Precast concrete research at the University of Nottingham

Concrete, Aug 2005 by Elliott, Kim, Ibrahim, Izni, Aziz, Farah Abdul, Robertshaw, Richard, Paull, Jonathan

The Structural Engineering Group in the School of Civil Engineering has a long tradition of carrying out large-scale testing in support of postgraduate research and commercial development. During the past 15 years the School has been the national leader in the field of precast and prestressed concrete structures, notably by the work carried out by Kim Elliott on precast concrete floor diaphragms(1) and semi-rigid connections(2), and by John Owen on the dynamic behaviour of concrete beams(3,4).

The precast work has twice been awarded the Institution of Structural Engineers' Henry Adams Award. Much of the research has been applied, due mainly to Kim Elliott's ongoing industrial involvement and his association with fib and British Precast. It has been dominated by:

* the structural interaction between precast concrete elements

* hybrid construction

* the relationship between the structural behaviour and manufacture.

At present, the 'precast' team includes six research students working on various aspects, well supported by a team of technicians and equipment. Recent refurbishment of the laboratory increased the size of the testing area to 15Om^sup 2^ and the capacity to 3000 tonnes, enabling specimens up to 12m long to be tested (see Figure 1 ). This paper provides an overview of four projects.

Structural toppings on precast concrete hollow-core floor slabs

Cast in-situ concrete toppings (see Figure 2) are added to hollow-core slabs to form the completed floor finish and to enhance the structural performance. The surface preparation is classified 'as-cast' and may contain surface laitance and other contaminants that could, in time, cause breakdown of the bond. Relative movement of the topping, and the injudicial placement of mesh reinforcement and construction joints, may all cause delamination, edge restraint, curvature and a loss of serviceability.

Instructions to site are currently limited to advisory notes(5) while design rules are mere extensions of isolated test results(6,7), which in no way reflect the real behaviour. The effects of shrinkage, creep and temperature variations are often neglected in design. The actual behaviour of composite floors has been examined to establish the relationship between performance and construction methods. Figure 3 shows a set of push-off tests on units supplied by Tarmac Topfloor, with varying surface conditions. In parallel, a 36m^sup 2^ full-scale composite slab has been constructed and a sustained load of 10kN/m^sup 2^ is being applied for 12 months (see Figure 4).

Catenary action in precast concrete structures

Precast concrete structures are generally designed as pinjointed braced frames. Moments are not transferred between elements unless continuity is added, usually by placing small quantities of reinforced concrete into the gaps and joints between units. However, designers may ignore continuity, as they prefer to use it only to satisfy the requirement for robustness. If an internal column is removed the excess forces in the system must be carried by catenary action. When the beam deflects, a tensile force is transmitted through the tie bar via projecting loops to the beam. The deflection will cause tension in the column and a new equilibrium state will develop. The result is a load redistribution that is difficult to predict accurately, often leading to an overly conservative design to account for the unpredictability.

This project aims to determine such catenary forces in tie bars, and to examine the role of the critical links that anchor the tie bars to the beam where there has been a loss of support. Precast concrete beams were made continuous by adding a reinforced concrete topping and were subjected to bending moments. Figure 5 shows the test arrangement; note that they were tested upside down for convenience so that the tie bar is actually at the bottom. The main conclusion is that catenary forces involve a complex relationship between beam geometry and tie details, but can nonetheless be determined. The maximum capacity in the tie bars is attained when the sagging deflection is approximately 0.2 � beam span.

Steel-fibre connections between steel beams and precast concrete hollow-core slabs

Hollow-core floor units are frequently made composite with structural steel beams by using shear studs in the narrow gap between the ends of the slabs and coupling steel bars concreted into slots formed in the slabs. The width of the gap and the position of the shear stud can vary considerably (see Figure 6). Previous work(8-10) established guidance rules for the effect of the gap and area of lateral tie bars, by providing reduction factors to the push-off capacity of the shear studs. The tie bars also enable horizontal diaphragm action, where the floor is required to act as a rigid plate between shear walls/cores.

Although the design is well established, there can be problems on site, mainly due to difficulties in placing the tie steel in the slots. The bars are often placed in the bottom among debris that can affect the bond achieved with the insitu concrete; ideally the tie steel should be placed at mid-depth. The optimum gap is rarely achieved, with the unit often bearing directly against the shear stud, as shown in Figure 6.