Polymer Modified Levelling and Wearing Screeds

Polymer modified screeds are typically specified when thin high performance screeds are required.  Wear resistant polymer modified screeds, as described in BS 8204 Part 3: Polymer modified cementitious levelling screeds and wearing screeds — Code of practice, are ideal for heavy duty industrial floors and polymer modified levelling screeds can be applied fully bonded to a concrete slab, unbonded on a concrete slab or other solid base or floating on a resilient layer, such as insulation board or acoustic underlay.

All thicknesses shown are the minimum recommended and allowance must be made for substrate and screed irregularity when specifying nominal screed thickness.

Characteristics of Polymer Modified Screeds

Polymer modification of screeds increases; flexural strength, tensile strength, compressive strength and durability, allowing screeds to be laid thinner.  Polymer modification reduces; water demand, permeability to water/ water vapour and shrinkage.  Beneficial effects of polymer modification of screeds increase as the polymer: cement ratio increases and when screeds are site batched, care must be taken to ensure that the polymer: cement ratio is adequate for the application and that the proportion of sand to cement is appropriate.  Site batched screeds should be batched by weight for accuracy of batch proportions, when batching by volume, shovels are unacceptable and batch boxes must be used.

Screed Compaction

It is always important to ensure adequate compaction of screeds and when laying floating screeds the resilience of insulation boards makes compaction more difficult, even in thinner screeds.  The addition of sufficient gauging liquid is vital for adequate compaction and to ensure that there is enough gauging liquid to fully hydrate the cement.  A simple test to determine the correct screed consistency is to make a ball of the mixed material, when the ball is squeezed there should be no free liquid expelled and when the ball is pulled apart, there should be no crumbling of the mortar.  The following is an image of a poorly compacted unmodified sand/ cement screed which was laid too dry.

Tolerances

Nominal screed thickness should be the sum of minimum thickness + sub-floor levels variation + screed surface regularity.  A screed laid to SR2 surface regularity tolerance may need to be ≥ 15mm thicker than the minimum thickness to avoid the risk of high spots in the sub-floor coinciding with low spots in screed level.

Ronafix Products for Polymer Modified Levelling and Wearing Screeds

Ronafix admixture is used in screeds, bedding mortars, weather resistant renders, tanking renders and concrete repairs.  The use of Ronafix admixture in sand/ cement screeds improves development of physical properties throughout the curing period, allowing pedestrian access after 24 hours and vehicle traffic after 2-3 days at 20⁰C.  In the longer term, testing of heat aged samples indicates that Ronafix screeds remain durable for more than 80 years.  Tensile strength of unmodified control samples after simulated 20 years of ageing had reduced to ˂ 1N/mm² and had become unmeasurably low after simulated 80 years ageing, but tensile strength of Ronafix modified samples remained higher than the original strength of unmodified control samples.  Flexural strength of Ronafix modified samples was unaffected by 80 years of heat ageing but a gradual reduction was seen in the unmodified sample and flexural strength was reduced by approximately 29% between 20 years and 80 years of ageing.  The addition of Ronafix admixture to sand/ cement mixtures considerably increases flexural and tensile strengths and the reduced water cement ratio of Ronafix modified mortars will in many cases improve compressive strength.   Increased physical properties allow Ronafix screeds to be laid thinner than conventional screeds.  Water and water vapour permeability of Ronafix modified mortars is low or extremely low, depending on the mix design used, making Ronafix modified screeds highly resistant to frost damage.  Typical shrinkage of Ronafix Mix A screed and its prepacked equivalent is 0.01%, similar to that of epoxy resins.  Typical shrinkage of Ronafix Mix A1, Mix F and Mix B1 screeds and pre-packed equivalents is 0.02%, considerably lower than unmodified screeds.  Ronafix modified screeds applied at 50mm thickness are sufficiently dry for application of resin floorings and waterproof membranes, resilient floorings, carpet, quarry tiles, ceramic tiles and stone tiles after approximately 10 days of air curing when cured at 20⁰C and approximately 60-65% relative humidity.  At 15mm thickness in the same conditions, Ronafix polymer modified screed will typically dry after 3 days air curing.

Mix designs of Ronafix site batched screeds and pre-packed screeds are similar, the differences are that Ronafix Pre-packed Screeds are batched in the factory to provide accuracy of batching and consistency of material quality; they contain CEM I 52,5 Rapid Hardening Portland Cement (which sets at a similar rate to other Portland cement grades but gains strength more quickly and attains higher ultimate strength)  and accurately graded kiln-dried aggregates.  Site batched screeds typically contain CEM II 42,5R, which has a higher crushed limestone or PFA content and screeding sand, graded from 0-4mm, with variability in proportions of aggregate fractions between dust and 4mm.  The water content of washed sands use in site batched screeds is variable and this can make it difficult for the screeder to regulate the total liquid content of the screed.  It is common for screeders to use shovels for “measuring” components when batching site mixes, which often results in overly sandy mixes.  Many screeders prefer to produce screed mixes which are too dry to adequately hydrate the cement, thus reducing strength and making compaction more difficult, especially when screeds are laid on resilient insulation boards.  Mix proportions in screeds are critical to adequate performance and when site batching mixes by volume rather than by weight, careful use of batch boxes and not shovels is essential.  Increasing the sand content and reducing the gauging liquid content can reduce the compressive strength of a screed mix design by 80%.  Pre-dilution of Ronafix admixture is likely to result in a lower polymer content if the water content of the sand is misjudged.