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Combining Structural and Thermal Performance in Floors

Traditional floor constructions consist of either a groundbearing or suspended concrete slab, or a suspended timber floor bearing on masonry. Tighter Part L requirements and the prevalence of off-site construction have swung the market towards precast concrete products – either beam and block or precast floor elements, writes Sean Downey, Team Manager – Engineering at the British Board of Agrément.

British Board of Agrément

Both have distinct advantages: beams and blocks are readily available as stock items from suppliers and can be installed with minimal plant requirements; precast elements can be rapidly installed and provide an immediate working platform for follow-on trades.

Until fairly recently, the division of duties was split clearly between structural and insulator components. The primary structural performance of the floor was provided by precast or cast in-situ concrete or suspended timber and the insulation added to meet the required floor U-value.

Increased thermal performance requirements and a drive for construction efficiency created an opportunity to redesign floor systems to meet more exacting standards and recruit the insulation into a dual-purpose role. No longer must it simply offer low thermal conductivity, the performance envelope has widened, and it must now also be sufficiently robust and stiff to contribute to the performance of the structure, resisting concentrated loads, uneven stress distributions, creep and short/long-term deflections – and at an attractive price point.

Beam and block

The most common form of structural insulated floor is ‘beam and poly block’, consisting of pre-stressed concrete beams, expanded or extruded polystyrene (EPS or XPS) infill system and a concrete topping. In many cases, the EPS or XPS will be load-bearing, transferring imposed loads from the concrete topping to the supporting beams. The mechanism through which the loads are transferred varies from system to system but, in most cases, the stiffness of the concrete beams and toppings relative to that of the insulation means that recruitment of the flexural and shear strength of insulation is not readily achieved and the insulation acts in direct compression in the region of the beam header. This gives rise to significant concentrated stresses to be resisted. As both EPS and XPS exhibit viscoelastic behaviour, deflections due to creep must be accounted for. The serviceability of the floor is further complicated by the need to preserve both the micro properties and thickness of the insulation needed to fulfil its thermal function. This is where the engineering trade-off is made between low-density, lower-cost and low-thermal conductivity materials and stiffer, dense grades of insulation, with a floor system often being optimised with a range of materials which puts the stiffer materials in the highly-stressed areas but minimises thermal bridging. Insulation materials applied in this manner require rigorous testing and analysis for the required serviceability and durability to be ascertained.

For beam and block floors, the most obvious example of this conflict between thermal and structural performance can be found at the floor edges. To achieve the lowest linear thermal transmittance (Ψ) value at the wall-floor junction, it is preferable to place the first-floor beam as far as possible from the wall. However, doing this creates a cantilevered section of insulation and concrete topping. The concrete typically has a limited depth of no more than 75mm and minimal reinforcement to resist the imposed stresses. Such a thin concrete element is also less tolerant to poor construction practice and needs careful pre- and post-pour attention to avoid problems with shrinkage, bleeding and thermal stresses, especially on exposed sites or during warm or windy weather. Nonetheless, when designed and executed correctly, this can provide a satisfactory floor solution with excellent thermal performance.

Fibre-reinforced concrete

Of significant concern is the replacement of traditional steel reinforcing mesh in concrete toppings with steel or polymer fibres, which can offer time and material savings. Design of fibre-reinforced concrete is not covered by current Eurocode standards, so achieving acceptance by warranty providers and building control professionals has been a difficult task for floor system providers. These fibre suppliers and warranty providers, in conjunction with the British Board of Agrément, have conducted, over several years, a multitude of full and small-scale load tests to develop, prove and refine their offerings. Certification is granted only to those systems considered to satisfactorily perform as a system and manufactured under the strictest quality controls to ensure constancy of performance.

Ensuring quality

The absence of definitive design standards for structural insulation materials in floors, and the complex interaction between the insulation and concrete components, increases the risk of failure due to product substitution. Such practice should be avoided at all costs. Independent certification of a floor system means little if there is not adequate ownership of the quality of the installation and the quality assurance procedures adopted on-site. Floors can be quickly and easily constructed, consist of materials with time-dependent performance characteristics and are often quickly concealed by finishes and the rest of the structure; a floor could be a hiding place for future problems. The requirement to adhere rigidly to the approved construction details for proprietary systems, with adequate inspection and sign-off by competent persons, cannot be stressed enough.

The potential for future problems has led to warranty providers being selective about which systems they will accept. This is best seen by the NHBC’s prohibition of microfibre toppings which are normally still certified by certification bodies and used by the construction industry at large. (In 2018, the NHBC prohibited the use of micro-polymer fibres in concrete toppings for all but a few systems on sites for which it offers warranties).

Hackitt review implications

One of the key challenges facing the construction industry, as highlighted in the Hackitt review, is the establishment of ownership over the design and procurement chain. While individual components will often be covered by their own British or European standards, this is not a guarantee that the assembled system will function satisfactorily as a whole and therefore does not cover aspects relating to design and installation.

Fit and safe

When innovative solutions are introduced, construction product procurement processes should be properly and clearly defined to ensure all flooring systems have the correct safety standards intact. The recognised technical expertise of certification bodies such as the BBA plays a vital role in gaining market acceptance of these innovative systems and provides reassurance to the end-user that the floor on which they are standing is, and will remain, fit for purpose for the life of the building.

British Board of Agrément

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