Photo: Pat Whelen

Can water rise through concrete slab?

What causes rising moisture? Rising moisture occurs when ambient temperatures rise, which draws moisture up from the ground and through the concrete. One way to avoid this is by placing a Damp Proof Membrane (DPM) between the concrete and the ground. This will prevent wet spots from forming.

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While rising moisture may not sound like a big deal, it does a lot more harm to your concrete than you realise. Read on to find out how you can protect your concrete from rising moisture.

What causes rising moisture?

Rising moisture occurs when ambient temperatures rise, which draws moisture up from the ground and through the concrete. One way to avoid this is by placing a Damp Proof Membrane (DPM) between the concrete and the ground. This will prevent wet spots from forming. Leaking pipes, that run through or under your concrete, can also cause rising moisture. So, it is a good idea to get a plumber to do a pressure test. This will ensure that they aren’t leaking through or under your concrete and prevent moisture from rising.

How to recognise rising moisture?

A clear indicator that your concrete has moisture damage, is the forming of damp patches or wet spots on the surface of your concrete. Wet spots not only damage the structural integrity of your concrete but can prevent protective coatings from sticking. This is why it is so important to test your concrete for moisture.

How to avoid damage from rising moisture?

The best way to avoid damage from rising moisture is to test your concrete for moisture before applying any coating. Otherwise, you could end up wasting a lot of time and money. There are a range of methods available to determine the moisture levels in your concrete flooring. According to the Cement Concrete & Aggregates Australia’s (CCAA) datasheet – Moisture in Concrete and Moisture-sensitive Finishes and Coatings; the three best quantitative methods to check moisture in your concrete slabs are:

1. The Relative Humidity (RH) Test

For this, you will need a hydrometer. You can get one that seals itself to the concrete or in probe form. For a probe, you will need to drill a hole into the concrete before testing. A RH test will measure the humidity between the instrument and the concrete. You will need to seal the instrument onto the surface and then leave for 16 hours. According to the standards stated in CCAA’s moisture in concrete datasheet, the cement is dry enough to apply a coating when the relative humidity is less than 70%.

2. The Electrical Resistance Test

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This test measures the electrical conductivity between two points. This is done using a resistivity meter and pins, probes, or electrodes, which are inserted into the concrete. The meter will indicate how much moisture is in the concrete. For an accurate reading, your resistivity meter should be calibrated for accuracy in the range of 4% to 9% moisture and be able to measure changes to the nearest 0.5%. If you drill holes into the concrete slab, you can measure the content of moisture in the entire slab; which can give you a more accurate reading. To pass standards stated in CCAA’s moisture in concrete datasheet, the moisture content cannot be higher than 5.5%.

3. The Plastic Sheet Test

If you do not have access to the above equipment, don’t stress. You can perform a moisture test with items that can be found in the average Australian’s tool shed. For this you will need some duct tape and a square plastic sheet (50cm x 50 cm).

Method:

Place the square sheet on your concrete floor. Seal onto the ground by lining the edges of the sheet with duct tape. Leave for 24 hours. Unseal the plastic sheet and check for moisture. If there is moisture on the plastic sheet, then moisture is rising through your concrete. While this method does not give you a quantifiable result, it provides a fair indication on the levels of moisture in your concrete. To find out more information about rising moisture in your concrete and how to protect your flooring, get in contact with us today!