The Screed Scientist has an Apprentice!

Posted on 15th December 2019

If you have joined our gang on the Screed Scientist website to learn a thing or two about screeding, well consider yourself lucky in that you might actually know more than our latest recruit ‘The Screed Apprentice’

It occurred to us that we were a bit heavy on the boffin side with The Screed Scientist, The Screed Doctor and The Underfloor Heating Man. Who was actually going to ask the questions? Who was actually going to get on with some real work? Could you see The Screed Doctor coiling up the screed pipes or running around shouting at the MEWP drivers not to crush the screed? No, of course not! The Dr will be too busy taking samples and validating crushing resistance of the aforementioned specimens.

The whole idea for this website was to educate the industry, to ask the obvious questions and not to be afraid to test conventional wisdom in an effort to get the right things done first time…..

So enter The Screed Apprentice!

Now that the introductions are over, we have some work for him. He needs to get himself over to the CSC Screeding yard to carry out some testing. As you know our whole western civilization is based upon innovation. If you innovate and bring new ideas to market then you can progress, if you don’t, then you risk getting wiped out by those who are prepared to operate in such a manner. Ok, ok, lets forget the lofty discussion on the global economics system and concentrate on pushing the boundaries of floor screeding. Can we take some conventional wisdom and look to gain some efficiencies? Now that is a relevant question and why you are here on our site.

So, back to the young gun:

“Mr The Screed Apprentice, your mission  (and you have no ability to refuse this mission due to your status as an apprentice) is to make up some samples at our yard for testing as follows:”

  • Make up 8 boxes in timber, using 25mm, 50mm & 75mm Insulation and applying a Isocrete K Screed at 60mm with varying mix designs.
  • The purpose is to see what ISCR (in situ crushing resistance) we could achieve and to see if there is an opportunity in the market for thinner screeds achieving greater strength. Could this help a few projects along?
  • Will it be possible to achieve Cat A, B & C, which could be applicable for markets in the residential to commercial sectors.

Again back to the whipper snapper and we will let you imagine the next bits…..

  • Where’s the start button on this Mortel Meister …..
  • Oi you horrible urchin!  Hard Hat?  Hi-vis jacket? (If he cant get it right on our own site how can we possibly let him lose on a client site?)
  • Sorry I forgot to add the K Screed…
  • What’s this spinning laser thing used for?
  • Oi, which part of “put yer goggles on” did you not understand ….

On a serious note:

While we are having a bit of fun with our new character we could not be more serious in our intentions. Innovation and details are key. If we can get our screed depth down then there are opportunities to add more insulation to improve the building envelopes’ energy efficiency and performance as well as lowering the cost of the deployed screed.

Would you like to know more?  Join our newsletter or give us a call and talk to the real Screed Scientist.

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Screed Protection Considerations

Posted on 26th October 2016

Protection to levelling floor screeds should be carried out as soon as possible after installation. This is especially true on large construction sites with where many tradespeople are potentially competing to access the available work areas.

  • The expected traffic type and volume should be considered to determine the type of protection required.
  • Instructions on heavy loadings on to the levelling floor screed should always be sort from a qualified structural engineer.

Taking adequate measures will ensure that the floor remains in perfect condition for the floor finisher to achieve best results.

Please see the image below, what could have possibly caused the screed crack here?

 

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Wearing (Granolithic) Screed, what is it and where best to use it.

Posted on 11th January 2016

Wearing screeds, widely known by the former name of granolithic screeds, are high-density toppings suitable for application to green or hard concrete slabs. Offering characteristics desirable for areas likely to be subjected to severe wear and heavy traffic, these toppings are the preferred choice for industrial flooring projects.

Granolithic screeds consist of cement, sand and crushed rock aggregate, such as granite, limestone and quartzite, in a 1:1:2 ratio. Since granolithic toppings have a higher resistance to abrasion and impact than traditional cement-sand screeds, they can be used in a variety of industrial applications as wearing surfaces without final floor coverings.

Traditionally, granolithic finishes are used in applications where a sound, consistent, hard-wearing yet inexpensive surface is required (e.g. service corridors, warehouses, plant rooms, power stations, weight-lifting areas, etc.). Granolithic toppings should be laid at 15 mm to 20 mm depth, directly on to fresh concrete before setting has commenced (monolithic construction), or to a thickness of 25 mm to 40 mm, bonded to hardened concrete slabs.

Granolithic screeds are not suitable for unbonded constructions. In applications where bonding is not possible, a new concrete slab typically 100 mm thick should be installed and the screed laid monolithically on top.

Though granolithic toppings can be used in conjunction with underfloor heating, extra caution is required as excessive drying, cracking and curling can occur, especially when the commissioning cycle of the underfloor heating system has not been performed correctly.

Since these screeds are typically mixed on site, specific issues such as incorrect mix proportions, batch-to-batch variations and inadequate mixing can lead to adhesion and compaction problems, making the screed more prone to cracking, curling and debonding from the base. To prevent screed failure, the screeder must comply with the recommended cement-aggregate-water proportions and use the right mixing techniques (e.g. forced-action mixing instead of hand mixing).

Additionally, the builder must ensure that the right mix proportions are provided for the concrete slab to develop the strength required (minimum 35N/mm2, as per BS 8204-1). In bonded constructions, proper preparation of the substrate is critical to guarantee optimal bonding and long-lasting performance.

Though the performance and lifespan of granolithic screeds depend on the type of aggregate used, screed thickness, quality of finishing, curing and drying, screeders can enhance resistance to cracking, abrasion and wear by adding polymers and PP fibres to the screed mix.

Another point worth mentioning is that granolithic toppings provide a rougher surface than other types of screeds. However, they tend to wear smooth and lose slip resistance over time. To improve the level of slip resistance, an abrasive grit can be incorporated into the screed. The screeds that have become slippery can be treated mechanically or chemically to provide a rougher surface.

Two major advantages of using these toppings are that they are unaffected by dampness and require minimum maintenance, which usually implies the removal of dirt with hot water and detergent, and repairs or replacements of damaged areas with epoxy resins or granolithic finishes.

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Screed Scientist Christmas 2015

Posted on 14th December 2015

MIND = Make It No Danger

It is that wonderful time of the year again. Playful snowflakes, the smell of pine from the Christmas tree, the festive spice of cinnamon, the suave flavour of oranges, stockings hung by the chimney…Yes, the Christmas break is approaching quickly and everyone is excited, including Santa, his reindeers and – as surprising as it may seem – I, The Screed Scientist.

Looking back at 2015, I can say that it has been an excellent year for the screeding industry, with many ambitious refurbishment projects and amazing structures taking shape. But this year I’ve also become aware of a very important aspect: with the construction industry expanding in the UK and companies getting busier than ever before, workers have ended up working at a faster pace and potentially paying less attention to safety.

If we add the influx of foreign workers to the picture, almost anyone could predict the dangers. Besides language barriers and differences between the UK’s competency assessment methods and those of other countries, many foreign workers come from countries that lack a mature health and safety culture and have yet to align their national health and safety regulations to the EU directives.

As an expert in screeding, I take health and safety very seriously. That’s why I would like to advise the people working in the construction field, whatever their role, to comply with the health and safety codes of practice that apply to this sector. Though laying floor screed may not seem dangerous, there are many hidden dangers lurking around construction sites, from the machines and tools we use to occupational diseases like hearing loss, respiratory conditions and musculoskeletal disorders (e.g. hand-arm vibration syndrome, vibration white finger, etc.).

In the UK, most contractors understand the importance of having health and safety systems in place. However, some people think that health and safety codes of practice have become an albatross around the neck of entrepreneurs. This is like a red rag to a bull for us, and we’ll do everything in our power to continue to drive a culture where health and safety are an integral part of every business decision we make. Here are two examples as to why health and safety measures should be implemented:

  • When working with cement-sand screeds, wearing protective clothing, including gloves, is a must. Since wet cement is corrosive, it can burn the skin and eyes. Cement powder contains silica, which can damage lungs if inhaled. Also, using waterproof knee pads can help prevent wet screed from being absorbed by workers’ trousers. When this happens, the bleed water held against the skin can cause severe caustic burns. Workers may not even realise that the burns are occurring until it’s way too late.
  • Though most screeding companies keep their equipment in good condition, some workers may ignore safety rules so that they can continue working. If a worker, for instance, removes the safety grill and the interlock (the device that locks the dangerous parts when the grill is removed) from the screed pump to work without obstruction, (in this case the pump will mix the screed as the worker continues to load the pump), he may potentially catch his hands in the rotating blades with horrendous injuries to follow.

Since work-related injuries and deaths are unacceptable, regardless of the industry, my goal for 2016 is to focus more on prevention and preparedness. Additionally, I promise to keep everyone involved and informed about every single aspect relating to the screed industry.

Together with my friends, The Underfloor Heating Man, The Screed Doctor and The Screed Apprentice, I’m raising a virtual toast to the health and well-being of you, our beloved customers, readers, subscribers, suppliers, colleagues and passer-byers.

Happy Holidays!

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Screed Reinforcement – What’s the Crack

Posted on 19th November 2015

Shrinking, curling and cracking can lead to problems with flooring. Using reinforcement may reduce their impact and lend greater strength to the floor.

Before we begin with this blog post, we need to be clear that there are many products, architect specifications and project related conditions that need to be considered and that you should always seek professional advice. Our intention here is to shed some light onto the topic and when we say “reinforcement” we do not mean “structural”.

The main purpose of reinforcement within screeds is to control shrinkage, curling, and cracking, which often appear during the process of drying, and give sub-floors extra toughness for greater resistance to impact. When a crack occurs, the reinforcement deflects the stress, slowing down crack propagation to minimise its width and spread. Screed reinforcement comes in two different forms: PP fibres and reinforcement mesh. Some screeding contractors may routinely use reinforcement in their jobs, unless there is a good reason not to.

PP Fibres

Polypropylene fibres, or PP fibres, are added to ensure that screeds will better support the stresses and micro cracking that can occur naturally during the drying period. Improving screed resistance to cracking, PP fibres prevent larger cracks from forming, minimising the danger of early screed failure. Evenly dispersed in screeds, PP fibres also increase screed resistance to abrasion and impact, whilst reducing problems at the surface along with the risk of delamination. Defined as the splitting of floor screeds into layers, delamination occurs when water and/or air is trapped inside screeds (e.g. when final floor finishes are installed before screeds dry optimally). PP fibres are usually used in bonded, unbonded, floating screed constructions and in screeds installed over underfloor heating pipes.

Reinforcement Mesh

Reinforcement mesh is used in cementitious screeds to deflect intrinsic stresses, which may lead to cracking during the drying stage, and in applications designed to endure heavy loads. According to BS 4483, D49 (and chicken wire in certain circumstances) can be applied to enhance the compression and flexural strength of screeds, increasing the bending moment (the point at which the screed bends as a result of external forces) and reducing the risk of shrinkage cracking around  pipes and day joints. To lower the risk of cracking, a minimum width of reinforcement mesh is necessary and the screed should be laid at a minimum depth above the pipework, the details of which will require careful specification and are subject to product datasheets. Although D49 mesh is usually recommended to be placed at day joints, our experts prefer brick ties, which not only are easier to use but also deliver a solid, durable construction that can withstand a wide range of tensions and loads.

One or several cracks rarely compromise the integrity of a floor screed. In fact, a screeder can successfully apply a series of corrective measures to repair cracks and deliver a high-quality sub-floor, according to specifications. However, using reinforcement can help avoid time-consuming, costly repairs.

The type of reinforcement should always be selected according to type of screed, method of mixing, screed depth, and expected loads. Reinforcement is used in traditional and some modified screeds but not in anhydrite formulations. Though PP fibres and reinforcement mesh can be used in a wide variety of applications, they cannot replace structural steel. In this case, recommendation from the structural engineer or architect is necessary.

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Relative Humidity – Do You Know What it Means for your Screed?

Posted on 16th October 2015

At first glance, relative humidity (RH) is a quite confusing topic. Since people use different terms such as moisture, vapour, water content, relative humidity, absolute humidity, actual humidity, specific humidity or just humidity to indicate how moist the air is, many of them feel confused when they come across the term “relative humidity”. In response to the questions I have received over the years, I decided to write an article that clears up the confusion surrounding relative humidity. Knowing exactly what RH is will allow people to better understand its effects on screed performance.

As the name implies, RH is just a relative value. To find out the actual humidity, RH must be considered in relation to the dew point temperature (the temperature at which the air is saturated with water vapour), wet bulb temperature (the lowest temperature that can be obtained by evaporating water into the air) and dry bulb temperature (the actual temperature of the air measured with a regular air thermometer), and compared with the maximum amount of water vapour the air could hold.

RH needs to be correlated with the air temperature because the moisture holding capability of the air varies with the temperature. For example, if the dry bulb temperature is 30°C and the dew point temperature is 15°C, RH will be 40 percent. If the dry bulb temperature is 25°C and the dew point temperature is 20°C, RH will rise to 70 percent, as shown by the psychrometric charts below.

RH 40%

RH 70%

Because humidity affects not only the properties of the air but also the characteristics of the materials that come in contact with the damp air, understanding the impact of RH on screeds is very important.

The water from the screed is evaporated into the air and transported by air currents. When the screed is laid in cold, damp weather or in confined spaces with poor ventilation, the air above it will stagnate and become saturated with water vapour. In this case, the screed will need more time to dry. One way to reduce the drying time is to ensure adequate ventilation and temperature. A higher rate of air flow will make possible a continuous exchange of moist air with dry air, accelerating the drying process. In addition, heating the screeded area will help increase the moisture absorption capacity of the air, reducing the drying time considerably.

The drying time depends not only on RH, ventilation and temperature but also on the type of screed. When traditional screeds are used, correct drying implies curing with polyethylene sheets to prevent water from dissipating too quickly from the surface and avoid cracking and curling. Conversely, proprietary screed formulations incorporate special additives that allow the water molecules to bond and the screeds to develop adequate strength and dry when the ambient conditions are not ideal.

High RH levels that may prevent the screed from drying optimally within the time frame specified require particular attention. When sensitive decorative treatments (e.g. vinyl tiles, hardwood floors, etc.) are installed onto damp screed, floor failure may occur. Therefore, ensuring that the screed is dry and ready to take on the final floor finish is critically important before proceeding with the installation of the final floor.

There are two methods professionals use to assess moisture content in screeds:

1. Tramex Meter

A Tramex meter provides a very convenient and fast method to measure the moisture content, but the results should only be used as guidance. Since the meter assesses the moisture only at the surface, the screed could be drier or wetter at the top than at the bottom.

2. Calcium Carbide Test

The Calcium Carbide Test is the most accurate method to assess moisture levels in screeds. Implying the use of a screed sample, this test allows specialists to calculate the actual moisture content along with the time the screed needs to dry completely.

*Recommended Moisture Levels According to the Screed Type

% Moisture
Not Heated Heated
Cement-Based Screed < 2 < 1.8
Anhydrite Screed < 0.8 < 0.5

In conclusion, all screeds should be allowed to dry for the specified time frame, irrespective of the screed formulation used. If RH levels are higher than 50 percent or the temperature is lower than 20°C, the drying time should be extended accordingly. In addition, carrying out a reliable moisture assessment test before the installation of the final floor finish is critical to avoid costly errors.

A Note on Psychrometric Charts

** Psychrometric charts are used to indicate graphically the parameters relating to the water content of the air. Typically, a psychrometric chart includes:

  • Dry bulb temperatures shown on the horizontal axis, as vertical lines.
  • Wet bulb temperatures shown as diagonal lines.
  • Dew point temperatures and humidity ratio shown along the vertical axis, as horizontal lines.
  • RH levels shown as curved exponential lines.

The intersection point of the dry bulb, wet bulb and dew point temperature readings with the nearest line of constant relative humidity indicates the RH level for the values specified.

Credits:

The psychometric charts created for this blog were made here:
http://flycarpet.net/en/psyonline 

Sources:

  1. Sometimes it’s relative: The weather doctor http://www.islandnet.com/~see/weather/elements/humidity.htm
  2. Psychrometric charts and air conditioning. https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10b.html
  3. Psychrometric chart use
    http://web.uconn.edu/poultry/NE-127/NewFiles/psychrometric_inset.html
  4. Observed Dew Point: University of Illinois http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/maps/sfcobs/dwp.rxml
  5. Engineering Toolbox: Dry wet bulb dew point air
    http://www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html 
  6. A beginners Guide to Humidity Measurement http://www.npl.co.uk/upload/pdf/Beginner’s%20guide%20to%20humidity%20measurement%20%28draft%20for%20comment%29.pdf
  7. Tomorrows Flooring: Banish Bubbles in Vinyl. http://content.yudu.com/web/1jybr/0A1vxp9/TFJan15/flash/resources/17.htm
  8. Screeds Flooring and finishes: Selection construction and maintenance: CIRIA press
  9. Floors and Flooring: PW Pye and HW harrison
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Tiling onto Cracked Screed – Addressing Underlying Causes of Tile Floor Failure

Posted on 29th September 2015

Do you need to lay or are you worried about large format tiles placed directly over cracked screeds? Have you heard about crack inducement joints? Read on.

As you can see large format tiles are being placed over cracks

Not dealing correctly with cracked screeds is one of the main causes of tile floor failure. Keep reading to find out how to prevent, address, and solve cracks in screeds the right way

“I’ve just renovated my home, including replacing floors and installing UFH. The contractor applied a layer of traditional cement-sand screed to a depth of 65mm and told me to wait 110 days for the screed to dry before calling the tiler. However, a few superficial cracks developed across the screed surface. Unconvinced that repairs were necessary, the tiler installed large format tiles over the cracked screed directly. After I used the UFH system a couple of times, my new tile floor cracked in a few areas and several tiles came out. Now, my floor is completely ruined. Is there anything I can do to fix it?“

Unfortunately, this has become a common occurrence in floor remodelling projects. One reason for this is that all newly installed screeds can potentially shrink and crack as they dry, even when ideal ambient conditions (20°C and 50% Relative Humidity) are provided. Cracks form especially because the excess water evaporates from the surface at a faster pace than it is replaced by the residual water trapped in the concrete slab or at stress points such as doorways and corners. When UFH is used, water evaporates at a more rapid pace, increasing the risk of cracking. When tiles are applied directly to a cracked screed, crack-induced tension transmits through the screed to the brittle tiles, causing them to crack and/or debond.

A crack in the screed exactly where you would expect it to occur

Again, the screed cracks at a stress point, typically in a door way

There are four main ways to prevent cracks from forming in screeds:

  1. choose the right type of screed; though traditional screed can be used in conjunction with UFH successfully, the most suitable screeds systems for UFH can be anhydrite or modified proprietary screeds;
  2. use crack inducement joints;
  3. dry and cure the screeds in the right way;
  4. commission the UFH system correctly before the installation of the final floor finish.

If cracks still develop, implementing adequate remedial actions is imperative to prevent the aforementioned unwanted consequences. An efficient way to repair cracks in screeds is to use epoxy resin injection. However, expert advice should be sought before opting for this repair method.

An example of a well placed crack inducement joint.

A crack inducement joint on a new build

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Our 5 Most Popular Pages This Month

Posted on 5th August 2015

The Screed Scientist website is great for expanding your screeding knowledge. And this month nearly 6000 visitors used it to help them with their projects. Some of our most popular articles have been related to Underfloor heating with many of our readers looking for detailed information on how to make sure that their underfloor heating will be laid to the highest spec.

1. The Difference Between Concrete and Screed

These terms can be confusing as many trades will use them interchangeably. Although their base materials may be the same, the difference in properties between these two materials is determined through variations in mixture and proportions. Knowing why these differences matter will help you understand what makes a solid foundation for your floor.

Read the article: The Difference between Concrete and Screed

2. Optimum Drying Times for UnderFloor Screeds

The choice of screed you use with underfoor heating will affect the depths, drying times, reinforcement and insulation required to make an efficient system. This article will guide you through the pros and cons of each.
Read the article: Optimum Drying times for UnderFloor Screeds

3. Install Rigid Floor Insulation

Preparing the floor to receive rigid floor insulation requires attention to detail to ensure adequate contact and coverage. Many of our readers are using this article as a primer for this task.

Read the article: Install Rigid Floor Insulation

4. How Important is it to Assess the Moisture Content in Screed


In our opinion very! Too many screeding failures are caused by assuming the screed is dry. This article will help you assess how to tell if your screed is ready.
Read the article: How important is it to assess the Moisture content in Screed

5. Floor Screeding

Floor screed right and wrong

This is our most popular information page, getting nearly 50% of all page views. It’s a great overview of types of screed and screed care. We think that this is a great starting point for building up your screed knowledge.
Read the article: Floor Screeding

If you’ve found these articles informative, we think you’ll really enjoy reading our guide to Screed Testing. Download it here and find out more about the ultimate way to get perfect screed.

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WHY SCREED TESTING CAN SAVE YOUR PROJECT TIME AND MONEY?

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What affects screed drying time?

Posted on 6th July 2015

All screed mixes contain more water in them than what is required to hydrate the cement. Just about 40-50% of the water added to a screed mix is used to hydrate the cement, whilst the remaining 50-60% takes up the role of making it workable. Once the screed is installed, this ‘excess’ construction water is no longer required in the screed and has to be ousted completely before laying the final floor finish. If left to stay within the screed, this residual water can be the cause of serious moisture problems and screed failure.

The Drying process

When a screeded surface is exposed to the atmosphere, its general tendency is to attain equilibrium with the atmosphere. The moisture in the screed moves from the base layers to the top by capillary action and diffusion, and escapes to the atmosphere through evaporation, if the atmospheric air is dry (not excessively humid).

At this stage, it is quite easy to mistake the screed to be dry, as the exposed top surface looks dry and even coloured, when the screed could still be harbouring moisture in the layers below. As it is often difficult to make a visual judgement, the best way to ensure the screed is dry throughout is to carry out a reliable moisture test such as the Calcium Carbide Test at the end of the prescribed drying period.

According to industry guidelines a 75mm traditional screed would require up to 110 days to dry at 20˚C and 50%RH at the rate of 1mm per day for the first 40mm and 0.5 mm thereafter. However, it is much shorter for modified screeds which use water reducing and additives and admixtures to reduce the water/cement ratio in the screed mix. But, no matter what the screed type is, it is important to make sure that all screeds are allowed to dry for the specified time prescribed by the manufacturer, even if the screed appears to be dry from the surface.

External factors affecting screed drying time:

Relative Humidity:

The moisture content or relative humidity of the atmosphere plays is a major factor that affects the drying time of the screed. By definition, Relative Humidity (RH) is a measure of the amount of water vapour present in the atmosphere relative to the amount of water vapour it can hold at that given temperature. For instance, a Relative Humidity of 50% would mean the atmosphere is capable of holding double that moisture before it reaches saturation.

The higher the RH, the lesser the amount of moisture the atmosphere will be able to take up from the screeded surface, and higher would be the time required for the screed to dry out to the required level.

Temperature:

Air temperature is another important factor that plays a major role in speeding up evaporation from the screed surface. Higher temperature (and lower humidity) will help to increase the rate of the drying by providing higher thermal energy to drive the evaporation process. Hence the rate of drying is higher in summer and is much slower on a cold day.

Rate of air flow:

Higher rate of air flow helps the screed to dry quickly as it prevents the moisture laden air from stagnating above the screed surface. When the air is still, the air above the screeded surface becomes saturated with moisture, bringing the drying process to a stop. Providing good ventilation can speed up the drying. But remember not to leave the screed exposed to rain or external moisture in the process.

Internal factors affecting screed drying times

Water/Cement Ratio:

The lower the initial water-cement ratio, the shorter the drying time required for the screed. But as reducing the water content can affect the workability of the screed and make the screed friable, the best solution is to add a water-reducing additive or admixture to the screed which can help to keep the water requirement to a minimum, while providing good workability and strength to the screed mix.

Thickness of the Screed:

The thickness of the screed is another major factor determining the drying time of the screed. As discussed earlier, for traditional screeds the general guideline is to allow 1 day per millimeter for the first 40mm, and 1 day for every 0.5mm thereafter. Given below is a table showing a rough estimate of screed drying times of traditional screed depending on thickness.

Screed Thickness Estimated Drying time
50mm 4-6 weeks
75mm 6-8 weeks
50mm screed plus
100mm concrete
with no DPM		 6-12 months

However, for fast drying screeds the drying time is much shorter (depending on the formulation used), and is also not generally as bound strictly by ambient humidity and temperature conditions.

When on a strict timeline, it is common practice to resort to force drying to speed up the drying process. But it has a major side effect – excessive drying shrinkage, which in turn can cause cracking and curling of the screed.

So if you are on a time bound project, your best bet would be to go for a proprietary screed which can give you a drying time that suits your project. But no matter what your screed type is, make sure you carry out a reliable moisture test at the end of the drying period and make sure the screed is dry for the final floor finish, for a small error in judgment can be a costly mistake where moisture is concerned.

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Preventing sulphate attacks on anhydrite screeds

Posted on 20th May 2015

Anhydrite/gypsum based screeds are popular for their free-flowing and self-levelling properties as well as ease of installation. They differ from traditional screeds in that they use a calcium sulphate binder instead of Portland cement in the screed mix. The calcium sulphate based binders allow the screed to be laid to a lower thickness than traditional cement-sand screeds. The thinner screed layer coupled with force drying after seven days can provide quicker drying times and lower drying shrinkage.

However, special care must be taken while tiling and installing the final floor finish on anhydrite screeds as they are inherently incompatible with cement and cement based adhesives. The sulphates in anhydrite screeds react with calcium aluminates present in cement (cement based adhesives) to produce a crystalline powder called ettringite.

The formation of ettringite can damage and weaken the screed and cause structural changes in the screed, cause delamination of tiles and other floor coverings and increase the risk of floor failure. This is usually aggravated by the presence of moisture – either from within the screed or from external sources such as spillage or leaks from plumbing, groundwater or rain penetration.

Prevention of Sulphate Attack

It is important to seek manufacturer’s advice and ensure that the adhesives used are compatible with anhydrite screeds. Cement-based adhesives should only be used after priming the screed surface with a suitable priming agent recommended by the manufacturer.

Sulphate resistant cement adhesives, gypsum based adhesives and decoupling membranes are also being used to protect screeds from sulphate attack. However, they can be expensive and should only be adopted after consulting with the manufacturer.

It is also important to make sure the screed is protected from all internal and external sources of moisture. The screeds should be allowed to dry for the prescribed drying time before laying the final floor finish, as any residual construction water that remains in the screed can further increase the risk of sulphate attack.

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