Frequently asked questions
Below are the answers to some of the most frequently asked
questions about Formtex®.
Q: How many times can I use Formtex®?
A: Formtex® can be used twice with optimal reduction of the w/c ratio in the outer zone resulting in a dense and corrosion resistant concrete structure with greatly extended lifetime. Formtex® can be used several times to limit the occurrence of blow-holes in the surface.
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Q: Should I wait longer than normal before removing the formwork when casting against Formtex®?
A: It is not necessary to wait longer than normal before
removing the formwork, but doing so will improve the results.
Formtex® absorbs and retains water so whilst in place it provides for good curing.
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Q: What kind of concrete surface can I expect when casting against Formtex®?
A: The basic function of Formtex® is to drain surplus water and air from the concrete surface.
• a dense and uniform surface with few if any blowholes
• a low w/c ratio gives a darker surface with some colour variation.
• hydrostatic pressure varies in accordance with height causing difference in colour. The bottom of the structure will be darker than the top.
This flash animation shows the process and the idea of Controlled Permeability Formwork.
Q: Can I use any concrete mix when casting against Formtex®?
A: Yes, Formtex® will drain off water from any type of concrete. The quantities drained off, relate to the w/c ratio in the concrete mix. The higher the w/c ratio in the concrete, the greater the drained quantity. Irrespective of the concrete’s w/c ratio, the w/c ratio in the finished concrete surface will be lower when casting against Formtex®.
Q: Why drain the concrete to lower the w/c ratio?
A: When specifying a w/c ratio there are many factors to consider, among these is workability. It is a fact that with a lower w/c ratio, you get a stronger, denser and more penetration resistant concrete. These are all properties the concrete needs in the critical concrete cover zone. In order for concrete to be properly placed and compacted, more water is required than is needed to hydrate the cement. The only way to ensure optimal w/c ratio in the concrete cover with properly compacted concrete is to drain water from the concrete during compaction.
Q: How do I establish that the desired effect has been achieved?
A: Visible drained water comes from somewhere! Laboratory tests have demonstrated that the water is drained from the
first 15-20 mm of the concrete cover. Through the concrete cover the w/c ratio increases to the concrete’s original w/c ratio. The increased strength of the surface can be measured with a Schmidt-hammer.
Q: What does the drained water look like? Are there cement particles in the water?
A: The drained water is clear and yellowish depending on the use of admixtures. As the fabric’s pores are smaller than the cement particles there are no cement particles in the drained water.
Q: Can I use Formtex® when casting vertical and horizontal structures?
A: Hydrostatic pressure forces surplus water out through the Formtex® filter. Formtex® will, therefore, drain significant quantities of water both when casting vertical, sloping and at the bottom of horizontal structures.