Cost and programme
Concrete frames can be constructed quickly and safely, and are competitive in most situations [25,26]. There are many aspects of cost to consider:
Driven by market forces, concrete frames are usually competitive. Studies [25,26] confirm that using concrete frames leads to marginally more economic buildings than those constructed with competing materials. Concrete frames also provide the inherent benefits of fire resistance, excellent acoustic and vibration performance, thermal mass and robustness – all at no extra cost. Specialist concrete frame contractors have expertise that can reduce costs and maximise value when their input is harnessed early in the design process. Whenever possible, consider early (specialist) contractor involvement (ECI).
As concrete is a heavy material, foundations to concrete framed buildings tend to be marginally more expensive than for those constructed of steel. However, this is more than offset by savings in other areas such as cladding, as illustrated below.
Cladding can represent up to 25% of the construction cost, so the shallower floor and services zone of concrete solutions leads to lower floor-to-floor height and hence lower cladding costs.
Sealing and fire stopping at partition heads is simplest when using flat soffits, saving up to 10% of the partitions package compared with that for options with downstand beams. Even when rectangular concrete downstand beams are used, there are still savings over profiled downstand steel beams.
In service cores, structural concrete walls often take the place of what would otherwise be non loadbearing stud partitions. However, the costs then show in the structural frame budget and savings in the partitions budget.
Services distribution below a profiled slab costs more and takes longer than below the flat soffit of a concrete flat slab: a premium of 2% on M&E costs has been reported .
All other things being equal, in-situ concrete construction’s ‘pay as you pour’ principle saves on finance costs – up to 0.3% of overall construction cost compared with steel-framed buildings .
Fabric energy storage means that concrete buildings that use their inherent thermal mass will have no or minimal air-handling plant. This reduces plant operating costs and maintenance requirements.
In overall terms, in-situ concrete-framed buildings are as fast to construct as steel-framed buildings: indeed, in some situations, they can be faster [25,26]. Sound planning will ensure that follow-on trades do not lag behind the structure. The following issues have an influence on programme times:
As may be deduced from Figure 9.2, it is common to install 500 m2 per crane per week, on reasonably large concrete flat slab projects. Even faster on-site programmes can be achieved by:
- Using greater resources.
- Post-tensioning of in-situ elements.
- Using precast elements or combinations of precast and in-situ (known as hybrid concrete construction).
- Rationalising reinforcement.
- Pre-fabricating reinforcement.
- Using proprietary reinforcement such as shear stud rails.
The prerequisite for fast construction in any material is buildability. This includes having a design discipline that provides simplification, standardisation, repetition and integration of design details.
Generally, in-situ projects require very short lead-in times. The use of precast elements requires longer lead-in periods to accommodate design development, coordination and, where necessary, precasting. Contractor-led designs will generally lead to shorter overall construction times but the contractor will need additional lead-in time to mobilise, consider options, develop designs and co-ordinate with designers and subcontractors. Figure 9.2 shows these effects and also shows the possible effect of using a specialist post-tensioning (P/T) contractor for specialist design.
|Typical speed of construction and lead-in times
Liaison with specialist contractors
The use of enlightened specifications and, where appropriate, a willingness to adopt specialist contractors’ methods, can have a significant effect on concrete construction programmes. Many contractors appreciate the opportunity to discuss buildability and influence designs for easier construction.
Improved speed of construction can be achieved by increasing resources. Whilst this option comes at a price, managing speed in this way is an attribute of concrete construction valued by many contractors.
Flat soffits allow maximum off-site fabrication of services, higher quality of work and quicker installation. Openings in concrete slabs for service risers can be simply accommodated during design. Small openings can usually be accommodated during construction.
The overall accuracy of concrete framed buildings is not markedly different from other forms of construction. BS 5606 gives 95% confidence limits as follows:
- Variation in plane for beams: concrete +/- 22 mm, steel +/- 20 mm
- Position in plan: concrete +/-12 mm, steel +/- 10 mm.
The use of in-situ concrete allows alteration at a very late stage. However, this attribute should not be abused or productivity will suffer.
Striking times and propping
Allowances for striking times and propping are a part of traditional in-situ concrete construction. When critical to programme, specialist contractors, with the co-operation of designers, can mitigate their effects.
New methods, such as climbing panel protection systems that enclose two or three floors of work areas, provide safe and secure working environments at height. Panel formwork systems, which can be assembled from below, dramatically reduce the risk of falls. Concrete structures provide a safe working platform and semi-enclosed conditions suitable for follow-on trades.
Modern methods of concrete construction can overcome the effects of wind, rain, snow, and hot or cold weather. Such events just need some planning and preparation.
25 GOODCHILD, C H. Cost Model Study, RCC/British Cement Association, 1993.
26 THE CONCRETE CENTRE. Cost Model Study – Commercial Buildings, CCIP 010, 2007; School Buildings, CCIP–011, 2008; Hospital Buildings, CCIP–012, 2008.
32 THE CONCRETE CENTRE. Concrete Framed Buildings - A guide to design and construction, 2016.