Posted: Thu Feb 02, 2012 9:11 pm
LLL - it goes back to a British Standard which has been handed down from specifier to specifier over the years. I'll see if I can dig it out. Screed is now predominently supplied to a European Standard, generally by flexural (bending) or compressive strength however the most common mix still equates to a 4:1. A 3:1 is rarely used because of the tendency to result in excessive shrinkage coupled with curling at the edges - these increase with an increase cement content. Most screeders don't like laying it either - it's too sticky.
The real acid test to determine the quality of in-situ screed is with the BRE Screed Tester (aka drop hammer). A 4kg weight is dropped through a height a 1 metre via a steel tube onto a 25mm diameter steel foot. After 4 drops a domestic (house) screed is considered acceptable if the indentation is no more than 5mm. This piece of kit really sorts out the good screed from the bad. We can supply an accurately batched 4:1 screed at the correct mositure content but if it's abused on site, i.e. poorly compacted and without any curing, particularly when the weather is very hot and dry, it can all go pear-shaped. In saying this, the drop hammer is not commonly used in domestic circumstances but more so on the larger commercial jobs (if specified by the architect/engineer or in the case of a dispute). Screeds for commercial properties/infrastructure buildings such as a hospital operating theatre have to suffer more stringent maximum indentation values, i.e. 4mm and 3mm respectively. Assessment by compressive strength (test cubes) can also be used if agreed.
The real acid test to determine the quality of in-situ screed is with the BRE Screed Tester (aka drop hammer). A 4kg weight is dropped through a height a 1 metre via a steel tube onto a 25mm diameter steel foot. After 4 drops a domestic (house) screed is considered acceptable if the indentation is no more than 5mm. This piece of kit really sorts out the good screed from the bad. We can supply an accurately batched 4:1 screed at the correct mositure content but if it's abused on site, i.e. poorly compacted and without any curing, particularly when the weather is very hot and dry, it can all go pear-shaped. In saying this, the drop hammer is not commonly used in domestic circumstances but more so on the larger commercial jobs (if specified by the architect/engineer or in the case of a dispute). Screeds for commercial properties/infrastructure buildings such as a hospital operating theatre have to suffer more stringent maximum indentation values, i.e. 4mm and 3mm respectively. Assessment by compressive strength (test cubes) can also be used if agreed.
