Concrete floors offer a wide range of advantages in terms of both construction efficiency and inherent performance benefits explains Andrew Minson, head of structures at the Concrete Centre.

INCREASINGLY, the potential of post-tensioned floors (PT) and suspended concrete floors is being realised in the UK.

Their use offers several benefits, not least of which is the fact that PT floor slabs are generally thinner than an ordinary reinforced concrete slab. They can also be up to 300mm thinner than a floor in a steel frame. This can minimise the floor to ceiling height to the extent of allowing an extra floor within the same building height. PT slabs can economically span further than a reinforced slab. This in turn reduces the required number of columns and foundations and increases flexibility for space planning. Flexibility is further enhanced by a PT slab being able to accommodate irregular grids.

The clear flat soffits of PT slabs enable complete flexibility of service layout. The absence of trimming beams around service cores avoids conflicts between services and structure. There is also flexibility in positioning holes through the slabs because tendons are widely spaced and can be positioned around openings.

In addition to these flexibility benefits, PT allows rapid construction. Thin slabs equals less concrete which equals fewer lorries. There is less reinforcement which reduces fixing time,
and early stressing of the concrete allows the formwork to be struck quickly.

There are two methods of PT: bonded and unbonded. With bonded systems, the prestressing tendons run through small continuous flat ducts that regrouted up after the tendons. The bonded systems generally develop high ultimate strengths. However, the bonded ducts are larger than for unbonded. This reduces the effective section depth for design purposes but there is less reliance on the anchorages after grouting.

With unbonded systems, the tendons run through a small protective sheath that allows the tendons to move independently from the concrete. They can be manufactured offsite thereby reducing the on-site programme. The tendons are more flexible and can be deflected in plan to be placed easily around holes. There is also no need for another trade to carry out the grouting.

PT slabs are generally more economic at spans greater than 7.5m. Typically, three main forms of construction are used: flat slab, band beams and ribbed slab. The minimum depth of PT flat slab, generally spanning between 6m and 13m, is controlled by deflection criteria and punching shear. Band beam and slab can provide large clear spans with the beams spanning up to 20m and the slab spanning up to 10m while still permitting relatively simple formwork. Ribbed slabs can span up to 10m and are more structurally efficient than the other options but because the formwork is more complicated, ribbed slabs may be less cost-effective.

Contrary to popular opinion, PT slabs are not harder to alter or demolish than other structural forms. Indeed, when it comes to minor alterations, PT slabs are often easier to work with compared to other
structural forms. They derive their tensile strength from high-strength steel tendons. Depending on the specific circumstances the concrete can generally be cut out between the tendons without the need for strengthening, this includes potential opening 750mm square.

Currently, bonded tendons are used for the majority of new PT construction in the UK. With this system the steel strand is bonded via the grout and duct to the concrete, so that any cut through the tendon has a local effect only. At a bond length away the tensile strength is unaffected. Experience has shown that there is no explosive release of energy when the concrete is broken out because the strand is fully grouted and bonded plus the concrete is broken out in relatively small areas. For major refurbishment projects new tendons and anchorages can be installed to work in combination with the existing post-tensioning.

Many of the older PT slabs in the UK were constructed using unbonded tendons. Although the techniques for altering these are similar to bonded PT there is an element of more necessary planning and potential disruption. This is because unbonded construction relies on anchorages at either end to transmit forces between the slab and tendons. Cutting the tendon releases the tension throughout its length, therefore, before any concrete breaks out the slab must be propped throughout the length of the strand to be cut and then detensioning of the strand should be carried out.

PT slabs are an efficient form of construction and offer a wide range of benefits. The use of PT slabs is predicted to increase as designers and engineers become more familiar with the construction method.

Although they have been around a lot longer than post-tensioned floors, precast floors continue to offer a modern construction technique.

There are three generic types of precast concrete floors: hollowcore, beam and block, and lattice girder.

Hollowcore consists of concrete elements cored along their length. They are generally available from 1200mm to 1500mm wide with depths ranging from between 100mm to 400mm, depending on span and loading conditions. Narrower units of 600mm to 750mm are also available. Beam and block uses inverted T-sections of beams from between 150mm and 225mm deep. The infill between the beams can consist of dense, medium or lightweight aggregate blocks, or where improved thermal performance is required, of expanded polystyrene blocks. These provide insulated permanent formwork for a structural concrete top layer.

Although commonly used at ground level for housing, this approach is also suited to upper floor levels for a wide range of building types. Lattice girder is used as part of a composite construction and is generally available in widths of up to 2400mm. The lattice girder comprises a reinforced concrete slab with projecting steel lattice girders. These provide an effective mechanical bond between the precast element and the applied concrete topping. Lattice girder is commonly used for car parks and offices.

Recent changes in Building Regulations, particularly to Parts E and L have underlined the benefits of concrete floors, especially their inherent ability to reduce impact and airborne noise. Initial concerns over beam and block’s ability to cost-effectively meet the new standards have been overcome by the provision of new Robust Standard Details. Sponsored by the Housebuilders Federation as a possible alternative to pre-completion testing, robust details (RD) were approved by the Government in 2004.

There are two RDs for beam and block as laid out in the latest Robust Details Handbook. These are:

• E-FC-6: Modified beam and block floor with Regupol E48 resilient layer and floating screed
• E-FC-7: Modified beam and block floor with tongue and groove board floating floor treatments.

Both approaches have RD status and have been found to exceed the sound insulation performance standards required by Approved Document E by a significant 5dB. A recent develop for precast floors is the use of T-beams infilled with high-duty expanded polystyrene panels and topped with C28/35 reinforced concrete structural screed. Potential cold bridging problems are avoided due to careful design of the panels. The performance fully complies with the requirements of Part L of the Building Regulations.

As with in situ floors, precast floors also offer inherent fire protection and flexibility for varied room and service layouts. Both also score highly with encouraging sustainable construction as their inherent thermal mass which absorbs daytime heat and then releases it later at night. This can reduce the need for air conditioning in the summer and heating in the winter when they are used as part of passively sustainable design and construction.

Concrete floors, whether in situ or precast, offer an unrivalled range of benefits. These benefits have been underlined by recent alterations to regulatory standards and by the increased awareness of the need to reduce the operational energy used to cool and heat our buildings.