On solid ground

Paul Gregory gets beneath the surface of flooring for housing with a guide to ground-supported slabs

Ground-supported floors rest on, and are supported by, the ground. Floors in houses and multiple dwellings are characterised by short spans and light loads, typically from occupants, furniture, partition walls and appliances. Residential floor guidance will also apply to other types of building, such as offices, where the floor is not subjected to large concentrated loads.

As well as the concrete structural slab, housing floors includes other layers such as a hardcore sub-base, damp-proof membrane, insulation layer, heating pipe system, screed and floor finish. In some parts of the UK, a gas membrane will also have to be incorporated to protect against the effects of radon gas or methane from the ground. Residential floors are usually finished with carpet, vinyl, ceramic tiles or wood and these are often laid on top of a levelling screed, though they could be laid directly on a suitably prepared concrete slab. Alternatively, the concrete slab could be finished to provide a decorative polished floor.

Concrete slab design

The design of most ground-supported floors in housing, offices and shops is based on experience rather than on calculations. If the ground or sub-base provides uniform support then reinforcement is not required to support the loads and the only calculations needed are those to determine the tension stresses caused by possible restraint to the shrinkage of the concrete and to control cracking.

Concrete shrinks after it has been cast. If the slab is prevented from shrinking as it dries, for example by friction underneath, cracks can form. Casting the slab on a plastic slip membrane or damp-proof membrane laid over a flat sub-base will reduce the risk of cracking and increase the distance required between movement joints. Isolation joints should be provided around the perimeter of a slab so that the slab is not restrained.

Reinforcement is often provided to control cracking due to shrinkage. The steel will not yield when the first crack forms, and rather than one large crack there will be a number of smaller ones, typically less than 0.3mm wide.

If a ground slab rests on the inner leaf of the external walls or on any internal walls, it should be designed as a suspended, rather than ground-supported, slab. A slab supported on poor ground, spanning over soft spots, should also be designed as a suspended slab.

Slab thickness

The Building Regulations and BS 8204-1 state that the concrete slab should be at least 100mm thick. Guidance in BS 8204-2 suggests that the slab might need to be 150mm thick to minimise the risk of curling. A 150mm slab might also be needed to satisfy sound insulation requirements between separating walls and to make the ground floor more resilient to the effects of flooding (see box, below).

Flood resilient flooring

DEFRA’s Property Flood Resilience Action Plan, launched at the end of 2015, seeks to make flood resilience standard practice within the next five years. It sets out measures to help prevent water ingress into a building or aid rapid recovery, for example allowing householders to simply wash out and disinfect after flooding, rather
than requiring a wholesale replacement of the building fabric.

In the latest guidance contained in BS 85500:2015, a reinforced ground-supported concrete slab, at least 150mm thick, is the preferred option for properties at risk of flooding, to prevent water and mud entering a subfloor void through ventilators and service entries.

Concrete floors can withstand pressure and ground-water flooding and so reduce the risk of water ingress into the building. If they do become wet, there is no risk of deformation or failure arising from the drying process.

The floor should be reinforced and designed to resist the anticipated upward forces of predicted flood events, which may require a greater thickness than 150mm depending upon the specific circumstances and location of insulation. Rigid closed cell insulation is recommended in every event, to retain its integrity and resist moisture absorption.

A minimum 1,200-gauge damp proof membrane (DPM) or another heavy-duty DPM is recommended, ideally bonded to the slab, and laid to the manufacturer’s recommendation. Joints should be overlapped and sealed or welded to provide a continuous water barrier, lapped and sealed with the horizontal damp proof course (DPC) in
the walls.

The pros and cons of the location of the insulation and DPM either above or below the slab are described in BS 85500.

Suitable floor finishes for buildings at risk of flooding include ceramic or concrete-based floor tiles, which are robust, water-resistant and easily cleaned after a flood. They should be fully bedded on a cement-based adhesive or bedding compound. Alternatively, an exposed concrete floor finish can be appropriate. Water absorption, and therefore drying times, can be reduced through the design of the concrete mix or with waterproofing sealants.

Joints and reinforcement

A joint is intended to accommodate the natural movements of a slab and hence limit any shrinkage stresses that may give rise to random cracking. To minimise the risk of uncontrolled cracking in unreinforced slabs, the joint spacing should not exceed 6m in either direction for a 150mm-thick slab and 4m for a 100mm-thick slab. The introduction of steel fabric reinforcement, commonly referred to as “mesh”, will allow greater distances between movement joints and should limit random cracking to a series of fine hair cracks.

The floor shape will affect the risk of cracking, and square shapes are best. For unreinforced slabs, the ratio of the length of the sides should preferably not be larger than 1:5. Where a bonded wearing screed is laid on the slab, there should be joints in the wearing screed to reduce the risk of debonding and curling. The distance between joints in the screed should be limited to 1.5 times its width and the bay size should not exceed 20m2 for a wearing screed thickness up to 30mm. The bay size should be reduced to 15m2 for thicker bonded screeds. The following applies to concrete floors of all kinds.

Concrete quality

Concrete in the UK is specified using BS 8500-1:2015. The process is explained in the document How to Design Concrete Structures using Eurocode 2, published by MPA The Concrete Centre.

BS 8204 Screeds, bases and in situ floorings, specifies minimum strength class and cement content requirements. There are also general recommendations for a visual concrete mix.

BS 8500 provides guidance to ensure that the concrete will be durable for its intended life, either 50 or 100 years. There are various exposure conditions that cause concrete to deteriorate and these must be determined for each project. For internal concrete on non-aggressive ground, the concrete will only need to satisfy the XC1 class for corrosion induced by carbonation and the minimum requirements for concrete in the ground.

If the slab is in contact with aggressive ground, the aggressive chemical environment for concrete (ACEC) exposure class should be identified by a site investigation report.

Surface finish

Finish should be one of the first decisions to maximise performance and economy. It should be appropriate to the service conditions, with particular reference to loading, impact, abrasion, chemical resistance, hygiene, dust prevention, slipperiness and decorative treatment.

Some applied finishes have special requirements for the concrete surface to which they are to be bonded. For example, a penetrating resin sealer may need a floated, not heavily trowelled concrete surface; with a separately bonded wearing screed, no floating or trowelling will be necessary, but the surface will need to be treated by mechanical equipment to provide the necessary bond.

Surface tolerances

BS 8204-1 and BS 8204-2 provide a classification for surface regularity for direct finished base slabs and wearing surfaces. The highest standard, SR1, should be used where thin flooring is to be applied and where the minimum irregularity is required of the finished floor. Conversely, the lowest standard may be selected where a thicker type of wearing surface is to be applied and where the regularity of the finished floor is not a significant factor.

The National Structural Concrete Specification (NSCS) defines four types of unformed finishes: basic, ordinary, plain and special.

Basic is normally applicable for areas to receive a levelling or wearing screed. A closed finish is produced by levelling, and use of a skip float or similar process. Float marks and ridges will occur. This is the default finish.

Ordinary is for areas that will receive flooring materials, types of false floor or other raised finish. It may not be suitable for thin vinyl flooring without grinding. If thin vinyl with minimal additional work is required, a plain finish should be specified. An ordinary finish is a level uniform surface typically produced by hand or power floating, but not skip float. The finish is to be free from ridges but fine float marks are expected. This may be acceptable for applying finishes such as tiles or carpet.

Plain is for use in areas without any other finish other than paint or surface coating, and provides a dense, smooth surface typically produced by hand or power trowelling. This may be suitable for directly trafficked surfaces.

Special is for industrial floors, areas of special trafficking or areas requiring a special texture (eg tamped or brushed), and can be produced by further working other finishes.

The NSCS follows the same test method as in BS 8204 but specifies different tolerances. If smaller tolerances are required, they must be stated in the NSCS Project Specification.

Surface finishing techniques

The prerequisite for good performance from a concrete floor surface is full compaction. This is usually fulfilled by means of a beam compactor fitted with a vibration unit. The action of surface vibration tends to draw water to the surface, and wet finishing may produce a weak layer at the top. If this layer is not removed, the hardened surface will have low durability and wear resistance, and a high risk of dusting. The following techniques may be used with most construction methods.

Hand trowelling Traditionally floors are finished by successive trowelling with steel trowels, with a delay of an hour or more between each trowelling to allow further moisture to evaporate. Any surplus mortar can be scraped from the surface as trowelling proceeds, and by exerting considerable pressure on the trowels to close the pores in the surface left by the evaporated moisture, a skilled person can produce an excellent floor.

Power floating A power float may be used as a preliminary to power trowelling to regulate and close the surface. Floating must not be started until surface moisture has evaporated and the concrete is stiff enough to take the weight of the machine, or a weak surface and poor levels will result.

Power trowelling No mortar is removed during power trowelling and timing is therefore critical if a hard-wearing surface is to be achieved. Surface moisture must evaporate, and the concrete stiffen further before the first trowelling operation begins. This may be sufficient for the direct application of thin sheet coverings, but for surfaces subject to heavy wear, two or more additional trowellings will be needed with the blades tilted at greater angles, after further moisture has evaporated.

Power grinding A power grinder may be used to produce either a hard-wearing surface, or one that is suitable for the direct application of thin sheet flooring or tiles. Grinding is a finishing technique and should not be used to correct gross irregularities in the surface. These should be removed by initial finishing while the concrete is still workable.

Hand floating Where a high-grade surface is not required and traffic is light, initial finishing with a skip float or hand wood float may be adequate, especially if thick flooring such as carpeting is to be laid.

Building Regulations 2010, Approved Document C: Site preparation and resistance to contaminants and moisture

BS 85500:2015: Flood resistant and resilient construction – guide to improving the flood performance of buildings

The Concrete Society Concrete Advice No. 45 Indoor decorative concrete floors, April 2012

BS 8204-1:2003+A1:2009: Screeds, bases and in-situ floorings – Part 1: Concrete bases and cementitious levelling screeds to receive floorings – Code of practice

BS 8204-2:2003+A1:2009: Screeds, bases and in-situ floorings – Part 3: Concrete wearing surfaces – Code of practice

BS 8204-6:2008+A1:2010 Screeds, bases and in-situ floorings – Part 6: Synthetic resin floorings – Code of practice

BS 8500-1:2015: Concrete – Complementary British Standard to BS EN 206 Part 1: Method of specifying and guidance for the specifier

BS 4483:2005: Steel fabric for the reinforcement of concrete - Specification

The Concrete Society Concrete Advice No. 22 Moisture in Concrete Floors, April 2005

National Structural Concrete Specification (NSCS), Fourth edition, The Concrete Centre, 2010

Photo: BRE and Delta Membranes​