field trip 2 - coreslab structures inc essay_final
TRANSCRIPT
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,
3
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
4
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
5
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
6
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
7
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.
8
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
9
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
10
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
11
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
12
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.
13