It is less than six months since construction finished on the seven storey insitu concrete frame at the BRE's large building test facility at
Cardington, but data is already pouring in. Although the performance tests are in their infancy, researchers are studying data on the project collected over the last year. That is because the purpose of the European Concrete Building Project is not just to study the behaviour of the building itself: at the heart of the project is a desire to improve the entire process of designing and
building in concrete.
The insitu frame building gives the industry a chance to develop best practice in design and construction, to improve relationships in the supply chain, and to employ innovative techniques throughout the process, as well as validating test and research data.
Dr Pal Chana of Imperial College is a member of the project implementation group. He says the key to the whole project is 'to optimise construction and add value to construction'. It also offers a valuable opportunity to contribute to the new concrete design Eurocode EC2, especially in the area of flat slab design.
Validation of laboratory tests and codes of practice is also a primary purpose of the project. As Chana says: 'We do a lot of tests in the lab, but how do they relate to a complete building? When we do get the chance to see how a building behaves - for example when one is demolished - we are always surprised by its strength, particularly insitu buildings. A lot of advances have been made in finite element analysis, but we have very little chance to calibrate that against real data.'
The project implementation group developed the initial design brief. The frame is nominally a standard seven storey insitu concrete framed office building which could be built in any town in the UK (Bedford, in this case). It measures three bays by four in plan, with 7.5m square bays, has a floor to ceiling height of 13.5m and two service cores - one at each end. But within this specification there are some innovative features. They are not just there for the sake of it, according to Chana, but to 'help the contractor, speed up construction and improve the process'.
These include thin plate floors designed to the limit of the most advanced methods allowed in EC2; uniformly sized columns, with those on the lower three floors formed with high strength (C85) concrete; a 'hybrid flat slab' roof made with structural concrete permanent formwork slabs 'glued' together with high strength 'Densit' concrete (see page 15); glued segmental precast stairs; pre-manufactured shearheads; reinforcement mats; and steel cross bracing.
The client group also wanted to be able to compare different design and construction methods, so the reinforcement detail is different in almost every floor. Some of the concrete was pumped and some placed using skips, one floor slab was designed without any concern for deflection, polypropylene fibres went into the high strength columns, and the floors were designed to a live load of 2.5kN/m2 rather than the 5kN/m2 usually specified for modern offices.
Process research began at the very start of the project. 'Once a building is designed, there's a limit to what the contractor can do to improve things,' says Chana, explaining why the contractor and suppliers were involved at the conceptual and design stages. Researchers, designers, the main contractor, formwork contractor and rebar and concrete suppliers all 'bought in' to the project at this early stage.
From then on, process research focused on using high performance formwork to speed up construction;
rationalising reinforcement by using prefabricated mats;
improving the rebar supply chain;
assessing the potential for early striking and loading of the floor slabs;
improving the quality of management to allow early acceptance of the concrete.
The results will be produced in the form of best practice guides, but lessons are already being learned, according to Chana. 'A key thing coming out of the process side is early striking,' he says. 'We have demonstrated that it is possible to strike the slabs at 24 hours. It was something we wanted to do on at least one slab and we've proved it is possible.'
Researchers used non-destructive testing methods to prove the concrete had reached sufficient strength to strike the formwork, as well as testing some of the 1,000 cubes taken during construction. Early striking and loading allows construction of the next floor to begin more quickly.
Chana says the project has also demonstrated that it is possible to specify high strength (C85) concrete for the column construction. Because this was successful, the columns in the building are all the same size, so the contractor could use the same formwork throughout - a major efficiency.
The entire construction process was recorded on video, and the HSE has used it for a safety research project. Chana admits this means the building went up in a very controlled environment, but says: 'It's a demonstration of best practice - what can be achieved given care and planning. It would be nice to take these lessons out into the real world.'
Now the building is complete, Chana says: 'The first objective for performance testing is to validate some of the research that has been done before. A lot of tests have been carried out in isolation, but they haven't been combined in one building before.'
Serviceability rather than ultimate load is the key factor in thin slab design, he says, and believes this will be proved by the ECBP research. Deflection could be a critical design criterion, and sandbags have already been placed on some floors to replicate live loads, enabling the researchers to monitor deflection over the next two years. The range of different reinforcement details will help with the deflection project.
The dynamic characteristics of the thin floors are also of great interest, as they are significantly more 'springy' than traditional deeper slabs. BRE's novel way of testing this was to bring in a group of students to dance to music on the frame's floors.
Lateral stiffness testing requires a different set of equipment: a mechanism for shaking the entire structure. It was used successfully on the adjacent steel structure during stiffness testing, and will be used on the concrete frame to assess the building's response to lateral movement, as well as to see how well the steel bracing works.
Insitu concrete buildings are traditionally designed for ultimate load conditions, but Chana believes this building will 'behave in a different way' to that which the codes predict. 'Serviceability will be the key factor,' he says. 'Ultimate load will just be a check.'
The building will certainly be loaded to its ultimate capacity at certain locations once the serviceability testing is complete, as well as undergoing a range of other loading tests. These include applying loads to generate cracking along the yield line, and loading around the column/slab connection to see how it fails in punching shear. This latter failure is of great interest following the collapse of a car park structure under punching shear.
Chana says the test will 'tell us what the strength is and also the ductility', and adds: 'I think we could learn a lot from this.'
The punching shear test will be carried out by physically pulling down on the slab around a column. Holes have been pre-drilled into some slabs to allow Macalloy bars to be threaded through. These will run down to ground level, where they will be loaded to create the pulling force. It is a technique currently being used on the steel frame structure.
The building will be tested for differential settlement or subsidence by jacking up or dropping some of the columns by as much as 100mm.
A few elements will eventually be tested to destruction to find out how the rest of the building behaves. This includes not only load testing, but also fire and gas explosions.
Chana says: 'The BRE will model the explosions, but they can't look at gas explosions in the general sense of taking a column out. Cardington isn't the place to do that. They will do localised explosions to see what happens with the partitions.'
Tests are likely to include small scale fires within office-sized compartments, as well as a full scale fire on half of the ground floor, with monitoring to test its effect on the building as a whole, the distribution of load under fire conditions, and the movement of smoke.
'It's a question of getting a fundamental understanding of how a real building would behave,' Chana says. The tests will also demonstrate how well the high strength concrete columns bear up under fire. The high density of the mix can result in poor performance, but some of the columns in this building contain polypropylene fibres which should melt during a fire and create voids to allow pressure to be released.
Future tests on the insitu building are likely to include projects involving cladding, repair and strengthening techniques and recycling and demolition, depending on the needs of the industry and research proposals. Plans are also being developed for the next structure in the concrete series.
'We've decided it would be great to take the lessons from this building on board, and not get stuck to any particular type of frame for the next one,' says Chana. 'Let's have a real brainstorm and come up with an innovative building that's suitable for the next millennium. Let's not be constrained by codes. Let's give the client a building that's economical, and takes account of issues like the environment and whole life costing. With this building we've taken standard technology to the limit. Now we can go a little bit further.'
At the same time, he thinks it is time to put some of the research into practice in a competitive building environment. The insitu building is 'a step change not a revolution', according to Chana, who believes it 'probably meets what Latham asked for in terms of a 30% increase in client value'.
But he says: 'The lessons of this building have to be applied to a real building to demonstrate the savings.'