I have recently been charged with the dubious task of trying to figure out/understand/comprehend/explain (choose one) the whole idea of the industry’s effort to prevent cracks in a concrete slab from showing up in the surface of a ceramic tile installation. How simple is it to explain the whole concept of preventing crack telegraphing? One of my sources suggests that by “using a configured membrane with free space on it’s underside, commonly known as an uncoupling membrane. This system consists of a rigid plastic membrane geometrically configured to provide a grid structure of cut-back cavities on top and interconnected channels of free-space underneath - with an anchoring fleece laminated to its underside. An uncoupling system separates and protects the finished surface from movement stresses through in-plane geometric flexibility while providing a non-compressible supporting layer.”
I wish I had thought of that. How could I be so stupid? What? Let me first tell you what I do know for sure. Concrete cracks. Thank you. You are now enlightened.
But seriously, there are two things about concrete that we all need to know: it gets hard and it cracks.
Concrete cracks during the initial phases of curing due to the evaporation of the water of convenience that is necessary to place concrete. While the Portland cement in the concrete mixture may only need a water/cement ratio of 0.25, we need to add usually twice that amount (or more unfortunately) in order to place and finish the concrete.
When this “water of convenience” evaporates from the slab, mass is lost (the concrete gets smaller), as such, shrinkage occurs. It’s like death and taxes: concrete will crack.
In fact we are so certain that concrete will crack that we can actually predict where cracks are going to occur before we even place the concrete. Even before the truck arrives on the jobsite, there is someone with a concrete saw waiting to place the saw cuts or “control joints” in the slab. These cuts are usually one-quarter the depth of the pour or about 1 to 1-1/4” deep into the concrete. By putting these cuts at regular intervals, 8- to 12-ft. exterior or 24 to 32-ft. on interior slabs, we are defining where the cracking will occur. A chain is only as strong as its weakest link and the concrete where the saw cut was placed is only three inches thick instead of the full depth of four inches, for example. Where the concrete is thinnest is where the crack will occur.
Control joints can best be defined as joints that we cut into the concrete slab to “control” where the slab will crack. A nice straight line with a crack below the surface is much more readily acceptable than a crack that meanders along a centerline in the surface of the concrete, both from aesthetic and performance standpoints
Another type of crack that shows up in concrete is a result of a one-time event, event such as settlement or deflection due to the addition of a significant dead load. If there is structural movement of the slab that occurs after the slab is poured, is due to a shift in or settling of the structural slab itself, a crack in the concrete will develop. The reason this occurs is because concrete tends to be very strong in compressive strength (3,000-4,000 psi is a common value for a commercial structural slab on grade), but it isn’t very “elastic.” In other words, a concrete slab can absorb much more force when it’s applied perpendicular to its plane, but it doesn’t stretch very well when pulled along this plane. If you ever heard of “modulus of elasticity,” concrete is a very high modulus, which means high strength without much stretch.
There is a third mechanism by which concrete can crack: changes in temperature in the concrete slab’s environment will result in dimensional changes in the concrete, often of sufficient magnitude to cause cracking due to the concrete’s coefficient of thermal expansion. “Well excuuuuse me!” What this means is that, like most materials, slabs expand when they warm up and contract when they cool down. (Except for water and that whole ice floats thing – but I digress.)
If a slab is poured and there are not adequate expansion or contraction joints installed, cracks will develop in the concrete. As the physical dimension of the slab changes from its poured-in-place temperature of 50°F, to a summertime temperature of 85°F, which then contracts when the building in acclimated to its operating temperature of 70°F. The placement of appropriate movement joints is an important part of the structural floor construction and equally critical to the performance of a ceramic tile floor.
Whether it shrinks when the temperature drops, or expands as the temperature rises, movement can occur in the slab and cause cracking.
Concrete cracking can also develop due to deflection from live loads. We’ve all stood on the second floor of the mall and felt the slab moving as a result of simple foot traffic. This movement is continuous and can contribute to the cracking in the concrete long after all of the trades have left the site.
There are other less critical types of cracking that occur in concrete such as “mud cracking.” This is when the evaporation of the “water of convenience” proceeds too rapidly, resulting in a surface that looks like a dry lakebed. This accelerated evaporation could be due to a lot of direct sunlight combined with a bit of a breeze.
This phenomenon also occurs at high altitudes even when there is indirect sun or a breeze because water evaporates more quickly at higher elevations.
Didn’t you always wonder why they change the cooking directions on packages of many recipes at higher elevations? Therefore this superficial mud-cracking does not affect the performance of the tile installation.
In review of the above, we have tried to describe the different ways that cracks can develop in concrete. We have seen that some cracks that develop might be active, meaning that they will open and close, while others are dormant and no further movement is expected. Understanding how they got there and figuring out which ones are going to continue to move and which are dormant will help us decide how to deal with them.