CRACK MONITOR FAQS

Frequently Asked Questions About Crack Monitoring

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HOW ACCURATE ARE CRACK MONITORS?

Crack monitors come in various styles and options depending on your requirements. Before accounting for temperature-related expansion and contraction of the substrate or the crack monitor itself, typical thermoplastic X-Y grid monitors have a ±1 mm visual resolution, with ±0.5 mm interpolated accuracy. Buildera CRACKMON® crack monitors include patent-pending TickPoints™ to improve measurement accuracy across the entire measurement grid, including even during angular crack rotation.

To further improve measurement accuracy, some crack monitors, such as the CRACKMON 5020AV include integrated caliper standoffs. When using a quality digital caliper, such as Mitutoyo® or similar, the measurement resolution is as fine as ±0.02 mm (±0.001"), with an accuracy and repeatability more typically of ±0.1 mm (±0.005").

What's the Best Way to Install Crack Monitors for Lasting Performance?

Returning to the job site to fix a botched installation is expensive. You can install most thermoplastic crack monitors using approved epoxy and/or mechanical fasteners. For best vandal resistance and to help prevent accidental movement of the crack monitor during its service life, Buildera always recommends using epoxy and fasteners when possible. In order of preference, use:

  1. Epoxy and tamper-resistant fasteners (best method)
  2. Epoxy and standard fasteners (excellent method)
  3. Epoxy only (very good method)
  4. Fasteners only (fair method—use only when epoxy is not available or may damage delicate surfaces. Due to potential fastener expansion or movement, Buildera does not recommend this method)

What Are Technical Considerations of Epoxy-Adhesive Selection for Crack Monitoring?

Selecting the right epoxy adhesive and following all installation procedures will help to ensure a proper bond between the crack monitor and substrate. Reports of failed installations have been traced to poor surface preparation, improper mixing, and/or use of non-approved adhesives. Epoxy chemistry is complex, highly specialized, and application-specific. Considerations include:

  • Types of surfaces to bond together
  • Epoxy compatibility with each material
  • Storage temperature
  • Installation temperature (ambient and surface)
  • Substrate moisture content
  • Epoxy gel time
  • Viscosity / non-sagging (thixotropic)
  • Mixing ratio (hardner to resin)
  • Tensile strength

Buildera has tested various epoxies and offers dual-syringe tubes under the STRUPOXY™ brand, available online. This thixotropic, non-sagging structural epoxy offers excellent bonding strength and tensile properties. It does not sag during installation, including on ceilings or vertical surfaces. The thick gel-like consistency fills voids in the substrate, and aggressively bonds to thermoplastic and metallic crack monitors. The epoxy sets up in five minutes and cures fully within 24 hours. When properly mixed and applied, it performs well outdoors and indoors, including in high humidity areas subject to moisture.

What is the Coefficient of Linear Thermal Expansion and Why Does it Matter?

The Coefficient of Linear Thermal Expansion (CoE) characterizes the lengthwise expansion and contraction of a material versus a step change in temperature. Mathematically:

α = ΔL / (L × ΔT)

where:

  • α = Coefficient of linear thermal expansion
  • ΔL is the change in length of the specimen
  • L is the original length of the specimen
  • ΔT is the temperature change during test

Most materials expand and contract during heating and cooling cycles, respectively. CoE is a unitless measure, but is generally expressed as micrometers-per-meter of change in length per degree Celsius. Some product and material data sheets specify CoE in parts per million, or ppm/ºC. Different materials have different thermal expansion coefficients. For most thermoplastic crack monitors made from acrylic or polycarbonate, the CoE for typical ambient temperatures ranges  30-70 μm/(m·°C). One micrometer is equivalent to one millionth of a meter, or 10E-6 meters. For most measurements, the measurement device CoE is negligible compared to other error sources, such as visual measurement accuracy, or expansion of the substrate itself.

Example 1:

What is the CoE of a 110 mm polycarbonate specimen that expands by 0.15 mm when the temperature changes from 10ºC to 30ºC?

  1. ΔL = 0.15 mm
  2. L = 110 mm
  3. ΔT = (30ºC - 10ºC) = 20ºC
  4. α = ΔL / (L × ΔT) = 0.15 mm / (110 mm × 20ºC)
    = 0.000068mm/(mm·°C)
    68 μm/(m·°C)
    = 68 ppm/°C

Example 2:

If concrete has an expansion coefficient α of 9.3 μm/(m·°C), how will the length of 5-meter wall change between 0ºC at night and 20ºC at day?

Rearranging the equation to solve for change in length ΔL:

  1. ΔL = α (L × ΔT)
  2. = 9.3 (5 × 20)
  3. = 930 μm
  4. = 0.930 cm (0.37")

Which Inudstry Standard is Used to Measure Coefficient of Expansion in Thermoplastics?

Materials used in Buildera CRACKMON 4020A and CRACKMON XL 15040A crack monitors adhere to ASTM D696 - 08e1, Standard Test Method for Coefficient of Linear Thermal Expansion of Plastics Between −30°C and 30°C with a Vitreous Silica Dilatometer

WHAT'S A CRACKPOINT™ AND WHEN WOULD I USE IT?

A CRACKPOINT™ is a small disk, typically milled from brass, stainless steel, or titanium. The disk includes a center detent for use with dial or digitial calipers. Use always in pairs, and place each CRACKPOINT approximately 3-6 inches across a crack. Measure periodically and note changes in crack width relative to the original measurement. Buildera mills the CRACKPOINT 60ti/90ti from pure titanium, and is extremely corrosion resistant—more so than even 316-grade stainless steel. Custom colors are available.

Use the CRACKPOINT any time precision crack-width measurements are required, with high resolution to 0.02 mm (0.001"). For best results, drill a shallow hole in the surface, deep enough to recess the CRACKPOINT flush with the surface. This is ideal for floors and walls in high-traffic areas. A patent-pending epoxy wells locks in the CRACKPOINT to any surface, using epoxy.