What is Screed in Concrete Construction? | Screed for Rehabilitation | Properties and Composition
The screed can be defined as a construction element in flooring system that is introduced to bring the floor to the desired thickness or desired height or to make it suitable for a specified flooring. The screed can be also categorized as a rehabilitation method.
The preblended mortar that is mixed with anhydrite -based binders and the cementitious binders will form the composition of screeds.
Objectives of Screed in Construction
- The screed must behave as a substrate that is suitable for the flooring type that is specified.
- The screed has to be laid in schedule
- The screed laid must be durable in all service conditions without any sort of compromising both internally and externally. This is valid for civil, industrial and commercial flooring applications.
Characteristics of Screeds
The screed must possess some unique characteristics in order to make it suitable to install it in the flooring system. These features will include:
- Sufficient Thickness
- Mechanical Resistance
- Properly Cured and Stable
- Crack Free
- Clean and Dry
Sufficient Thickness of the Screed: The thickness of the screed that is installed will depend on the type of screed, type of flooring, the type of traffic and the intensity of the traffic acting.
Mechanical Resistance of the Screed: The screed must gain sufficient thickness with mechanical strength in order to safely support the floor structure coming over it. The mechanical strength of a screed that is installed for flooring in the case of domestic application will have a minimum value of 20MPa. This value can be 30MPa in the case of Industrial floor installation.
Compactness of the Screed: The screed must possess compactness throughout the thickness of the same. Any occurrence of crumble areas or inconsistency resembles the lack of mechanical resistance. This will result in the detachment or the breakage of the flooring. These areas have to be properly assessed and repaired based on the extent of the defect.
Properly cured and Dimensionally Stable Screeds: Adequate time has to be given for the installed screed for proper curing and completion of most of the shrinkage issues. Only after this provision, the installation of the floor has to be conducted. The curing cycle of the screed will make them prone to hygrometric shrinkage. This is as a result of evaporation of water and the drying off. This effect will cause curling or cracking as shown in figure-2 and figure-3.
|Fig.1. Deformation Caused in Flooring due to "Curling"|
|Fig.2. Cracks formed in Floors|
The cracks that are formed after the installation of flooring system will result in the detachment or the damage of the floor, as shown in figure-4.
|Fig.3. Serious Damage Caused due to Curling after the Installation of Floors|
Note: The time for curing for traditional mix of sand and cement is around 7 -10 days per centimeter of thickness for a good condition of weather. The screed that is made from the cementitious material can be longer say more than 1 month. The use of special admixture or special binders will help in reducing the curing times to a large extent.
Crack Free Screeds: A number of reasons can result in cracks in screeds. These reasons will include hygrometric shrinkage, use of fine aggregates in a large amount, a large amount of cement, too much water in the mix. All the cracks seen in the screed must be sealed monolithically before the application of the floor over it. The epoxy resins like EPORIP or EPOJET can be used to seal the cracks. The presence of hairline cracks will ask for the laying of an anti-fracture membrane. This can be MAPETEX.
Clean and Dry: The screed surface has to be clean free from dirt, dust and detached areas. Rubbles or any other material have to be removed and cleaned from the surface before the installation of the floors. These materials will hinder the adhesion between the screed and the floor.
|Fig.4. Detached Floor System From Screed due to Improper Adhesion|
The residual humidity of the screed has to be checked so that it conforms to the maximum level for the type of screed system that is installed. This is because the flooring is highly sensitive to the humidity.
|Fig.5: Carbide Hygrometer to Measure the Relative Humidity in Screeds|
For anhydrite screeds, the value of relative humidity must be lesser than 0.5%, irrespective of the type of floor cover installed. For screeds of the cementicious type, the value of R.H is lesser than 2% for wooden floors and for PVC material floor types it is less than 2.5 to 3%.
The smoothness of the Screed: The grade of finish of the screed is chosen based the type of flooring that is installed. This will decide the level of roughness or the level of smoothness. The rough surface help the floor to dry out easily and promotes good adhesion properties with the floor system. In the case of installation of resilient flooring there comes the need for a smooth finish.
Flatness of the Screed: By laying a straightedge at least 2m long in all the direction on the screed surface will let us measure the flatness of the screed. The max: tolerance with this particular straight edge can be 2mm. If the flatness is not in tolerance, the surface must be leveled properly before installation of the flooring system.
|Fig.6: Method that is used to Check the Flatness of the Flooring|
Article No: 147