FAR EASTERN UNIVERSITY
INSTITUTE OF ARCHITECTURE AND FINE ARTS
Nicanor Reyes St., Sampaloc, Manila
RESEARCH DATA
on
CONCRETE: CAST-IN PLACE AND PRE-CAST, SLAB TYPES
AND FLOOR SYSTEMS
for
MIXED-USE BUILDING PROJECT
Castillo, Mark Andrew A.
De dios, John Lormer DG.
BGTECH541 – AR047
Submitted to:
Arch. Rhea R. Olimpo
August 1, 2015
TABLE OF CONTENTS
Definition of Terms
1
Cast-in Place and Pre-Cast Concrete:
Overview: What are these?
2
The Verdict: Cast-in Place VS Pre-Cast
2
Useful Information about Cast-in Place Method
3
Useful Information about Pre-Cast Method
7
Floor and Roof Systems:
Metal Floor and Roof Decking
Concrete Floor Systems
11
13
Flat Slab
15
Flat Plate
16
Waffle Slab
17
Ribbed Floor Slab
18
Lift Slab
19
Span Stress Floor System
20
Slip Form Method
21
Types of Slabs:
References
22
Cast-in Place Concrete and Pre-Cast Concrete
Overview: What are these?
A. Cast-in Place
Cast-in
Place
(CIP)
concrete walls and slabs are made with readymix concrete placed into removable forms
(wooden partitions) erected on site. Historically,
this has been one of the most common forms of
building basement walls. The same techniques
used below grade can be repeated with abovegrade walls to form the first floor and upper
levels of homes.
Early forays into this technology were done more than 100 years ago
by Thomas Edison. He saw the benefit of building homes with concrete well before it was
widely understood. As technology developed, improvements in forming systems and
insulation materials increased the ease and appeal of using removable forms for single-family
construction. These systems are strong. Their inherent thermal mass, coupled with
appropriate insulation, makes them quite energy efficient. Traditional finishes can be applied
to interior and exterior faces, so the buildings look similar to frame construction, although the
walls are usually thicker.
B. Pre-cast
Precast concrete is a construction product
produced by casting concrete in a reusable mold or "form"
which is then cured in a controlled environment, transported to
the construction site and lifted into place. In contrast, standard
concrete is poured into site-specific forms and cured on site.
Precast stone is distinguished from precast concrete by using a
fine aggregate in the mixture, so the final product approaches
the appearance of naturally occurring rock or stone.
By producing precast concrete in a
controlled environment (typically referred to as a precast plant),
the precast concrete is afforded the opportunity to properly cure
and be closely monitored by plant employees. Utilizing a
Precast Concrete system offers many potential advantages over site casting of concrete. The
production process for Precast Concrete is performed on ground level, which helps with
safety throughout a project. There is a greater control of the quality of materials and
workmanship in a precast plant rather than on a construction site. Financially, the forms used
in a precast plant may be reused hundreds to thousands of times before they have to be
replaced, which allow cost of formwork per unit to be lower than for site-cast production.
Cast-in Place Concrete and Pre-Cast Concrete
The Verdict: Cast-in Place VS Pre-Cast
With the difference of the Cast-in Place and Pre-Cast concrete
discussed on the previous page, we have come up with a solid observation on what is more
reliable and advantage-able in building structures fast and easy; and it is Pre-Cast. To sum it
up, Cast-in Place gets the job done and it is considered as the traditional method in forming
the desired design of the building by means of removable temporary partitions. But Pre-Cast
steps out of the way with its modular and easy-to-install maintenance. With pre-made forms,
building your design is just an assembly away.
And for the record, here’s a table providing the differences of Pre-Cast
and Cast-in Place and how Pre-Cast outperforms Cast-in Place as provided by Advance
Concrete Products, Co.:
Figure 3: Table showing why Pre-Cast outperforms Cast-in Place
Cast-in Place Concrete and Pre-Cast Concrete
Useful Information about Cast-in Place Method
Cast-in Place Concrete and Pre-Cast Concrete
Useful Information about Pre-Cast Method
Floor and Roof System
Metal Floor and Roof Decking
Floor and Roof System
Concrete Floor Systems
Types of Slabs
Flat Slab
A flat slab is a one-way or two-way system with thickenings in the slab
at the columns and load bearing walls called ‘drop panels’ Figure 9. Drop panels act as Tbeams over the supports. They increase the shear capacity and the stiffness of the floor
system under vertical loads, thus increasing the economical span range. This form of
construction has become less popular in recent years because of the limit on economical
spans of about 9.5 m for reinforced slabs and about 12 m for pre-stressed slabs. Reinforced
flat slabs may need to be sensibly pre-cambered (not overdone) to control deflection.
The plan dimensions of the drop panels are a minimum of 1/3 of the
span in the direction under consideration, usually rounded to the nearest 100 mm. The overall
depth of the drop panel is typically taken as 1.75 to 2 times the depth of the slab, again
rounded to suit timber sizes or the nearest 25 mm.
Advantages:
-
Simple formwork.
No beams—simplifying under-floor services outside the drops.
Minimum structural depth.
Usually does not require shear reinforcement at the columns.
Disadvantages:
-
Medium spans.
Generally not suitable for supporting brittle (masonry) partitions.
Drop panels may interfere with larger mechanical ducting.
Vertical penetrations need to avoid area around columns.
For reinforced flat slabs, deflection at the middle strip may be critical.
Figure 4: Flat Slab on-site and framework
Types of Slabs
Flat Plate
A flat plate is a one- or two-way system usually supported directly on
columns or load-bearing walls. It is one of the most common forms of construction of floors
in buildings. The principal feature of the flat plate floor is a uniform or near-uniform
thickness with a flat soffit which requires only simple formwork and is easy to construct. The
floor allows great flexibility for locating horizontal services above a suspended ceiling or in a
bulkhead. The economical span of a flat plate for low to medium loads is usually limited by
the need to control long-term deflection and may need to be sensibly pre-cambered (not
overdone) or pre-stressed.
An economical span for a reinforced flat plate is of the order of 6 to 8
m and for pre-stressed flat plates is in the range of 8 to 12 m. The span ‘L’ of a reinforced
concrete flat-plate is approximately D x 28 for simply supported, D x 30 for an end span of a
continuous system, to D x 32 for internal continuous spans. The economical span of a flat
plate can be extended by pre-stressing to approximately D x 30, D x 37 and D x 40
respectively, where D is the depth of slab.
Advantages:
-
Simple formwork and suitable for direct fix or sprayed ceiling.
No beams—simplifying under-floor services.
Disadvantages:
-
Medium spans.
Limited lateral load capacity as part of a moment frame.
May need shear heads or shear reinforcement at the columns or larger columns for
shear.
Long-term deflection may be controlling factor.
May not be suitable for supporting brittle (masonry) partitions.
May not be suitable for heavy loads.
Figure 5: Flat Plate on-site and framework
Types of Slabs
Waffle Slab (2-Way Joists)
Ribbed floors consisting of equally spaced ribs are usually supported
directly by columns. They are either one-way spanning systems known as ribbed slab or a
two-way ribbed system known as a waffle slab. This form of construction is not very
common because of the formwork costs and the low fire rating. A 120-mm-thick slab with a
minimum rib thickness of 125 mm for continuous ribs is required to achieve a 2-hour fire
rating. A rib thickness of greater than 125 mm is usually required to accommodate tensile and
shear reinforcement. Ribbed slabs are suitable for medium to heavy loads, can span
reasonable distances, are very stiff and particularly suitable where the soffit is exposed.
Slab depths typically vary from 75 to 125 mm and rib widths from 125
to 200 mm. Rib spacing of 600 to 1500 mm can be used. The overall depth of the floor
typically varies from 300 to 600 mm with overall spans of up to 15 m if reinforced, longer if
post-tensioned. The use of ribs to the soffit of the slab reduces the quantity of concrete and
reinforcement and also the weight of the floor. The saving of materials will be offset by the
complication in formwork and placing of reinforcement. However, formwork complication is
minimized by use of standard, modular, reusable formwork, usually made from
polypropylene or fiberglass and with tapered sides to allow stripping.
Advantages:
-
Savings on weight and materials.
Long spans.
Attractive soffit appearance if exposed.
Economical when reusable formwork pans used.
Vertical penetrations between ribs are easy.
Disadvantages:
-
Depth of slab between the ribs may control the fire rating.
Requires special or proprietary formwork.
Greater floor-to-floor height.
Large vertical penetrations are more difficult to handle.
Figure 6: Waffle Slab on-site and framework
Types of Slabs
Ribbed Floor Slab (1-Way Joist)
Introducing voids to the soffit of a slab reduces dead weight and
increases the efficiency of the concrete section. A slightly deeper section is required but these
stiffer floors facilitate longer spans and provision of holes. Economic in the range 8 to 12 m.
the saving of materials tends to be offset by some complication in formwork. The advent of
expanded polystyrene moulds has made the choice of trough profile infinite and largely
superseded the use of standard T moulds. Ribs should be at least 125 mm wide to suit
reinforcement detailing.
The rib-slab is special in that it provides a lighter and stiffer slab than
an equivalent flat slab, reducing the extent of foundations since the ultimate load is reduced.
They provide a very good form where slab vibration is an issue.
They are also known as waffle slabs, because they look like waffles
with rows of beams running underneath them. They have advantages of saving on material,
and providing long spans.
Advantages:
-
Medium to long spans.
Lightweight.
Holes in topping easily accommodated.
Large holes can be accommodated.
Profile may be expressed architecturally, or used for heat transfer in passive cooling.
Disadvantages:
-
Higher formwork costs than for other slab systems.
Slightly greater floor thicknesses.
Slower.
Figure 7: Ribbed Floor Slab on-site and framework
Types of Slabs
Lift Slab
Lift slab construction is a method of constructing concrete buildings by
casting the floor or roof slab on top of the previous slab and then raising (jacking) the slab up
with hydraulic jacks, so being cheaper and faster as not requiring boxing and supports for
casting in situ. The method was first used at Trinity University in San Antonio, Texas during
the construction of Northup Hall in 1952.
Johnstone Hall, a Clemson University dormitory in Clemson, South
Carolina, was erected using this method in 1954. Several of the blocks have now been
demolished, and campus legend says that that two other similar structures built elsewhere
collapsed before completion.
Lift slab construction was also involved in the L'Ambiance Plaza
collapse in Bridgeport, Connecticut, in 1987, and resulted in a nationwide federal
investigation into this construction technique in the United States, and Connecticut imposed a
temporary moratorium on lift slab construction. A patent was issued to Tom Slick for this
construction method, called the "Youtz-Slick" method, in 1955.
Advantages:
-
Easier maintenance and lesser manpower.
Cheaper against the other types of slabs.
Is widely common with modular structures.
Disadvantages:
-
Earthquake-prone.
Slabs are fastened only by bolts; if not maintained properly, might cause accidents
that lead to structure failure.
Load capacity is limited.
Figure 8: Lift Slab on-site and framework
Types of Slabs
Span Stress Floor System
Different buildings have different requirements, so not surprisingly
there is no 'one size fits all' most appropriate solution. Clearly the requirements vary
depending on the type of use, but there are also some more subtle issues to consider and these
are highlighted below. It should not be forgotten that when considering intended use, it may
be appropriate to pay attention to a different use in the future - many steel solutions offer
flexibility that can result in high levels of sustainability over the lifetime of a building.
As a rule of thumb designers should adopt the simplest solution that
will meet the project requirements. Generally speaking the simplest solution will also be the
most common, and familiarity will facilitate the design, fabrication and erection processes as
no new learning is involved.
Within the context of steel floor systems, simple also means less labor
and cost. For example, the simplest solution of a downstand solid web I-section beam as
opposed to a truss means; fewer structural elements, less fabrication , fewer surfaces to be
fire protected and less time to design.
Floor Stiffness:
Stiffness is needed to ensure that a floor behaves correctly from a
dynamic point of view, thereby assuring user comfort. This is a complex subject, as the real
issue is how the floor responds (in terms of acceleration), and that is a function of a number
of variables including stiffness and the mass that is mobilized. The traditional approach,
which is recognized as being crude, for designing a floor to respond acceptably is to check its
natural frequency and compare that with a limiting value (which is a function of the floor
mass). The required behavior depends on the function for a given building/room. Some uses
are less tolerant to floor movements (e.g. an operating theatre). Some uses (e.g. a gymnasium
within an office) are more likely to cause problems and warrant particular attention.
Figure 9: Span Stress Floor System
Types of Slabs
Slip Form Method
Slip forming, continuous poured, continuously formed, or slipform
construction is a construction method in which concrete is poured into a continuously moving
form. Slip forming is used for tall structures (such as bridges, towers, buildings, and dams),
as well as horizontal structures, such as roadways. Slipforming enables continuous, noninterrupted, cast-in-place "flawless" (i.e. no joints) concrete structures which have superior
performance characteristics to piecewise construction using discrete form elements. Slip
forming relies on the quick-setting properties of concrete, and requires a balance between
quick-setting capacity and workability. Concrete needs to be workable enough to be placed
into the form and consolidated (via vibration), yet quick-setting enough to emerge from the
form with strength. This strength is needed because the freshly set concrete must not only
permit the form to "slip" by the concrete without disturbing it, but also support the pressure
of the new concrete as well as resist collapse caused by the vibration of the compaction
machinery.
In vertical slip forming the concrete form may be surrounded by a
platform on which workers stand, placing steel reinforcing rods into the concrete and
ensuring a smooth pour. Together, the concrete form and working platform are raised by
means of hydraulic jacks. Generally, the slipform rises at a rate which permits the concrete to
harden by the time it emerges from the bottom of the form.
In horizontal slip forming for pavement and traffic separation walls
concrete is laid down, vibrated, worked, and settled in place while the form itself slowly
moves ahead. This method was initially devised and utilized in Interstate Highway
construction initiated by the Eisenhower administration during the 1950s.
Figure 10: Slip Form Method framework
REFERENCES:
Written Literature:
Architectural Graphic Standards: Student Edition, Ramsey/Sleeper, 1994, John Wiley &
Sons, Inc.
Merriam-Webster Dictionary, Mr. and Mrs. Merriam-Webster, 1980, WV, FLR
Civil Digital, WordPress, 2015, WordPress Publishing and EBooks Corporation
Advance Concrete Products, Inc., online brochures, 2011, Magna Press Limited
Online Literature:
www.civildigital.com
www.formstress.co.nz
en.wikipedia.org
Definition of Terms
forays - a sudden attack or incursion into enemy territory, especially to obtain something; a
raid.
inherent - existing in something as a permanent, essential, or characteristic attribute.
formwork - is the term given to either temporary or permanent molds into which concrete or
similar materials are poured. In the context of concrete construction, the falsework supports
the shuttering moulds.
beam - a long, sturdy piece of squared timber or metal spanning an opening or part of a
building, usually to support the roof or floor above.
flat slab - in similarity with a slab; a concrete panel for use with elevating a structure.
slab - a large, thick, flat piece of stone, concrete, or wood, typically rectangular.
joist - a length of timber or steel supporting part of the structure of a building, typically
arranged in parallel series to support a floor or ceiling.
purlins - a horizontal beam along the length of a roof, resting on a main rafter and supporting
the common rafters or boards.
stiffness - is the rigidity of an object — the extent to which it resists deformation in response
to an applied force. The complementary concept is flexibility or pliability: the more flexible
an object is, the less stiff it is.
framework - an essential supporting structure of a building, vehicle, or object.
pre-stressed - strengthened by the application of stress during manufacture, especially (of
concrete) by means of rods or wires inserted under tension before the material is set.
penetrations - the action or process of making a way through or into something; the
perceptive understanding of complex matters.
timber - wood prepared for use in building and carpentry.