How to Select a Stone Joint Sealant

The movement ability of a sealant is crucial. It needs to be able to handle the expected movement for an acceptable amount of time.

Water Resistance

Stone Joint Sealant need to resist the effects of water and vapor on their performance. This is especially important in a working sealant which will be exposed to the elements including rain, dew, snow, fog and sea spray.

The modulus of the sealant is a critical property that determines the ability of the sealant to withstand stress (strain) on its bond to substrates. The higher the modulus the stronger the bond and resistance to movement.

A sealant must be capable of taking on its cured state movement (extension and compression) without damage to the substrates or joint sealant. The movement capability of a sealant is determined by its flexural strength and compressive properties as measured by ASTM C719. This value will need to be coordinated with the expected joint movements in a project. This will allow for an appropriate choice of sealant and joint design. It will also prevent premature failure due to the inability of the sealant to take on its flexural stresses.

UV Resistance

A sealant is exposed to the elements when it's installed and must withstand accelerated weathering in order to last as long as its expected useful life. This includes exposure to ultraviolet (UV) radiation, ozone, and acid rain.

Back-up materials can restrict the sealant depth, support the sealant and facilitate tooling, and help resist indentation and sag. They can also function as a bond breaker, preventing the sealant from adhering to the back of the joint, which would limit movement capability. Common backup materials include butyl tape, polyethylene, and coated paper.

A sealant's movement capabilities must match up with the design movements of the joint it will be installed in. This requires careful consideration of the joint design and its installed conditions. Some examples of movements include seismic movement, live load, elastic frame shortening, concrete shrinkage and thermal expansion. Movement capability must be evaluated, designed for and accommodated.

Chemical Resistance

A sealant's chemical resistance is an important factor in the selection of a product for a particular joint. This relates to a number of factors including the ability of a sealant to resist changes in its own chemical makeup that can result from temperature shifts and other environmental conditions.

The physical properties of a sealant will also change with time and temperature. These effects can be difficult to predict. It is therefore important to understand the environment in which a sealant will be installed and to specify the appropriate material.

There are several different specifications for elastomeric joint sealants. The most rigorous is ASTM C920. This includes tests for single component (ready to use) and multicomponent sealants; different grades such as P = pourable, NS = gunnable, non-sag and NS-S for sanitary joints; different movement classes and more. This is the most stringent of all specifications and requires a significant investment in equipment. It requires the sealant to be put through 10 cycles of extension and compression.

Durability

The durability of a sealant depends on several factors. The most important is the ability of the sealant to handle the movement anticipated for a particular joint under field conditions and not experience adhesive or cohesive failure. Movement capability is typically evaluated using a test method such as ASTM C719.

The conditions at the time of installation also have an impact. The installer must be able to access the joints to place the sealant and backer rods. Additionally the temperature must be at or near the midpoint of the movement range of the sealant.

It is important that the scope of the sealant work be fully described on the drawings and in the specification. This can help eliminate implementation mistakes such as a failure to specify the proper backer rod size or joint opening width. It also helps to ensure that the sealant and backer rods have arrived at the job site in sufficient quantity.