Chapter 1 – Basic Function and Modes of Failure

Modern commercial buildings rely heavily on joint sealants to prevent water and air entry. While residential buildings commonly use water-shedding techniques like sloped roofs, lap siding, and overlapping flashings, many commercial designs don't, providing little or no barrier to leakage when the joint sealant fails.

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Product Characteristics

If the exterior shell of a building could be built without joints, and cracks wouldn't develop later, joint sealing could be eliminated. However, most modern homogeneous rigid exterior substrates are purposely jointed, to allow movement without damage to the material. The two principal causes of movement are thermal expansion and contraction, and seismic movement. Some substrates can be overlapped to allow rainwater to run off while allowing movement -- these usually won't need sealing (e.g. traditional shingling), or will incorporate the seal into the product design (e.g. metal panels with edge joints designed to prevent water infiltration). Other combinations of exterior materials are simply different and, as a result, seldom form a watertight joint without the addition of a sealer. The principal exterior substrates that need to be sealed are exterior wall joints, joints around door and window frames, concrete paving joints, metal flashings, joints in roofing, and seismic movement joints.

Indoor joints don't usually go through the thermal fluctuations that exterior joints do, but interior materials are often jointed for other reasons. Such joints are usually sealed up to keep dirt (and other things) out and to make them look better. Interior joints in wet areas are sealed to keep out water, and other interior joints are sealed to reduce sound transmission through cracks and holes. The principal interior substrates that are sealed are gypsum board, plaster, floor control and expansion joints, kitchen and bathroom wet joints.

Besides the natural effects of weathering and human occupancy, there are many points in the design and construction process where bad judgment or bad behavior results in inevitable sealant failure.

Basic Functions of Joint Sealants and Other Joint Treatments

  • Keeping water out -- precipitation, water used indoors.
  • Preventing freeze-thaw damage -- even if the water itself did not damage anything.
  • Keeping water in -- tanks, basins, pools, fountains, etc.
  • Preventing air passage (enclosure air barrier).
  • Blocking flanking sound paths.

Some consequences of joint sealant failure are described in ASTM C1472:  

"Sealant joint failure can result in increased building energy usage due to air infiltration or exfiltration, water infiltration, and deterioration of building systems and materials. Infiltrating water can cause spalling of porous and friable building materials such as concrete, brick, and stone; corrosion of ferrous metals; and decomposition of organic materials, among other effects. Personal injury can result from a fall incurred due to a wetted interior surface as a result of a failed sealant joint. Building indoor air quality can be affected due to organic growth in concealed and damp areas. Deterioration is often difficult and very costly to repair, with the cost of repair work usually greatly exceeding the original cost of the sealant joint work."

Modes of Failure of Joint Sealants

Adhesion Failure:  To achieve its function, a joint sealant must stick to both sides of the joint. When a joint's width increases after sealant installation (usually due to thermal contraction of the substrates), more stress is applied to the sealant. That is, the pull exerted on the bond line (the adhesion surface) increases. Adhesion that was adequate at a narrower joint width may no longer be sufficient. The sealant detaches from the substrate. The fault causing the detachment can be:

  • In the sealant itself -- it's not sticky enough. That can be due to poor chemistry or poor mixing of a multi-part sealant. It's also possible that the sealant was adequate initially but passage of time has reduced its adhesion capacity. No known joint sealant will last much more than 20 years.
  • In the substrate -- it's simply difficult to stick anything to. A primer might have made it possible but was not used.
  • In the cleaning of the joint. Dirt and moisture on the surface almost always impair adhesion significantly but that might not be apparent until the joint width increases.

Cohesion Failure:  When a cured sealant is stretched, it can only stretch so much. If the sealant doesn't lose adhesion, the sealant could be stressed beyond its elongation capacity, and the sealant will tear apart rather than detach from the sides of the joint. The sealant is simply incapable of withstanding the stretching that occurs. The fault could be:

  • In the sealant itself -- it's not stretchy enough. As for adhesion failure, this could be due to poor sealant selection or poor mixing of a multipart sealant, and it's also possible that the sealant was initially adequate but passage of time reduced its elasticity.
  • In the installation.  
    • To stretch properly, a sealant must be adhered to only two sides of the joint. A material used to prevent bonding to the third side usually must be used -- the two principal types are backer rods or bond breaker tape.  
    • There must be enough adhesion area on each side to resist the stresses.
    • The cross-section must be shaped like an hour-glass, so that when it stretches the cross-section at the center gets shallower. If the cross-section gets too shallow, it's no longer strong enough to resist stretching.  

 joint diagram

  • Due to damage after installation. For instance, if the surface of the sealant is damaged, such as by sharp grit being ground in under traffic, a small tear can start and then gets worse with stretching. Other causes of damage include "bubbling" (gas escaping from backer rods or the substrate), organic growth such as mildew, algae, etc., and deterioration of the material due to weathering ("chalking", "crazing/alligatoring" ).

Substrate Failure:  Some joint substrates are not strong enough to resist the stress exerted by a stretched sealant -- part of the substrate breaks off (as in a concrete spall) but is still attached to the sealant. Typically these substrates are relatively weak materials like masonry or plastics, but this could also happen to damaged edges of stronger materials like concrete. There is no way to prevent this via sealant selection or installation -- the substrate simply must be repaired to adequate strength.

Aesthetic "Failures":  These do not necessarily cause the sealant to fail its basic function, but they are usually undesirable and include color change due to UV exposure; airborne dust that sticks to the exposed surface but is washed off by rain, causing dirty streaks or staining of surfaces below.

Movement is the Cause of Failure

In all modes of failure, except aesthetic and simple aging, movement is the cause of the failure. The sides of the joint are going to move in relation to each other, because they have been designed to do so. In almost all climates, there is some annual thermal cycle -- the range of thermal differences will certainly cause movement and there are other factors as well. Joint design must take also into account construction tolerances, that may make the actual joint width larger or smaller than expected.

In practice, it's possible to do everything right -- design the joint correctly and select a suitable sealant -- and still have a failure because the installation occurred at one of the thermal extremes. Joint sealants are evaluated for their ability to withstand extension (stretching) and compression (squashing), measured from the initial "at rest" condition that the sealant is installed, as a percentage of the initial condition. For example, +/-25% movement capability means that the sealant can be expected to stretch to 25% wider than its installed width and compress to 25% narrower. Imagine the two sides of the joint in the diagram above moving away from each other. That happens when the building material gets colder -- the material "block" on each side contracts, moving away from the other "block", and making the joint wider. Suppose the joint in the diagram is 1/2 inch wide when the temperature is midway between the thermal extremes. Say it widens to 3/4 inch, when coldest, and narrows to 1/4 inch, at warmest. (Caution:  this is just for illustration, not an example of how to calculate movement.) If the sealant is installed at the thermal midpoint, the total extension it must withstand is 50% and the total compression is also 50%. Now imagine that the joint in the diagram was sealed when the joint was at its narrowest (1/4 inch) -- at the hottest time of the year. When everything subsequently gets colder, the thermal contraction widens the joint to 3/4 inch. The total extension in this case is 66.6%, with no compression at all. If a sealant having 50% movement capability in extension was used in the first case, there would be no extension failure (all other factors being optimum). But in the second case, with more extension, the same sealant is likely to fail.  


  • 1. The principal reason for joint movement is thermal expansion and contraction:  True / False.
  • 2. Joints that don't move don't need sealing:  True / False.
  • 3. A water-shedding design, like lap siding or shingling, is an effective substitute for joint sealing.  True / False
  • 4. Select all of the following modes of joint sealant failure that movement is the cause:
    • Color change.
    • Tearing of the body of the sealant.
    • Sealant detaching from sides of joint.
    • Tearing away from the sides of the joint taking non-sealant material with it.
  • 5. Tearing in the body of the sealant is an example of which mode of failure?
    • Adhesion failure.
    • Cohesion failure.
    • Substrate failure.
    • Aesthetic failure.
  • 6. Tearing away from the sides of the joint taking non-sealant material with it is an example of which mode of failure?
    • Adhesion failure.
    • Cohesion failure.
    • Substrate failure.
    • Aesthetic failure.
  • 7. Color change is an example of which mode of failure?
    • Adhesion failure.
    • Cohesion failure.
    • Substrate failure.
    • Aesthetic failure.
  • 8. Sealant detaching from sides of joint is an example of which mode of failure?
    • Adhesion failure.
    • Cohesion failure.
    • Substrate failure.
    • Aesthetic failure.
  • 9. Which of the following is not a potential reason for adhesion failure?
    • No bond breaker was used.
    • Sealant wasn't sticky enough.
    • The joint surfaces were dirty.
    • No primer was used.
  • 10. Which of the following is not a potential reason for cohesion failure?
  • The joint extension exceeds the sealant's capability to stretch.
  • The joint compression exceeds the sealant's capability to compress.
  • The sealant gets damaged by traffic.
  • The sealant was bonded on three sides.
  • 11. Exterior joints usually get wider when the ambient temperature goes up.  True / False
  • 12. Movement capability of +/-25%, as defined for joint sealants, means:
  • The sealant has overall 50% movement capacity.
  • The sealant should not be used in joints with less movement than +/-25%.
  • The sealant can be expected to stretch to 25% wider than its installed width and compress to 25% narrower.
  • 13. Which of the following is the most unpredictable variable in joint design?
  • Whether the contractor is going to purchase the correct sealant for the joint.
  • The amount of joint movement that will actually occur.
  • The point in the thermal cycle at which the installation will actually occur.

Next - Chapter 2 - Product Characteristics