Introduction

Coreslab Structures Inc. is a leading manufacturer of precast hollow core

concrete slabs in North America. We were given the opportunity to tour their

Ontario manufacturing facility.

1) Coreslab Structures Inc.’s Key Precast Products

Coreslab manufactures four different thicknesses of prefabricated hollow-

core concrete slabs; 8 inch, 10 inch, 12 inch, and 14 inch thicknesses are

available. The slabs are not so much different in their shapes as they are in there

thicknesses. These slabs, which can span up to fifty feet unsupported, are

produced in a plant with machines that control the thickness as well as the

diameter of the hollow cores inside each slab. The facility has 7 beds to produce

the slabs, with each bed yielding 500’ of product. The 7 separate beds also give

Coreslab Structures the versatility to produce 7 unique hollowcore slabs at a

time; no need to delay customer orders while other orders finish. Coreslab

Structures also does any and all transportation, installation and cutting of their

hollowcore products, to ensure it is correctly and efficiently done.

Although precast hollow core concrete slabs are the company’s main

focus, Coreslab Structures also offers two types of wet cast concrete beams, the

“ell” beam, which comes in 18” and 24” wide sizes, and the “inverted tee” beam,

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which comes in 24” and 36” wide sizes. Coreslab is capable of producing up to

seventy feet worth of each product per run. This is achieved with forms that are

already created and stored in the yard of the plant. These two different structural

shapes offer structural engineers additional options in the way they create and

support their structures.

Coreslab also creates wet casts of any specific shape or thickness, to a

certain degree, for the customer, as shown in figure 1.1. Coreslab creates the

unique forms out of welded

plates, and after being cast

and cured, the concrete slab

is shipped to the site and

installed by the Coreslab

installation crew

immediately.

2) Precast Hollow Core Concrete Slab

Coreslab’s main product is precast hollow core concrete slabs with

pretensioned reinforcing steel strands. This product is allowing the construction

industry to build larger concrete structures faster and more efficient than ever

before.

i) Applications and Spans

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Figure 1.1: Welded forms for Coreslab’s wet cast products

A major reason why hollow core concrete slab is becoming popular with

builders, is the long spans it can cover. Coreslab provides four different

thicknesses, and many re-bar counts and sizes to suit your project. Each

thickness and re-bar combination has its own maximum span and load that it can

carry. 8” thick core slab can span a maximum of 33 feet with a maximum service

load of 79lbs/ft2. 10” thick core slab can span a maximum of 43 feet with a

maximum service load of 62lbs/ft2. Though both 12” and 14” core slab can span

50 feet, the 12” slab can only hold a service load of 67lbs/ft2, while the 14” slab

can hold an impressive 129lbs/ft2. Every thickness of core slab has a wide range

of uses, including hotels, school, office buildings, and even residential

applications.

ii) Product Analysis as a Floor System

Coreslab’s precast hollow core concrete slabs have many advantages

compared to traditional cast-in-place alternatives. The main advantage is that the

slabs are produced in a controlled environment. Producing the slabs in a plant

allows Coreslab to provide the utmost quality control for their products. They

control the temperature moisture that the concrete is exposed to while curing,

which allow the concrete to achieve its maximum strength. Also, during

production, fifteen 2” by 2” hollowcore samples are taken daily to be broken for

strength tests by a compression tester. Quality control testing is also performed

on the sand and aggregate used in the concrete mixes to ensure compliance with

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CSA guidelines. An air test and slump test is also performed during each wet-

cast pouring; six 4” by 8” cylinders are taken as samples for strength tests.

Hollow core slabs are also very efficient; their hollow cores reduce weight

and improve performance levels. Also, installation can be performed in almost

any weather conditions. This is of significant advantage during the winter; cast-in-

place alternatives struggle with installation in winter weather, while Coreslab

continues without lost productivity. Coreslab installation crews can install 6000 to

9000 square feet of product per day, making installation quick and efficient. Also,

electrical and mechanical utilities can be placed in the hollow spaces within the

slabs. There are even cases of the hollow cores being used as a integral part of

the HVAC system, with hot and cold air running through the cores. Other

advantages include very low sound transmission, no floor squeaking compared

to wooden systems, and low maintenance.

One feature of Coreslab’s products which may be considered as both an

advantage and disadvantage is Coreslab’s complete production, transportation,

and handling of their products. Coreslab is responsible for cutting any holes or

special features in the slabs. Others are not authorized to cut or drill holes in the

slabs since they are so structurally complex; an ill-placed drill by a plumber could

compromise the structural integrity of the slab. They also transport the slabs to

the site and install them at a mutually agreed upon time; discussions take place

ahead of time with the contractor to ensure maximum efficiency. In most cases

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this seems like an advantage, however it also brings to light the versatility and

flexibility of cast-in-place alternatives. If an on-site problem arises, such as

conflicting mechanical and structural plans, a revision can be quickly issued and,

if a cast-in-place floor system is used, the forms and reinforcing can be modified

on site and the concrete can be poured. However, a hollow core slab is precast,

and modifying it in the required way may not be possible; in this case an order

must be made for a new hollow core slab.

Another disadvantage of the hollow core slab is the inability to cantilever

more than 5’. In general, continuous support is necessary for hollow core

concrete slabs, but reinforcing can be added in the top section of the hollow core

slab to allow for a small cantilever; this cantilever cannot exceed 5’. For larger

cantilevers, the engineer is required to use solid concrete slabs for balconies and

associated situations. Lastly, the installation environment is limited for hollow

core slab; without proper site access, namely crane access to the required

installation area, hollow core slab is not an option.

iii) Designing with Hollow Core Slab

In order to design using hollow core concrete slabs, a structural engineer

must consult Coreslab’s design tables. Design tables are categorized by

thickness of the slab. Figure 2.1 shows a 300mm thick hollow core slab load

table. Column A lists the number of 13mm diameter reinforcing steel strands

present in the slab. Row A lists the length of the span. Based on the thickness of

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the hollow core slab, the number of reinforcing steel strands and the length of the

span, the table shown in figure 2.1 provides us with the uniformly distributed

superimposed service load that a particular hollow core slab can carry. For

example, from the table below, we can determine that for a span of 11.0m, a

300mm (12”) thick slab with eight 13mm reinforcing steel strands, a total

uniformly distributed service load of 6.7 kPa can be achieved.

An obvious deduction from this table is that an increase in the number of

reinforcing steel strands will increase the UDL that a certain thickness of hollow

core can carry over the same span. When designing our floor system, we would

look first at the required span and UDL that the floor must carry. Based on these

two elements, we can consult Coreslab’s design tables to find the most cost

effective and efficient hollow core slab to use. The required deflection values for

our floor will also be a determining factor when choosing the proper hollow core

slab.

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Column A Row A

Figure 2.1

iv) Hollow Core Slab Manufacturing Process

The manufacturing process for precast hollow core concrete slabs can be

separated into two distinct sections, the batch plant and the hollow core

production. The batch plant is where the concrete mixes used in the hollow core

slabs are created. Coreslab Structures uses two different cements, Type 30 and

flyash. Type 30, supplied by Essroc, is stored in two 60,000 kg bins, while flyash,

supplied by Lafarge, is stored in one 60,000 kg bin. Type 30 is predominately

used due to its high early strength yield, which allows the hollow core slabs to be

removed from the production beds as soon as possible. The aggregate, 14 mm

stone, and sand are each stored in separate dual 120,000 kg bins. Any

admixtures used in the concrete mix are supplied by Axim. A delivery bullet,

which travels at 300ft/min., supplies the concrete mixer, which turn produces 1-

1/3 m3 of concrete every

3 minutes. Figure 2.2

shows the delivery bullet

and concrete mixer.

Hollow core slab

production occurs on 7

beds 520 feet in length,

which allow 500 feet of

product to be poured on

each bed. The resulting production is 2,000 square feet of hollow core slab per

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Figure 2.2: The delivery bullet is the yellow machine on tracks, the concrete mixer is orange, and the extruder is the blue machine producing the hollow core slab.

bed. Extruders, which are the machines that form the concrete into hollow core

slabs, run in tandem, allowing the concrete mixer to supply two extruders at

once. Figure 2.2 shows the concrete mixer supplying an extruder.

Coreslab Structures produces four unique depths of hollow core slab - 8”,

10”, 12” and 14” – and the extruder forms each depth of slab at a different speed;

the thicker slabs contain a higher volume of concrete, causing the extruder to

move slower. Coreslab does not commonly produce 14” thick hollow core slabs;

the bulk of their business deals with 8”, 10”, and 12” thick hollow core slabs. The

extruder runs at 48-52 inches/minute, 44-46 inches/minute, 38-42 inches/minute

for 8”, 10” and 12” slabs respectively. It takes approximately 1 hour, 40 minutes

to pour a bed of 8” hollow core slab, and approximately 2 hours, 40 minutes to

pour a bed of 12” hollow core slab.

Before the extruder forms the

hollow core slab on the beds, pre-

stressed steel reinforcing strands are

arranged on the beds. The strands on

the bed, shown in figure 2.3, which

vary in thickness and number of

strands depending on the customer’s

order, are stressed using a micro-

motion stressing pump and jack system; this system digitally displays the

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Figure 2.3: Pre-stressed steel reinforcing strands.

pressure and elongation of the strand. For example, a ½” strand is typically

stressed to 27,500 lbs, and has an elongation of 32-1/2”, or 815 mm.

After the extruder forms the

hollow core slab, workers apply a

thin layer on concrete to the top to

smooth out the surface. When the

hollow core slabs are dry, they are

cut to the customer specific

lengths, crane lifted and stored

outside the manufacturing facility

until delivery and installation. Figure

2.4 shows this process.

v) Detail Work

When using Coreslab

products, there are areas where

special details are required. As

mentioned earlier, hollow core slab

can only be cantilevered 5’

maximum; if a larger cantilever is required, a solid concrete slab must be used,

which Coreslab also produces. Also, when designing the structure, one must be

aware of the concrete topping, if any, that will be added to the top of the hollow

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Figure 2.4: Top – Workers rig a cut section of slab to the overhead crane.Bottom – Hollow core slabs are stored onsite until transportation to the site.

core slabs to achieve a level, aesthetically pleasing floor. When specifying the

offset of doors placed on the hollow core slab, the concrete topping, usually 2”

thick, must be taken into account. Coreslab provides a large amount of

documentation for their hollow core floor systems; included in this documentation

are detail drawings of connections with different types of structural systems, wall

systems, etc. These documents should be thoroughly reviewed during the

structural design process.

vi) Cost Analysis and Comparison

Coreslab Structures has a range of products and many factors are

involved in pricing their hollow core slab product. It is difficult to give an

approximate cost for the hollow core slab without knowing many associated, job-

specific factors, such as job site accessibility. However, when Coreslab is

approached with a specific job to price, they are able to more accurately price the

job compared to cast-in-place estimates. This is mainly due to installation costs

and scheduling; cast-in-place installation is dependent on more factors - such as

weather conditions – than hollow core installation. For example, a cast-in-place

floor slab estimate can be rendered useless by a bout of bad weather delaying

installation.

Although it is difficult to give a price for hollow core slab, a rough estimate

would be one dollar per inch thick, per square foot. For example, an 8 inch hollow

core slab is approximately $8.00 per square foot. Coreslab is unable to place

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slabs down unless the area has adequate crane accessibility; if this crane access

is limited, the costs associated with hollow core installation rise. Typical cast in

place concrete averages one dollar per inch for a square foot as well, however

this number is most likely closer to the actual cost than the Coreslab estimate. To

properly estimate a job’s cost, Coreslab Structures must have all the information

about the structure and the site.

Conclusion & Key Advantages

The advantages of precast hollow core concrete slabs are numerous, as

discussed earlier, and we are of the opinion that these advantages far outweigh

the disadvantages. The main advantages of hollow core slab are the ease of

installation, the precision and quality of factory-produced product, and the

lightweight and efficient properties of hollow core. Of course, hollow core is not

for every structure and situation; a careful analysis of the structure, the site, and

other governing factors will help determine if precast hollow core concrete slab is

right for your project.

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