Aggregates are one of the major constituents of a screed mix, forming nearly 5-80% of its mix volume. They impart strength and reinforcement to the overall screed structure, and constitute the major bulk of the screed.
The European Standards (EN13139) and the British Standards (BS8204) provide specific guidelines regarding the choice of aggregates for specific screeding purposes. Right from grading to the shape, texture and inertness of the aggregate material, there are several quality parameters that can influence the quality and cost of production of a screed.
Grading is the process of classifying aggregates into uniform lots based on particle size. Traditionally, they are classified into three major groups depending on the approximate size of the particles:
i) Fine Aggregates
The term ‘fine aggregate’ is often synonymously used with sand in the industry. As per the European Standards, fine aggregate refers to aggregate material that are less than or equal to 4mm in size. These are the aggregates generally recommended for use in standard screed mixes.
ii) Coarse Aggregates
These are the aggregates that are larger than 4mm in size. Coarse aggregates are further classified into:
- Graded Aggregates: The aggregate lot consists of particles of different sizes ranging between the maximum and minimum specified sizes.
- Single Size aggregates: The entire aggregate lot is of a single specified size.
iii) All-in Aggregates
All-in aggregates are classified as a mixture of both coarse and fine aggregates.
Given below is a table comparing the British and European Standard specifications for the different aggregate grades:
|Aggregate Type||Example||Test Sieve Size (British Standard)||Test Sieve Size (European Standard)|
|Coarse aggregate||Gravel||Aggregate retained on a 5 mm test sieve||Upper (D) sieve size > 4mmLower (d ) sieve size >= 2 mm|
|Fine aggregate||Sand||Aggregate passing 5.0 mm test sieve but retained on a75μm (micrometre) test sieve||Upper (D) sieve size <= 4mmLower (d) sieve size = 0 (always)|
|All-in aggregates||Mixtures of sand and gravel||Lower (d) sieve size = 0 (always)||Lower (d) sieve size = 0 (always)|
|Fines||Particle size fraction of an aggregate||Aggregate passing the 75 μm (micrometre)test sieve||Aggregate Passing the .063 mm test sieve|
The recommended mix proportion for a standard screed is – 1 part cement: 4.5 parts aggregates.
For levelling screeds, the specified aggregate requirement is 0-4mm washed sharp sand, whereas for heavy duty screeds the specification is 6-10mm coarse aggregates, for achieving the best results.
In fine concrete screeds, up to 25% of 0-4mm of the aggregate is replaced with a single sized aggregate in a range between 6 and 10mm. Though the mix ratio is specified as 1:1:3, it can be adjusted to facilitate troweling.
Shape and Texture
The shape and texture of the aggregate also influences the working properties of the screed mix, as well as the cost of the production.
Rough, angular and elongated aggregates leave more gap (void) between particles, leading to higher consumption of cement, and consequently increased cost of production. Rounded, smooth and compact aggregates help to achieve the expected workability without increasing the proportions of water and cement in the mix.
Inert, Clean and Free from Contaminants
The European Standards BS EN13139 specifies that screeding aggregates should be clean and free of contaminants, chemicals or other particulate matter as they can have a significant effect on the strength and quality of the screed. As per the European Standards, the maximum permissible limit of impurities such as clay, silt and other fine particles is 3% for screeding aggregates.
The aggregates should also be free of other deleterious materials such as chlorides, sulfates, lignite, iron pyrites and mica. For screeding using aggregates larger than 4mm, it is recommended to assess the aggregate for shell content. Several test methods such as the methylene blue test and sand equivalence test are available to assess the proportion of harmful impurities in the aggregates.
Choosing the right aggregates can improve the quality of the screed and help in achieving the desired workability without compromising on costs.