Tilerworld forum

What’s the difference between ... mature and green screeds?

No matter what the tiling project, one factor is constant: the customer will want it completed as quickly as possible. When the job involves tiling onto a freshly-laid screed, this can raise a challenge. To a customer’s inexperienced eye, it may look as though the floor is ready for tiling. “If it is dry enough to walk on,” they ask, “why is it not dry enough to tile on?”

This gives the tiler the chance to demonstrate their professionalism, by explaining that there is a formal British Standard, BS 5385 Parts 1-5, giving recommendations of minimum drying times for constructional backgrounds and bases. These vary, according to the surface:

  • Cement:sand screeds – 3 weeks
  • Concrete – 6 weeks
  • Cement:sand render – 2 weeks (3 weeks in swimming pools)

           
These times relate to the optimum drying conditions, which are generally 20° C at 65% relative humidity. A longer drying period would be necessary in cold or damp conditions. The aim is to reduce the risk of shrinkage effecting the tiling layer during drying, a common cause of movement. Movement stresses built up within the tiling layer can result in loss of adhesion, bulging of the tiles or cracking of the tiles and grout.

Drying shrinkage is a reduction in size, which occurs with most materials when their moisture content is reduced. With cement:sand screeds and concrete, there are three phases of shrinkage.

Plastic shrinkage takes place in the wet/fluid state and is not normally detrimental to adhesion. Hydration shrinkage can occur as cement hydrates are being formed, leading the system to contract. This is also the phase at which expansive cements deposit hydrates that can cause shrinkage of the overall system.
Drying shrinkage takes place during and after the cement matrix has been formed. Water is lost in the system either through the hydration process or evaporation to the atmosphere. This loss of water causes shrinkage and can take up to six weeks to stabilise.

BS 5385: Part 3 2007, Appendix C.2.6  explains that screeds dry out and shrink, with resultant compression, so, “The screed should be kept covered with waterproof sheeting for at least seven days after laying to prevent drying out.” The Standard further adds that, “After the seven days curing period, levelling screeds should be subjected to continuous air drying for at least a further two weeks before tiling is started.”

It is better to keep the screed covered for seven days to allow strength development and to reduce differential drying shrinkage between the top and bottom of the screed. The possibility of adhesion failure between the adhesive and the screed is not the only risk from movement stress; lateral compression stresses, or squeezing, can result in lifting, bulging or tenting of tiling, so that the tiles no longer fit into the space they once occupied.

BAL Green Screed Adhesive combats these risks and allows tiling to begin much earlier. Tests have shown that it shrinks with the screed/concrete to minimise differential shrinkage, caused by different rates of shrinkage between the adhesive and screed. Developed for good bond strength and flexibility, it also maintains good compressive strength, reducing potential risks associated with impact damage in heavily trafficked areas. Its deformability enables absorption of movement stresses from the screed, so that it exhibits almost zero differential stress.

In practice, a tiler fixing ceramic tiles can save 20 days when tiling to new cement:sand screeds. Just one day’s drying, at 20° C, is needed. For new concrete, up to 35 days can be saved, with a drying time of just one week, at 20° C, being required.

Such savings are not available with every ‘flexible’ adhesive. Microstrain tests have shown that deformability or flexibility alone is not enough to reduce the amount of differential shrinkage stress between the adhesive and the concrete/screed substrate.

As a further benefit, screed strength is improved, because water trapped in the screed enables maximum hydration of the cement. Once water has been lost, the hydration process will stop, so leaving screeds uncovered early on could reduce the final strength of the screed.

Finally, the choice of grout needs careful consideration. A flexible grout is needed due to the movement involved, but the potential for efflorescence (the formulation of salts leading to surface staining) is also increased as the screed dries out. A CG2F grout, such BAL Microcolour Wide Joint Grout would be ideal as the product is specifically formulated to reduce significantly the potential for efflorescence.

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For further advice and precise information, visit the BAL website at
www.bal-adhesives.com

or call 0845 600 1222

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