There are literally tens, if not hundreds of factors that promote cracking in masonry and concrete. Yet, it is uncommonly rare for one factor to be the sole cause of the problem. Most of the time cracking occurs because the forces attempting to crack the material exceed its tensile strength. This applies generally to cementitious materials, rather than aggregate or brick itself. In the former the tensile strength is always relatively low, much more so at early ages. Raymond J. Schultz once likened the phenomenon of cracking with a race between the development of tensile strength and the development of the forces that will produce the cracking.
On the other hand, when cracking occurs in the brick unit itself, it is more likely to be due to excessive general or localized compression (such as that generated by the wrong mortar mix for the particular brick).
Mix design and behavior
Both masonry and concrete are composite materials that behave well in compression and relatively poorly in tension. The trick then is to design components that are in compression all or at least most of the time, and to keep the unavoidable tension events as rare as possible and the spikes low enough as to avoid not only immediate and total failure, but also any significant cracking which typically gets worse in time through cycles of restricted movement, water penetration and repeated freezing and thawing.
For Materials Engineers who design concrete mixes, it is a known fact that using large, sound aggregate one increases the final compressive strength of the individual mix. Like everything else in life, this increase comes at a cost: reduced workability and productivity. In masonry, this is actually a given: brick work is a special and specific case of concrete, with very uniform (often one single size), large aggregate, buttered with mortar, rather than cement paste. The final compressive strength of masonry is not that of the weakest component, but rather (through an intricate process) a synergetic result of the combination of brick and mortar. Actually, if the paste (cement and fine aggregate) in concrete has a very high compressive strength, the concrete may loose some of its elasticity, and the rock component may be crushed. Same holds true for masonry: if the mortar has too high a compressive strength when compared with that of brick, it will crush the masonry units. This brings us to lime in mortar: not only it will improve workability, but also it will increase the elasticity of the composite material and improve bond/adherence.
Cracking encompasses relatively narrow fissures (1/64"-1/2" wide) in masonry units, mortar, or grout alone or in any combination.
Cracking may be caused by a variety of conditions, such as improper material choices (primarily the wrong masonry unit/mortar combination), wrong field installation (plugged weepholes or hinged/slide masonry anchors loaded with mortar to the point of impairing the free movement), oversized anchors, structural settlement, etc.
Cracking is the most common and most visible form of cladding problem, in a range encompassing everything from cosmetic defects to total failure. Sometimes the location, orientation and form of the crack give an indication of the cause of the failure. It is generally known that cracks can be minimized or even eliminated by using proper masonry detailing, design and construction. This can be accomplished by introducing bed-joint reinforcement, properly sized and spaced movement joints, adequate support and anchoring, good artisanship and supervision, combined with better than just adequate mortar mix designs, thus ensuring improved flexural bond and reduced shrinkage. Instead of these, owners and designers attempt to achieve very high compressive strengths. This is usually counter-productive, as rarely does masonry fail in compression.
Masonry deterioration may start from something as insignificant as small cracks in some of the individual units (often a minor esthetical rather than structural problem), then progress through cycles of termal movement alone, or in combination with freezing and thawing cycles, to wider cracks over a larger area, which sooner or later may become structurally significant, especially if not mapped and monitored early in the process.
Causes, symptoms, mechanism, and effects of failure in masonry cladding
Cracking may be less likely to occur if there is:
The best way of mitigating problems is avoiding them in the first place and while not every hazard can be foreseen, many can. Please note that the degree of success with the following steps will vary with numerous factors. Even so, each of the following steps, when taken seriously and done correctly, will bring dramatic improvements in any masonry project life cycle:
...more to follow sooner or later
As stated earlier, most of the time concrete cracks because its tensile strength is lower than the tensile stresses it is subjected to. There are numerous factors that contribute (usually by acting in combination) to the actual fracture. They can be grouped based on their the occurence time as before, during or after the hardening process.
In the beginning
Hardening (and therefore strength development) is more or less a continuous process. Fresh concrete is plastic. Depending on the mix design and concreting conditions, it will start to lose slump and plasticity in a matter of anywhere from minutes to days, though commonly it is a matter of hours. During this time the concrete starts to change chemically through a reaction called hydration. This process of hydration continues to occur even after the concrete hardens, anywhere from a few days to weeks after you pour the concrete. Of course, at these early hardening stages relatively has little if any measurable strength. As such, even small forces or movements are capable of breaking the still forming crystaline structure and either produce permanent cracks, or establish weakened planes that will likely open into cracks at later stages, under larger load or movement conditions. These weakened planes may actually be generated as minuscule cracks that self-heal as long as hydration continues strongly.
Later, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction which causes concrete to go from the liquid or plastic state to a solid state requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete. You can make sure that the necessary water is available for this reaction by adequately curing the slab. The use of liquid curing compounds, covering the slab with plastic, wet burlap, and other methods can be used to cure concrete.
Shrinkage is a primary cause of cracking. As concrete hardens and drys it shrinks. This is due to the loss, thru evaporation, of excess mixing water. Thus, in most cases, the wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.
Concrete does not require much water to achieve maximum strength. In fact, a wide majority of concrete used in residential work, in many cases, has too much water. This water is added to make the concrete easier to install. It is a labor saving device. This excess water can not only promote cracking, it can severely weaken the concrete.
Back to: Building Pathology Home Page or to the
Building Pathology site map
Back to: Masonry table of content
Back to the Construction Durability Glossary
Back to the Construction Pathology Glossary
Site design by the author himself. Strange hobby, isn't it?