This chapter corresponds to Section 8 in EN 1994-2, which has the following clauses:
. General Clause 8.1
. Actions Clause 8.2
. Design, analysis and detailing of the bridge slab Clause 8.3
. Interface between steel beam and concrete slab Clause 8.4
8.1. General
Clause 8.1(1) states the scope of Section 8: precast deck slabs of reinforced or prestressed concrete which are either:
. partial thickness, acting as permanent participating formwork to the in-situ concrete topping, or
. full thickness, where only a small quantity of concrete needs to be cast in situ to join the precast units together. Figure 8.1 illustrates a typical deck of this type.
Precast slabs within the scope of Section 8 should be fully composite with the steel beam – clause 8.1(2). Non-participating permanent formwork is not covered, for it is difficult both to prevent such formwork from being stressed by imposed loading, and to ensure its durability.
Clause 8.1(1)
Clause 8.1(2)
Precast or in situ edge beam
Precast or in situ edge beam
Transverse joint
Pocket for shear connectors
Precast deck slab unit spanning multiple girders Projecting
reinforcement not shown
Fig. 8.1. Typical full-thickness precast concrete deck
Clause 8.1(3)is a reminder that the designer should check the sensitivity of the detailing to tolerances and specify stricter values than those required by EN 1992 (through EN 13670) if necessary. Key issues to consider include:
. detailing of the precast slabs at pockets to ensure that each pocket is correctly located over the steel beam, that projecting transverse reinforcement will not clash with the shear connection, and that there is sufficient space for concreting (clause 8.4.3(2))
. detailing of stitch reinforcement between adjacent precast slabs to ensure that bars do not clash and to satisfy clause 8.3(1) on continuity
. tolerances on overall geometry of each precast unit so that, where required, abutting units are sufficiently parallel to each other to avoid the need for additional sealing from underneath. The tolerances for steelwork are also important, and are referred to in clause 8.4.1.
8.2. Actions
Clause 8.2(1) warns that the design of precast deck slabs should consider the actions arising from the proposed construction method as well as the actions given in EN 1991-1-6.30
8.3. Design, analysis and detailing of the bridge slab
Even where full-thickness slabs are used, some interaction with in-situ concrete occurs at joints, so clause 8.3(1) is relevant to both types of precast concrete slab. Its requirement for the deck to be designed as continuous in both directions applies to the finished structure.
It does not mean that the reinforcement in partial-thickness precast slabs or planks must be continuous. That would exclude the use of ‘Omnia’-type planks, shown in Fig. 8.2. Precast planks of this sort span simply-supported between adjacent steel beams and are joined with in-situ concrete over the tops of the beams. The main reinforcement in the planks is not continuous across these joints, but the reinforcement in the in-situ concrete is. In the other direction, the planks abut as shown, so that only a small part of the thickness of the slab is discontinuous in compression. Continuity of reinforcement is again achieved in the slab but not in the planks. The resulting slab (part precast, part in situ) is continuous in both directions.
EN 1992-1-1 clause 6.2.5 is relevant for the horizontal interface between the precast and in situ concrete. Examples of bridges of this type are given in Ref. 108.
To allow precast slab units to be laid continuously across the steel beams, shear connection usually needs to be concentrated in groups with appropriate positioning of pockets in the precast slab as illustrated in Fig. 8.1. Clause 8.3(2) therefore refers to clause 6.6.5.5(4) for the use of stud connectors in groups. Clause 8.3(3) makes reference to clause 6.6.1.2.
This allows some degree of averaging of the shear flow over a length, which facilitates standardisation of the details of the shear connection and the pockets.
Clause 8.1(3)
Clause 8.2(1)
Clause 8.3(1)
Clause 8.3(2) Clause 8.3(3)
Precast plank In-situ concrete
Flange of steel beam
Fig. 8.2. Typical partial-thickness precast concrete planks
8.4. Interface between steel beam and concrete slab
Clause 8.4.1(1) refers to bedding, such as the placing of the slabs on a layer of mortar.
Sealing of the interface between steel beam and precast beam is needed both to protect the steel flange from corrosion and to prevent leakage of grout when the pockets are concreted.
Where a precast unit is supported by more than two beams, bedding may also be needed to ensure that load is shared between the beams as intended.
‘Bedding’ in clause 8.4.1(1) appears to mean a gap-filling material capable of transferring vertical compression. Where it is intended not to use it, the clause requires special tolerances to be specified for the steelwork to minimise the effects of uneven contact between slab and steel flange.
This does not solve the problems of corrosion and grout leakage, for which a compressible sealing strip could be applied to the edges of the flange and around the pocket. There would then still be no direct protection of the top flange by in-situ concrete (other than at a pocket) and so clause 8.4.2(1) requires that a top flange without bedding be given the same corrosion protection as the rest of the beam, apart from the site-applied top coat.
If a non-loadbearing anti-corrosion bedding is provided, then the slab should be designed for the transfer of vertical loads only at the positions of the pockets. It would be prudent also to assume that clause 8.4.1(1) on special tolerances still applies.
Clause 8.4.3gives provisions for the shear connection and transverse reinforcement, sup-plementing Sections 6 and 7. Clause 8.4.3(2) emphasises the need for both suitable concrete mix design and appropriate clearance between shear connectors and precast concrete, allow-ing for tolerances, in order to enable in-situ concrete to be fully compacted. Clause 8.4.3(3) highlights the need to detail reinforcement appropriately adjacent to groups of connectors.
This is discussed with the comments on clause 6.6.5.5(4).
EN 1994-2 gives no specific guidance on the detailing of the transverse and longitudinal joints between precast deck units. Transverse joints between full-depth precast slabs at the intermediate supports of continuous bridges are particularly critical. Here, the slab reinforce-ment must transmit the tension caused by both global hogging moreinforce-ments and the bending moment from local loading. To allow for full laps in the reinforcement, a clear gap between units would need to be large and a problem arises as to how to form the soffit to the joint. One potential solution is to reduce the gap by using interlocking looped bars protruding from each end of adjoining slab units. Such a splicing detail is not covered in EN 1992-2, other than in the strut-and-tie rules. Experience has shown that even if satisfactory ultimate performance can be established by calculation, tests may be required to demonstrate acceptable performance at the serviceability limit state and under fatigue loading.
The publication Precast Concrete Decks for Composite Highway Bridges109gives further guidance on the detailing of longitudinal and transverse joints for a variety of bridge types.
Clause 8.4.1(1)
Clause 8.4.2(1)
Clause 8.4.3 Clause 8.4.3(2) Clause 8.4.3(3)