Beam and block floor construction

[edit] Introduction

Beam and block is also referred to in some countries as ‘rib and block’ or ‘lintel and block’. The technique is used to create in-situ, suspended concrete floors (ground or upper) in concrete or masonry buildings and has become popular in residential construction.

[edit] Composition

Beam and block floors incorporate clay or concrete blocks, either solid or hollow (sometimes referred to as ‘pots’), supported on a series of parallel, typically pre-cast, pre-stressed concrete beams or ribs. Once the blocks are placed to infill between the parallel concrete beams, a continuous working surface is created.

There are several ways to achieve this. One involves inverted T-beams that incorporate continuous ledges on their lower sections that give the blocks full support. Beams are typically 130-250mm-deep and can be made to span up to 6m. The beam profile will depend on the span, the shape of the block and the loading requirements.

With the beams in place and supported at either end, laying the infill blocks is a quick process: assuming the blocks are to hand, a worker can drop one into place every five to ten seconds. A typical current block specification is to have dimensions of 440 x 215 x 100mm thickness, 3.5N/mm2 or 7N/mm2 minimum compressive strength, and able to support a minimum transverse load of 3.5kN on a 420mm span.

The advantage of beam and block is that no shuttering (support) is required on the underside of the floor as the beams are supported at each end, either on internal load bearing walls or on perimeter walls. Once the filler blocks are installed, a continuous working surface is created upon which further work can take place safely. Often, a sand and cement grout is brushed over the top surface to fill any gaps and prevent insects and vermin from entering, as well as to assist with air tightness.

[edit] Choice of floor finish

This may be followed by either timber deck flooring (eg flooring grade OSB, ply, etc) or a poured structural concrete topping (or screed). Alternatively, insulation slabs may be laid over a beam/block floor followed by a damp-proof membrane (if it is a ground floor) or vapour barrier followed by the structural concrete topping. The topping may also incorporate steel mesh reinforcement for greater strength. Once cured, the topping will allow the floor to take its full working load.

Precast beams are very popular as they allow the floor to be assembled relatively quickly to create a usable working platform. The beam’s cross-section can vary and will depend on the manufacturer. It will also determine the resulting soffit which will either be closed, in which case a relatively continuous soffit will be produced, or it will be open and will require an applied ceiling: this might be plaster or enclosure by a suspended ceiling.

Where a very lightweight, insulated floor slab is required, the concrete hollow blocks can be replaced with rigid insulation slabs.

[edit] Advantages and disadvantages

The advantages of beam and block floors are:

• Easy to handle
• Can make use of unskilled labour
• Provide a durable concrete floor system
• Can be used for suspended ground and upper floors
• Economical due to the off-site manufacture of beams and blocks
• Hollow blocks give a lighter construction than in situ-poured concrete slabs and can be quicker as they do not rely on the curing time for a poured concrete slab
• Can be used for suspended floors just above the ground where, for example, the site slopes; the ground has poor bearing capacity; there is a high water-table or toxic chemicals are present in the ground.
• Can reduce site delays as the method is not weather dependent.
• Provides good fire resistance and noise reduction
• Shuttering is not normally required.
• Hollows can be used for running services such as underfloor heating.

Disadvantages include:

• Not suited to irregular plan shapes that require a large number of special-shaped units: the key to economy is standardisation
• Can be difficult to provide a monolithic structure, as providing an efficient, rigid junction between the floor and the supporting beams or walls may be problematic.

NB Offsite residential construction, Glossary of terms, published by Buildoffsite in 2018, defines beam and block floor as: ‘Extruded or wetcast prestressed beams between 150 mm and 225 mm deep, spaced to suit the applied loading and spans, together with blocks of various types. These may be purpose-made blocks with rebates to suit the shape of the beams (‘tray blocks’) or may be standard concrete masonry blocks which have been tested and certified for use in floors. Also commonly used are specially shaped extruded or expanded polystyrene blocks, which provide a high degree of insulation for ground floors.’

[edit] Related articles on Designing Buildings

• Blinding.
• Coal holes, pavement lights, kerbs and utilities and wood-block paving.
• Compartment floor.
• Floating floor.
• Floor definition.
• Floor insulation.
• Floor loading.
• Flooring defects.
• Floors of the great medieval churches.
• Insulation for ground floors.
• Plenum.
• Raised floor.
• Separating floor.
• Sistering floor joists.
• Slab.
• Sprung floor.
• Suspended ceiling.
• Suspended timber floor.
• Types of flooring.

[edit] External references

• ‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007).
• Forterra Building Products Ltd