Measuring cracks with tell tales

By The Helpful Engineer / On / In Existing Building Issues


Crack Monitor

Engineers can be requested to assess existing cracking in buildings. When doing this it is important to establish whether the crack is increasing in size. The best method for measuring crack movements is by use of a crack monitor or ‘tell-tale’. There are various types:

Crack monitors types. The most popular type is made with two slips of overlapping plastic plates with a red cross on one and a marked grid on the other, see picture above and below. Once this monitor is fixed in position an initial reading is taken. Then more readings are taken at regular intervals to establish if movement is occurring and if so, at what rate. By comparing the different rates of movement for cracks at different locations in a building, the cause and location of the weakness can be established. The monitors should be left in place for as long as possible to establish whether the crack is moving or not, and for a minimum of eight weeks.

There are other types of monitors which are sometimes used, for instance one type is simply a piece of glass which is fixed across the crack. If the glass is broken it is a sign that the crack has moved. However I don’t like this type as the glass can be broken by vandals and it can’t establish the rate of movement.

Other methods of crack measurement. There are other means of measuring crack width movement, these include rulers with various crack widths marked on to allow comparison, another type is visual crack microscope device and finally cracks can be measured by simply fixing metal studs either side of the crack and using a vernier calipers to measure the distance. A sensible approach is necessary when using microscopes and vernier calipers, the extremely small dimensional changes in crack widths which they can measure could simply occur due to daily temperature changes. If using very fine measuring devices the measurements should probably be taken at the same time of the day or at least check the temperature each time a measurement is taken.

If the end of the crack is visible, which is more likely for smaller cracks, this can be physically marked along with a date. Be warned however this approach isn’t advisable on ships. A chef on an oil tanker used this method when he discovered a fracture on the floor of his ship; he marked the development of the crack for weeks. Unfortunately before he brought it to the attention of the ship owners, the boat split in two and sank. But the markings and dates he made along the length of the crack were discovered in the wreckage by the accident investigation team and were useful in establishing the cause and rate of development of the fracture.

Installing a crack monitor. A crack monitor is relatively easy to install. Tools required are an electric drill, screw driver, fixings, a knife and of course the crack monitor. Some people use epoxy resin to fix the monitor in place rather than screws, but I prefer not to as the monitor may only be fixed to the paint on the wall, which could peel away. See photographs below.

 

1. Crack to wall with holes drilled for crack monitor
2. Monitor, screws on, cutting tape
3. Screws tightened, crack monitor installation complete. The final tightening of the screws can knock the axis of alignment off, as in the case above. This is fine once the initial reading is carefully recorded. Never assume it ha been erected at zero zero.

 

 

3 thoughts on “Measuring cracks with tell tales

  1. Thank you, a very accurate description. Behind the words am feeling a lot of experience. I might add that modern monitors have metal studs. This enables accurate observations using electronic calipers.
    Use “crack monitor Zi-2.2” for monitoring cracks. They are sold on ebey and sent to many countries.

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