screeds

huangyhg

screeds
i have a project which calls for a 100mm screed to be placed over a strucutral slab.  the screed will used by forklifts etc and the design strength is to be 50mpa.  the spec recommends that it is placed in two layers with mesh reinfrcement.
are there any definitive guidelines: from what i've found, the mesh would be near to the top of the top layer with suitable cover to control early thermal drying cracks.  the mix could be a 1:3 cement :10mm max agregate mix and we are considering two 50mm layers to build the 100mm screed.  we will use a system of joints.
i have seen a few screed failures and the resaons were mix design and workmanship. anybody have and views or advice regarding the above or joint layout curing times etc.?
thanks nb
assuming the "screed" you are referencing is the same as a topping slab in the us, you have to make the decision of whether this is to be a bonded screed or an unbonded screed.  assuming a bonded screed and minimal reinforcement other than crack control, i would turn my attention to the concrete mix design.  the 10mm top aggregate size will increase the cement requirement, thus increasing the shrinkage potential.  for a 100mm screed, you can go to a 20mm or 25mm maximum aggregate size but then you should place in one layer, not two.  since wire mesh is difficult to work with in any section, i would consider using fiber reinforcement for crack control.  the cracks that develop will likely be from drying shrinkage, not thermal unless you place just before a quick temperature drop.
for jointing, assuming that the substrate is a reinforced concrete slab, you should match the existing joints and then consider cutting all joint spacing in half once again.  this is because reinforced concrete is typically jointed at greater intervals than non-reinforced concrete, so if you place a non-reinforced slab on top of a reinforced slab, you must decrease the joint spacing, but you must also match the existing joint spacing because of the potential (and probability) of reflective cracking.  saw cut the joints to at least 20mm, preferably 25mm within 6 to 8 hours after placement of the concrete.  fill all sawcut joints with a flexible sealant after you are sure that cracks have occurred.
with regard to the fiber, i would suggest one of the steel fibers (bekaert dramix or mitchell fibercon).  polypropylene fibers can work, but i have had better results from the steel fibers.
good luck.
many thanks i will take it under advisement.  unfortunately the specification insists on two layers with a mesh.
your comment about additional cement increasing shrinkage is interesting: is there any experience of mixing pfa (flyash) with the cement to reduce the heat generation?
adding flyash will reduce the heat of hydration as well as reduce the available alkali for potential reactivity if you have suspect aggregates.  i have not seen flyash to reduce the shrinkage a great deal, but it likely has some effect on this.
since you must stick with the two layer placement, i would suggest that you still consider a 20mm top size aggregate.  secondly, use flat mats, not rolled wire for the welded wire fabric placement.  this will make life a bit easier for the placement crew which will likely get you a better result.
also, don't allow vibration in the lower layer, but require vibration as the upper layer is placed.  this will allow better mixing of the two layers.
good luck.