Concrete/cement screed

What makes concrete screed different from other screeds?

Concrete (Portland-cement-based) screed is a dense, moisture-resistant floor layer widely chosen for bathrooms, kitchens, balconies, and other wet applications where anhydrite or traditional sand-cement screeds would fail. It is poured as a slurry of Portland cement, sand, aggregates, and water, curing through hydration to form a strong, durable mass. Unlike anhydrite screeds, which rely on crystallisation and are unsuitable for wet environments, concrete screeds are formulated specifically to endure moisture exposure and repeated thermal cycling over radiant floor heating systems. The trade-off is higher shrinkage during curing, which requires careful joint planning and moisture management to prevent visible cracking in large floor areas.

What are the strength classes and mix design rules?

Concrete screeds are classified by their 28-day compressive strength, measured in MPa (megapascals). A C25/30 mix (25 MPa cylinder strength, 30 MPa cube strength) is adequate for most residential floors under normal loading. For systems with embedded underfloor heating, point loads from furniture, or high thermal stress, a C30/37 or C35/43 mix is specified to reduce shrinkage cracking and improve durability. The water-to-cement (w/c) ratio is critical: lower ratios (0.45 to 0.50) produce stronger, less porous concrete with reduced drying shrinkage, but require careful curing and adequate time for hydration. Screeds are usually specified on-site with a compressive strength statement (e.g. "C30/37 concrete screed, 50 mm thick, poured to falls") and verified by crushing test samples at 7 and 28 days. Higher strength grades also improve resistance to chloride ingress in coastal or salted environments and reduce creep under sustained load.

How thick should concrete screed be over underfloor heating?

Thickness depends on the heating system design, pipe diameter, and desired thermal response. For typical residential radiant systems with DN16 (16 mm) or DN20 (20 mm) pipes spaced 150 mm to 300 mm apart, a 50 mm screed is standard. Pipes must be embedded at least 20 mm below the finished surface; some designs specify 30 to 40 mm cover for additional thermal mass and impact resistance. A thicker screed (65 mm) slows the heating system response and increases thermal lag, improving comfort by averaging temperature swings but reducing responsiveness to demand changes. A thinner screed (40 mm) allows faster heat release but increases risk of surface temperature non-uniformity ("striping") if pipe spacing widens. Bonded screeds (poured directly on the concrete structural slab without a separating layer) can be reduced to 35 to 40 mm if the base is very stiff; floating screeds, sitting on resilient isolation layers, must be at least 50 mm to avoid puncture. Always follow the heating system designer's specification for thickness and reinforcement.

What role does reinforcement play in concrete screeds?

Welded wire mesh (typically A98, diameter 4 mm, 200 mm x 200 mm spacing) or polypropylene fibre reinforcement controls crack distribution. The mesh does not prevent the concrete from shrinking; instead, it arrests crack propagation and distributes tensile stress, turning a few wide cracks into many fine hairline cracks that are barely visible and do not trap water or damage finishes. Mesh is positioned in the upper half of the screed thickness (ideally 30 to 40 mm from the finished surface) to resist bending stresses caused by shrinkage curling. In floating screeds, mesh is essential because the resilient base allows the slab to curl more freely; in thin bonded screeds over very stiff concrete, the structural base often provides adequate restraint, and mesh requirements are reduced. Polypropylene fibre (typically 12 to 15 kg/m³ dosage) is an alternative for thin, relatively small areas; it is easier to pump and distributes more evenly than mesh but may not be recognised by all waterproofing or adhesive manufacturers, so always verify compatibility before specifying.

How does concrete screed cure and why is moisture management critical?

Concrete cures through hydration: Portland cement reacts with water to form calcium silicate hydrate and other compounds, gradually developing strength over weeks. Early-stage curing (first 7 days) determines long-term durability; rapid drying during this phase creates internal stresses and increases shrinkage cracking. Curing proceeds fastest at warm temperatures (15 to 25 °C) and is slowed by cold or windy conditions. Concrete is walkable after 3 to 7 days (at C25/30 strength, roughly 70% of final strength) but continues to dry internally for weeks. The residual moisture content must fall below 1.5 to 2% (depending on the finish material) before vapour-tight floor finishes are applied. Traditional sand-cement screeds are dried by ventilation alone, taking 4 to 6 weeks or longer; fast-setting proprietary concrete screeds with additives can reach sufficient dryness in 2 to 3 weeks. Moisture trapped under a vinyl, epoxy, or similar finish will migrate into the adhesive layer, causing delamination and mould growth. When underfloor heating is present, the system must be progressively heat-cycled after curing reaches 7 days: gradually increased over 3 to 5 days to avoid thermal shock, then held at operating temperature while the screed continues to outgas moisture, which is exhausted through ventilation. This heat-up cycle is mandatory; skipping it or rushing it is a leading cause of later failure.

What causes shrinkage cracking and how is it controlled?

Concrete shrinks as water evaporates from the freshly set mass, with edges drying before the core. This uneven shrinkage creates tensile stress parallel to the surface, causing the edges to curl and internal tensile cracks to form, often at 3 to 6 metre intervals in large bays. The crack width and visibility depend on the water content, cement type, aggregates, and curing conditions. Portland cement screeds shrink 0.3 to 0.6%, roughly twice the shrinkage of anhydrite screeds. Control is achieved through movement joints spaced to limit unsupported bay size: typically 4 to 6 metres for floating screeds, 6 to 8 metres for bonded screeds on very stiff bases, and tighter spacing (3 to 4 metres) if high moisture loading is expected after construction. Joints must extend through the full thickness of the screed and be sealed with compressible material (cork, polyurethane foam, or plastic-foam backer rod) that accommodates 5 to 10% linear movement. Low water-to-cement ratios (0.48 to 0.52), air entrainment (small entrained bubbles that reduce internal stress), and proper curing (preventing rapid surface drying) all reduce cracking. Retarders (additives that slow the set time) allow longer working time and more uniform curing. Specialist concrete screeds formulated for low shrinkage use selected aggregates and sometimes polymeric additives; these command a cost premium but are worthwhile in large open-plan areas prone to visible cracking.

Why is concrete screed better than anhydrite in wet and exterior applications?

Anhydrite (calcium sulfate) screeds are superior for underfloor heating in dry residential settings because they shrink far less and allow larger bays without movement joints. However, anhydrite softens and loses strength if exposed to sustained moisture, making it unsuitable for bathrooms, kitchens, balconies, and exterior floors. Concrete, bound by Portland cement, resists water ingress and actually strengthens when properly cured in damp conditions. This makes concrete the mandatory choice for any application where water contact is possible. Anhydrite is also incompatible with Portland-cement-based tile adhesives in wet areas because calcium sulfate and Portland cement form expansive ettringite crystals in the presence of moisture, causing cracking and tile debonding. Concrete screeds avoid this risk. In exterior applications (covered balconies, transition thresholds, patios in climates with freeze-thaw cycling), concrete's durability and impermeability outweigh the drawback of higher shrinkage cracking. However, concrete must be air-entrained (3 to 5% entrained air) and sealed with a hydrophobic treatment if it will experience prolonged wet conditions or de-icing salt exposure. The general screed article covers the broader context of screed types and acoustic isolation; the anhydrite screed article provides technical depth on the calcium-sulfate alternative.

How does thermal mass in concrete screed affect comfort and energy performance?

Concrete's high density (approximately 2400 kg/m³) and specific heat capacity (around 0.9 kJ/kg.K) give it substantial thermal mass. When embedded with radiant heating pipes, the screed acts as a thermal buffer, absorbing heat from the system and releasing it gradually to the room, smoothing temperature swings and reducing the frequency of heating pump cycles. This improves thermal comfort by reducing "hunting" (rapid on-off cycling in response to small temperature changes) and can reduce energy consumption in well-controlled systems by 5 to 10%. The thermal mass effect is stronger with thicker screeds (65 mm vs 40 mm) and lower water-to-cement ratios (dense concrete conducts heat better than porous concrete). In passive-house standards and ultra-low-energy buildings, concrete screeds are often deliberately specified for their thermal storage contribution to the overall building thermal time constant.