Due to a high number of screed failures in the UK market, ready mix suppliers have taken the opportunity to offer ready mixed screeds in order to guarantee the mix proportions and charge a premium for the service. The truth of the matter, as a screeding specialist, the number of negatives far out way this methodology and perhaps the former could be reserved for those that do not specialise.
We attempt to answer the most frequently asked question of how do you gauge the mix?
We can clearly demonstrate that if the screed is forced action mixed, generally in a screed pump which has the same capability of a ready mix plant, there should be no reason for failure if the correct proportions are followed. The cement, preferably CEM 1 class 42.5N is gauged into a 260 liter mixing vessel and filled with sand complying with BS EN13139 0-4mm grade and add any proprietary products to the pre-agreed mix design. This gives the complete control of quality, quantity, mix and water content back to the specialist in order to achieve optimum performance. Due to the general mix parameter of 1:3-4.5 the above method is perfectly satisfactory and testing can always be carried out to ensure that the screed is fit for purpose. The recognised testing method is a BRE drop hammer test in compliance with the current standard of BS8204 1 2003 and perhaps this should form part of the screed package. Ready mixed screeds would not necessarily pass the test as compaction and workability play a huge part.
The common problem for failure is the lack of design and understanding of screed characteristics and there intended use. A range of final floor finishes and construction types at varying depths should be considered and all relevant codes of practice and standards should be understood. A specialist floor screeding contractor should have this level of knowledge and the Main Contractor together with their design team should disclose as much information as possible to reduce risk of any potential design faults.
We move on to address the main benefits of site mixing in preference to ready mixed screeds as follows:
Site mix benefits
- Added control of starting and finishing points.
- Guarantee of day joints coinciding with pre-agreed locations.
- Ability to adjust the water to facilitate trowelling and achieve a good finish.
- Minimal waste as the screeder has complete control of mixed quantities.
- Guaranteed to be able to start at required time as sand and cement can be delivered in advance, giving better efficiency.
- Easy to manage change requests and varying depths.
- Improves carbon footprint due to waste reduction.
- Many additives and cement replacements require site mixing and ready mix is not an option.
- More competitive pricing structure.
- Easy to cancel / delay if unforeseen circumstances arise.
- Specialist should comply with a quality management plan.
- Specialist is fully trained and competent.
- Screed is always fresh and installed at optimum time reducing risk of re-agitated mix and dry jointing giving a friable surface and poor compaction.
Ready Mixed Negatives
- No control over plant breakdown.
- Many logistical problems to guarantee site delivery on time, normally causes delay and increased waste.
- If there is a variation of screed depth then additional loads might not be an option or attract a premium cost for small loads.
- High amount of waste if the screed is thinner than expected and no other areas are available. High amount of waste if next joint location cannot be achieved.
- In the summer the screed is too dry and in the winter it’s too wet. Water in the sand is not measured before it’s mixed and differs in weather conditions.
- Day joints would be random and no control to comply with pre-agreed locations.
- Not guaranteed to contain correct retarders and mix design, risk of human error due to varied mix design requirements by different clients.
- Poor compaction if the screed is too dry.
- Poor finish if the screed is too wet and increased shrinkage due to high water / cement ratio.
- High cost to purchase and high cost to discard.
It is therefore our opinion that specialist floor screeding contractors should be given the opportunity to site mix as a preferred method and Main Contractors should take advantage of the skill sets offered.
Overall benefit is better efficiency, better quality control, more cost effective, better waste management and lower carbon footprint.
We are further driving the reduction in carbon footprint by offering modified screed systems that allow thinner section and directly reduce cement usage.
Sustainability publication coming soon.