Hollow core slabs

Prestressed hollow core slabs are the most widely used type of precast flooring. This success is due to the highly efficient design and production methods, choice of unit depth and capacity, smooth underside and structural efficiency.

The hollow core slabs are manufactured on long-line beds according to the customer’s placed layout of the slabs with the indicated holes. The slabs are cut to length using a circular saw. A square end is standard but skew or cranked ends, which are necessary in a non-rectangular framing plan, may be specified. Longitudinal cutting is possible for match plates.

All the products are approved of the Certification Centre of Construction products in Lithuania.

Technical characteristics

The hollow core slabs are produced according to the indicated dimensions at the customer request. The main parameters of the hollow core slabs are placed below in the table.

Profile	Width, mm	Height, mm	Weight, kg/m²	Weight (joints filled), kg/m²	Max length of span, m
HCS 180	1200	180	287	302	9.38
HCS 200	1200	200	316	332	9.50
HCS 250	1200	250	380	401	12.20
HCS 300	1200	300	411	436	14.50
HCS 400	1200	400	496	532	16.75

Note: max length of span is placed taking into consideration the minimum loading of roof is 3 kN/m².

The curves placed below give the load bearing capacity with a limitation of the deflection under variable loading to 1/800 of the span.

Prestressed concrete units are subjected to precamber, depending on the magnitude and centroid of the prestressing force, modulus of rigidity of the cross section and length of the unit. The design should take account of the precamber in determining the thickness of the topping and screeds and the final levels after finishing - for example, for door thresholds, etc.

Standard profiles

The nominal width of the units is 1200 mm, inclusive of the longitudinal joint. The various cross sections are given alongside. The edges of the slabs are profiled to ensure an adequate transfer of horizontal and vertical shear between adjacent units. The standard profiles have a fire resistance of 60 to 120 minutes. The latter is obtained by raising the level of the tendons.

Hollow core floors are normally used without structural topping. However, in the case of frequent changes of load or important point loads, a topping may be indicated. The thickness should be at least 40 mm, concrete quality C 30/37.

Openings

Holes in hollow core floors are made as indicated in the figure. The dimensions are limited to the values given in the table. Small holes may be formed at the center of the longitudinal voids. The maximum size is limited to the width of the void. Holes are normally made in the fresh concrete during the production process. The edges of the openings are rough.

l/b	HCS 180 - 300	HCS 400
Corner (1) Front (2) Edges (3)	600 / 400 600 / 400 1000 / 400	600 / 300 600 /200 1000 / 300
Center (4) round holes square openings	Core minus 20 mm 1000 / 400	Ø 135 1000 / 200

Handling and transport

Handling, loading and storage arrangements on delivery should be such that the hollow core slabs are not subjected to forces and stresses that have not been catered for in the design. The units should have semi-soft (e.g. wood) bearers placed at the slab ends. Where they are stacked one above the other, the bearers should align over each other.

When stacking units on the ground on site, the guidelines will be similar to the above. The ground should be firm and the bearers horizontal, such that no differential settlement may take place and cause spurious forces and stresses in the components. During handling, provisions shall be taken to ensure safe manipulation, for example safety chains under the slab.

Hollow core slabs are hoisted with specially designed clamps hanging on a steel spreader beam. The use of a sling alone is strictly forbidden.

Assembling

The erection of the hollow core floor slabs should be done according to the instructions of the design engineer. If needed, “Betonika” can second him to supervise the construction methods. “Betonika” will supply written statements of the principles of site erection, methods of making structural joints and materials specification on request.

The nominal bearing length of simply supported hollow core floor units is given in the table. Neoprene strips ensure a uniform bearing.

Supporting material	Slab thickness	Support length a
		Max length	Minimum effective length
Concrete or metal	≤ 250 mm ≥ 300 mm	70 mm 100 mm	50 mm 80 mm
Bricks	≤ 250 mm ≥ 300 mm	100 mm 120 mm	80 mm 100 mm

Floor planks

This typical semi-prefabricated floor system consists of precast floor-plates that are combined with an in-site reinforced concrete topping to achieve a robust solid composite floor. The precast plate units are 0,6 to 2,4 m wide and 40 to 100 mm thick. The lengths are adjusted to the floor spans. Floor plates are made either in reinforced or prestressed concrete. The underside has smooth finish.

To ensure a good interaction between precast plates and in-site concrete, the plates are in most cases provided with protruding lattice girders. During transport and erection the reinforcement results in an improved stability and stiffness of the units. The floor plates normally need propping during construction at a spacing of 1.5 to 3.5 m depending on the upper flange of the girder.

The essential advantages of this system, compared to traditional cast-in-site floors are the following:

Apart form the props, no moulds have to be used;
Most of the reinforcement is already incorporated in the prefabricated plank.

Self-weight of floor planks is higher than for hollow core or ribbed floors.