Hybrid Concrete Construction

South Africa has been slow to realise the benefits of hybrid concrete construction (HCC), despite the widely appreciated construction benefits. One of the barriers to the use of HCC has been the lack of comprehensive guidance. Hybrid concrete construction can be described as being 'best of both worlds'. It marries together the advantages of precast and insitu concrete construction with often significant benefits. For example, the adoption of a hybrid concrete frame instead of a composite steel frame on a shell-and core office project in central London resulted in construction savings of 29 percent and a 13 percent increase in net lettable floor area.

The time is right for hybrid construction. Reports such as Accelerating Change from the Strategic Forum for Construction and the Egan Rethinking Construction report have focused attention on the need for the UK construction industry to move on from its inherent conservatism and modernise and increase efficiency. The business environment of the UK construction industry is changing. If the industry is to answer its critics and modernise, then it has to examine the potential of different construction techniques and contractual arrangements.

In terms of costs, insitu reinforced concrete is commonly viewed as being the most economic framing option while precast concrete promotes speed and factory quality. Combining the two as a hybrid frame results in even greater construction speed, quality and overall economy. Traditional formwork typically accounts for up to 40 percent of an insitu frame costs. These costs can be significantly reduced by increasing the use of precast concrete which has no on-site formwork requirement. This reduces the duration of operations critical to the overall construction programme. Precasting is not constrained by site progress or conditions and can continue independently of on-site operations. Some HCC techniques can remove the need for follow-on trades such as ceilings and finishes. This allows for an even faster programme. HCC also encourages speed of construction by promoting increased buildability, which should be a fundamental design objective.

Concrete produces robust, and adaptable buildings that are inherently fire resistant, vibration free and quiet. Exposure of the hybrid concrete frame can be used to exploit concrete's inherent thermal properties in naturally ventilated, low-energy buildings. The finish and shape of the exposed units can also assist with even distribution of lighting levels and the reduction of noise levels. Long spans can be easily achieved using large units or by pre-stressing or post-tensioning.

HCC is about providing best value. It is not necessarily about first cost, although this alone can result in hybrid concrete construction being chosen. Gains from improved buildability on site soon overtake any material cost differences. Inherent benefits, such as occupier comfort and increased efficiency, lead to potentially massive cost benefits in comparison with other structural approaches. For the full potential of economy, safety, speed, buildability and performance to be realised then HCC should be considered at the beginning of the design process. The new best practice guidance shows how that full potential can be achieved.

BEAM AND BLOCK
A common example of HCC would be the “ Beam and Block”  method of construction. Beam and block slabs are made up of precast concrete rectangular shaped beams. The most common beam spacings being 560, 600 and 650mm. A non structural topping should have a minimum strength of 25 MPa at 28 days and a minimum thickness of 40 , mm, or 1/10 multiplied by the clear distance between the beams. Welded mesh reinforcement is placed in this topping to control possible shrinkage cracks. The filler blocks are available in different heights ranging from 60 mm to 350 mm which produces an overall depth of slab from 110 mm to 400 mm, or more if double blocks are used. This type of slab requires temporary supports at approximately 1.5 m centres, but certain systems can also be designed to eliminate the need for props. The advantages of this type of slab are as follows:

Design guidelines

Beam and block system slabs are designed as a series of ‘T’ sections with the in situ cast concrete providingthe compression flange and the recast beam the tension reinforcement. The beams and the composite slab are designed for specific spans and loads and are reinforced accordingly, (complying with the relevant Code of Practice.) Two or more beams may be placed together to accommodate concentrated line loads parallel to the span. If necessary, blocks may be omitted over the support to increase the shear capacity.

PANEL AND TOPPING

The panel consists of a precast reinforced or prestressed slab utilized essentially as a permanent shutter with tensile reinforcement included, acting compositely with an in situ structural topping. The panels may be produced in relatively narrow widths to facilitate manual erection or in a wide configuration (up to 2,4 metres) offering greater speed but requiring mechanical handling. In some situations, reinforcement in the form of a lattice girder may be incorporated to provide stiffness during handling. Reinforcement is placed in the topping over supports and over the joints between precast units.
By varying the tensile reinforcement in the panel and/or the strength and thickness of the topping concrete, different loadings and spans can be accommodated. Projecting steel is not normally required at the precast/in situ concrete interface in solid composite construction. Design can be simply supported or fully continuous.
Depending on the the system, selected treatment to the soffit may or may not be required.

CASE STUDY

VOLKSWAGEN PAINTSHOP- UITENHAGE

OWNER: VW South Africa

ENGINEER: ARQ Consulting Engineers

PRINCIPAL AGENT: Sigma Consulting

PROJECT MANAGER: Peter Meidlinger,
                              Grinaker-LTA Civil Engineering

CONTRACTOR: Grinaker-LTA Civil Engineering and 
                       Grinaker-LTA Building Eastern Cape

The reasons for using Hybrid construction included speed, and access to below the slab - no time for curing and stripping conventional formwork.