the energy to push water through the cracks giving rise to additional capillary opportunities. To emphasis, the concrete subjected to hydrostatic pressure must be below the water table otherwise hydrostatic pressure does not exist!If there is uninterrupted permeability barrier, hydrostatic pressure will have no effect.
POROSITY, PERMEABILITY AND FRACTURES
Porosity is simply void space in the concrete which is filled with either or water. Porosity in concrete caries but is generally (<10%). Porosity in beach sand is about 40%. Porosity in granite is <3%. The lower the porosity, the more difficult it is to transmit water through it. Permeability is the interconnectedness of the porosity. The greater the permeability the more easily water can flow through the concrete. Porosity and permeability normally track each other. The average concrete has very low permeability which is measured in millidarcies. Beach sand has several thousand millidarcies. Concrete has <10 millidarcies sometimes more. So, from the description of the transmission capabilities of concrete it seems as if it shouldn’t let any fluid pass. But alas we don’t live in perfect world. The making of concrete is not consistent batch to batch or within a singular batch. Discontinuous in material, chemistry and water distributions always exist which alters the chemical process of concrete curing. The human factor is normally the cause of inconsistencies as well as good or bad luck.
The inhomogeneities created in the slurry and in the curing process cause mechanical stress within the hardening concrete. For instance, these flaws will cause variable rates and degrees of hydration within the concrete. This implies that differential volume changes occur while the concrete is curing. If that’s the case, then there must exist stress in the concrete. If the stress is greater than the tensile strength of the concrete at any time during the curing process, microfractures or shrinkage cracks will relieve ambient stress conditions by adjusting the total volume. Should these extend through the slab vertically, then a porosity and permeability pathway would exist for amy moisture to ‘wick’-up – the perfect capillary avenue or opportunity.
Or course, there are other obvious opportunities such as imperfectly sealing joints and structural breaks by a multitude of causes. Each of these discontinuities are potential loci for penetration of water from the base of the cement to the adhesive layer and beyond. Additionally, concrete does have intrinsic porosity and capillary pathways which allow the transmission of water.
How does water reach the capillary opportunities?
- Direct contact of water to the concrete (with or without hydrostatic pressure) exposes water to the pores and cracks which is all that is needed to start capillary action. Water need only be puddled and in direct contact with the base of the slab.
- Condensation at the moist sub-slab/soil interface. Should the temperature of the concrete be below the dew point at the given humidity, then water will condense on the base of the concrete and will wick-up the pore via capillary.
- In situ water. The third source would be the water trapped within the concrete itself. If mixed and cured properly, this might not be a problem.
- The fourth method is a little more obscure and is related to the movement of vapor molecules in very small spaces and van der Waals forces. In very small spaces such as capillary terminations or the nooks and crannies of a very rough surface vapor molecules have a greater probability to attach to each other. It’s kind of like billiard balls on a very small pool table. When molecules are this close, van der Waals forces take over and form a condensation can occur. It is unclear how important this is.
EFFECT OF EVAPORATION
When water is wicked to the surface of the concrete, it will evaporate naturally. This will evacuate the capillary and allow additional water to move into the void. This process will continue until there is no longer a source of water or a permeability is emplaced to cut off the surface from the source of water.
THE THEORETICAL DEBONDING PROCESS
It is believed that the conditions for debonding exist when the flooring product is first laid down. Rather than being strictly a debonding process, perhaps a better description might be “The no-bonding process”. The conditions exist such that bonding to the concrete is imperfect from the beginning. (FIGURES 5, 6).
In order to form a perfect bond between adhesive and concrete, the concrete surface must be dry, or better stated, not “waterwet”. If any part of the surface is water wet, the adhesive will not bond to the water instead of the concrete unless the water is displaced. Of course, there is no bond in these areas. As can be surmised, it is imperative that the surface of the concrete must not be water wet at the microscopic level!It may not be possible to detect visually which parts of the floor are still water wet on the microscopic level. Areas may be water wet for a variety of reasons each of which was stated previously.
- Excess of water of hydration within the slurry.
- Shrinkage cracks opening permeability (capillary) pathways to the surface of the concrete.
- Structural cracks/imperfectly sealed joints providing capillary pathways to the concrete surface.
APPLICATIONS OF FLOORING PRODUCT
The functions of the adhesive is to chemically bond to the concrete – which includes all types of solid materials within the concrete matrix such as lime, quartz sand, and clay (if any). If there is even a layer of one molecule of water between the adhesive and the concrete, a poor hand will form. Normally, a waiting period exists between the laying of adhesive and the laying of the concrete such that is assumed that the concrete is dry. Obviously, it isn’t always the case otherwise every such flooring installation would not fail.
EFFECT OF TIME
Let’s assume that concrete has been laid and given “enough” time to cure and dry such that it will accept adhesive. The adhesive is laid and bonds with 95% of the concrete perfectly which was non-water wet. Over 5% of the surface, the conditions stated above exist where it was water-wet but went undetected at the time of application (FIGURES 5, 6). Within this 5%, the adhesive finds a way to bond to 40% of the areas which leaves 60% unbonded. It can be assumed that the bonding strength of the adhesive to the concrete is greater than that of the water to the concrete. However, we must now consider the dynamics of a heterogeneous system – bonded non-water wet and unbonded water wet areas. In particular, this system is exposed to two stresses – chemical and mechanical.
Chemical Effects. It is assumed that the adhesive is inert and thus unreactive to water. However, is inert when exposed to the chemistry of the water of hydration of the concrete slurry which is normally very alkaline? Does the curing process concentrate the alkalinity or perhaps salts in the water not hydrated and exposed to the adhesive? If the adhesive is not inert to such conditions, debonding will occur.
Mechanical Effects. There are three sources of mechanical forces which may cause the debonding of the 60% mentioned above. First the work done by water and capillary action. Perhaps there are microscopic areas where the bond to the concrete us very tenuous. Capillary action of water, where water’s adhesion to the concrete locally is great than that if the adhesive, may displace the adhesive form the concrete a molecule at a time. Given enough time, perhaps a year or two, this could have a definite consequence, The second source of mechanical stress is from temperature changes of the entire floor. If the ambient temperature and humidity changes significantly and regularly, expansion and contraction in the flooring product, i.e., vinyl tile, cause stresses which will be concentrated in areas that are free to move – even if only in small areas. (FIGURE 6). The 5% mentioned above will therefore be subjected to an exaggerated amount of lateral stress which will cause further weakening of any bonds that might already exist. Over time, enough of these bonds will break and allow the relief of the horizontal stress to be converted to vertical movement – the only way it can go – which will form a buckle or warp in the flooring. The third source of mechanical stress comes from typical use. Should advanced usage, such as moving heavy equipment, be concentrated over already weakly bonded areas, further weakening may take place.
The chemical action of the water hydrations of the concrete and the expansion and contraction of the entire floor concentrating stress in the weakly bonded areas are the two processes likely to advance the preconditions of a weakly bonded flooring product. These processes would have no opportunity if the preconditions and initial bonding were perfect.
It is obvious that the condition of the concrete before a flooring product is laid sets up the durability and integrity of the adhesive bond. The less water wet the surface, the more likely it is that a “perfect” bond will exist between the adhesive and the concrete and subsequently the flooring product. What can be done to prevent water from wetting the floor even on a microscopic level? Is it possible? Well obviously, there would be perfectly bonded floors. Most of the problems occur when the concrete is in contact with a constant source of water. Water access must be denied to the concrete surface. Given the opportunity, water will move through material that it ‘wets’ instantly through capillary. The key to stopping this is to plug all of the opportunities, that is, pore spaces, micro-fractures, fractures and joints. Plastic vapor barriers and epoxy coatings have been used – not always successfully. The former, if done rigorously, will tend to leak and allow condensation and/or contact at the base of the concrete above the soil. The latter will have the same problems encountered when applying adhesive.
The aim of CONCUREFlooring Sealant and Admixture is to fill all of the pore spaces with calcium silicate hydrate. THE CHS is formed when the CONCURE Flooring Sealant and Admixture comes in contact with the deleterious particle called PORTLANDITE which is formed by hydration. By plugging all porosity and permeability in the concrete, that is, creating a permeability barrier (versus a vapor barrier), all opportunity for a capillary process are eliminated. By making the concrete intrinsically impermeable, it may not be necessary to apply other external preventative methods, or perhaps such methods would become more robust in effect. The CONCURE products, if applied properly, will prevent water penetration through minor cracks and inherent porosity regardless of the source of the water. The processes of capillary action and hydro-static pressure will be halted. Excess water of hydration from an improperly mixed or green concrete will be stabilized and locked in place. At that point, the biggest worry should be the integrity of the seals at joints and other structural intersections and the larger, obvious to the eye macro-fractures which can be treated appropriately with by standard concrete repair methods.
CONCLUSIONS AND RECOMMENDATIONS
The aim of this paper was to discuss the natural processes that might cause the debonding of flooring products. Capillary action of water through concrete via porosity and fracture volumes is the process by which water penetrates concrete and causes initial imperfect bond between concrete and adhesive when the adhesive attempts to bond to a water wet surface even at the molecular level. It should be the aim, therefore, to shut off or isolate these void volumes which will make the concrete surface non-water wet. The CONCURE product line will create conditions such that capillary actions is prevented rendering a perfect surface to apply flooring products or any coating system.