- Report
- FR-011
- Published
- Written by
- Tomás Brandão
- Reading time
- 02 min / 356 words
Drift in a ten-year slab
Ten years of deflection readings from a school in Uppsala show a concrete slab that kept moving long after the model said it would stop.
Section 01
A slab that kept moving
In 2015 we fitted displacement sensors to the first-floor slab of a school in Uppsala, our first long-term monitoring contract. The slab spans nine metres in post-tensioned concrete, and the structural model predicted that creep would add about six millimetres of deflection in the first three years before levelling off. For three years the readings matched the prediction almost exactly. Then they kept going. By 2020 the slab had moved eleven millimetres, and nobody had yet asked whether that mattered.
Section 02
Ten years of readings
The answer came from setting the readings against everything else we record. The extra movement followed how the school was used, not the weather or the calendar. It accelerated during two summers when the building stood empty with its ventilation switched off, and slowed again once normal humidity returned. Concrete dries and shrinks from its surface. In a slab that dries unevenly from one side, that difference shows up as curvature, and curvature shows up as deflection at midspan. The structure was never at risk, but the model had left out the life of the building.
Fig. 011.1/ Midspan deflection
Unit / mm
| Period | Measured | Design model |
|---|---|---|
| 2015 | 0 mm | 0 mm |
| 2016 | 3.4 mm | 3.6 mm |
| 2017 | 5.1 mm | 5.2 mm |
| 2018 | 6 mm | 5.9 mm |
| 2019 | 8.2 mm | 6.2 mm |
| 2020 | 11 mm | 6.4 mm |
| 2021 | 11.6 mm | 6.5 mm |
| 2022 | 13.9 mm | 6.6 mm |
| 2023 | 14.3 mm | 6.6 mm |
| 2024 | 14.6 mm | 6.7 mm |
| 2025 | 14.8 mm | 6.7 mm |
Section 03
Why it matters for timber
We now model long-term deflection with the operation of the building in the loop: seasonal humidity, periods out of use, the hours the ventilation actually runs. That matters even more for the timber and hybrid floors that make up most of our current work, because timber responds to moisture faster and more strongly than concrete. A ten-year record from one school slab now sets the monitoring plan for every floor we design. The cheapest sensor on that project has turned out to be the most valuable.
Section 04
What we changed
Splitting the measured movement by cause gave a clear answer. Of the 14.8 millimetres recorded after ten years, 6.7 came from the creep the model predicted, 5.1 from uneven drying in the two empty summers and 2.4 from ordinary seasonal humidity. Less than a millimetre is left unexplained. The school now keeps its ventilation running at a low setting through every summer break, and the slab has not moved more than 0.2 millimetres since. For new floors we write that rule into the operating manual before handover, not after the readings start to drift.
Fig. 011.2/ Deflection by cause after ten years
Unit / mm
- Creep, as modelled6.7 mm
- Drying while empty5.1 mm
- Seasonal humidity2.4 mm
- Unexplained0.6 mm
