Upload
globalfriendship
View
222
Download
0
Embed Size (px)
Citation preview
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
1/16
RISK & OPPORTUNITIES ASSOCIATED WITH LIGHTWEIGHT CONCRETE
STRUCTURES
A Paper presented
By
Uttam Nangre-Patil & Mahadeo Nalawade
INTRODUCTION
Concrete design has evolved rapidly in the last 30 years. Construction
technology has seen the introduction of a variety of concrete products
to the market as well as an increased use of supplementary
cementitious materials and recently blended cements. Emphasis has
been placed on creating more durable concrete through changes to the
mix constituents and proportions, including the aggregates, admixtures
and the water-cement ratio. This evolution, along with improved
reinforcing steel strength and the use of lightweight fiber reinforcement
steel has lead to modifications in design philosophy - most notably the
use of thinner structural members.
Lightweight concrete can be defined as a type of concrete which
includes an expanding agent in that it increases the volume of the
mixture while giving additional qualities such as nailibility and lessened
the dead weight. It is lighter than the conventional concrete with a dry
density of 300 kg/m3 up to 1840 kg/m3; 87 to 23% lighter. It was first
introduced by the Romans in the second century where The Pantheon
has been constructed using pumice, the most common type of
aggregate used in that particular year. From there on, the use of
lightweight concrete has been widely spread across other countries
such as USA, United Kingdom and Sweden. The main specialties of
lightweight concrete are its low density and thermal conductivity. Its
advantages are that there is a reduction of dead load, faster building
rates in construction and lower haulage and handling costs. The
building of The Pantheon of lightweight concrete material is stil l
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
2/16
standing eminently in Rome until now. It shows that the lighter
materials can be used in concrete construction and has an economical
advantage.
But despite all these advantages there are research which shows
the risk associated with use of lightweight concrete structures. To
highlight the issue we would like to discuss bellow the results of
the tests done by researcher on lightweight concrete.
RISKS..
Strength & Density Comparison: The purpose of this test is to identify
the performance of aerated lightweight concrete in term of density and
compressive strength. The result is presented in Table 1. Based on, it
can
Density(Kg/M3)
CompressiveStrength
(Kn/M2)1470 2.52
1720 5.5
1770 10.34
1780 9.19
1810 13.12
1820 11.87
1840 13.21
1840 16.78
1920 16.73
1990 16.58
2040 17.27
2040 12.18
2050 9.35
2060 22.99
Table 1 : Density of Hardened Concrete and Compressive Strength at 28 days
be seen that compressive strength for aerated lightweight concrete are
low for lower density mixture. The increment of voids throughout the
sample caused by the foam in the mixture will lower the density. As a
0
500
1000
1500
2000
2500
1 2 3 4 5 6 7 8 9 10 11 12 13 14
DensityKg/M3
Compressive Strength Kn/M2
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
3/16
result, compressive strength will also decrease with the increment of
those voids. The required compressive strength of lightweight concrete
is 3.45 MPa at 28 days as a non load bearing wall. The compressive
strengths obtained from these mixtures carried out are higher than 3.45
MPa and therefore it is acceptable to be produced as non-load bearing
structure. However, the compressive strength for the mixture with
density of 2050 kg/m3 is slightly low compared with density of 2040
kg/m3. This is due to the compaction problem during mixing process.
The final mixture is quite dry and since compaction is not perfectly
done, samples are not well compacted. This has resulted the
compressive strength to be lower than the mixture with lower density.
As been discussed before, tr ial and error method was used in
determining the most suitable mixture in preparing research samples.
Fourteen trial mixes have been prepared during the research and from
the results, the mixture with the highest compressive strength with low
density will be used for further investigation. Compressive strength of
aerated lightweight concrete is determined on the 7, 14, 21 and 28
days for each sample. There were three samples for each test and the
results would be taken as the average of these three. Fewer variables
had been set for different mixture, this variable would be changed
accordingly while the others were fixed to forecast their effect on the
mixture. Percentage of foam, foam agent and water, cement and sand
ratio were the variables made during the mixing process. For example,
three mixtures were prepared to determine the effect of different foam
agent and water, cement and sand ratio. The percentage of foamapplied is fixed for three mixtures and the difference in the results
would occur because of the foam agent and water ratio. All the results
were based on the 75% foam injected in the mixture. Figure 2 shows
the compressive strength of aerated lightweight concrete according to
the percentage of foam in each mixture. It can be seen that the mixture
with 25% of foam is higher than the compressive strength of 100%
foam. This is because, with higher percentage of foam, voids
throughout the sample will be increased, and as has been discussed
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
4/16
ealier, this would result in the decrease of the compressive strength.
Compressive strength of mixture with 50% foam is slightly higher than
mixture with 75% foam. The density of 25%, 50%, 75%, and 100% of
foam is 2040 kg/m3, 1820 kg/m3, 1810 kg/m3, and 1470 kg/m3
respectively. The density of 50% and 75% of foam mixture is the same
as been showed in Figure 2, compressive strength for this two mixture
did not differ much. But it can be seen that there is a difference
between 25% of foam mixture and 100% of foam mixture. The density
of 25% of foam mixture is 27% higher as compared to 100% of foam
mixture and seen in Table 2, the compressive strength is 85.4% higher
at 28 days. For a 25% mixture the compressive strength is 17.27 MPa
and for 100% mixture is 2.52 MPa.
DaysCompressive Strength ( Kn/m2)
At 25% Foam At 50% Foam At 75% Foam At 100% Foam
7 32.20 9.45 8.12 1.43
14 14.68 8.88 11.02 2.44
21 16.41 14.42 11.96 2.23
28 17.27 11.87 13.12 2.52
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
5/16
As per above test i t is seems that the reduction in density or the
addition of voids in concrete would effect on the strength of the
concrete. Secondly there are some other research shows that the major
disadvantages of lightweight concrete have been the inability to provide
consistent compressive strengths and density throughout the entire
area. Lightweight concrete derives its compressive strength from the
molecules in the foam additive that when properly mixed form
around the cement and serve as aggregate does in standard concrete
mixes. If the foam additive is not properly mixed, there is a probability
of foam collapse, which weakens the products compressive strength.
Secondly if the lightweight concrete produced with using the lightweight
aggregates then uniform workability of the mix is more difficult to
maintain with due to their general high absorption and the wide
variation in rate of absorption from particle to particle. And also
because the coarse aggregates are lighter than the concrete mass,
they tend to float to the surface when improperly placed. There is also
more possibility of lightweight mixes tend to entrap air and honeycomb
more than normal weight concretes. There are researches going on to
minimise these risks to increase the use of lightweight concrete in
structures. For highrise structures, it has been suggested that the use
of lightweight concrete shall be restricted for slabs and horizontal
components and regular concrete for vertical components like columns
and shear walls to mitigate the risk associated with lightweight
concretes.
There are so many ways developed to minimise the risk andimprove the performance of the lightweight concrete and it has
been successfully used for construction of landmark structures
across the globe. But in the Indian scenario as our construction
industry is facing various challenges including the quality of
labour force, use of technology, machineries and equipments, we
need to be more cautious while using innovative materials and
technologies. Now let us know some of great opportunities, we
Indians can explore in which these above mentioned risks are
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
6/16
minimised with using controlled process environment. Concrete
Cloth and pre-cast concrete components with lightweight concrete
with glass fibre are two best options available for us which change
paradigm of Indian Infrastructure scenario.
OPPORTUNITIES..
Basically one of the biggest disadvantages of normal conventional
concrete is its self weight of about 2200 to 2600 Kg/m3 which is so
high and attempt have been made in past to reduce the self weight of
the concrete and to increase the efficiency of the concrete as a
structural material. Therefore day by day the util ization of normalconcrete in building across the globe is going down due to its
inflexibility, material cost and the associated cost of labour for handling
the materials. The weight of building on foundations is important factor
considered while designing the structures particularly in case of weak
soil and highrise structures. We know that a solid ordinary concrete
made of only fly ash, Portland cement and aggregates can gives the
strength of 55to62 N/mm2. This strength is much more than the
required strength for most of the structural applications. So the need
for going for developing alternative ways to lighten the strength of
concrete as well as make it l ightweight with keeping desired properties
required for most of the structural applications. But developing a viable
lightweight structural concrete with least amount-of materials and
manufacturing cost is a complex science as its not that easy to fulfi l l
all the desired parameters. With using innovations various products
have been used across the globe which are not only enhancing the
quality of structures but reducing the time and cost to execute the
projects. Let us discuss the two wonderful application of technologies,
Concrete Cloth and Pre-cast Fiber Reinforced Concrete structures.
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
7/16
AMAZING INOVATION- CONCRETE CLOTH
Concrete is a freshly mixed material, which can be moulded into
required shape. There are many advantages of concrete, but there is
one drawback is that, it is not flexible, when it is hardened. To
overcome through this drawback of concrete, a new construction
material was developed by British Engineering Company called
Concrete Canvas. Concrete cloth (CC) is a unique proprietary material.
It has a very wide range of applications throughout the building & civil
engineering industry. Concrete cloth is a flexible; cement impregnated
fabric that hardens when hydrated to form a thin, durable, water & fire
proof concrete layer. CC allows concrete construction without the need
for plant or mixing equipment. Simply position the canvas & just add
water. CC has a design life of above 10 years and is significantly
quicker and less expensive to install compared to conventional
concrete. CC consists of a 3- dimensional fiber matrix containing a
specially formulated dry Concrete mix. A PVC backing on one surface
of the cloth ensures the material is completely waterproof, while
hydrophilic fibers (Polyethylene and Polypropylene yarns) on the
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
8/16
opposite surface aid hydration by drawing water into the mixture. The
material can be hydrated either by spraying or by being fully immersed
in water. It can be easily nailed, stapled through or coated with an
adhesive for easy attachment to other surfaces. Once set, the fibers
reinforce the concrete, preventing crack propagation & providing a safe
plastic failure mode. CC is available in 5, 8 & 13 mm thicknesses
Fig. Concrete Cloth Section
Specifications For CC
CC TypeThickness
in mm
Roll
Width
in mm
Dry
Weight
(Kg/Sqm)
Batched
Roll
Coverage
(Sqm)
Batched
Roll
Length(
m)
Bulk Roll
Coverages
(Sqm)
Bulk Roll
Length (m)
CC5 5 1000 7.00 10.00 10.00 200.00 200.00
CC8 8 1100 12.00 5.00 4.50 125.00 113.60CC13 13 1100 19.00 N/A N?A 80.00 72.70
CC Material Properties
Strength: Very high early strength is a fundamental characteristic of
CC. Typical strengths and physical characteristics are as follows:
Compressive Strength
- 10 day compressive failure stress (MPa) 40
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
9/16
- 10 day compressive Youngs modulus (MPa) 1500
Bending tests
- 10 day bending failure stress (MPa) 3.4
- 10 day bending Youngs modulus (MPa) 180
Abrasion Resistance
- CC lost 60% less weight than marble over 1000 cycles.
Tensile Test
- Similar to twice that of OPC Max 0.10 gm/cm 2
CBR Puncture Resistance
- Min. Push-through force 2.69 kN
- Max. Deflection at Peak 38mm
Resistance to Imposed Loads on Vehicle Traffic Areas - Gross weight of 2 axle vehicle 30 to 160 kN- Uniformly distributed load not exceeding 5kN/sqm
Method of Hydration : CC can be hydrated using saline or non saline
water. The minimum ratio of water to CC is 1:2 by weight. CC cannot
be over hydrated so an excess is recommended. The recommende d
methods are: In a hot/arid environment, re-wet the material 2 - 4 hours
after the initial hydration.
F i g . S p r a y T h e F i b e r S u r f a c e W i t h W a t e r U n t i l I t F e e l s W e t T o T o u c h F o r S e v e r a l M i n u t e s A f t e r S p r a y i n g .
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
10/16
Cutting of CC : A disposable blade is the most suitable tool for cutt ing
CC before it is hydrated or set. When cutting dry CC, a 20mm
allowance should be left from the cut edge due to lost fi l l. This can be
avoided by wetting the CC prior to cutting.CC can also be cut using
handheld self sharpening powered disc cutters.
Angle Gi rder Di sc cut ter
Applications of CC : .Some of the applications of the CC are as follows
:
Fig. Slope Protection
Application Products
Dust Suppression CC5
Foundation Binding CC5
Weather proofing/ slope
stabilization CC5
Ditch Lining CC5,CC8, CC13
Bund Lining CC5,CC8, CC13
Sandbag/Gobian
ReinforcementCC8 & CC13
Trackway/Flooring CC8 & CC13
Pipe Protection CC5,CC8, CC13
Cable Covering CC13
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
11/16
Fig.: Ground Resurfacing Fig.: Mining Applications
Fig: Bund Lining Fig.: Sandbag Reinforcement
GLASS FIBERE-REINFORCED PRE-CAST CONCRETE COMPONANTS (GRFC)
Glass fiberreinforced concrete, commonly known as GFRC, is a composite
concrete product fabricated by many precast concrete manufacturers in many
developed countries. It consists of a portland-cement-based composite that is
reinforced with an absolute, minimum of 4% by weight of alkali-resistant glass fibers
to total mix, which are randomly dispersed through the material. The fibers serve as
reinforcement to enhance the concretes flexural, tensile, and impact strength. The
low weight of GFRC panels decreases superimposed loads on the buildings
structural framing and foundation, providing potential savings in multistory
construction and in areas with poor supporting soil. Its light weight also makes it
ideal for use on low-rise frame buildings where heavier cladding systems would
increase the size of framing members required.
A variety of precast concrete components can be used in creating a completepassive-design system for a building. Foremost among these are:
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
12/16
Hollow-core slabs, which serve as combined floor/ceiling systems and can also be
used as wall panels in either vertical or horizontal configurations.
Wall panels, which offer high fire ratings and work with other components to create
a noncombustible envelope. Insulated sandwich wall panels can also be used.
Double tees, which can be used similar to hollow-core planks for roofs, ceilings,
floors, or wall panels.
Columns and beams, which create a framework that will resist intense heat and will
not add fuel to a fire.
A total-precast concrete system provides an effective design for minimizing fire
damage and containing the effects within the smallest space possible for the longest
time.
The Use of all Pre-Cast Structure including tees, beams, columns, slab & walls
Precast concrete components provide a variety of savings to a project in ways that
are not always considered when looking at upfront costing versus other materials,
including masonry and curtain wall. These savings include:
Speed. Precast concrete components provide a variety of ways to speed the
construction process, from design through fabrication and erection. These
efficiencies can shave as much as one-third of the time needed for construction,
meeting tight deadlines and generating revenues quicker. Time can be saved
through:
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
13/16
The fabrication process. Precast concrete components can be fabricated while
permitting and foundation work progress, so they are ready to begin erection as soon
as foundations are complete. As a single-source supplier for a large portion of the
structural system, precasters help maintain the critical path scheduling.
The erection process. Foundations can be placed one day and precast concrete
loadbearing or non-loadbearing panels can be erected as soon as the foundations
have cured sufficiently. Wall panels, double tees, and hollowcore planking also erect
quickly, often cutting weeks or months from the schedule. This speed allows
construction to get into the dry quicker. The fast enclosure also lessens concern for
weather or material damage during erection, reducing the contractors risks and
costs.
The finishing process. Precast concrete insulated sandwich panels create a
finished interior wall that avoids the time and cost of furring and drywalling.
Architectural panels can have a variety of colors and textures cast into them,
including several in one panel, eliminating the need to field-set trim pieces or paint
the faade after the structure is built.Design Economy. The custom, sculptured designs that are possible with precast
concrete may be achieved within a limited budget by selecting economical
aggregates and textures combined with repetitive units and effective production and
erection details. By reusing the same dimensions for components, the same molds
can be used, minimizing the total number needed and the changes between casting.
Efficiency is created by making it possible for similar, if not identical, shapes to be
produced from the same basic (master) mold and by minimizing the time required to
disassemble a mold and reassemble it for the next piece.
Hybrid post-tensioned precast frame: This method has the precast concrete
beams connected to multistory columns by unbonded, post-tensioned strands that
run through a duct in the center of the beam and through the columns. Mild steel
reinforcement is placed in ducts at the top and bottom of the beam, which is sleeved
through the column and grouted. The reinforcement yields alternately in tension and
compression and provides energy dissipation, while the post-tensioning strands
essentially act as rubber bands that help right the structure after the seismic event
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
14/16
ends. There are no column corbels, with the vertical shear resistance provided by
the post-tensioning strand. The post-tensioning steel balances the mild steel
reinforcement so the frame re-centers after flexing during a seismic event.
Pre-Cast Concrete Components Manufacturing Assembly Line
A Pre-tensioned precast frame, which is applied at locations where the most
economical connection method features one-story columns with multispan beams.
The multispan beams are cast with partially deboned pre-tensioning strand set on
the columns. The columns reinforcing steel extends through the sleeves inside the
beams. Reinforcing-bar splices ensure continuity above the beam. As the frame
displaces laterally, the de-bonded strand remains elastic. While the system
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
15/16
dissipates relatively less energy than other systems, it re-centres the structure after a
major seismic event.
Day 1 -Foundation Ready Day 7 Taking Shape
Day 15- Interior Shear Walls Day 25- Shear Structure
Day 44- I am ready
Finally we would like to close this article with one wonderful note
in which we can see that how human imagination is working for
innovations to create wonderful structures not only on earth but on
Moon.
7/29/2019 Risk & Opportunities Associated with Lightweight Concrete Structures
16/16
Lunar Concrete: Mr. Larry Beyer of University of Pittsburg
conceived idea of lightweight concrete formed from lunar regolith
which is called as Lunacrete or Mooncrete. It can be produced on
moon with using cementitious material on the moon and regolith as
aggregates with water and sulfur. Water can be produced there
with mix of oxygen with hydrogen produced from lunar soil. This
concrete will be useful for future permanent construction on the
moon saving lot of cost for transportation from earth.
*******************************************************************************