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.

Pre-stressed concrete hollow core slabs used for building floors

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:
  1. Improving the contribution of each kilo of cement and each kilo of steel to the structural demand in a building.
  2. 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:

Pre-stressed concrete hollow core slabs used for building floorsFigure 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.

Pre-stressed concrete hollow core slabs used for building floorsFigure 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.

Pre-stressed concrete hollow core slabs used for building floorsa), 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.

Pre-stressed concrete hollow core slabs used for building floorsFigure 4: Hollow core slabs produced with exposed strands at both ends

Pre-stressed concrete hollow core slabs used for building floorsFigure 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.

Pre-stressed concrete hollow core slabs used for building floorsFigure 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.

Pre-stressed concrete hollow core slabs used for building floors

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.
Elematic's Large Precast Concrete Plant for My Home GRAVA Project
Hyderabad is witnessing the construction of Asia's tallest towers with a planned built-up area totalling 3.7 million sqm. The two 180-meter towers, each comprising 43 storeys, will be accompanied by six more similar towers. With each tower

Read more ...

Benefits of Shear Keyed Surfaces in Precast Building
Depending on the finish of the concrete surface, different coefficients are assigned by international codes in the calculation of shear resistance. The American Concrete Institute Code ACI318, Nov.19, states the following: Chapter 16.2.1.1 – Transfer of forces by

Read more ...

Revolutionizing Precast Industry with Advanced Tracking of Precast Concrete Elements
The precast concrete industry is undergoing a transformative change with innovative software solutions that offer enhanced tracking of precast concrete elements. One of these pioneering software solutions is StruSoft’s Impact Precast, which aims

Read more ...

Emerging Trends & Growth of the Prefab Industry
Prefabricated technologies are becoming increasingly popular in the construction industry, particularly for large-scale construction. As prefabricated structures continue to evolve and improve, they are being used to construct warehouses, factories

Read more ...

Precast RCC Structures versus Pre-Engineered Steel Structures
A comparative study on Industrial Building Adopting Structural Frames as Precast RCC and Structural Steel Frames with Prestressed Precast Hollow Core Slabs as Flooring Slabs, presented by C. A. Prasad, Director, METEY Engineering Consultancy, Hyderabad

Read more ...

My Home Mega Project Shows Potential of Precast Construction Technology in India
Storey by storey, Asia’s tallest precast towers with a total planned built-up area of 3.7 million sqm are rising towards the sky in Hyderabad. These two 180-meter towers of 43-stories each will be followed by six more similar towers. With each tower

Read more ...

Mekuba: A Leader in Mould Releasing Agents for Concrete
With over 47 years of experience in the petrochemicals industry, and an expert in the formulation and production of mould-releasing agents, Mekuba Petro Products is growing rapidly in the construction chemicals segment with a daily production

Read more ...

Dextra Groutec Coupler - A Unique Solution for Reliable Joints in Precast
Dextra has added yet another customer-centric, innovative, and custom engineered product - the Groutec Coupler - an efficient product that has been qualified and tested successfully on several major infrastructure projects in the recent past. With the

Read more ...

India an Important Market for TOPWERK
Sachin Shetty, India Head – Operation & Sales, and Rajesh Jha, India Head - Sales & Market Development, TOPWERK INDIA, discuss the potential of precast concrete construction in India, localization of non-critical components to bring down cost, and the possibilities of digitalization

Read more ...

Vollert: Building A Stronger India with Precast
Vollert India has recently expanded its production capacity in Sikandrabad (UP) India. As a global leader in precast concrete components, systems, and machine solutions, Vollert’s investment in India indicates a growing interest among German companies

Read more ...

Precast India Connector - Helping Precasters Overcome Cost & Time Overruns
Precast India Connections, a new entity of Precast India Infrastructures, has innovated and patented a method of achieving a Dowel-Sleeve connection that functions even in the horizontal direction. Kapilesh Bhate, MD, Precast India Connections, shares

Read more ...

Vollert India Expands Production Capacity
The expansion of the Vollert plant in India marks an important milestone in the company’s commitment to provide high-quality products to customers. With the increased production capacity, the company aims to expand its market share and strengthen

Read more ...

Brimax AAC: An Indian Lighthouse Project from HESS AAC SYSTEMS, Netherlands
Brimax AAC Products LLP contracted Hess AAC Systems to supply a new AAC plant in Vadodara, India, with a capacity of 680 cbm/day (expandable up to 900 cbm/day with reinforcement for panel production). This order strengthens Hess´ position as

Read more ...

Vollert India Expands Production Capacity to Cater to Infra Development Sector
Vollert India, based in Sikandrabad, near Greater Noida (UP) since 2017, is strengthening its commitment to India; it is now expanding its production capacity of precast machinery and components to meet the greater demand from India’s fast

Read more ...

Elematic Precast Technology Solutions for Building, Industrial and Infra segments
Elematic offers a comprehensive range of precast technology solutions for the Building, Industrial and Infra segments. Elematic precast technology enables clients to automate the production of walls, slabs, columns, beams, and stairs, providing an efficient

Read more ...

Advanced Concrete Curing Systems from Kraft Curing
Kraft Curing Systems GmbH is offering advanced concrete curing systems that optimize the hardening process of concrete. Kraft Curing’s Advanced Concrete Curing System provides numerous solutions, from vapor-based (steam) systems mainly for the precast

Read more ...

Why Automating Production is the Solution
Automating the production processes will lead to reduced wastage, fewer errors, more precision, greater safety, and higher quality of the end-products, and in a shorter period of time. First, we need to understand where we stand today as regards industrialisation

Read more ...

Elematic India confident of the future of Precast Business
Elematic India sees growing demand for its precast plants in India due to the growth in precast construction across segments. What gives the company an edge is its ability to support the customer through the entire value chain. Says Sridhar Rao, Sales

Read more ...

F.B.I. Tasbud Partners With Progress Group
Progress Group is helping companies like Tasbud stay ahead in a changing market with automation and software integration. Automated machinery to produce precast elements as well as the reinforcement and the integration of the fitting software are

Read more ...