How To Determine The Width And Depth Of A Sealant Joint?

This can be a civil engineering question with broad answers because it involves a few factors such as purpose of the joints, anticipated joint movements caused by thermal expansion and contraction, vibrations, settlement, loading, impact, or concrete shrinkage. These joint movements would also be affected significantly by CTE (Coefficient of Thermal Expansion) of the adjacent materials, which then affects the choice of sealants – that is why there are sealants with different “movement capability” properties- ±12.5%, ±50%, or even +100%/-50%. All these factors have implications on the distance between sealant joints, and then sealant width and depth. Determining the correct width and depth of a sealant joint is crucial for ensuring the sealant performs effectively, providing a durable and flexible seal.

Engineers must decide on:

• number of joints in the structure
• distance and location of joints
• width of the joints (sealant depth will be based on joint width, see below)

based on the following criteria:

• amount of movement expected
• movement capability of the joint sealant
• other practical considerations

Determining Joint Width

BS 6093 Standard provides guidelines for the design of joints in building construction to ensure that they accommodate movement and maintain a watertight seal. The calculation of joint width based on BS 6093 involves determining the movement accommodation factor (MAF, i.e. sealant movement capability) and using it to ensure the joint can handle the expected movement without compromising the sealant’s performance.

Key Concepts for Joint Width Calculation

1. Movement Accommodation Factor (MAF):
   • The MAF is a percentage that indicates how much movement the joint sealant can accommodate. It is a critical factor in determining the joint width.
   • Also known as Movement Capability
   • Can be obtained on TDS of sealant

2. Movement (M, total anticipated movement):

See “How to anticipate the length of joint movement” for detailed steps on obtaining M by using CTE.

1. Joint Width (W) Calculation Formula:
   • The formula to calculate the joint width W is typically given as:

W =M / MAF + M

Example Calculation

To illustrate how to calculate joint width, let’s work through an example:

Given:

• Anticipated Movement (M): 12 mm
• Movement Accommodation Factor (MAF): ±50% (or 0.5 as a decimal)

Calculation Steps:

1. • W=M / MAF + M
   • W = 12mm / 0.5 + 12mm = 36mm

So, the calculated joint width W is 36 mm.

Summary of Joint Width Calculation

1. Determine Anticipated Movement (M) of joint width from its original state: it involves calculation with CTE. See guide here: “How to anticipate the length of joint movement caused by temperature changes”
2. Determine the Movement Accommodation Factor (MAF, or Movement Capability): This is typically provided by the sealant manufacturer or specified in the design requirements. Typically, it is ±25% or ±50%
3. Convert MAF to Decimal (if necessary): If MAF is given as a percentage, convert it to decimal form. ±225% = 0.25, ±50% = 0.5
4. Calculate the Joint Width: Use the formula W=M/MAF + M to find the required joint width that can accommodate the anticipated movement.

Practical Considerations

• Movement Types: Consider both thermal and structural movements when determining the anticipated movement.
• Design Requirements: Ensure that the calculated joint width aligns with the design and structural requirements of the building.
• Minimum Width: The minimum width of a sealant joint is usually around 6 mm. This ensures that the sealant can properly bond and maintain flexibility.
• Maximum Width: The maximum width can vary, but it’s typically up to 35 mm for standard sealants.

By following these steps and using the appropriate formula, you can determine the joint width required to ensure the effectiveness and durability of the sealant in accommodating movement.

  Determining Joint Sealant Depth

 General Rule of Thumb:

• The joint sealant depth is determined by sealant width and type of adjacent substrates.
• Sealant bead cannot be too thick because it will create unnecessary stress on bonding line -adhesion, cohesion, or material failure may occur.
• Sealant bead also cannot be too thin because there may not be enough bonded substrates, which can cause adhesion failure.

1. Width-to-Depth Ratio (per ASTM C 1193-16):

 Porous Substrates (e.g. concrete, masonry, stone, etc.):

For joint width between 6 to 13mm (0.25” to 0.5”), width:depth ratio is 1:1

For joint width between 13 to 25mm (0.5” to 1”), width:depth ratio is 2:1

For joint width between 25 to 50mm (1” to 2”), sealant depth should NOT be greater than 13mm (0.5”).

Non-Porous Substrates (e.g. metal, glass, etc.)

For joint width between 6 to 13mm (0.25” to 0.5”), sealant depth should be 6mm

For joint width between 13 to 25mm (0.5” to 1”), sealant depth should be NO MORE than one-half the width, up to a maximum of 9mm (0.375”)

For joint width between 25 to 50mm (1” to 2”), sealant depth should NOT be greater than 9mm (0.375”).

 Minimum Depth:

• The minimum depth of the sealant should be at least 6 mm (0.24 inches). This ensures proper adhesion and performance.

1. Backer Rod Use:

• For joints with a depth greater than 10 mm (0.4 inches), a backer rod is used to control the depth and provide a firm base for the sealant. The backer rod should be compressed slightly to fit snugly in the joint. If using backer rod is not possible, use bond breaker tape.