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A concrete screed may be defined as a construction element applied in a range of thicknesses whose objective is to obtain the installation surface of the concrete flooring to the design height and to deliver a surface appropriate for establishing the established flooring.
Concrete Screed in construction is a flat board employed to sleek the concrete after it has been established over a surface.
Screeds are produced from pre-blended mortar which is combined with cementitious binders or anhydrite-based binders.
They are set as guides for straight edges which assists in bringing the surface of the floor of the concrete to the preferred elevation. The screed must be adequately rigid in nature.
This allows them to withstand the stresses and the misshaping that is induced at the time of the spreading and leveling of the floor topping. The metal strips or the pipe placed not more than 10 ft apart create effective screeds.
1. Specifications of Concrete Screeds
Specifications of a good screed are mentioned below:
a. Behave as a substrate that’s necessary for flooring.
b. It’ll be generally laid on schedule.
c. It’s sturdy and durable under various harsh conditions.
d. The service conditions specified can be external or internal for either civil, commercial, or industrial flooring purposes, etc.
2. Characteristics of Concrete Screed
The characteristics of concrete screed are as follows:
a. Sufficient Thickness
The thickness of the screed relies on the kind of screed designed to establish, the thickness of the floor and its kind, and the intensity of traffic that is calculated to drop over the floor.
b. Mechanical Resistance
To support the final load (service load) along with the subsidy of the load from the type of floor, specific mechanical resistance must be achieved by the screed fixed.
Generally, it is suggested that the screed utilized for flooring for domestic intents must have a lowest strength of 20 MPa. In the case of industrial objectives, the lowest strength must be 30 MPa.
The screed must be packed and homogeneous on the total surface and throughout the entire thickness.
If the screed displays layers or regions that are crumbled or have descending consistency, it is a cue of poor mechanical characteristics. This weakness will outcome in the breakage or the separation of the flooring.
d. Concrete Screed Curing and Dimension
Before the installation of the floor, it is necessary to inspect that the screed is appropriately positioned and cured. This right curing will let to meet the shrinkages in it if any. During the curing cycles of concrete screeds, they are prone to hygrometric shrinkage.
This hygrometric shrinkage may be expected to be part of the evaporation of the mixing water or the drying approach. This shrinkage will outcome in the curling or cracking of the screed.
If these cracks are constructed just after the installation of the floor, then there is a chance of floor split or separation.
e. Crack Free Screed
The cracks in concrete screed can be induced due to various elements. One of which is hygrometric shrinkage. Other reasons are due to the existence of a considerable quantity of water in the mix.
The usage of aggregates that are very fine or mixed with too largely cement content also outcome in cracks. It is suggested that all the cracks should be monolithically sealed. This sealing is accomplished with high accuracy by standards of epoxy resins.
These operations are taken out before the installation of the floor. To neglect hairline cracks in the surface of the screed, it is suggested to complete the usefulness of the anti-fracture membrane.
The surface of the screed must be swept completely. Any existence of dust, dirt, rubble, separate areas, or any other material or substance on the surface of the screed must be cleared before the installation of the flooring.
Dirty surfaces will outcome in the prevention of adhesion between the floor and the screed.
The level of residual humidity must be reviewed on the screed surface. This amount must serve with the ultimate level for that kind of floor covering and should be consistent throughout the entire thickness of the screed. This is mainly taken out during the installation of flooring that is susceptible to humidity.
For anhydrite screeds, the level of residual humidity must be smaller than 0.5%. An electric or carbide hydrometer is employed to evaluate the residual humidity in a screed.
The flatness of the screed is controlled by spreading a straight border minimum of 2 m long in all directions on the surface of the screed. The ultimately acceptable tolerance with this straight edge is described to be 2 mm.
If the flatness is not within the forbearance, then the surface must be grounded off employing an appropriate product, before moving out the floor installation
The appropriateness of the grade of finish of the surface and the level of roughness largely relies on the type of flooring that is determined to be established.
For the reflective finishing, it is recommended to involve a skimming product that is created particularly for this objective.
3. Composition of Concrete Screed
The composition of the concrete screed is as follows:
a. Admixtures of water & powdered superplasticizers, cement, & appropriate aggregates.
b. Special binders (water & proper aggregates).
c. Blends of aggregates in a granulometric curve to make the screeds.
d. Special pre-blended mortars to combine with water.
e. Most manufacturers bestow a 1:3 to 1:4.5 ratio of cement to sand in the screed.
f. Amplified screeds embrace supplements to bump up the quality of the screed.
g. This yields quicker drying spans & surplus sturdiness.
h. Manufacturers also donate a lot of self-compacting screeds which are anhydrite compounds kept on a CaSO4 binder.
9. CaSO4 screeds can’t be neglected but aren’t best for soggy regions like wet rooms, steam rooms & pool fringes.
4. Defects of Concrete Screed
The weaknesses of concrete screed are mentioned below:
ii. Surface dust or bleeding
iii. Crumbly surface
iv. Fractures around the pipework
v. Localized crumbly surface
vi. Rising dampness
vii. Insufficient flatness
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