Management and economic analysis of an experimental study for
strengthening and rehabilitation of reinforced concrete frame with
CFRP
F. Rangelova
Department of Construction Management and Economics, University of Architecture, Civil Engineering and
Geodesy (UACG), Sofia, Bulgaria, fantina_fce@uacg.bg
E. Abdulahad
Department of Solid Structures, University of Architecture, Civil Engineering and Geodesy (UACG), Sofia,
Bulgaria, georgosing@gmail.com
D. Panichkov
Department of Solid Structures, University of Architecture, Civil Engineering and Geodesy (UACG), Sofia,
Bulgaria
ABSTRACT: The Bulgarian building heritage’s evaluation showed that large percent of the buildings needed
to be rehabilitated. Thereby, the development of effective and affordable rehabilitation techniques is an urgent
need.
The Fiber Reinforced Polymer (FRP) composites can provide rehabilitation alternative for unreinforced masonry walls. In addition to their outstanding mechanical properties, the advantages of the FRP composites versus conventional materials for rehabilitation and strengthening of structural elements include: unchanged dynamic properties of the structures because little addition of weight and stiffness, lower installation cost,
improved corrosion resistance, onsite flexibility of use, minimum changes in the member size after repair, minimized disturbance to the structures’ occupants, minimized loss of usable space during the rehabilitation.
The main objectives of the presented and analyzed experimental study is to be develop an advanced method
for strengthening and rehabilitation of reinforced concrete frame with CFRP Composites, standing new and
pendant research topic and activities for Bulgaria.
1 INTRODUCTION
The Bulgarian building heritage’s evaluation showed
that large percent of the buildings needed to be rehabilitated. Thereby, the development of effective and
affordable rehabilitation techniques is an urgent need.
The strengthening of concrete structures with externally bonded steel reinforcement is already a wellknown technique for many years. The basic principle
of externally bonded reinforcement is very simple.
Additional reinforcement, in most cases for carrying
tensile forces, is added to the structure by bonding it
onto the structure’s elements. Originally only steel
plates were used, but with the development of new
high grade FRP (fibre reinforced polymers) materials
(e.g. in the aerospace industry), new materials became available to be used for structural strengthening
purposes.
In management of civil engineering structures,
lack of decision support tools, which identify the
whole of life benefit/cost of using smart materials and
technologies such as Fiber Reinforced Polymer
(FRP) composites, hinders adoption of such smart
technologies. A newly commenced research project
the Bulgarian CRC for Construction Innovation was
primarily being undertaken at Structural Laboratory
at the UACG is aimed at addressing this gap in
knowledge by developing an integrated framework
for the use of FRP composites in rehabilitation of
civil engineering reinforced concrete and masonry
structures.
Fiber reinforced polymer (FRP) is a composite
material made by combining two or more materials to
give a new combination of properties. However, FRP
is different from other composites in that its constituent materials are different at the molecular level and
are mechanically separable. The mechanical and
physical properties of FRP are controlled by its constituent properties and by structural configurations at
micro level. Therefore, the design and analysis of any
FRP structural member requires a good knowledge
of the material properties, which are dependent on
the manufacturing process and the properties of constituent materials.
FRP composite is a two phased material, hence its
anisotropic properties. It is composed of fiber and
matrix, which are bonded at interface. Each of these
different phases has to perform its required function
based on mechanical properties, so that the composite system performs satisfactorily as a whole. In this
case, the reinforcing fiber provides FRP composite
with strength and stiffness, while the matrix gives rigidity and environmental protection. In addition to
their outstanding mechanical properties, the advantages of the FRP composites versus conventional
materials for rehabilitation and strengthening of
structural elements include: unchanged dynamic
properties of the structures because little addition of
weight and stiffness, lower installation cost, improved corrosion resistance, onsite flexibility of use,
minimum changes in the member size after repair,
minimized disturbance to the structures’ occupants,
minimized loss of usable space during the rehabilitation. Different fibre types can be used, but most
commonly carbon, glass or aramid are applied The
corresponding composites are indicated as CFRP,
GFRP or AFRP. The good mechanical properties of
the FRP materials are only available in the fibre
direction.
Mainly, two types of FRP reinforcement are available on the market: hardened plates and flexible
sheets (wet lay up). The end result, the fibres embedded in a resin matrix is often indicated by the term
“laminate”. In the early stage, the hardened FRP
plates were autoclaved, which limited the available
length. Later on, FRP plates produced without autoclaving came on the market. These plates are available in different thicknesses, widths and stiffness and
with nearly unlimited length. The application of these
hardened FRP plates is analogous to the application
of steel plates. Another type of FRP reinforcement is
the UD-sheets, which are very flexible and can easily
be cut by means of scissors, figure 1. At application,
the flexible sheets are impregnated with the right ratio of epoxy resin components and the chemical reaction starts. When the first layer has hardened enough,
the second layer can be applied in the same manner.
The sheets are completely impregnated on the site.
Both types of FRP, the sheets and the plates, are
available on roll, which means that they are available
in any length, whereas the steel plate length is limited
in practice to 6 meters. The FRP materials consist of
continuous fibres embedded within a thermosetting
resin system. The resin is required to bond the fibres
together and transmit loads between fibres. Some
mechanical properties such as in-plane and
interlaminar shear are highly resin dependent, whilst
others such as longitudinal strength and stiffness are
highly fibre dependent.
The most common resins used in FRP are polyester based, which are economical and of low-moderate
strength. Numerous grades of polyester resin are
available, but the most common consist of either
orthophthalic or isophthalic saturated acids.
Orthophthalic resins are more economical but exhibit
low mechanical properties and chemical resistance
and are less likely to be suitable for the reinforcement
of concrete structures, where good resistance to alkaline environments and low shrinkage may be required.
Vinylester and epoxy based resins offer improved
mechanical properties but with increased cost. However, if high performance fibres such as aramid or
carbon are being used, the resin will only form a very
small portion of the total cost. This is another reason
why for structural purposes nearly always an epoxy
resin will be used.
Carbon fiber is higher-performance fiber available
for civil engineering application. They are manufactured by controlled pyrolysis and crystallization of
organic precursors at temperatures above 2000 oc. In
this process, carbon crystallites are produced and
orientated along the fiber length. There are three
choices of precursor used in manufacturing process
of carbon fibers-rayon precursors, polyacrylonitrile
(PAN) precursors, and pitch precursor. PAN precursors are the major precursors for commercial carbon
fibers. It yields about 50% of original fiber mass.
Pitch precursors also have high carbon yield at lower
cost. However, they have less uniformity of manufactured carbon fibers. Carbon fibers have high elastic
modulus and fatigue strength than those of glass fibers. Considering service life, studies suggests that
carbon fiber reinforced polymers have more potential
than aramid and glass fibers.
The main objectives of the presented and analyzed
experimental study is to be develop an advanced
method for strengthening and rehabilitation of reinforced concrete frame with CFRP Composites,
standing new and pendant research topic and activities for Bulgaria. Regardless of the well known advantages of FRP one critical issue need to be justified. The important issue that must be determined is
the competitiveness of FRP strengthening method on
a cost basis in the future, compare to conventional
methods such as strengthening with steel.
Life Cycle Cost is probably the best process to answer that issue. Life Cycle Cost of FRP strengthening method includes the Initial Costs, Maintenance/Inspection/Repair Costs, and Disposal Costs.
The use of FRP composites as a strengthening solution for reinforced concrete frame is expected to increase service life and lower maintenance costs. The
main problem encountered is the initial costs of FRP
strengthening method are significantly higher than
those from steel. Hence, the initial costs of FRP
strengthening must be reduced to be cost competitive
with the steel on a life cycle cost basis.
The initial future costs can be estimated by utilizing improvement (learning) curve theory and various
improvement models to predict future costs are under development. The various models apply the improvement theory with different bases and the results
obtained are varied. The preliminary results indicate
that CFRP strengthening method become economically feasible.
2 EXPERIMENTAL SET UP
For the goals of the experiment there were prepared
two reinforced concrete beams, particularly connected with two columns (frames).
The beams is reinforced with 2N10 bottom reinforcement and 2N10 top reinforcement, and stirrups
Ø6,5 by 20 cm. The columns are reinforced with
4N14 and stirrups Ø6,5 by 20 cm.
Between the columns and beams are implemented
a masonry for realization of the real task of the experimental study. The shoulder with columns and
beam masonry is loaded with horizontal and vertical
force.
After frames’ collapse they were strengthened:
one of them with CFRP system, and one with shaped
steel and steel plates and tires:
Figure 1. Reinforced Concrete Frame strengthened with CFRP
200
20
2
1
2
175
25
1
1-1 2-2
25
шини-5mm
L 50/50/5
25
Figure 2. Reinforced Concrete Frame strengthened with Steel
Figure 3. Reinforced Concrete Frame strengthened with CFRP
In the experimental work the follow FRP materials
and systems were applied:
MEGAWRAP 200 – one directional carbon fibers
laminate with follow technical characteristics:
- carbon fibers’ weight – 200 g/m2 ;
- total carbon fibers’ weight - 224 g/ m2 ;
- thickness – 0,11 mm;
- tension strength ffib - 3800 MPa ;
- module of elasticity Efib - 235 GPa;
- deformation by breakdown εfib - 1,5%;
- density 1,81 g/m3.
The mechanical properties of the carbon fibers are
average quantities, received experimentally by tests
for tension strength by ASTMD 4018-81
EPOMAX – LD
Two-component epoxy resin is appropriate for carbon and synthetic fibers, which are used for static
strengthening or reinforced concrete elements’ correction:
- flexural strength – 44,6 MPa;
- elastic extension – 1,7 %;
- Stress strength – 90 MPa;
- Tension strength – 70 MPa;
- Module of elasticity – 2500 MPa;
- Adhesion strength - >4 N/mm2.
The experiment was implemented in the Structural
Laboratory to the Department of Solid Structures at
the University of Architecture, Civil Engineering and
Geodesy (UACG), Sofia, Bulgaria with assistance of
“ISOMAT INTERNATIONAL” Ltd. and “A&K
Engineering” Ltd.
The CFRP strengthening was performed with application of 2,5 м2 carbon fiber laminate with thickness
t = 0,11mm, and with 2,5 kg epoxy resin (1kg/m2).
The steel strengthening was performed with application of 35 numbers tires with dimensions: 150/50/5
mm, 11 m shaped steel - L50/50/5, edge-iron plates
with А = 900см2, d = 5mm and 108 dubels - 50/5
mm.
There was implementing and about 3 m2 guniting
with thickness 2 cm.
learning curves have been applied to all types of
work from simple tasks to complex jobs. Improvement curves are a more appropriate name for learning curves. Improvement rate is the complement of
learning rate; thus if the learning rate is 90 percent,
the improvement is 10 percent.
The implemented experimental investigations give
the reason to be assumed that the strengthening with
the CFRP’s is effective. It means that the character
of the work of the reinforced concrete elements is the
same. Only the bearing capacity of the reinforced
concrete elements increasing, and guarantee rehabilitation of a part of the constructive physical wear.
3 COST ANALYSIS
There are two approaches one could use for cost
analysis, initial costs and life cycle costs. Basically,
initial costs are a subset of life cycle cost. When initial cost is the major cost component, life cycle costing results will be similar to considering only initial
costs. However, when inspection, maintenance, and
disposal costs become dominant, life cycle costing
should be utilized.
3.1 Initial coast
Initial costs include the material cost, component
manufacturing, fabrication, assembly, shipment, installation and testing costs. They reflect the largest
costs in most strengthening methods and are appropriate for a majority of the applications.
When comparing conventional and composite
strengthening methods on the basis of initial costs,
it’s clear that the direct initial costs favor conventional one. The higher initial cost of CFRP strengthening method is expected due to the high fiber and
resins costs. However, the maintenance, rehabilitation, demolition, and indirect costs favor composite
strengthening. Projects with long lives require that
life cycle costing be utilized, as polymer strengthening should have reduced rehabilitation and maintenance costs. In order to be competitive, it is felt that
the initial costs of CFRP strengthening must be approximately € 40 per square meter to be competitive
with steel strengthening solution.
3.1.1 Improvement Curves
Learning curve was first applied in the aircraft industry, and translated into an empirical theory in 1925.
In 1936 T. P. Wright disclosed the results of empirical tests of the learning curve and described a basic
theory for obtaining cost estimates based on repetitive production of airplane assemblies. Since then,
Figure 4. Typical Learning Curve (Xanthakos, 1995)
3.2 Life Cycle Costs
Whole of Life cycle cost analysis (WLCCA) is an
evaluation method, which uses an economic analysis
technique that allows comparison of investment alternatives having different cost streams. WLCCA
evaluates each alternative by estimating the costs and
timing of the cost over a selected analysis period and
converting these costs to economically comparable
values considering time-value of money over predicted whole of life cycle.
The analysis results can be presented in several
different ways, but the most commonly used indicator in construction asset management is net present
value of the investment option. The net present value
of an investment alternative is equal to the sum of all
costs and benefits associated with the alternatives
discounted to today’s values (Darter and Smith,
2003).
Making a decision for selection of the rehabilitation method will be done by minimizing the life cycle
costs. Such a decision analysis is referred as a whole
of life cycle costing, cost-benefit or cost-benefit-risk
analysis. Life cycle costs will assess the cost effectiveness of design decisions, quality of construction
or inspection, maintenance and repair strategies
(Stewart, 2001).
Life Cycle Costing (LCC) is defined as "The total
cost of the system or product under study over its
complete life cycle or the duration of the period of
study, whichever is the shorter". The study period
of LCC is defined as the length of time over which an
investment is evaluated. It depends on time horizon
of investor or expected life of system. Three key
dates of study period are base date (beginning of
study period), service date (beginning of operational
period), and end date (end of study period).
The six main steps in an LCC analysis are: (1)
Identify feasible project alternatives (2) Establish
common assumptions (3) Identify relevant project
costs (4) Convert all money amounts to present value
(5) Compute and compare LCCs of alternatives, and
(6) Interpret results. In order to get appropriate
analysis, assumptions should be clearly defined; the
most common ones are the definition of Life, Costs,
Initial costs, Discounting and Inflation, Taxation, and
Benefits.
Since FRP is a new-technology material, it is required to compare this technology with the conventional technologies. Ehlen and Marshall recommend
the following steps for calculating the life cycle cost
of a new-technology material vis-à-vis a conventional
material. Those steps are:
o Define the project objective and minimum
performance requirements;
o Identify the alternatives for achieving the objectives;
o Establish the basic assumptions for the analysis;
o Identify, estimate, and determine the timing
of all relevant costs;
o Compute the LCC for each alternative;
o Perform sensitivity analysis by re-computing
the LCC for each alternative using different
assumptions;
o Compare the alternative’s LCCs for each set
of assumptions;
o Consider the other project effects;
o Select the best alternative.
In each alternative the user should use the same
fixed discount rate and the same study period. Implicit in any LCC analysis is the assumption that every proposed alternative will satisfy the minimum performance requirements of the project.
These
requirements include structural, safety, reliability, environmental and specific building code requirements.
The Perform sensitivity analysis by recomputing the
LCC for each alternative using different assumption
is the life cycle cost method is a fundamental part of
assessing new construction material. The costs and
technical performance of new materials are intrinsically uncertain; and this method must address this
uncertainty. The inherent cost uncertainty of materials and designs that are not in mainstream use can be
handled with Monte Carlo simulation.
Further, those researchers suggested using the
LCC classification scheme when evaluating newtechnology material, mainly to make sure that all
costs associated with the project are taken into account in each alternative. Three level cost classification proposed include: Level 1: Costs by LCC Category (typically used are construction, operation/
maintenance/ repair, and disposal); Level 2: Costs by
the Entity that Bears the Cost (agency costs, user
costs, and third-party costs), and Level 3: Costs by
Elemental Breakdown (elemental costs, nonelemental costs, new-technology introduction costs).
The life cycle cost of an alternative is represented
by either: (1) Present Worth Cost or (2) Equivalent
Uniform Annual Cost. Another approach might be
used is Benefit/Cost Ratio.
The equation to calculate the life-cycle cost of an
alternative using the first approach is as follows:
LCC (PV) = PVIC + PVOMR + PVD
PVIC = Present Value of Initial Costs
PVOMR = Present Value of Operation, Maintenance, and Repair costs
PVD
= Present Value of Disposal costs
The costs associated in a rehabilitation project
may initially include:
o Initial cost;
o Maintenance, monitoring and repair cost;
o Extra user cost;
o Estimated cost of failure
All of these costs are valued in resource cost terms
(i.e. Market prices + subsidies - taxes). If monitoring,
repair, extra user cost are considered as the maintenance cost.
Benefit Cost Ratio method in principle examines
the extra benefits of advancing one improvement level to the next divided by the corresponding extra
costs. Cost-Benefit Comparison is suitable for comparing alternatives with equivalent life expectancy,
performance and maintenance. If significant differences are expected in one of these factors, first cost
analysis will not give a true comparison of cost effectiveness of the various alternatives. Life cycle cost
analysis is similar to first cost analysis except that future costs are also considered. Typically, future costs
include maintenance, future rehabilitation expenditures, and probable replacement costs. In life cycle
cost analysis, future costs must be discounted to present worth before they are combined with present
(immediate) costs.
3.3 Initial Cost Feasibility Study
When evaluating first-cost in relation to traditional
methods, strengthened CFRP technology can be
more expensive, but the key advantages of FRP
strengthening are often overlooked in relation to
their high material and manufacturing costs. FRP
strengthening can be cost-effective in the long run. A
direct comparison of the unit price basis may not be
appropriate, but rather an overall project and lifecycle cost.
There are arguments that initial costs of new technologies decrease with time, as their use become
more extensive and accepted. On the analogy of
Xanthakos (1995), the strengthening method that use
FRP are expected to have higher initial costs than
traditional strengthened reinforced concrete frame,
due to high cost of fiber and resins. However, this initial cost will decrease as more frames are repaired
according to the Learning Curve theory. “The Learning Curve theory predicts that, as experience builds
up, the cost will decrease in an exponential manner.
Typical costs start high, but drop steeply when methods and materials become more cost effective as the
product matures. Over time, the large inefficiencies
are removed from the process and the costs stabilize.”
The economic analysis of presented experimental
study show that the reinforced concrete frame’s
strengthening with conventional steel is cost about
€35, 48 per square meter, compared to the reinforced
concrete frame’s strengthening with advanced CFRP
composite system, which is cost about € 48,35 per
square meter (Table 1.)
detour costs and inconveniences. Also, FRP material
installation is quite simple; the material is light and
can be installed using hand tools. There is a high likelihood that, once trained, the technology can be applied by state employees. Though gains in structural
strength may not be a concern for rehabilitation, construction strengthened using the FRP method will
experience increases in strength and therefore have
higher capacities. Since the costs of the FRP
strengthening solution included contracting fees
(profit, overhead, etc.), the application of the FRP
may become significantly less expensive once this
strengthening is performed exclusively by state “inhouse” forces. Repairs using this method are easier
and faster to implement. Also, these strengthening
tend to be more durable than those using conventional methods, allowing for less frequent re-repairs.
Table
1. Initial cost of applied strengthening methods
______________________________________________
Materials/
Steel Method _____________
CFRP System
____________
Work
BG
Lev
Euro
BG
Lev Euro
______________________________________________
Shaped steel
62,37 33,88
Steel plates
14,76
9,47
Steel tires
9,98
5,10
Dubles
54,00 27,61
Labor
32,40 16,57
36,00
18,41
CFRP
175,00
89,47
Epoxy resin
25,43
13,00
TOTAL
173,51 88,71
236,43 120,88
_____________________________________________
5 ADVANTAGES OF FRP LAMINATES
COMPARED TO CONVENTIONAL
MATERIALS
4 ANALYSIS AND CONCLUTIONS
The implemented experimental investigations give
the reason to be assumed that the strengthening with
the CFRP’s is effective. It means that the character
of the work of the reinforced concrete elements is the
same. Only the bearing capacity of the reinforced
concrete elements increasing, and guarantee rehabilitation of a part of the constructive physical wear.
Economics analysis shown that the cost of CFRP
strengthening method is competitive with the cost of
the traditional method, and when “life-cycle”
strengthening is included, the FRP method is less expensive. Also, it is likely that once the FRP strengthening solution becomes more widely accepted and
used, both material cost and experience in installation
will improve (Learning Curve theory).
Setting aside cost differentials, the FRP strengthening
method has many other advantages over the conventional strengthening method. The most pronounced
advantage is speed of application. The FRP method
may allow for the construction to remain in exploitation during strengthening. Also, the installation and
curing time for the FRP strengthening solution is
quite short in comparison to the more complex forming traditional strengthening with steel. Thus the rapid application of FRP repair can lead to reductions in
The actual advantages of FRP laminated composites
to conventional materials should be estimated from
both technical and economic points of view.
From technical point of view, composite materials
have significant advantages:
o Laminated composites offer significant
weight saving over existing metals. Composites can provide structures that are 25-45 %
lighter than the conventional structures designed to meet the same functional requirements;
o Unidirectional fiber composites have specific
tensile strength (ratio of material strength to
density) about 4 to 6 times greater than that
of steel and aluminum;
o Unidirectional fiber composites have specific
modulus (ratio of material stiffness to density) about 3 to 5 times greater than that of
steel and aluminum;
o Fatigue endurance limit of composites may
approach 60% of their ultimate tensile
strength. For steel and aluminum, this value is
considerably lower;
o Corrosion resistance of fiber composites leads
to reduced life cycle cost.
From an economic point of view, the main factors
contributing to their competitiveness with respect to
conventional materials are time saving, flexibility,
low labor costs, low tooling and machinery costs on
the construction site because of the light weight and
manageability of tools, possibility of restoring a
structure without interrupting its utilization by users
and durability.
In spite of many advantages of FRP laminated
composites over traditional materials complex me-
chanics involved in laminated fibrous composites
poses new challenges for the construction industry.
6 APPLICATION OF FRP LAMINATED
COMPOSITES IN STRUCTURAL
ENGINEERING
Structural safety is always a crucial aspect, especially
in seismic areas where social and economic concerns
are very high. More resources are being devoted to
the retrofit and upgrade of existing structures. The
widespread use of the FRP system is in the restoration and seismic strengthening of historical masonry
buildings. The use of laminated composites is thus
becoming more widespread as an optimal innovative
system able to reduce the seismic vulnerability of reinforced concrete structures. It ensures an effective
confinement of concrete.
The use of laminated composites is also very effective in case of urgency, for safety and temporary
preservation of structures damaged during special
events.
Structures having strategic interest and identified
as sensitive objectives (in case of explosion risk, terrorist or attempted attacks, etc), the adoption of laminated composites can help to limit damages to persons and structures.
FRP laminated composites can be efficiently used
where a fast installation is a crucial factor, such as
during military/army operations etc.
The use of FRP laminated composites in the aerospace and automobile industries is now wellestablished and gaining momentum in structural applications such as buildings, bridges and marine applications. The use of FRP laminated composite in
structural applications is discussed along with significant masonry, reinforced concrete building elements
and bridge superstructure developments.
The outlook for FRP laminated composites is very
promising in structural engineering. FRP laminated
composites offer a number of advantages over traditional materials and as such can be a viable alternate
solution in civil engineering structures.
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