Masonry and concrete: a primer of sorts

Introduction

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.

Masonry cracking

Overview

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

• cause: deformation of reinforced concrete frame
  symptoms: crushing, spalling or cracking of veneer; possible buckling of the cladding.
  failure mechanism: shrinkage and creep of frame imposes compressive load on cladding.
• cause: expansion of clay brick masonry
  symptoms: vertical cracks near corners and offsets; oversailing and diagonal cracking of parapets; bowing and arching of walls; damage to adjacent members.
  failure mechanism: moisture-induced expansion of brick masonry against restraints induces significant compressive forces in the wall.
• cause: thermal movement of building elements
  symptoms: horizontal, vertical and/or diagonal cracks near steel beams and concrete slabs. Cracks open and close with temperature changes.
  failure mechanism: tensile, compressive and shear stresses are induced in the cladding by thermal movement of attached elements.
• cause: post tensioning/prestressing movement of building elements
  symptoms: damage to adjacent members.
  failure mechanism: ...
• cause: movement/sagging of supports
  symptoms: vertical cracks near supporting elements (beams, slabs, etc.); cracks wider at the lower ends; also some diagonal and (seldom) possible horizontal cracks; crack width may increases with time even under constant loads.
  failure mechanism: excessive deflection of supporting slab subjects cladding to in-plane bending.
• cause: movement/sagging of cantilevered elements such as balconies
  symptoms: vertical cracks near supporting elements (beams, slabs, etc.); cracks wider at the lower ends; also some diagonal; crack width may increases with time even under constant loads.
  failure mechanism: torsion of face beam that runs parallel with building envelope and supports masonry above. This masonry above the beam could end up in shear and flexure. If there is masonry below the beam and it is not clearly separated from the beam, besides shear and flexure stresses, it could be compressed to the point of spalling or even total failure.
• cause: movement of foundations
  symptoms: vertical and diagonal cracks; usually cracks wider at one end.
  failure mechanism: uneven settlement of foundation subjects building and cladding to shear and flexural loading.
• cause: poor shelf angle detail
  symptoms: spalling or crushing of veneer at shelf angle locations; buckling of veneer.
  failure mechanism: contraction of frame plus expansion of veneer induce large compressive stresses in the veneer, in the absence of proper movement joints.
• cause: shrinkage of concrete masonry
  symptoms: cracks of rather even widths around thickness and height changes and openings; cracks are vertical and/or diagonal; cracks evenly space along long wall.
  failure mechanism: tensile rupture under tensile stress which is generated when shrinkage is restrained.
• cause: mortar shrinkage
  symptoms: ...
  failure mechanism: ...
• cause: inadequate tie/anchor system
  symptoms: buckling of veneer under vertical loads (ie its own weight, unless upper shelf angle deficiencies or poor structural design add weight from the masonry above; wind collapse of veneer; cracking at tie/anchor locations; punch-out at tie/anchor locations.
  failure mechanism: tie system has inadequate strength to provide lateral bracing to cladding, or withstand suction force of wind; tie corrosion generates expansive forces in joints.
• cause: poor water path provisions (ie flashing, weephole, and vapor barrier system) and details
  symptoms:dampness at floor levels; efflorescence; spalling of units and heaving of veneer in cold weather.
  failure mechanism: water is not quickly drained away from cavity, and freezes on flashing, inside the mortar or grout mass, and in units; vapor condenses on cold inside face of veneer.

Cracking prevention

Cracking may be less likely to occur if there is:

• provision of control (contraction) or expansion joints for horizontal movement
• bedjoint reinforcement (ladder or truss ties) to reduce shrinkage cracking
• soft joints to provide elastic and creep deformations of a structural frame
• slip planes to accommodate differential thermal movements of non-masonry products.

Avoiding disasters

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:

• defective or no flashings, damp proof courses
• improper paint or sealer materials used
• lack of control/expansion joints to avoid wall damage
• foundation wall settlement.
• defective or no wall capping at the top of a wall
• defective hose bibs, drains, eavestroughs
• no wall drainage ie: weepholes, vents, troughs
• foundation wall does not extend above grade line
• lack of vapour barriers on the warm side of the wall
• no caulking or flashing where there is a change of product line to allow for differential movement
• landscape grading does not slope away from building

...more to follow sooner or later

Overview

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.

Site design by the author himself. Strange hobby, isn't it? E-mail: me