Benefits and Advantage

Prestressed Hollow core Floors are highly engineered structural products manufactured under factory controlled conditions.
The top-15 advantages as mentioned but the International prestressed hollow core association of the product are:

  • Reduced self-weight

  • Provides versatility for designers

  • Wide range of applications

    Suitable for the residential, healthcare, education, industrial and commercial markets.

  • Long spans without intermediate supports

  • High load capacity

  • Efficient span/depth-ratio leading to reduced storey heights

  • Provision of a sale working platform

  • Excellent fire resistance

  • Excellent acoustic insulation and thermal properties

  • Green product

    Reduced use of raw material

  • Economical solution

  • Reduction in number of site personnel

  • High speed of erection

  • Easily modified to enable heating and cooling of a building even without burning fossil fuels

  • Easily modified to be highly efficient in distributing fresh and warm air, electrical wiring, plumbing and sprinkler services through a building

Other Inherent Benefits

Prestressed Hollow core slabs have longitudinal cores of which the main purpose is to reduce the weight of the floor. When hollow core slabs are used in floor applications, it acts as a load-bearing panel which can reduce the cost of framing by providing a column-free perimeter. Layout flexibility can be greatly increased depending on floor planning as there is potential for reducing the number and/or size of interior columns. However, if there is a desire to reduce interior columns, there are elements available in different depths in order to satisfy the various performance needs for greater spans and loading. This results in a more efficient and less costly construction. Cost savings are greatest for low-to mid-rise structures of three to 10 stories with a large ratio of wall-to-floor area.

Precast concrete is non-combustible with inherent fire-resistant capability that helps protect personnel, equipment and the building itself. It also eliminates the need and cost of additional fireproofing measures.

Precast concrete is an environmentally sound material. It is produced from natural materials and no toxic substances are produced in its production or use. Besides that, Concrete requires only small amounts of energy to produce. Temperature swings are reduced thanks to the thermal mass of concrete which results in great saves in energy year-round.

When expansion work on buildings is to be carried out, precast sections can be typically reused. Nonload-bearing panels on the end simply are disconnected from the framing and additional panels and framing are added on each side. The end panels can be replaced with the new additions in place. Concrete strikes a perfect balance between meeting today’s needs and natural resources for tomorrow , hence regarding up well to sustainability.
As a single unit, precast panels provide one source for supplying the entire exterior wall system. When load-bearing precast structural floors along with panels are specified, it concentrates the complete shell with one certified and reliable producer. This approach ensures complete responsibility and accuracy for meeting design specifications with only one supplier, reducing the number of subcontractors and minimizes trade coordination. In turn, it minimizes errors and miscommunication. Also, the producer’s competent staff of plant engineers is available to assist the design team.

Application

Residential

Commercial

Figure 2: Roofing Solution
Figure 1: Flooring Solution

Product Specifications

120mm Hollow Core Slab

Topping No.of Wires Safe Superimposed Ervice Loads (kN/m2)
Simple Span (m)
3
3.3
3.6
3.9
4.2
4.5
4.8
5.1
5.4
5.6
6
0cm
9 no 5mm PC wire
17.16
13.31
10.37
8.09
6.28
4.82
3.62
2.63
–
–
–
12 no 5mm PC wire
–
–
15.31
12.30
9.91
7.98
6.40
5.09
3.99
3.07
2.27
9 no 7mm PC wire
–
–
–
–
–
13.11
10.91
9.09
7.56
6.27
5.16
5 cm
9 no 5mm PC wire
30.49
24.32
19.63
15.98
13.08
10.74
8.82
7.24
–
–
–
12 no 5mm PC wire
–
–
27.03
22.29
18.52
15.48
12.99
10.93
9.20
7.74
6.49
9 no 7mm PC wire
–
–
–
–
–
23.77
20.28
17.39
14.96
12.91
11.16

Table 1 : Load Table For 120mm Hollow Core Slab (900mm width)

Section Properties
Topping (cm)
5
A (mm2)
69600
113.600
I (x106 mm4)
113.6
317.60
Yb (mm)
60
93
Yt (mm)
60
77
Zb (x106 mm3)
1.898
3.423
Zt (x106 mm3)
1.898
4.124
w (kPa)
1.86
3.03
e (mm)
35
68
bw (mm)
2.72
272

Table 1(a): Section properties for 120mm hollow core slab

150mm Hollow Core Slab

Topping No.of Wires Safe Superimposed Ervice Loads (kN/m2)
Simple Span (m)
4.2
4.5
4.8
5.1
5.4
5.7
6
6.3
6.6
6.9
7.2
7.5
0cm
10 no 5mm PC wire
11.81
9.59
7.77
6.27
5.01
3.94
3.02
–
–
–
–
–
7 no 7mm PC wire
17.25
14.33
11.94
9.96
8.30
6.89
5.69
4.66
3.76
2.98
–
–
10 no 7mm PC wire
–
–
18.19
15.49
13.23
11.32
9.69
8.29
7.07
6.01
5.07
4.25
7 no 9.5mm PC wire
–
–
–
15.49
13.23
11.32
9.69
8.29
7.07
6.01
5.07
4.25
5 cm
10 no 5mm PC wire
19.07
15.91
13.33
11.19
9.39
7.88
6.58
–
–
–
–
–
7 no 7mm PC wire
27.23
23.02
19.58
16.72
14.33
12.31
10.58
9.09
7.80
–
–
–
10 no 7mm PC wire
–
–
28.95
25.03
21.74
18.95
16.58
14.53
12.76
11.21
9.85
8.66
7 no 9.5mm PC strand
–
–
–
25.03
21.74
18.95
16.58
14.53
12.76
11.21
9.85
8.66

Table 2 : Load Table For 150mm Hollow Core Slab (900mm width)

Section Properties
Topping (cm)
5
A (mm2)
83400
127800
I (x106 mm4)
216.60
514.20
Yb (mm)
75
110
Yt (mm)
75
90
Zb (x106 mm3)
2.884
4.692
Zt (x106 mm3)
2.884
5.681
w (kPa)
2.22
3.408
e (mm)
50
85
bw (mm)
268
268

Table 2(a): Section properties for 150mm hollow core slab

200mm Hollow Core Slab

Topping No.of Wires Safe Superimposed Ervice Loads (kN/m2)
Simple Span (m)
5.1
5.4
5.7
6
6.3
6.6
6.9
7.2
7.5
7.8
8.1
8.4
8.7
9.0
9.3
9.6
9.9
10.2
10.5
0cm
6 no 9.5mm PC strand
20.69
17.80
15.35
13.26
11.46
9.91
8.85
7.35
6.30
5.37
4.54
3.79
–
–
–
–
–
–
–
8 no 9.5mm PC strand
–
–
21.75
19.04
16.70
14.68
12.92
11.37
10.00
8.79
7.71
6.74
5.87
5.09
4.38
3.74
–
–
–
6 no 12.7mm PC strand
–
–
–
–
23.76
21.11
18.80
16.77
14.98
13.39
11.97
10.71
9.57
8.55
7.62
6.78
6.01
5.31
–
8 no 12.7mm PC strand
–
–
–
–
–
–
–
22.79
20.52
18.52
16.73
15.13
13.69
12.04
11.22
10.16
9.19
8.31
7.50
5 cm
6 no 9.5mm PC strand
28.68
24.93
21.75
19.04
16.70
14.68
12.92
11.37
10.00
8.79
7.71
6.74
–
–
–
–
–
–
–
8 no 9.5mm PC strand
–
–
30.12
26.59
23.55
20.92
18.63
16.61
14.83
13.26
11.85
10.59
9.46
8.44
7.52
6.69
–
–
–
6 no 12.7mm PC strand
–
–
–
–
33.43
29.92
26.86
24.17
21.80
19.70
17.82
16.15
14.64
13.28
12.05
10.94
9.92
9.00
–
8 no 12.7mm PC strand
–
–
–
–
–
–
–
32.20
29.20
26.54
24.16
22.04
20.14
18.42
16.86
15.45
14.17
13.00
11.92

Table 3: Load Table For 200mm Hollow Core Slab (900mm width)

Section Properties
Topping (cm)
5
A (mm2)
107500
151500
I (x106 mm4)
511.5
1008.5
Yb (mm)
100
136
Yt (mm)
100
114
Zb (x106 mm3)
5.116
7.3960
Zt (x106 mm3)
5.116
8.8725
w (kPa)
2.87
4.04
e (mm)
75
111
bw (mm)
75
215

Table 3(a) : Section properties for 200mm Hollow Core Slab

Handling & Opening Guidelines

Handling and transport

Hollow core slabs are extremely strong when supported in their intended manner; however, if they are not handed and stored properly, they can be damaged. It is imperative that the supervisor of the field crew understands the proper procedures as described herein to avoid damage and accidents due to mishandling.
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.

Assembling

The erection of the hollow core floor slabs should be done according to the instructions of the design engineer. If needed, METRIC can send personnel to supervise the construction methods. METRIC 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.

Supporting material

Slab thickness

Support length a

 

 

Max length

Minimum effective length

Concrete or metal

≤ 250 mm

70 mm 100 mm

50 mm 80 mm

Bricks 

≤ 250 mm

100 mm 120 mm

80 mm 100 mm

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

Corner (1) 
Front     (2) 
Edges (3)

600 / 400 
600 / 400 
1000 / 400

Center    (4)  
    – round holes
    – square openings

Core minus 20 mm 
1000 / 400

Locating Openings

Openings are a critical component to the design and layout of  hollowcore plank. It is extremely important to give METRIC all required opening sizes  and locations as early as possible  in the design process. It is also recommended that the locations of penetrations are properly coordinated early enough so that the precast can be properly designed. However, it is possible that some openings may need to be moved or re-sized for structural reasons.
The best place to locate large openings is near the bearing point. This location has the least design impact to the floor system. Large openings near the bearing may require solid concrete areas that are done either in the plant prior to shipping or in the field after installation. Large openings near the mid-span of the plank will require more prestressing and will impact camber. 
Small core drilled openings can be placed anywhere provided they are located in the hollow void of the plank and are small enough that they do not cut any prestressing strand.  Any core drilled openings that cut through the strand must be accounted for in the design of the plank and carefully monitored in the field.  Any core drilled opening that cuts prestressing strand that was not accounted for in design  needs the immediate review of METRIC’s engineer. For multiple cores concentrated in one area, it is recommended that these are aligned and bunched together in order to reduce the amount of coring perpendicular to the spans.

Important Considerations for Opening

  • Keep openings away from areas where the plank supports wall and point loads.
  • Headers are only intended to assist the construction process, and should not be misconstrued as the primary support component of the opening.
  • Minimize the length of the opening transverse to the plank span. A longer narrower opening is almost always better than a shorter wider one. Minimize the amount of corner overcut. Excessive overcutting may cut more strand than is feasible. In some cases this may required coring of the corners at a large opening to ensure that there is no over cut potential.
  • Layout all openings prior to cutting and verify if additional strands are going to be cut.
  • Continually monitor other trades that are doing core drilling in the plank. A series of core drills may inadvertently cut consecutive strand if not properly located. Verify that every penetration is actually needed prior to cutting. Filling in of unnecessary holes can be costly and may have structural implications.

Effect of Openings

Openings may be provided in hollow core systems by saw cutting after a deck is installed and grouted, by shoring and saw cutting, by forming or sawing the openings in the plant or by installing short slabs with steel headers. In laying out openings for a project, the least structural effect will be obtained by orienting the longest dimension of an opening parallel to a span, or by coring small holes to cut the fewest prestressing strands, or when several openings must be provided, aligning the openings parallel to the span to again cut the least number of prestressing strands.
Penetrations can affect the following design aspects: thickness; amount of prestressing; fire ratings; camber; composite topping; solid cores; redistribution of loading; steel headers;and layout of plank. 

Field Cutting

1. For ease of field cutting openings try to go through hollowcores.
2. Do NOT strike or cut any prestressing strands without permission from METRIC and the Architect/Engineer.
3. There will be some spalling at the exit point of the tool through the deck.

Figure 3 : (L) When field cutting openings up to 1” diameter a power drill or a star drill can be used. (R) When field cutting openings 1” to 6” diameter a core drill or a cutoff saw can be used.

Hanging other Constructions from Hollow Core Slab

Figure 4 : Example of utility placement in Hollow core slab.
Metric Factory Photo

About & Prestressing

A precast prestressed hollow core slab achieves its phenomenal attributes from prestressing and it’s voids.
Prestressing allows for longer spans, shallow depth and the ability to carry heavier loads compared to normal steel reinforced sections of equal cross section. This also allows for better space planning and a lower floor/floor height. Moreover, lengthening the span is more economical with fewer slabs to make and install. The slab thickness, the location and number of prestress strands dictates the load bearing capacity in a prestressed precast hollow core slab.

Camber

A camber, or upward deflection, in the hollow core slab is the after effects of prestressing. This camber causes the slab to be in tension. By doing so, the camber counteracts the effects of compression in loading, hence reducing deflection. However, it should be noted that the camber present in our prestressed hollow core slabs are of a very minute degree and is unnoticeable.

Slab Voids

The purpose of the continuous voids is to reduce the self weight of the slab. The reduction in self weight increases the ultimate load of the slab, and reduces cost at the same time. Moreover, with proper and adequate coordination and planning, the voids can be used to conceal electrical wiring or mechanical ducts. For example, lighting circuits can be routed through the cores which can allow for fixtures to be mounted in an exposed ceiling slab without unsightly surface mounted conduits.

Load Distribution

Load distribution behaves similarly to a monolithic slab when the system of individual slabs are installed and grouted together at the keyways. This beneficial behavior allows the slabs to act together and displace loads to adjacent slabs. This is achieved as hollow core slabs are designed to act as one way, individual, simple span slabs. A major benefit of the slabs acting together is the ability to transfer forces from one slab to another. However, non-uniform loading occurs in the form of line loads, concentrated loads, or load concentrations at openings in most hollow core slab deck applications. Though, the ability for the connected slabs to interact with one another allows these load concentrations to be shared by numerous slabs depending on the system

Bearing Supports for Hollow Core

Many different types of supports designed to handle the dead load and live load from the hollow core slab can be utilized. For example, precast beams, precast walls, poured concrete beams and walls, masonry walls, insulated concrete forming system walls, wood and steel stud walls and structural steel beams are all suitable for use with hollow core as load bearing systems.

Holes and Openings

Holes and openings may be provided in the hollow core system by numerous ways 1) after a deck is installed and grouted – by shoring and saw cutting 2) In the plant – forming or sawing or by installing short slabs with steel headers It should be noted that in any holes and openings should be cut parallel to a span so that the least number of prestressing strands are cut, and hence reducing the compromise of structural integrity.

Manufacturing

In manufacturing of our prestressed hollow core slab, the drawing out, fixing and prestressing of strands are the first steps in the production. Strands are used because of it is easier to handle. Strands are stressed individually. Casting of our hollow core slab is done by the utilization of an extruder. Extrusion is characterized by high compaction with very low moisture content of mixes. Hollow core slabs casting is carried out on long-line casting beds. The conventional method is to run the machines on steel beds up to 200 m long, equipped with stressing abutments. Travel speed will depend on quality of raw materials and depth of slab. More than single shift operation is possible, if the correct curing conditions are available.

After the slabs have been cured and cut to dimensions, stacks of slabs are removed to stockpiling by an overhead crane.

Normal production width is 900mm. Normal slab depths go from 120 mm to 500 mm.  Angle cuts are possible but cost more. It is desirable to have the plan dimensions fit the slab module, non-module plan dimensions can be accommodated using partial width slabs.

The main basic operations in the manufacturing of prestressed hollow core slab are:

The main basic operations in the manufacturing of prestressed hollow core slab are:

  • Production planning
  • Preparation of casting beds
  • Drawing out and fixing prestressing strands and wires
  • Prestressing
  • Feeding the concrete
  • Casting the total slab length on the bed
  • Plotting and identifying the slabs
  • Removing notches, openings etc in the wet state
  • Covering the slabs, to retain moisture during curing
  • Curing the total slab length
  • Soft detensioning of the strands or wires
  • Cutting the slabs to the required lengths and angles
  • Sample deflection testing
  • Creating stacks of slabs
  • Removing stacks to the stockyard
  • Delivery to site and erection on the building
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