Prensoland Modern Precast Floors Appearance of the iHCS

Pre-stressed concrete hollow core slabs used for building floors are unidirectional members; they are laid one next to the other until the complete carpet area is covered. In the presence of lateral loads like wind, or due to seismic episodes, they behave as a single unit, in what is called diaphragm action.

The elements available on a floor to refrain the relative movement along their span are:

perimetral reinforcement of the floor
shear mobilized at the longitudinal joint between adjacent hollow core slabs
compression layer of concrete with screeding to work in composite action with the hollow slabs
ties with the supporting horizontal members.

Out of the four mentioned elements, all but the first one, are intrinsic to the production of precast slabs. In modern precast, instead of using add-on concrete and steel reinforcement, the hollow core slabs carry within themselves the features for an improved monolithic behavior at the joints and connections with other precast elements.

The concept is to embed structural performances on the precast concrete components. As a consequence of this driving approach, the structures can be leaner, and their masses reduced. The evolution to modern precast has resulted in:

Improving the contribution of each kilo of cement and each kilo of steel to the structural demand in a building.
Optimising the design and production of the hollow core slabs used on floors, so that they are not banalized pretending that what matters only is the fact that their weight is less than a solid cast in-situ slab.

Shear mobilized at the longitudinal joint between adjacent slabs on a floor:

Figure 1: Different cord of stress for HCS and iHCS

There are fundamental differences in the hollow core slabs behavior on precast floors depending on the characteristics of the side surfaces.

Factors influencing the shear between concretes cast at different times are:

cohesion of the concrete
eventual presence of forces normal to their interface surface
dowel action in case there is steel reinforcement protruding from one of the concretes and inserted in the other.

For example, the hollow core slabs produced with slip formers are extruders that are classified as smooth. The Eurocode 2 on concrete structures and the American code ACI318 assigns different shear capacity to the concrete interface surfaces, depending on whether they are smooth or intentionally roughened.

The highest shear capacity is obtained with concrete when the interface concrete surface adopts a shear-keyed geometry. This applies to any precast element on molds such as column pockets, walls, and even beams.

Nowadays, the hollow core slabs can be side indented as well, provided a plastic concrete mix design is used in their pre-stressed continuous casting process.

Figure 2: Hollow core slab with shear keyed efficient side surface

The benefits brought by side indenting on the pre-stressed concrete hollow core slabs are:

there is a transmission of additional shear at the longitudinal joint without increasing the production cost.
in seismic zones, the shear key constitutes a lateral collapse mechanism with large inelastic energy absorption capacity.

The shear keys possible failure modes are shown in figure 3:
In a) it is observed that the resulting surfaces after the collapse of the shear key mobilize as much shear as a just installed at site smooth side hollow core slab.

a), b) & c) Figure 3: Collapse forms of a shear key

There is an equivalence between the additional shear brought by the side indenting on the hollow core slabs and the thickness of the compression layer laid on top of them. In fact, when the sides are shear-keyed, the thickness of the compression can be reduced or this one just removed. Should the removal of the compression layer lead to insufficient bending moment or shear capacity, the thickness of the hollow core slabs can be increased, which would always be beneficial compared to laying a solid topping.

Compression layer: The friction brought by the topping on a hollow core slab follows the equation:

V = bw * d * fv

Where:

bw is the width of the interface

d is the thickness of the compression layer, and

fv is the shear stress given the concrete quality.

The top surfaces of the slabs are often roughened to ensure the bonding with the concrete topping, else the composite action is not verified; hence, the moment of inertia and the resisting moment are affected.

Ties with the horizontal supporting members:
Using a plastic mix design, the pre-stressed concrete hollow core slabs can be produced with the strands protruding a predetermined length from their ends. These tracks of strands function in dowel action refraining the tensile stress across the longitudinal joint between adjacent slabs, as represented in Fig. 3 c) where the concrete at the joint is riding on the shear key and therefore this mechanism is not developing its full capacity.

Figure 4: Hollow core slabs produced with exposed strands at both ends

Figure 5: Upper face of the cores opened and re-filled for the connection with negatives

The exposed wires feature enables strand patterns with higher steel cross section. This is a higher bending capacity for a given hollow core slab thickness. Heavily reinforced HCS may crack during the flush cut operation due to internal shear gradients.

When instead of placing the hollow core slabs simply supported on both ends, bi-articulated, the cores are opened for an installation in continuity or pinned at a beam, there is a negative moment at the supports. The value depends on the number of cores opened at the upper face of the slab and partially re-filled in-situ with concrete.

The moment and shear resistance at service and at ultimate limit states increase up to 30% with the same cross section and reinforcement strand pattern. The fire protection is also about 30% higher due to the bars placed at the openings. In seismic design of prefabricated structures with hollow core slabs, higher ‘q’ factor values are obtained due to the increased structural ductility and energy dissipation capacity.

Figure 6: Given a HCS thickness, the number of cores affects the integration in the building system, without an unnecessary variation in weight

In modern precast, the producers of pre-stressed concrete hollow core slabs can easily adapt to the business design with fractional width slabs and a seamless choice of thicknesses. The dedicated thickness molds are rapidly depreciated with the materials saving, and they contribute to avoiding the wastage of the finished products leftovers.

Conclusion
In the same way that the development of the concrete admixtures are helping towards a gradual substitution of the cement CEM I by a less clinker containing CEM II, the water reducers and super plasticizers contribute to the emerging use of plastic concrete in the production of pre-stressed concrete hollow core slabs. This concrete property endows the HCS with superior structural performance along a leaner building workflow. In seismic prone areas, the side indenting mechanism is recognized by most of the International Building Codes as suitable to an increased monolithic building execution.

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