Quality advantages of site mix over ready mix screed

It can be clearly demonstrated that if the site mix 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 litre mixing vessel and filled with sand complying with BS EN13139 0-4mm grade plus the addition of any proprietary products to the pre-agreed mix design.

This gives complete control of quality, quantity, mix and water content to the screed specialist in order to achieve optimum performance. Due to the general mix parameter of 1:3-4.5 this method is completely 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 major quality benefits of mix screed over ready mix screed are:

• better efficiencies
• increased quality control
• more cost effective
• better waste management
• lower carbon footprint

Site mix advantages

• complete control over start times as sand and cement can be delivered in advance
• minimal waste as the screeder has total control of the mixed quantities resulting in improved carbon footprint
• screed is permanently fresh and installed in optimum time
• easy to manage change requests and varying depths
• added control of starting and finishing points
• reduced risk of re-agitated mix and dry jointing which leads to poor compaction
• allows site mixture of additives and cement replacements which is not possible with ready-mix
• more competitive pricing structure
• very easy to postpone/cancel due to unforeseen circumstances

Ready mix disadvantages

• no control over plant breakdown
• many logistical hurdles to overcome in order to guarantee delivery on time to site
• High volume of waste if screed -
  • is thinner than anticipated and no other areas are available to use the screed
  • if next joint location cannot be achieved
• seasonal changes to the consistency  of the screed – in summer too dry; in winter too wet
• poor compaction if screed is too dry
• poor finish if screed is too wet plus increased shrinkage due to high water/cement ratio
• day joints would be random with no control to comply with pre-agreed locations
• not guaranteed to contain correct retarders and mix design due to risk of human error caused by varied mix design requirements by different clients
• additional loads might not be an option if there is a variation of screed depth or will attract a premium cost for small loads
• high cost to purchase plus high cost to discard