chapter one
ASSESSMENT OF STONE DUST IN PRODUCTION
OF HIGH PERFORMANCE CONCRETE
JOFFREY CHERUIYOT
MASTER OF SCIENCE
(Construction Engineering &Management)
JOMO KENYATTA UNIVERSITY OF
AGRICULTURE AND TECHNOLOGY
2015
Assessment of stone dust in production of high performance Concrete
Joffrey Cheruiyot
A thesis submitted in partial fulfillment for the degree of Master of
Science in ConstructionEngineering & Management in the
Jomo Kenyatta University of
Agriculture and Technology
2015
i
DECLARATION
This thesis is my original work and has not been presented for a degree in any other university.
Signature: _________________________ Date: ____________________
Joffrey Cheruiyot
This thesis has been submitted for examination with our approval as the university supervisors
Signature: ___________________________ Date: ___________________
Dr. Eng. Sylvester Ocheing Abuodha
UoN, Kenya
Signature: __________________________ Date: ____________________
Eng. Charles K. Kabubo
JKUAT, Kenya ii
DEDICATION
This work is dedicated to my wife, Beatrice Cheruiyot, my mother, Mrs. Ruth Langat, my children Christian Kiptoo and Ryan Kipropfor giving me easy moments during my studies. For their encouragement and support during my three years of study. iii
ACKNOWLEDGEMENTS
I wish to sincerely thank my supervisors, Engineer Kabubo, and Dr. Engineer Abuodha for their support and correction of my reports. I also extend my gratitude to Jomo
Kenyatta University for giving me the chance to study, research and financially supporting the research. To Kamami and all structural laboratory staff, for giving their time and support during the study. Bamburi special products, specifically Jerita Okoti for providing admixtures and sharing their experience in its use. Finally, Mary Wachira of JKUAT library postgraduate section for their guidance in formatting and reference and citation. iv
TABLE OF CONTENTS
LIST OF ABBREVIATIONS/ACRONYMS .......................................................... X
2.1 Materials usedin production of HPC ...................................................... 10
v
2.3 Theoretical review/Conceptual framework ............................................ 13
3.2 Design And Manufacture And Tests Of Concrete (HPC) .................................... 16
3.3 Structural Design of 10 Storey Office Structure .................................... 19
Suitability of stone dust in production of HPC ...................................... 22
Design Mixes and results ....................................................................... 24
Comparison of stone dust HPC with traditional concrete ...................... 29
Structural design to BS 8110 (1997) ...................................................... 29
4.3 Increased floor space as a result of reduced member sizes .......................... 31
SUMMARY, CONCLUSIONS AND RECCOMENDATIONS .......................... 34
vi
vii
LIST OF TABLES
Crushed stone dust grading results ...................................................... 23
Design mixes and laboratory results .................................................... 25
Structural member sizes obtained from the structural design .............. 29
Area freed from use of smaller members and estimated returns ......... 32
Cost benefit analysis presentation ....................................................... 33
viii
LIST OF FIGURES
Theoretical framework .................................................................... 13
Layout of the structure (Detailed drawings found at the appendix)
Material properties for stone dust used for concrete design ........... 23
Stress-strain curve for HPC (150mmx250mm cubes) .................... 28
ix
HPC
CBA
LIST OF ABBREVIATIONS/ACRONYMS
High Performance Concrete
Cost Benefit Analysis
W/C Water Cement Ratio
NPV Net Present Value
IRR Internal Rate of Return ( appreciated as, r in the calculation )
LCCA Life Cycle Cost Analysis
CF
1
The period of one net cash inflow
FM Fineness Modulus
SG Specific Gravity
WA Water Absorption
RD Relative Density
CEM I 42.5R Portland cement to Kenya Bureau of Standards (Also PC 42.5 R)
CEM I 52.5RPortland cement to Kenya Bureau of Standards (Also PC 52.5 R)
KES Kenya Shillings
M c
=Mass of cement
D c
=Density of cement
M s
=Mass of stone dust
D s
=Density of stone dust
M w
=Mass of water
D
W=
Density of water x
ABSTRACT
This study evaluated the suitability of stone dust in the design and production High
Performance Concrete (HPC). Stone dust from the quarry was sampled, tested and used to produce HPC mix. Structural design was done using the new HPC in comparison with conventional concrete classes currently in the market (Class 25, 30 and 35). Comparison was done using Cost Benefit Analysis (CBA) taking into consideration project full cycle costs. Economic evaluation of HPC was done using Internal Rate of Return (IRR) and
Net Present Value (NPV). The research addressed two problems; improving the quality and strength of concrete which results in more economical structural member sizes when used in design. This in effect increases working (lettable) space per floor in high-rise buildings. Use of stone dust which is a by-product of stone crushing and rarely used as a construction material will enhance environmental conservation. Environmental conservation was achieved in two ways; use of the rarely used by product of stone crushing process and reduced or eliminated use of river sand which over time has led to river degradation.
Laboratory tests were done to establish the properties of the designed concrete, compressive strength, and modulus of elasticity. Structural analysis using BS 8110:1997 was done for a 10 storey office building to establish structural member sizes. Members obtained from concrete classes 25, 30, 35 and the new compressive strength from HPC
(Class 80) were compared. Analysis was done for structural members’ sizes and area freed as a result of designing with HPC as well as the weight of reinforcement steel used.
To justify the use of HPC, CBA was used to compare high initial capital investment and benefits resulting from increased work space created. The minimum class of concrete used in design was limited to class 25 N/mm
2 which formed the base of the design. The research shows that it is possible to manufacture high strength concrete using locally available stone dust. The concrete made using stone dust sampled from a quarry in Athi xi
River, Kenya, achieved a nominal strength of 87.6 N/mm
2
at a water cement ratio of
0.32. A plasticizer and water reducing admixture was used to reduce water cement ratio in the concrete mix and improve workability. Structural analysis of a 10 storey structure with columns spaced at 8 meters center to center was designed using the four classes of concrete (25, 30, 35 & 80) and results compared. There was a reduction of column sections sizes from 0.9 m wide to 0.50m (over 45%) when concrete class changes from class 25 to class 80 creating over 3% of the total space area per floor.
Cost benefit analysis using Net Present Value (NPV) and Internal Rate of Return (IRR) presented business case for the use of HPC. Using concrete class 80, the IRR was calculated at 3% and NPV being 8% more than total initial investment. The steel reinforcement increased by 8.64%using class 30, 11.68% using class 35 and reduced by
8.37% at class 80. The base class for comparison was class 25 which is commonly used for structural design in Kenya. This study provides useful information to design
Engineers and Architects and informs future design of reinforced concrete structures. xii
CHAPTER ONE
INTRODUCTION
1.1 Background of the study
High Performance Concrete (HPC) has been used for more than 40 years. High-strength concrete itself has been enhanced in terms of performance and placeability through a lot of useful developments to the stage that now many applications have been done in North
America, Europe, Japan and other countries (Kuwai, 2001). Even though applications of
HPC have been greatly used in Japan, Europe and United States of America, little has been done here in Kenya. Research findings, and cost benefits analysis done for the 100 year design life using HPC will provide the engineering practitioners the necessary information for their design. Use of stone dust will also enhance environmental conservation particularly where river sand use will be reduced.
According to Mungai et al. (2000) sand harvesting has resulted in destruction of underground aquifers and loss of safe water. The scooping process has over time adversely affected surface water quality and quantity and damaged the aquatic ecosystem. Developing a concrete devoid of river sand will greatly enhance environmental conservation and reverse effects of sand harvesting in many river beds in
Kenya. HPC is an important structural material and merits careful attention because, although it is more expensive than conventional concrete per cubic metre, it can carry a given load more economically (Addis, 1991).
Thus producing HPC will greatly solve the problem of huge structural members, column spacing and increase on lettable space.
1.2 Statement of the Problem
Current reinforced structural design using classes; 25, 30 or 35 Mpa concrete results in large structural members which are costly and reduce working spaces per floor. Not only does a reinforced concrete member utilize bigger floor space, they also carry fewer loads per unit volume. Designing with huge structural members implies that more materials used for a given members and hence being unsustainable in the long run with depletion
1
of natural construction materials. Low strength concrete coupled with poor construction quality checks have also led to structural collapse of buildings. It is imperative that new mixes with higher strengths be introduced to solve the problem.
Using river sand as fine aggregates has caused significant problems in the mining areas.
Sand acts as a safe aquifer for water flowing below and through it. Removal of sand results in destruction of underground aquifers and loss of safe water. Sand scooping adversely affects surface water quality and quantity and damages the aquatic ecosystem.
Haulage of sand by heavy trucks causes environmental degradation by accelerating soil erosion and affecting soil stability in the mining areas. Storage of sand causes destruction of surface areas through clearing of vegetation and uses land that could otherwise be used for agriculture. Stone dust is least used as fine aggregate in Kenya, due to its high water demand as a result most of the materials is stockpiled as waste in the quarries. Use of stone dust which is often a waste product during production of aggregates will enhance environmental conservation in two ways; eliminating river sand harvesting and providing better use of stone dust which is usually dumped at the aggregate quarries.
1.3 Justification
This research investigated the suitability of stone dust (quarry waste) in manufacture of high performance concrete. Production of aggregates results in huge volumes of poorly graded waste material with predominantly fine particles. The material is avoided in the production of structural concrete because of its water demand and thus affecting Water
Cement Ratio (W/C). Quarry waste has a great potential in high performance concrete production owing to its high content of fine particles.
Use of high-strength concrete resists loads that cannot be supported by normal-strength concrete. Not only does high strength concrete allow for more applications, it also increases the strength per unit cost, per unit weight, and per unit volume as well. HPC typically has an increased modulus of elasticity, which increases stability and reduces deflections. Modulus of elasticity was obtained using a compressive strength machine and strain gauge in 150mm cubes.
2
Even though applications of HPC have been greatly used in Japan, Europe and United
States of America (Kuwai, 2001), little has been done here in Kenya. Research findings, and cost benefits analysis done for the 100 year design life using HPC will provide the engineering practitioners the necessary information for their design. Use of stone dust will also enhance environmental conservation particularly where river sand use will be reduced or eliminated.
1.4 Objectives
1.4.1
Main objective
Evaluate the technical and economic suitability of stone dust (quarry waste) in design and production of high performance concrete.
1.4.2
Specific objectives
1) To determine the material properties of stone dust
2) To design, produce, and test high performance concrete from the stone dust,
Cement(CEM I 42.5R), and plasticizer
3) Evaluate the cost benefits of HPC use in reinforced concrete design
1.4.3
Research Questions
1) Can high strength concrete be developed using locally available stone dust and ordinary Portland cement (CEM 42.5R)?
2) Can High Performance Concrete use in structural design reduce structural members and result in increased lettable space?
1.5 Scope
The study evaluated the suitability of stone dust in production of high performance concrete. The suitability was studied by performing material tests, design of concrete using the stone dust, cement and an admixture. Fresh and hardened concrete tests were
3
performed results analyzed. Material properties of stone dust included; grading, relative density, water absorption, and fineness modulus. Concrete tests included; slump, compressive strength and modulus of elasticity. Compressive strength of the concrete was used in design of a model ten storey structure and compared with design from class
25, 30, and 35. The structural design was done using code of practice for design and construction BS. 8110:1997. to justify HPC use, cost benefit analysis was done utilizing internal rate of return and calculation of net present value. The cost benefits was computed using the total life costs of the structure, cost of reinforced concrete frame
(initial investment) and returns expected from the lease of extra space created. The tests were done in JKUAT Civil Engineering Laboratory as well as Bamburi Special Products laboratory. The research covered the costs of transporting materials from site which were given for free from the quarry, purchase of cement, admixture, strain gauges, and material tests for the cost of test done outside of JKUAT laboratory.
1.6 Limitations
This research evaluated design and production of HPC using stone dust obtained from stone crushing quarries in Athi River. Whereas there are several quarries within Athi
River and also several parts of the country, it was only possible to analyze well sampled material from one quarry because of cost involved and time available for research.
However the sample collected and data obtained is representative of the stone dust currently being produced in Kenya if it fits within the grading envelope, have same or lower water absorption and with same fineness modulus.
Laboratory tests were done for fresh and hardened concrete using British Standard BS
EN 12390-1 (1985), testing hardened concrete. Concrete was not tested on site because constructing a model structure that enables full scale tests is expensive and require over
10 years to be able to monitor its performance over time. Compressive strengths of obtained HPC were high and could not be crushed using the laboratory compressive strength machine available in JKUAT. Thus samples were taken to Bamburi laboratory for compressive strength tests. Bamburi Laboratory did not have strain gauges and data loggers and therefore more concrete cubes were cast to test modulus of elasticity at
4
JKUAT laboratory. Modulus of elasticity is analyzed within the 60% of compressive force on the cubes and thus the machine in the laboratory was adequate. The number of molds in the laboratory were limited, thus all the trial mixes could not be done at once.
Two trial mixes were done at any given time, allowing for molds to be recovered for the next test and therefore the study took longer than earlier estimated.
A10 storey structure was used for design to evaluate the cost benefits of the use of the developed HPC. The findings are comparable to any other reinforced concrete design, high rise and tall structures as well as bridges which use common design principles.
Towards the end of laboratory work, Bamburi Cement Company advertised in the dailies availability of Portland cement class 52.5 (CEM I 52.5R). Time did not allow tests on the new higher strength cement on HPC production.
5
CHAPTER TWO
LITERATURE REVIEW
2.1
Theoretical Review
Concrete has been used in civil engineering projects for more than 100 years. Concrete construction technology and use has evolved and become highly sophisticated through the years. Good-quality concrete can sustain various loads, including earthquakes, and is quite durable in all weather conditions. However, poor-quality concrete can cause damage to or the collapse of a structure even before construction is complete (Shyh-
Chyang, 2007). Collapsing structures has been common in Kenya in the recent past leading to loss of lives and property.
High performance concrete (HPC) is defined as concrete whose strength and durability is greater than those of “normally” obtained concrete (Addis and Owens, 2001).
According to United States of America Strategic Highway Research Program on high performance concrete, HPC is defined as having 4 hour compressive strength of greater than or equal to 17.2Megapascals (MPa), 24 hour compressive strength of greater than
34.5 MPa and 28 day compressive strength of greater than 68.9 MPa (Zia et al., 1991).
High strength concrete (HSC) is an example of HPC and according to Japanese
Standards of Civil Engineers, high strength concrete is any mix with strengths of between 58.0 MPa to 78.5 MPa (Russell & Moreno, 1997). Although more expensive, per cubic meter, than conventional concrete, HPC will carry a given load more economically (Russell & Moreno, 1997).
2.2
Types of High Performance Concrete
High-strength concrete which is an example of High Performance Concrete (HPC) has been enhanced in terms of performance and placeability (Kuwai, 2001).HPC production processes have gone through useful developments enabling many applications in construction (Jinai, 2005). Self-consolidating concrete (SCC), a type of High-
Performance Concrete (HPC) that is able to flow and consolidate under its own weight,
6
completely fill the formwork even in the presence of dense reinforcement, whilst maintaining homogeneity, and without the need for any additional compaction. SCC mixtures designated for prestressed applications should be highly workable to flow easily through restricted spacing and completely encapsulate reinforcements without any mechanical vibration (Wu-Jian, et.al. 2014).
Other High Performance Concretes (HPC) has been enhanced using additives such as silica fumes and iron blast furnace slug. The effects of silica fume on the properties of plastic and hardened concrete has been researched and its applications tried and used over time. Silica fume imparts significant improvement to the strength and durability of concrete; and the availability of this material together with high-range water reducers
(superplasticizers) has been largely responsible for the development of high-strength and high-performance concretes (Thomas, et. al., 1998).
Silica fume has been used in the Canadian cement and concrete industry for over 15 years. Early use was driven by economy, since concrete of a given strength grade could be produced at lower cementitious material content (and cost) when silica fume was incorporated in the mix due to the initial low selling price of the material (Thomas, et. al., 1998).The beneficial effects of silica fume on concrete properties rely on adequate dispersion of the silica fume particles throughout the concrete mix. This is achieved when suitable product form, sufficient levels of water-reducing ad-mixtures (or superplasticizers), and suitable mix proportioning and mixing procedures are used. The performance of concrete, in terms of its placeability, physical properties, and its durability, can also be enhanced by the use of slag-blended cements or separately added ground granulated blast-furnace slag. It also has advantages for architectural purposes due to the whiteness it imparts to concrete. Properly proportioned and cured slag concretes will control deleterious alkali–silica reactions, impart sulphate resistance, greatly reduce chloride ingress, and reduce heat of hydration (Hooton, 2000).
Iron blast-furnace slag, when granulated (i.e., quenched) and ground to cement fineness has been used as a primary or secondary binder to produce concretes for over 100 years
7
in Europe. Use of slag as an ingredient in quality concrete has expanded rapidly since the 1950s and has spread to Australia, the Pacific Rim, North America, and parts of
Africa (Hooton, 2000).The first major use of separated ground slag as a supplementary cementing material in Canada, was in 1976 with the opening of the Standard Slag
Cement plant in Fruitland (now Lafarge), in Ontario (Hooton, 2000).
The replacement level of Portland cement depends on both climate and application. The choice of producing slags in blended cements or as a separate ingredient to the concrete mixer is based on economics and local practice — both can be used to make quality concrete. In North America the trend has been to add slag separately at the concrete plants blast furnaces and other furnaces used to produce iron from iron ore in the presence of added limestone or dolomite fluxes produce a molten slag, which floats above the molten iron and is tapped off separately. This 1500–1600°C molten slag has about 30–40% Silicon dioxide (SiO
2
) and about 40% Calcium monoxide (CaO), which is close in composition to Portland cement. If cooled slowly in pits, the slag crystallizes intomelilite or merwinite minerals and, although useful as a concrete aggregate or road base, it possesses little hydraulic value. However, if quenched rapidly, as by granulation in water (or by pelletizing), it forms a glass, which when dried and ground is latently hydraulic (i.e., it just needs an alkaline environment to hydrate, but does not require the lime that pozzolans such as silica fume and Class F fly ash do (Hooton, 2000).
According to America Concrete Institute, (1997) there has been many application of
HPC since its introduction. HPC with compressive strength of 62 MPa was used in columns, shear walls and transfer girders of the Water Tower Place in Chicago in
1975(Jinai, 2005). Many applications of HPC in projects, ranging from transmission poles to one of the tallest building on earth (KLCC Twin Tower in Kuala Lumpur,
Malaysia) with concrete strength reaching up to 131 MPa in the Union Square building in Seattle, Washington have been reported (Jinai, 2005).
High-strength concretes are being increasingly used in the columns of high-rise buildings. Analytical studies of the slenderness effects in these columns have been very
8
limited. The behavior of slender columns with normal- and high-strength concretes can be easily studied using finite element programs (Antoine, 1991).
It is likely that in the near future, the high-strength concrete will be prevailing and increasing in use all over the world. Very little application of HPC has been realized here in Kenya, with most multistory buildings using traditional class 25-35 MPa concrete. The newly completed KCB towers in Upper Hill used class 35 for all the structural members. With the current trend of construction development which has increased over time, there is need to develop economic and environmental friendly construction materials.
There is considerable work done so far on high performance concrete internationally and research has been done on stone dust (sometimes referred to as quarry dust) here in
Kenya (Kiliswa, 2011). Stone dust is classified as material obtained from stone crushing process, and due to its fineness rarely used in concrete design because of high water cement ratio requirement (Addis, 1991; Kiliswa, 2011) studied the effect of quarry dust on the strengths and permeability of normal concrete (class 20 and 25). From his results he concluded that concrete designed with quarry dust as a substitute of river sand had plastic and hardened properties equal to or greater than traditional mixes (aggregate, river sand and cement).
Aggregate sizes have a significant influence in properties of both plastic and hardened concrete (Özbay, 2010). Mechanical and permeability properties of concretes are influenced by the water–cement ratio, the cement–aggregate ratio, the bond between mortar and aggregate, and the grading, shape, strength, and size of the aggregates.
In general, durability will result if the concrete has a low water to cement ratio, has achieved adequate thermal and moisture curing, and has achieved a discontinuous capillary pore structure free of significant micro and macro defects (DeSouza, et. al.,
1997).
Özbay(2010), in his study established that decreasing the maximum aggregate size increased the water permeability, rapid chloride permeability, water absorption, but decreased the compressive and split tensile strengths of concrete and concrete -equivalent
9
mortars. In his research, the proportioned concrete mixture having a water–cement ratio of 0.45 and cement content of 450 kg/m3 with a maximum aggregate size of 22.4 mm.
Then, keeping the total aggregate surface area constant, three mixtures were proportioned with the same water–cement ratio, but the maximum aggregate size decreased to 16, 8, and 4 mm using the concrete -equivalent mortar method. Mechanical properties including compressive strength, splitting tensile strength and transport properties including rapid chloride permeability, water absorption, and water permeability tests were performed at 7 and 28 days.
The zone of cement paste that is affected by the aggregate particle has been defined as the interfacial transition zone (Basheer et al., 2005). Moreover, most of the important properties of hardened concrete are related to the quantity and characteristics of various types of pores in the cement paste and aggregate components of the concrete. For example, the engineering properties of concrete, such as strength, durability, shrinkage, and transport, are directly influenced or controlled by the number, type, and size of pores present.
Through the use of high-strength concrete floor, areas can be increased because it allows the sectional area of building columns to be reduced (Jinnai, 2005). Whereas there are documented application in Japan, Europe and America (Kuwai, 2001; Jinai, 2005;
Aminul, 2008), Kenyan construction industry has not embraced the technology. This research results shows percentage savings gained from lettable space in highrise buildings when HPC is used in the design.
2.1
Materials used in production of HPC
The performance of HPC is specific to the combination of materials used to their proportions in the mix (Addis & Owens, 2001).The exact chemical composition of cement is critical for HPC. This is because of strong interactions between the aluminates, the ferroaluminate phases, the calcium sulphate phases, and the superplasticizer molecules (Russell & Moreno, 1997). Fine aggregates do not require high fines content since it is already provided by the cementitous materials. A fineness
10
modulus of 2.7 to 3.0 should be suitable for production of HPC (Addis & Alexander,
1990). According to Addis (1991) tests done in South African aggregates showed the optimum nominal size for coarse aggregates was found to be 19 to 26.5 mm. Other literature however shows preference for relatively small aggregate sizes ranging from 10 to 15 mm (Addis & Owens, 2001).
Smaller aggregate sizes produce higher concrete strengths because of less severe concentrations of stress around the particles, which are caused by differences between the elastic moduli of the paste and the aggregate. Many studies have shown that crushed stone produces higher strengths than rounded gravel (Mohammad & Mansur, 2009).
The most likely reason for this is the greater mechanical bond, which can develop with angular particles. However, accentuated angularity is to be avoided because of the attendant high water requirement and reduced workability. Chemical admixtures are required for production of HPC. Chemical admixtures are defined as materials other than hydraulic cement, water aggregates and supplementary cementing materials, which are used as ingredients of concrete or mortar and added to the batch immediately before or during mixing. The most important ant admixtures are plasticizers and water reducers.
A good quality superplaticizer is an essential ingredient of HPC; super plasticizers should be based on Sulphonated Melamine Formaldehyde (SMF) or Sulphonated
Napththalene Formaldehyde (SNF) (Addis & Owens, 2001). Addis and Owens (2001) also noted that most important parameters arenaphtaline and melamine in the superpasticizers. The degree of sulphonation can be as high as 90% in a good superplasticizer or as low as 50% in a poor one (Skalny & Mindes, 1998). Up to 90% water reduction can be achieved with a well-controlled sulphonation process and 50% achieved when there is no control process. Sulphonated naphthalene formahayldehyde and sulphonated melamine formaldehyde oligomeric condensates are the most common active parameters in a superplasticizer. The materials were first developed as dye stuff dispersants for the textile industry, but they are also excellent cement dispersants.
Superplasticizing molecules have the same basic structure and action as water reducers but of much higher purity and hence do not result in deleterious effects.
11
Water reducers on the other hand act by bonding to the surface of cement particles and neutralizing the electrostatic charges, thus deflocculating the particles. The water reducing agent is later debonded from the surface of the cement particles as the cement hydration process proceeds. There are no chemical reaction between the agents and cement as the process is purely physical. Water reducing agents are large organic molecule with many active groups which includes; hydroxyl, amnino, carboxyl and sulphates (Skalny & Mindes, 1998). All these groups are able to accept or donate ions essential for bonding to the surface of cement particles and for neutralizing electrostatic charges.
2.2
Cost Benefits Analysis
Internal rate of return (IRR) is calculated based on current project costs, and projected earnings from the business. The internal rate of return on an investment or project is the
"annualized effective compounded return rate" or "rate of return" that makes the net present value of all cash flows (both positive and negative) from a particular investment equal to zero.
) ………………………………………………… (2.1)
In the equation; Ct represents the net cash inflow, Co is the initial investment, r is the discount rate, and t is the time in years
At IRR the Net Present Value (NPV) is zero, thus: NPV = 0; or PV of future cash flows
− Initial Investment = 0; Presented in an equation form as;
CF
1
( 1 + r )
1
CF
2
+
( 1 + r )
2
CF
3
+
( 1 + r )
3
+ ... − Initial Investment = 0………2.2)
In the equation; r is the internal rate of return, CF
1
is the period one net cash inflow,
CF2 is the period two net cash inflow,CF
3
is the period three net cash inflow, and so on...
12
Since it is not possible to isolate the variable r (internal rate of return) on one side of the above equation, r is calculated by iteration.
Another evaluation method alternative to CBA is Life Cycle Cost Analysis (LCCA) which evaluates the economic performance of a building over its entire life. Sometimes known as “whole cost accounting” or “total cost of ownership,” LCCA balances initial monetary investment with the long-term expense of owning and operating the building.
LCCA compares to cost of renting as opposed to owning a property. Finishes such as tiling, temporary walling, painting which requires periodic maintenance and replacement are easily computed using LCCA and compared to the option of renting.
2.3
Theoretical review/Conceptual framework
Material properties ofstone dust
Concrete class
(Variables)
25 Mpa
30 Mpa
35 Mpa
New HPC
HPC (>59Mpa)
Structural design and
CBA
Compressive strength>59Mpa
Output
Cost effectiveclass ofconcrete
Suitability and cost effectiveness of HPC
Figure2.1; Theoretical framework
Concrete production
and
Testing
13
The study was done in two parts; establishing the suitability of stone dust in HPC production and determining the economic value of HPC use in reinforced concrete design. In the first part, the dependent variable was high performance concrete defined by compressive strength of over 59 Mpa, while the variables were water cement ratio in the concrete mix. The stone dust sample was subjected to various tests to obtain its properties but only one sample was analyzed. Using stone dusts (with defined properties), Cement (CEM I 42.5R) and a plasticizer (Sika® ViscoCrete®-HE), Water cement ratios were varied and different mix designs obtained. Concrete mixes with compressive strengths above 59 Mpa were considered HPC. The mix design with the highest compressive strength was used in reinforced concrete design to obtain structural members.
In the second part of the study reinforced concrete design of the 10 storey structure was done. The dependent variable was the most economic structural frame designed while the variables were concrete classes 25, 30, 35 and new HPC. The total cost of the structural frame was calculated after the design was completed. Additionally total usable floor area was calculated taking into consideration the area taken up by the structural members mainly columns and beams. Economic evaluation was done using CBA to justify the benefits of HPC use despite the initial cost of investment.
2.3
Research Gaps
There has been lots of research on High Performance Concrete (HPC) in the world and a significant number in South Africa (Addis & Owens, 2001).Locally there has been little research done using available materials in production of HPC. Many studies have analyzed stone dust in production of normal classes of concrete. (Kiliswa, 2011) from the University of Nairobi investigated the use of stone dust as a replacement of river sand for class 20 and 25 concrete. In his research he found out that the concrete with stone dust had similar plastic and hardened concrete and therefore could be used as a substitute. In the recent past, there has been increased use of washed and well graded stone dust in production of concrete class 30 and 35. These research evaluated suitability
14
of locally available materials in production of HPC. It also evaluated the cost of producing the HPC as well as the cost benefits gained from utilizing HPC in construction. The economic aspect of use of high performance concretes have not been analyzed here in Kenya. Cost benefit analysis utilized NPV and IRR to compare the economic viability of HPC over conventional concretes.
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CHAPTER THREE
MATERIALS AND METHODS
3.1
Materials Selection and tests
Stone dust was collected from Athi River crushing plant for testing after stock pile sampling. Stone dust was sampled using the methods described in BS 812-102 (1989); testing aggregates, methods for sampling. The selection procedures in the standard provided a practical approach for ensuring that construction material samples were obtained in a random manner. Since the quarry had large stock piles of the material due to its low consumption rate, deliberate action was taken to sample from each pile. The stone dust collected was very dusty and heavy due to its high density. Portland cement
(CEM I 42.5R) from Bamburi Cement was used in concrete design. Cement used was purchased from the hardware and therefore could not be subjected to sampling.
Deliberate action was taken to purchase cement which had been delivered from the manufacturer within the week to ensure maximum quality. The admixture Sika®
ViscoCrete®-HE, a water reducer and superplasticizer was also purchased off- shelves.
The admixture used is based on Melamine naphthalene formulation. The admixture came on 5 liter container and its mixing done by volume of water reduced.
ViscoCrete®-HE is manufactured by a South African based company with two outlets in
Nairobi. Bamburi special products have been using their admixtures in concrete manufacture.
3.2
Design and Manufacture and Tests of Concrete (HPC)
3.2.1
Concrete Design
American Concrete Institute (ACI) methods of concrete mix design method ACI 211.1-
91 (1997) was used. The method is based on the following;
(i) The strength, at a given age, of fully compacted concrete, cured under standard condition, is governed by water: cement ratio (W/C) and type of cementious material used.
16
(ii) The amount of water required per unit volume of concrete for a given consistency and with a given material is substantially constant regardless of cement content, W/C or proportions of aggregate and cement. The main factors determining the amount of water are aggregate properties, cement properties and maximum stone size.
(iii)For any particular concrete mix and combination of materials, there is an optimum stone content which depends on size, shape and compacted bulk density of the stone, fineness modulus of sand and desired consistence of concrete.
(iv) The volume of compacted concrete produced by any combination of materials is equal to the sum of the absolute volumes of cement and aggregates plus the volume of water and that of entrapped or entrained air. The absolute volume of material is the total volume of solid matter in all the particles, and is calculated from the mass and the particle relative density
Where V is absolute volume, M is mass of the material and D is the density of the material
3.2.2
Calculation of Material Masses
Strength and of concrete and stone size was specified. Slump is specified as dependent on method of concrete transportation, placing and compacting. The type of cement, maximum water cement ratio, (W/C) and minimum cement content also specified
Characterization of cement; strength performance, particle relative density and possible effect on workability of concrete. Density of concrete and other characteristics were obtained from the manufacture’s manual and Calculation of the cement content using the following equation;
…………………………………………….. (3.2)
Where C is the cement content in Kgs, W is the water content also in Kgs
17
The next step is to calculate the stone dust content, assuming full compaction of concrete. The volume of concrete is equal to the sum of absolute volumes of cement, stone dust and water.
For 1 m3 of concrete (ignoring air content)
In the equation, M c
is the mass of cement, D c
is the density of cement, M s
is the mass of stone dust, D s
is the density of stone dust, M w
is the mass of water, and
D
W is the density of water
Using excel sheets and by iteration masses of all the materials were obtained
3.2.3
Batching
Batching was done by mass for cement and stone dust while admixtures in liquid were measured in litres using the conversion factor from the Sika® ViscoCrete®-HE data sheet provided. Mixing was done in a tilting drum mixer, the use of free fall mixers and hand mixing is not permitted (Addis, 2001).Sika® ViscoCrete®-HE a high range water reducing and superplasticizing admixture was used as a water reducing agent to increase plasticity of fresh concrete. A dosage of 0.6-0.8% of the admixture by weight of cement was used as per the technical specifications given for the admixture dosage.
3.2.4
Mixing Method
Concrete was mixed in a tilting drum mixer (laboratory type). The following mixing sequence was adopted: stone dust and cement were mixed for 2 minutes; water was added during mixing and continuously mixed for two more minutes; mixing was stopped, admixture added an additional 3 minutes of mixing allowed. The methodology adopted was proposed by Aminul (2008) in his research mix design of high performance concrete, National Institute of Technology Silchar, India. Slump test, compressive strength, and modulus of elasticity were tested and results analyzed to get the structural properties of obtained HPC in accordance with BS EN 12390-1(2002).Crushing machine and strain gauge ware used to obtain modulus of elasticity.
18
3.2.5
Testing of trial mixes
Trial mixes were done in eight batches, the mixes in the first two batches being a test with poor results and the six getting desired results. The first two batches had a water cement ratio of 0.45 and 0.25, the latter being dry and unworkable and the former being having high water content. Two batches of mix had water cement ratio of 0.42 with different percentages of admixture, the others each with water cement ratio of 0.35, 0.38,
0.32 and 0.29. From each batch, 5 cubes were cast, one for one 1 day test, two for 7 day tests and two for 28 day tests. Additional cubes were done for the batch with W/C ratio of 0.32 which were used for modulus of elasticity.
3.3
Structural Design of 10 Storey Office Structure
A typical high rise office structure (10 floors) was analyzed using code of practice for design and construction BS 8110, (1997). Structural member sizes were then determined from several classes of concrete25, 30, 35 and 80, the latter being HPC strength obtained. Ultimate Limit State (ULS) design philosophy was used while Serviceability
Limit States (SLS) was used to check for serviceability. The design ensured that the structure, or part of it under consideration, did not deflect excessively causing unsightly cracking and loss of durability. The Figure 3.1 shows the general layout of the office structure designed, the architect was specific with the column spans at 8 m center to center. Thus the structure was designed with ribbed slab, 4 m ribs supported by an8m spine beam simply supported from the main frame moving across. The structure was supported by 2 span frames on the shorter dimension and 4 span frames on the longer side.
19
Figure 3.1: Layout of the structure (Detailed drawings found at the appendix)
20
3.4
Cost Benefit Analysis
The cost of production of HPC (Class 80) was calculated based on optimum design mix obtained from the laboratory tests. The market cost of materials (Stone dust, Bamburi
CEM I 42.5R, Sika® ViscoCrete®-HE) was factored, cost of concrete manufacture estimated at 30% of the total cost and 10% profit margin added. The cost was then compared with the concrete costs of class 25, 30, and 35 as available in the market. To take into account project life cycle costs, Cost Benefit Analysis (CBA) was undertaken to evaluate the benefit accrued as a result of increased lettable space over the initial investment incurred. CBA was used to determine the soundness of the investment/decision (justification/feasibility), and provide a basis for comparing concrete classes. Total costs comparisons obtained for each concrete class against the total expected benefits accrued from the extra lettable space were analyzed.
21
CHAPTER FOUR
RESULTS AND DISCUSSION
4.1
Suitability of stone dust in production of HPC
4.1.1
Material Properties
Crushed stones was sampled from Mlolongo Quarry and taken to the laboratory for tests.
Grading, Fineness Modulus (FM), Specific Gravity (SG) and Water Absorption (WA) were determined. The material collected was very fine, left in the quarry as waste material and not useful. However when grading was done it easily fits the grading envelop as per BS 882 for sand grading. The sampled material was graded from 75 µm to 10mm particle size. The samples used had water absorption ratio of 5.8%, relative density of 2.389 and Fineness Modulus of 3.0. The material used in the study had a fineness modulus that compared to sands studied in South Africa by Addis and Owens
(2001). In their study they recommended aggregates with fineness modulus of between
2.7 to 3.0. Smaller particle sizes produce higher concrete strengths because of less severe concentration of stress around particles caused by differences between the elastic moduli of the past and the aggregate. The water absorption of the material was also quite high at 5.8%, but this was factored when designing the concrete mix by adding a similar percentage. The Table 4.1 and Figure 4.1 summarize the properties of the material used for test.
22
Table 4.1: Crushed stone dust grading results
INDIVIDUAL GRADING CURVES
% Passing by weight to BS 882
Sieve sizes CRUSHED SAND Achieved Lower Limit (LL) Upper Limit(UL)
10mm 100 100 100
5mm
2.36µm
1.18µm
600μm
300μm
150µm
75µm
100.0
71.6
49.0
34.8
25.8
18.7
12.0
89
65
45
25
5
0
0
100
100
100
80
48
20
16
The green and red lines represent the upper limits as per BS specification on crushed sand
Figure 4.1: Material properties for stone dust used for concrete design
23
4.1.2
Design Mixes and results
8 trial mixes were run; 6 producing concrete of good consistency and cohesiveness. 2 mixes were discarded with one being too fluid and the other too dry and unworkable. 2 of the 6 trial runs that were successfully had a water cement ratio ranging from 0.42 to
0.29. Two mixes had W/c ratio of 0.42 (one with 4.76% of admixture and the other with
3.57%), the other four had W/C ratios of 0.38, 0.35, 0.32, and 0.29. Beyond W/C of
0.29 the mixes became dry and unworkable (Figure 4.2). The 6 mixes had good consistency and cohesiveness, as there were no segregation and bleeding observed even when vibrating concrete in the molds. Water-cement ratio for HPC according to Russel and others (1977) fell within 0.42 to 0.25 in a study of South Africa stone dust. HPC manufactured from Athi River stone dust produced workable concrete with W/C ranging from 0.42 to 0.29 and maximum strength achieved at W/C of 0.32. Thus stone dust sample collected well compared to the South African stone dust researched by Russell in
1977.
From the results a maximum compressive strength of 92.7 N/mm
2 corresponding to water cement ratio of 0.32 and a minimum compressive strength of 62N/mm 2 corresponding to water-cement ration of 0.35 were obtained. Factoring in standard deviation limited to 3.5 based on the laboratory results, and using the concrete formulae;
In the equation, fk is the characteristic strength of concrete, fc is the target strength of concrete and Sσ is the standard deviation of freshly manufactured concrete
Therefore using a characteristic strength of 92.7Mpa, standard deviation of 3.5, 87.8
MPa target strength is obtained. To take care of additional risks including inconsistent concrete manufacture, the safe strength used as HPC was 80 MPa. This new HPC was compared to the common concrete class 35, 30 and 25 N/mm
2
in reinforced concrete design. Concrete mixes were tested at one (1) day, seven (7) day and twenty eight (28) days. The compressive strength results are summarized in figure 4.2 and follow the
24
normal trend of concrete where strength increases as water cement ratio reduces. The maximum strength was achieved at a water cement ratio of 0.32. Figure 4.2 shows the relationship between water-cement ratio and compressive strength.
Figure 4.2: Relationship between water cement ratio and compressive strength of concrete.
Table 4.2 summarizes the mix designs and the results achieved at 1 day compressive strength for 150mm cubes, 7 day test and 28 day tests. It also shows the slump achieved and the mix design details.
Table 4.2: Design mixes and laboratory results
Cement
CEMI 42.5R
A
B
C
D
E
Cube Age ref (days)
1
CEMI 42.5R A
7
7
28
28
1
Mix details
Cement=595Kgs
S-dust=1000Kgs
Water=250Kgs
Admix=4.76% w/c ratio=0.42
Slump Density mm Kg/m 3
200
Cement=595Kgs 180
2187
2200
2200
2230
2230
2200
Strength Average
(N/mm 2 )
N/mm 2
27.5 27.5
60.1
59.0
58.0
74.5
73.5
72.0
25.0 25.0
25
CEMI 42.5R
A
B
C
D
E
F
B
C
D
E
CEMI 42.5R
A
B
C
D
E
F
A
CEMI 42.5R
B
C
D
E
F
CEMI 42.5R
A
B
C
D
E
F
7
7
28
7
28
28
28
28
28
28
28
28
28
7
28
7
7
28
28
7
7
28
28
1
1
7
7
28
S-dust =1318Kgs water=250Kgs
Admix =3.57% w/c ratio=0.42
Cement=595Kgs
S-dust =1318Kgs
Water=211Kgs
Admix =3.57% w/c ratio=0.35
Cement=657.9Kgs
S-dust =1225Kgs
Water=198.4Kgs
Admix =5.25%
240 w/c ratio=0.38
Cement=781Kgs
S-dust =1170Kgs
Water=198.4Kgs
Admix =6.25% w/c ratio=0.32
125
220
Cement=862.1Kgs
S-dust =1100Kgs
Water=198.4Kgs
Admix =6.90%
200 w/c ratio=0.29
2210
2230
2230
2240
2195
2200
2170
2170
2160
2210 62.5
2,232.3 49.7
2,210.4 44.8
2,226.1 74.3
2,233.5 75.2
2,224.0 74.2
2,249.2 75.0
2,258.4 64.5
2,254.8 66.8
2,321.2 92.3
2,325.9 92.6
2,337.5 93.0
2,330.1 92.9
2,273.8 60.0
2,285.3 63.7
2,341.0 90.1
2,346.7 90.0
2,337.5 89.7
2,340.1 89.8
56.5
55.0
72.5
72.0
24.0
24.0
47.0
48.0
62.0
60.0
72.0
24.0
47.5
62.0
48.0
75.0
65.5
93.0
62.0
90.0
The first two mixes had a similar water cement ratio of 0.42 but varying the percentage of the admixture (combination of plasticizer and a water reducer), the results were different but comparable. The strength obtained was 72 Mpa for the mix with 3.57% admixture compared to 73.5 Mpa for the mix with 4.76%. The slump too changed from
26
20mm to 180 when admixture was reduced from 4.76% to 3.57%. The result shows it is possible to maintain the same slump while varying the water content by changing the percentage of the admixture, or maintain the same strength and vary the slump. When the W/C ratio was varied significantly to 0.35, the slump dropped to 125mm and strength of 62 Mpa achieved. The trial run with W/C ratio of 0.38 produced compressive strength of 75 Mpa and a slump of 240mm. Thus as W/C ratio reduced, the compressive strength increased to a maximum of 93 Mpa at W/C ratio of 0.32. The trend of increased strength when W/C is reduced is similar to that of conventional concretes. The strength reduced for the next batch when W/C ratio was further reduced to 0.29, though still significant at 90 Mpa at a slump of 200mm. Beyond W/C ratio of 0.29, concrete mixes became dry and unworkable. Russell (1977) in his study had recommended W/C ratio up to 0.25. The stone dust tested in Kenya had a high water absorption rates and therefore could not manufacture concrete beyond W/C ratio of 0.29. The other factors could be as a result of the admixture used in the study which may not be the most superior in the market. The admixture used is combination of a plasticizer and a water reducer. Water reducing agent acts by bonding to the surfaces of cement particles and neutralizing the electrostatic charges thus deflocculating the cement particles. The water reducing agent is later debonded from the surface of the cement particles as cement hydration process proceeds. Therefore there is no chemical reaction between the agent and the cement.
The plasticizer on the other hand has sulphonated naphthalene formaldehyde and sulphonated melamine formaldehyde oligomeric condensates as their major agents. The plasticizing molecules have same basic structure as the water reducers but has much higher purity and do not result in side effects.
The mode of failure for the concrete cubes was similar to that of conventional concrete.
The cube after crushing remains with a prism similar to that of normal concrete cubes. A tensile- shear crack mechanism controlled the failure which occurred in the direction parallel to the applied load.
4.1.3
Modulus of Elasticity
27
The average modulus of elasticity obtained from the results was 49.4 GPa, well above the normal concrete range of 20-30GPa. Figure 4.3 shows stress strain curve for the
HPC 150mm by 150mm cubes tested using a compressive strength machine and a strain gauge. Due to the limitation of the available crushing machine in the laboratory (100T capacity), data was collected on the first 60% of compressive force. The data was useful because it is on this range that modulus of elasticity is analyzed. The concrete produced is therefore much more robust compared to normally produced concrete. Aggregates have higher elastic modulus than the cement paste and therefore elastic modulus of concrete lie between that of the cement paste and that of aggregates. HPC having a much higher modulus is thus more economical in construction, though expensive per unit volume, it give much higher strength per unit volume and per unit weight.
Figure 4.3: Stress-strain curve for HPC (150mmx250mm cubes)
28
9
10
4.2
Comparison of stone dust HPC with traditional concrete
4.2.1
Structural design to BS 8110 (1997)
A typical structure was analyzed to obtain all the structural members. The analyzed building had10storeys and a size of 38.6m by 22.6 m with floor spans of 8m by 8 m.
Since the floor spans are large, ribbed slab was designed for the 8 m by 8m; the other remaining small floor spans was designed with 150mm two way spanning slab. The structural elements design included structural main frame which is two spans with a distance of 8m between the columns. Secondary frame, also four spans of 8 meters each, ribbed slab with a spine beam, columns and foundation bases. Structural members were obtained as outlined in the table 4.3.
Table 4.3: Structural member sizes obtained from the structural design
#
1
2
3
4
5
6
7
8
Structural member
Columns(C1)-internal
3 m high
Columns-external (C2)
3 m high
Column base(B1) 3.5 by 3.5m
Column base(B2) 2.5 by 2.5m
Beams (ribbed slab)-
4m long- 320 No
Spine beam -8m long
Beam 01 -8m long- 4
No
Beam 02 -8m long-10
No
Beam 03 -8m long-12
N0
Slab thickness, h f h f h b h b h h f b h b h b
Size
(mm)
Concrete Class
25 30 35 b h b
900
900
700
700
600
600 h h f
600 500
600 500
450
450
350 300 400
300 250
125 125
200 175
300 300
400 400
200 200
450 450
200 200
600 600
200 200
450 450
150 125
200
450
200
450
200
350
100
300
125
150
300
400
80
500
500
300
300
450
200
300
200
425
200
250
75
250
125
125
300
400
29
The volumes of concrete and steel reinforcement were also calculated from the members and it was established that concrete volume reduces by 29.9% when concrete strength was increased from Class 25 to Class 80. Figure 4.4 shows the volume of concrete in structural members and the total reinforcement required for the structure. There was a reduction of volume of concrete used for structural members from 1873.95M
3 to
1,216.13 M 3 when concrete is changed from class 25 to class80 representing 35%.
Whereas this concrete is expensive it is more economical per unit area, per unit volume and per unit load it carries.
Figure 4.4
: Comparison between class and volume of concrete
Steel reinforcement significantly rose for concrete Class 30 and 35 and significantly drops for Class 80. Class 25 concrete had a total of 35,464Kgs compared to 39,736 Kgs and 39,778 Kgs for concrete Classes 30 and 35 respectively. Total reinforcement for class 80 was 34,083 Kgs. Though reinforcement is dependent on the class of concrete
30
the figures cannot be scientifically compared because the structural member sizes were not kept constant. Each of the structural member size was optimized to find a good balance of size and reinforcement steel. Figure 4.5 shows the total weight of reinforcement obtained for the difference classes of concrete.
Figure 4.5: Reinforcement steel obtained from designing using different classes of concrete
4.3 Increased floor space as a result of reduced member sizes
Floor area was calculated factoring in the area taken by the columns and it was established that additional 1.5% (13.4 m
2
of 874m
2
) of the floor area was added per floor when concrete strength was increased from class 25 to class 80. This is a result of reduced column sizes from 900mm to 500mm for internal columns and 600mm to
31
300mm for internal columns. The main beam also reduced from 600mm to 425mm giving extra headroom of 225mm per floor, if the structure is more than 15 storeys, there will be an extra floor created for a similar height of building done using class 25. Table
4.4 summarizes freed space in the structure. The floor area brings in additional income from the lease throughout the life cycle of the structure.
Table 4.4: Area freed from use of smaller members and estimated returns
Concrete Class
N/mm 2
25
30
35
80
Total Additional Area (m2)
Benefits- returns/m2
(KES)
- -
67 143,215.60
96 206,484.40
134 287,830.00
Class 25 is the minimum class of concrete used for design and the base used for analysis.
The cost of hiring space at CBD in Nairobi is estimated at KES 2152/m 2 per month according webmetrics, one of the leading Kenya’s property market research firms.
(Kaptich, 2011)
4.4
Cost Benefit Analysis
Internal rate of return (IRR) was calculated based on current concrete costs, cost of steel and projected earnings from the extra space obtained from reducing the structural members. The internal rate of return on an investment or project is the "annualized effective compounded return rate" or "rate of return" that makes the net present value of all cash flows (both positive and negative) from a particular investment equal to zero.
The IRR for class 80 N/mm
2
was found
to be 5% which reduces to 4% for both classes 35 and 30 and 0% for class 25.
Table 4.5 summarizes the Net Present Value and Internal Rate of Return. The results shows that whereas the initial cost investments for concrete Class 80 concrete is the
32
highest, the NPV is lower and comparable with using class 35. The IRR for using class
80 is 3% and thus shows that it makes business sense to use class 80 concrete. Thus it is prudent to invest KES. 29,111,182.79 at present as opposed to KES. 22,814,698 and get additional return of KES. 287,830 monthly from the extra space created. The analysis is even more relevant to investors who have little space and wants to gain maximum return from the space.
Table 4.5; Cost benefit analysis presentation
Concrete Class
35
80
N/mm 2
25
30
Current Price for construction
(KES)
22,814,698.66
25,906,853.52
28,362,648.86
29,111,182.79
Net Present Value
(NPV)
22,814,698.66
25,278,222.59
27,220,323.25
26,962,578.56
Internal
Rate of
Return
(IRR)
0%
1.44%
2.15%
3.06%
The cost of concrete used at the time was based on the market rates with class 25 at KES
12,000/m
3
, class 30 at KES 15,000/m
3
, class 35 at 18,000/m3 and class 80 at KES
22,000/m
3
. Though the HPC utilizes higher cement volume per m3 (780Kgs/m
3
), the cost of the stone dust is cheap (KES 900/tonne) bringing the costs down. In the study under review only the cost of structural frame was analyzed. The framed structure will remain as constructed throughout the life of the building. Other building fixtures such partitioning, tiling and painting were not included in the analysis. These fixtures are items that are replaced periodically or require routine maintenance. To analyze the total cost of running a building over time, life cycle costing analysis is utilized. Thus using internal rate of return and net present value was more practical in analyzing the framed structure over life cycle cost analysis which is used for comparing the cost of lease or owning the property.
33
CHAPTER FIVE
SUMMARY, CONCLUSIONS AND RECCOMENDATIONS
5.1
Summary
Quarry dust obtained from Mlolongo quarry graded between 75 µm to 10 mm with a relative density of 2.389 made a good high strength concrete of over 80 N/mm
2
at a water cement ratio of 0.32. The concrete manufactured used Sika® ViscoCrete®-HE admixture which is a plasticizer and water reducer to minimize the water requirement and achieve the required slump. Stone dust is much cheaper than both course aggregate and river sand and therefore minimize the cost of high performance concrete which has high cement volume per tonne (780kg/m
3
from the mix design).
Design of structures using high performance concrete, reduces significantly the structural member sizes, steel reinforcement and increases the available usable space in the structure. The limitations for HPC use for reducing structural members are modulus of elasticity, particularly, for slender columns and shear reinforcements for foundation footing and heavily loaded beams where shear is a big factor.
There is significant return on investment when High Performance Concrete is used for structural members (in the case of construction of 10 storey structure, the IRR was
3.06%). Use of HPC present additional advantages; in the case where the client has a small space and needs bigger floor area or in the case where the floor areas have big spans (architectural requirements of spaced columns).
5.2
Conclusion
From the research the following conclusion were made;
1.
Manufacture of high strength concrete using locally available stone dust is possible with strengths of over 80 N/mm
2
with a modulus of elasticity of 49.4
GPa achieved using stone
34
2.
Use of high strength concrete significantly reduces structural members (columns sizes and beam depths). Internal Column sizes for the structure reduced from 0.9 m wide for class 25 to 0.50m for Class 80 representing 45% reduction. External
Columns reduced from 0.6m to 0.4 of meter designed using Class 25 and 80, respectively, equivalent to 33.3% reduction. The sizes are, however, limited by modulus of elasticity, particularly, for slender columns and shear reinforcement for heavily loaded columns and foundation pads.
3.
Additional 234 M
2
was created in the structure when column sizes were reduced from 0.9m to 0.5m. The extra space at the present value generates KES. 287,830 per month as benefits to the investor. The increase presents a business case when life cycle costs of the structure are considered.
4.
The internal rate of return for class 30 and 35 was found to be1.44% and 2.15%, respectively, and that of Class 80 as3.06%. Using Class 25 the initial investment for the structural frame considered was found to be KES22,814,698 compared with KES 29,111,182.79 for class 80.The net present value for the class 25 was found to be KES 22,814,698.66 compared to KES 26,962,578.56 for Class
80.This is important for clients who have smaller development space and want to maximize it or for structure(s) that have bigger floor spans.
5.3
Recommendations
1.
The construction industry needs to consider use of high strength concrete in the current market as it will greatly improve the construction standards. There is enough justification for its use in high storey structures especially in rapidly congested areas of the cities where land has reduced.
2.
Use of HPC will also help architects improve their designs particularly where they require big spaces or minimum columns at the centre of the structure.
3.
Use of stone dust will also reduce river sand use which has in the recent past presented serious negative environmental impacts in the rivers where they are
35
mined. Dumping of stone dusts which usually becomes unsightly will be reduced.
4.
Further research need to be conducted for the new type of cement currently in the market, CEM 52.5 R.
36
REFERENCES
Addis, B. & Owens, G (2001), Fulton’s concrete technology (8th edition) .
Midrand,
South Africa: Cement & concrete institute,
Addis, B.J. (1991). Properties of high-strength concrete made with South African
Materials. Unpublished PhD thesis, Johannesburg: University of Witwatersand.
Addis. B.J. & M.G. Alexander (1990). A method of proportioning trial mixes for high strength concrete, High Strength Concrete.
Second international symposium,
Berkeley, California , (pp 227-278). ACI special Publication 121
ACI Committee. (1991). Standard Practice for Selecting Proportions for Normal,
Heavyweight, and Mass Concrete (ACI 211.1–91). In American Concrete
Institute .
Aminul, I.L. (2008). Mix Design of High Performance Concrete, Silchar, India:
National Institute of Technology
Hooton, R. D. (2000). Canadian use of ground granulated blast-furnace slag as a supplementary cementing material for enhanced performance of concrete.
Canadian Journal of Civil Engineering , 27 (4), 754-760.
Jinnai, H., Kuroiwa, S., Watanabe, S., Namiki, S., & Hayakawa, M. (2005).
Development and construction record on high-strength concrete with the compressive strength exceeding 150 MPa. ACI Special Publication , 228 .
Kaptich, L. (2011, July 2). Office Space Price Comparison, in Nairobi and across East
Africa.
Retrieved September 15, 2014, from http://kaptichlouis.blogspot.com
Kawai, T. (2005). State-of-the-art report on high-strength concrete in Japan–Recent developments and applications. In JSCE/VIFCEA Joint Seminar on Concrete
Engineering (pp. 8-9).
Kiliswa M. (2011). Effect of quarry dust on the strengths and permeability of concrete; unpublished Msc Thesis University of Nairobi, Nairobi: UON
Lahoud, A. E. (1991). Slenderness effects in high-strength concrete columns. Canadian
Journal of Civil Engineering , 18 (5), 765-771.
Lin, S. C. (2007). Monitoring of concrete building construction. Canadian Journal of
Civil Engineering , 34 (10), 1334-1351.
Long, W. J., Khayat, K. H., Lemieux, G., Hwang, S. D., & Han, N. X. (2014).
Performance-based specifications of workability characteristics of prestressed, precast self-consolidating concrete—A North American prospective. Materials ,
7 (4), 2474-2489.
Mehta, P. K., & Aitcin, P. C. (1990). Microstructural basis of selection of materials and mix proportions for high-strength concrete. ACI Special Publication , 121 .
37
Mungai, D.N, Gichuki F.N.,Gachene C.K.K., (2000). Environmental and land-use consequences of sand harvesting in Masinga division , Nairobi: University of
Nairobi.
Özbay, E. (2010). Influence of aggregate size on the mechanical and transport properties of concretes and concrete-equivalent mortars. Canadian Journal of Civil
Engineering , 37 (10), 1303-1314.
Rashid, M. A., & Mansur, M. A. (2009). Considerations in producing high strength concrete. Journal of civil engineering (IEB) , 37 (1), 53-63.
Russell, H. G., Anderson, A. R., Banning, J. O., Cantor, I. G., Carrasquillo, R. L., Cook,
J. E., ... & Guennewig, T. G. (1977). State-of-the-Art Report on High-Strength
Concrete. Chicago Committee on High-Rise Buildings . Chicago: ACI
Saravanan, J., Suguna, K., & Raghunath, P. N. (2014). Slenderness effect on high strength concrete columns confined with GFRP wraps. Indian Journal of
Engineering and Materials Sciences , 21 (1), 67-74.
Shyh-Chyang, L. (2007). Monitoring of concrete building construction, Canadian
Journal of Civil Engineering, 2007, 34(10),1334-1351, 10.1139/l07-058
Skalny, J. and S. Mindess (1998). Materials Science of Concrete, Westerville, Ohio:
American Ceramic Society.
Thomas M.D.A.,Cail K., and Hooton R.D.,(1998). Development and field applications of silica fume concrete in Canada: a retrospective. Canadian Journal of Civil
Engineering, 25(3), 391-400
Zia, P.,Leming M.L,Ahmad S.H., (1991). Mechanical Behavior of High Strength
Concrete, Strategic Highway Research Program, High performance concretes, a state of the art report , Washington DC: SHRP National Research Council.
APPENDICES
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