Concrete screed is a construction component placed in a variety of thicknesses that aims to convey the installation surface of the concrete floor to the design height and provide an appropriate surface for installing the desired flooring.
In concrete construction, the screed is a flat board that is used to smoothen the concrete after it is placed on the surface.
Screeding is defined as the method of cutting excess wet concrete to permit the higher surface of the slab to achieve the proper grade and smoothness.
Manufacturing process of Concrete Screed:
Screeds are comprised of pre-blended mortar which is blended with cementitious binders or anhydrite-based binders.
They set as guides for straight edges which helps to convey the surface of the concrete flooring to the desired height.
The screed should be sufficiently inflexible in nature.
This helps them to resist the stress and deformation that happens during the spreading and leveling of floor toppings.
The distance of the metal strip or pipe does not exceed 10 feet which makes effective screeds.
Characteristics of Concrete Screed:
The screed is used for the installation of the floor should have certain technical and performance characteristics, which are as follows:
1. Adequate thickness:
The thickness of the screeds is determined by the kind of screed planned to install, thickness, type, and the intensity of traffic that is projected to fall on the floor.
2. Mechanical Resistance:
To maintain the ultimate load (service load) along with the contribution of the load by the type of floor, certain mechanical resistance have to be achieved by the chosen screed.
Usually, it is recommended that the screed employed in flooring for domestic purposes should have a minimal power of 20MPa.
In the case of industrial purpose, the minimal energy should be 30MPa.
The concrete screed must be compact and homogeneous over the entire floor throughout the thickness.
If the screed exhibits layers or areas which are crumbles or low stability, this is a signal of poor mechanical characteristics.
The floor will break or collapse because of this defect.
4. Concrete treatment and dimensions:
Before installing the floor, it’s essential to examine that the screed is correctly placed and cured.
This proper treatment will permit complete shrinkage in it.
During curing cycles of concrete screed, they are vulnerable to hygrometric shrinkage, this hygrometric shrinkage may be the result of evaporation of the combined water, or to the drying process.
This shrinkage will end in curling or cracking of the screeds.
If these cracks form simply after the installation of the floor, there is a possibility of the floor being damaged or broken.
5. Crack Free Screed:
Cracks in concrete screeds will be caused by many components such as hygrometric shrinkage, other reasons are due to the presence of large amounts of water within the mixture.
Cracks are also caused by the use of aggregates blended with too effective or too much cement content.
It is suggested that all cracks need to be sealed intact, this sealing is performed with excessive precision through epoxy resins.
These procedures are performed prior to the installation of the floor.
To avoid hairline cracks within the screed surface, the usage of anti-fracture membranes is advisable.
The screed surface has to be completely cleaned.
Any presence of dust, dirt, debris, isolated areas, or any other materials or substance on the surface has to be removed before the installation of the floor.
Faux surfaces will result in the prevention of adhesion between the floor and the concrete screed.
Residual humidity levels on the screed surface must be checked.
This value should correspond to the utmost level for that type of floor covering and it must be uniform throughout the entire thickness of the screeds.
This is especially accomplished during the installation of flooring that is sensitive to moisture.
For anhydrite screed, the residual humidity level should be less than 0.5%.
An electric or carbide hydrometer is used to measure residual humidity in a concrete screed.
The flattening of the screed is checked by laying a straight edge a minimum of 2 meters long in all directions on the screed floor.
The maximum allowable tolerance along this straight edge is specified to be 2 mm.
If the flatness is not within the tolerance, the surface must be leveled using a suitable product before installing the floor
The suitability of the grade of surface finish and the level of roughness primarily depends on the type of flooring that is chosen to be installed.
For reflective finishing, it is advisable to use a skimming product designed particularly for this purpose.
Composition of Concrete Screed:
The main components of concrete exhibiting good efficiency characteristics are given below:
- Admixtures for mixing it with water, liquid, powder superplasticizers, cement, and suitable aggregates.
- Special binder to combine with water and suitable aggregates.
- The mixture of aggregates in a granulometric curve to form screeds.
- Special pre-mixed mortar for mixing with water.
Defects in Concrete Screed:
The most typical defects seen in the screed are mentioned below:
- Surface dust or bleeding.
- Crumbling surface.
- Fracture around pipework.
- Localized dislocated surface.
Also read: Concrete Spalling, Concrete forms & Concrete Joints
Difference between Concrete and Concrete screed:
|Application/use||A smooth, flat layer or sub-floor that is laid at the base of the floor to take over the final floor.||For structural use as masonry and final wearing surface.|
|Mixing ratio||1: 3-5 (cement: sand) 1: 3: 1 (cement: 0-4 mm sharp: sand: 6-10 mm coarse aggregate) for heavy-duty.||1: 2: 3 (cement: sand: coarse aggregate).|
|Cement grade||Ordinary 42.5N Portland cement & if 32.5N Portland cement, add an additional 10% cement to the original mixing ratio.||A wide variety of cement grades based on design specification.|
|Types of Aggregate||Cementations levelling screed: 6 for 0-4 mm & for heavy-to-heavy screeds: 6-10 mm fast sand washed for single-size aggregates.||Fine Aggregate: natural sand or mixtures with sizes less than 5 mm.|
Coarse aggregate: gravel, crushed gravel, crushed stone or crushed concrete, air-cooled blast furnace slag not exceeding the 9.5– 37.5 mm dimensions as that in the screed.
|Water||Clear potable water.||Clear potable water.|
|Mix consistency||Semi-dry for cementitious screeds and free flowing for anhydrite screeds.||The rate of deceleration depends on the application and mixture design.|
|Delivery method||Pneumatic pump or forced action mixer for cementitious screeds and worm pump/grouting pump for anhydrite screeds||Piston pump or direct discharge from ready mix trucks.|
Concrete and screed are essentially made of basic elements such as cement, aggregates, and water but are different in the size of the aggregate, the cement grade, the mix consistency, and their intended application or use.