What is Screed in Concrete Construction?
Screed in concrete construction is a flat board used to smoothen the concrete after it has been placed over a surface. Properties and composition of screed in construction is discussed.
Screed is a construction element laid in a range of thickness and its purpose is to bring the installation surface for the flooring to the design height and to provide a surface suitable for installing the specified flooring.
Screed in concrete construction can be defined as a flat board or a purpose that is made of an aluminum tool, which is used to smoothen the concrete after it has been placed over a surface.
Screeds are made from pre-blended mortar which is mixed with cementitious binders or anhydrite based binders. They are set as guides for straight edges which helps in bringing the surface of the floor of concrete to the desired elevation.
The screed must be sufficiently rigid in nature. This helps them to resist the stresses and the distortion that is caused during the spreading and leveling of the floor topping. The metal strips or the pipe spaced not more than 10 feet apart make effective screeds.
The following specifications are guaranteed by an effective screed:
1. Behave as substrate that is essential for installing the specified flooring
2. It will be laid on schedule
3. Will show durability under various service conditions
The service conditions specified may be external or internal for either civil, commercial or industrial flooring purpose etc.
Screeding can be defined as the process of cutting off the excess wet concrete to bring the top surface of a slab to the proper grade and smoothness.
Characteristics of Screed in Concrete Construction
Screed must possess certain technical and performance characteristics for it to be used for floor installation. The properties of screed are:
1. Sufficient Thickness
The thickness of the screed depends on the type of screed planned to install, the thickness of floor and its type, and the intensity of traffic that is estimated to fall over the floor.
2. Mechanical Resistance of Screed
To sustain the final load (service load) along with the contribution of the load from the type of floor, certain mechanical resistance must be gained by the screed selected.
Generally, it is recommended that the screed employed for a flooring for the domestic purpose must have a minimum strength of 20MPa. In case of industrial purpose, the minimum strength must be 30MPa.
3. Compactness of Screed
The screed must be compact and homogeneous on the whole surface and throughout the whole thickness. If the screed shows layers or areas that are crumbles or lower consistency, it is a sign of poor mechanical characteristics. This defect will result in the breakage or the detachment of the flooring.
4. Screed must be Properly Cured and Dimensionally Stable
Before the installation of the floor, it is essential to check that the screed is properly placed and cured. This proper curing will let to complete the shrinkages in it if any.
During the curing cycles of screed, they are prone to hygrometric shrinkage. This hygrometric shrinkage may be due to part of evaporation of the mixing water or the drying process. This shrinkage will result in the curling or the cracking of screeds.
If these cracks are formed just after the installation of the floor, then there is the possibility of floor detachment or damage.
5. Crack Free Screed
The cracks in screeds can be caused due to several factors. One of which is hygrometric shrinkage. Other reasons are due to the presence of large amount water in the mix. The use of aggregates which are very fine or mix with too much cement content also results in cracks.
It is advised that all the cracks must be monolithically sealed. This sealing is done with high precision by means of epoxy resins. These procedures are carried out before the installation of the floor.
To avoid hairline cracks in the surface of the screeds, it is recommended to make use of the anti-fracture membrane.
The surface of the screed must be cleaned perfectly. Any presence of dust, dirt, rubble, detached areas or any other material or substance on the surface of the screed must be removed before the installation of flooring.
Unclean surfaces will result in the prevention of adhesion between the floor and the screed.
The level of residual humidity must be checked in the screed surface. This value must conform with the maximum level for that type of floor covering and must be uniform throughout the whole thickness of the screed. This is especially carried out during the installation of flooring that is sensitive o humidity.
For anhydrite screeds, the level of residual humidity must be less than 0.5%. An electric or carbide hydrometer is used to measure the residual humidity in a screed.
The flatness of the screed is checked by laying a straight edge at least 2m long in all the directions on the surface of the screed. The maximum acceptable tolerance with this straight edge is specified to be 2mm.
If the flatness is not within the tolerance, then the surface must be leveled off using a suitable product, before carrying out the floor installation
The suitability of grade of finish of the surface and the level of roughness mainly depends on the type of flooring that is chosen to be installed. For the reflective finishing, it is advised to apply a skimming product that is designed specifically for this purpose.
The Composition of Screed Concrete
The major composition of screeds showing good performance characteristics are mentioned below:
Admixtures for mixing it with water, liquid and powdered superplasticizers, cement, and suitable aggregates.
Special binders to mix with water and suitable aggregates.
Mixes of aggregates in a granulometric curve to make the screeds.
Special pre-blended mortars to mix with water.
Defects in Screeds
The most common defects that are seen in screed are mentioned below:
Surface dust or bleeding
Fractures around the pipe work
Localized crumbly surface