This study introduces the need, difficulties associated with and solutions for implementing the use of High Strength Concrete in Peshawar and challenging areas in general. It does so by using Peshawar as a case study and generalizing its characteristics to any other such regions in the world.
It reviews current standard methods for making High Strength Concrete in the world and investigates those used in the target region. Then compares them to find the difficulties that stand as a hindrance in achieving High Strength Concrete in said area, naming them as challenges. It uses the PEC (Pakistan Engineering Council) contractors’ categorization to provide guidelines for each category to tackle the challenge that hinders it the most. Furthermore, it provides a set of actions for PEC to follow for a successful implementation of High Strength Concrete in the area. It also gives a review of the process for use in other challenging environments, also giving examples of such environments in Pakistan and other parts of the world.
Table of Contents
Table of Contents
List of Figures
List of Tables
Abbreviations
Abstract
Chapter 1. Introduction
1.1 The Construction Industry in Peshawar
1.2 Limitations of the current practices
1.2.1 Standard Methods of Design and Practice
1.2.2 Standard Process of Construction
1.2.3 Standard Construction Practices
1.2.4 What happens in Peshawar?
1.2.5 The Problem
1.3 High Strength Concrete
1.3.1 Education of the General Public
1.3.2 Using materials with higher strength
1.3.3 Conclusion
1.3.4 Further Discussion
1.4 Problem Statement (A Challenging Environment)
1.5 Peshawar As A Case Study
1.6 Scope and Significance
1.6.1 Significance
1.6.2 Scope
1.7 Approach
Chapter 2. Literature Review
2.1 Concrete
2.1.1 Concrete
2.1.2 Portland cement as Hydraulic Binder
2.1.3 Aggregates
2.1.4 Cement Hydration and Concrete formation
2.1.5 Fresh Concrete and its transport, placement problems
2.1.6 Concrete Strength and Failure Mechanism
2.1.7 Factors affecting the strength of concrete
2.1.8 Reinforced Concrete
2.1.9 Importance
2.2 High Strength Concrete (363, 2010)
2.2.1 Definition
2.2.2 Materials for Production
2.2.3 Proportioning
2.2.4 Fresh HSC
2.3 Specifications
2.3.1 Cement
2.3.2 Course and Fine Aggregates
2.3.3 Silica Fume
2.3.4 HRWR
2.4 A New Method Of Mixing
2.4.1 Mechanism of Failure in Concrete
2.4.2 Delaying Bond Failure
2.4.3 Silica Fume
2.4.4 Need for Dispersion
2.4.5 New Method of Mixing (Lewis, 2019)
Chapter 3. Methodology
3.1 Defining HSC In Peshawar
3.1.1 Data of Cylinder Tests
3.1.2 Laboratory Tests
3.1.3 Borderline
3.2 Availability of Materials
3.2.1 Cement
3.2.2 Sand
3.2.3 Aggregate
3.2.4 Silica Fume
3.2.5 HRWR
3.3 Construction Practices in Peshawar (Zada, 2019)
3.3.1 Two Categories
3.3.2 Mix Design or Proportioning
3.3.3 Fresh HSC
3.3.4 Gallery
3.4 Challenges
3.4.1 Formulation of Strength Required (318, 2011)
3.4.2 Aggregate Ratios
3.4.3 Use of SCMs and Admixtures
3.4.4 Quality Control and Testing
3.4.5 Batching Plants and SD
3.4.6 Industry Acceptance
3.5 Categorization
3.5.1 Categories and Solutions to Challenges for Each
3.6 Role of the Authorities
3.6.1 Stage-I
3.6.2 Stage-II
Chapter 4. Conclusion
4.1 Expanding Horizons
4.2 Examples
4.2.1 Lahore (Space)
4.2.2 Gilgit (Soil)
4.2.3 Kashghar (Conflict)
4.3 Conclusion
References
Appendix
Buildings in Peshawar
Mix Design for Experiments
1. Required Average Laboratory Compression Strength
2. Selection of Maximum size aggregate
3. Selection of Optimum Course Aggregate
4. Estimating Mixing water content and Air content
5. Selection of W/C+B
6. Proportioning basic mixture with cement as only cementitious material
7. Proportioning Companion Mixture using cement and Silica fume
8. Ratio
9. Adjustments to trail mixture proportion
Weights and Volumes of Different Components
Cylinder Test Data for KPK
Data (psi units)
Proof of Authenticity
Is a Higher Strength Worth It?
1. A System Dynamics Model
2. The feedback and how the loops work
3. Evidence of Its Authenticity (363, 2010)
4. Conclusion
5. Policy
Acknowledgement
All praise and glory to God, the most gracious, compassionate and merciful who provided us with the ability to complete this feat
We owe our deep and sincere thanks to our parents; who raised and supported us, our teachers; who reared and educated us and to the People of Pakistan; who provided us with the environment and resources to achieve this feat.
We also thank our supervisor Dr. Qaiser Ali who guided us during the entire project and provided us with advice on issues we would have never considered on our own.
We would also like to thank the staff and faculty of University of Engineering and Technology, Peshawar and its Civil Engineering department specifically who provided us with support and guidance during our laboratory works.
In addition we thank Mr. Robert Lewis of Ferroglobe PLC for his cooperation in explaining and providing the new and enhanced method of concrete mixing.
Finally we would like to thank and appreciate the help of C-1 contractor Mr. Saeed Zada of Rehman Construction Co. who provided us with details regarding construction practices in Peshawar.
List of Figures
Figure 1- Peshawar and Its Location
Figure 2- Failure Mechanism in Concrete (Wight, et al., 2012, Figure 3.1)
Figure 3- Concrete Batching Plant, with the Silo’s
Figure 4- Truck Mixer for Mixing and Transportation
Figure 5- ASTM C150, Standard Cement Composition Requirements
Figure 6- ASTM C33, Limits for Deleterious Substances in Fine Aggregate
Figure 7- ASTM C33, Limits for Deleterious Substances in Course Aggregate
Figure 8- ASTM C33, Fine Aggregate Grading
Figure 9- Course Aggregate Grading
Figure 10- ASTM 1240, Silica Fume Chemical Requirements
Figure 11- ASTM 1240, Silica Fume Physical Requirements
Figure 12- Stress Strain Curves of Concrete with Different Strengths (Wight, et al., 2012)
Figure 13- Silica fume Micro pellets
Figure 14- Burj Khalifa, Abu Dubai
Figure 15- Tarapur Nuclear Power Plant, India
Figure 16- Oman Airport, Muscat
Figure 17- Tsing Ma Bridge, Hong Kong
Figure 18- Strength Frequency
Figure 19- Class Frequency Plot with Average Strength
Figure 20- Laboratory Test Results
Figure 21- River Sand, Observe the sandy wet texture
Figure 22- Fin, or 9mm down
Figure 23- Example of a Small Project, notice the mason working with the concrete in the hand buggy
Figure 24- Transportation and Mixing in a Big Project, Notice the mixer just emptied into the bucket
Figure 25- Example of a Big Project
Figure 26- Hunza valley, The Sedimentary Soil near the river is clearly visible
Figure 27- Badshahi Masjid in a Congested Lahore
Figure 28- Tight Security in Kashghar
Figure 29- Local Neighborhood in Peshawar near Charsadda Road
Figure 30- Tehkal Peshawar, Commercial Hub
Figure 31- Model Behavior
Figure 32- Final Model
List of Tables
Table 1- Compounds Present in Portland cement, (Neville, 2004)
Table 2- HRWR Specifications
Table 3- Calculating Average Strength
Table 4- Properties of the Data
Table 5- Volume of Components
Table 6- Weight of Selected Components
Table 7- Weight of Cementitious Materials for Silica Fume
Table 8- Volume of Cementitious Materials
Table 9- Modified Volume of Components
Table 10- Modified Weight of all Components
Abbreviations
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Abstract
This study introduces the need, difficulties associated with and solutions for implementing the use of High Strength Concrete in Peshawar and Challenging areas in general. It does so by using Peshawar as a case study and generalizing its characteristics to any other such regions in the world. It reviews current standard methods for making High Strength Concrete in the world and investigates those used in the target region. Then compares them to find the difficulties that stand as a hindrance in achieving High Strength Concrete in said area, naming them as challenges. It uses the PEC contractors’ categorization to provide guidelines for each category to tackle the challenge that hinders it the most. Furthermore, it provides a set of actions for PEC to follow for a successful implementation of High Strength Concrete in the area.
It concludes with a review of the process for use in other challenging environments, also giving examples of such environments in Pakistan and other parts of the world.
Keywords: Challenging areas, Peshawar, High Strength Concrete, PEC and Implementations
Chapter 1. Introduction
1.1 The Construction Industry in Peshawar
Peshawar is the capital of the province KPK of the country Pakistan. Being the capital of the province and the first major city after entering the country from the Afghan border, it holds major potential for development and hence has a booming business in the construction industry. Buildings for all types of purposes are made in the city. 5-10 storey buildings up until the beginning of the 21st century used to be a very big deal. However, only recently, there has been a boom of these 5-10 storey “plazas” all over the city. Even in commercial value neighborhoods, 4-6 storey residential-commercial buildings have become the norm1.
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Figure 1- Peshawar and Its Location (©2019, Microsoft Corporation, Maps. Edited by (Shafiq , et al., 2019))
Unfortunately, even though the city has stepped into a new stage of development. The way buildings are constructed has not changed accordingly. Poor workmanship, Shabby Supervision, Obsolete theories are still practiced without any prospect for change
1.2 Limitations of the current practices
As the scale of a structure increases, the risk associated with it increases accordingly. Therefore the design protocols, the materials selection, construction procedures and everything else in the process becomes more and more strict. However, in Peshawar, this change has not been significant. Especially during the construction phase.
1.2.1 Standard Methods of Design and Practice
It was previously discussed that the need for excessive care in construction increases as the scale and the risk associated increases. This excessive care has been addressed by both Private International organizations and Government organizations of countries. Amongst these to name a few are;
- American Concrete Institute: (ACI) The American Concrete Institute is a non-profit technical society and standards developing organization.
- ASTM International: Known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services.
- The American Society of Civil Engineers: (ASCE) is a tax-exempt professional body founded in 1852 to represent members of the civil engineering profession worldwide. Headquartered in Reston, Virginia, it is the oldest national engineering society in the United States.
- British Standards: The British Standards Institution (BSI) was established in 1901 as the Engineering Standards Committee.
Each of these organizations issue standards that cover different aspects of the construction process including design codes, batching, mixing, formwork, etc. requirements. However, their primary use is during the design and planning phase of the construction project.
1.2.2 Standard Process of Construction
Standard construction procedures are specified by state authorities and are usually statutory orders. These procedures consist of two main parts, one is the entire administrative and inter-party regulation set by the state, and the other is the sets of methods and protocols for actually constructing the structure i.e. like the limit on the depth of foundation for a class of structures. These are made to ensure that professionals like engineers and architects are actively involved in each step of the process. For the current context, the first part or administrative set will be explained so as to emphasize the need for this project.
The main steps for construction of a building include;
1. The client consults an architect for the concept of the structure, regardless of the scale.
2. A consultant is hired to design the structure and provide structural drawings, bill of quantity, etc.
3. A contractor is hired to construct the structure according to the drawings and details provided by the consultant.
4. The contractor constructs the structure under the client or the consultants’ supervision according to the State Building Code i.e. The Building Code of Pakistan for example.
5. After inspection from the client, Quality control or Consultant, the contractor hands over the finished structure over to the client.
During this process, contracts between the client and the architect, consultant and contractors are written and cover any missing details.
1.2.3 Standard Construction Practices
In the previous section, it was stated that there are two parts in construction, the administrative set, and the construction set. Construction practices include limits and guidelines for actual construction including specifications. The standard set of construction practices are provided by “The Building Code of Pakistan” in Pakistan.
Their use,
However, is not followed; instead, the local masons use methods from as far back as the Mughal Empire to as new as the ones specified in the building code. Which they actually picked up from bigger scale projects when they worked in them.
1.2.4 What happens in Peshawar?
As it was previously pointed out that even though the scale of projects in Peshawar has increased, the associated caution and seriousness have not. This section provides a comparison of how things are in Peshawar on the administrative stage.
2-6 Storey Buildings
A typical approach of construction for residential and 2-6 storey buildings in Peshawar consists of;
1. Making a plan for the building, which is usually made by the client in tandem with the contractor.
2. The plan is passed from the local town administration, however, only architectural aspects are reviewed.
3. The client and the contractor agree on a schedule, make a bill of quantity and a method of payment.
4. The contractor hires masons of daily wages and carries out the construction.
5. The building after completion is handed over to the client.
In this entire process, there is no;
- Solid Interference from the Engineer
- Insurance of the building
- Strict Supervision on site
5 and above storey Buildings
These types are usually made for commercial or public use and their construction is taken somewhat seriously;
1. The client approaches a consultant on his/her own discretion, the consultant provides EPC (Engineering, Procurement, and Construction) services for the client.
2. The Building is insured and payments are done.
This process, as can be seen, is more stringent compared to the previous method and may be seen as an adequate approach. However, as the masons employed for the construction remain the same. The engineer has to make sure the design is adequate and ends up making it bulky (Excessive Dimensions) or more expensive because of masons’ incompetence.
1.2.5 The Problem
The reader can see the differences, in Peshawar we have;
1. No Involvement of Engineers in small scale projects, but since the number of these structures far exceed those of bigger ones. They pose a bigger threat to human safety
2. Bigger structures suffer from less space and more cost as compared to smaller ones.
3. The use of construction practices from different stages of history makes things very complicated for a proper engineered structural construction
1.3 High Strength Concrete
The problems associated with both the approaches stated previously, termed as limitations can be solved in two ways.
1.3.1 Education of the General Public
Education of the general public could be carried out to increase awareness regarding the importance of standard methods and protocols. However, carrying this out will require extensive awareness programs and a lot of time to result in any fruit at all. This could bring Peshawar out of the so-called third rate city status, but its accomplishment probability is too less.
1.3.2 Using materials with higher strength
Or the strength of the materials used in construction could be increased without changing the current practices or carrying out education campaigns. Generally, the most common material used in construction is concrete and all the other materials like steel bricks, etc. are directly or indirectly dependent upon the strength of concrete used. Increasing the strength of concrete will allow us for;
- Providing safer structures for the same conditions
- Decreasing dimensions of all elements
- Increasing more space
- Lesser costs
1.3.3 Conclusion
Considering the pros and cons of the two approaches, it is obvious that increasing the strength of the materials would be the best approach to solving the problem at hand. Therefore in light of the above, the project will focus on achieving higher strength concrete in the target locality. This will be beneficial for both cases of construction approaches stated above;
- For 2-6 story projects, the structures will be considered safe even with the poor type of construction practices currently used.
- For 5 and above story projects, the structure will be able to provide more space by reduced dimensions of its elements.
Implementation of this suggestion ought, to begin with, the 5+ story structures since huge investment is involved in these types. The work will be taken seriously and it would work as an excellent example of its usefulness to the rest of the community, helping propagate its use.
1.3.4 Further Discussion
If the reader is wondering whether increasing the strength will actually decrease the cost of the structure. He/ she may want to consider reading the topic “Is a Higher Strength Worth it?” in the appendix.
1.4 Problem Statement (A Challenging Environment)
Peshawar is not the only city suffering from the scenario stated above. Almost all third world countries suffer from the same fate, as a matter of fact, they are called as such because of the way they lack in infrastructure and education to name a few. Why is it like this? The same properties that define them as the third world cause these problems to generate. Poor education, lack of rule of law and other factors of similar nature are the root of the problem. However, it’s not like we or any other community is destined to be like this forever. If each and every department were to play their part, this fate can be changed. That’s where this research comes in, the aim is to achieve infrastructure that is on par with the developed world without the involvement of long term solutions that heavily depend on other departments. Therefore a new term will be defined; challenging environment, which can be stated as
A Challenging Environment refers to an environment that would render the use of standard protocols and methods impractical.
Also our next statement
Enhancing material strength will allow for safe construction in a challenging environment
Here is the dilemma, however, enhancing material strength requires the use of standard protocols and methods too, and therefore either the existing protocols will need to be amended for use in the said environment or a set of new methods that would be applicable will need to be provided. Therefore the aim of this project is
To provide new or amend existing methods and protocols for achieving High Strength Concrete in a Challenging Environment, one in which the said cannot be practically implemented
1.5 Peshawar As A Case Study
The city of Peshawar, as stated previously is in need of massive infrastructure development to keep up with its ever-growing industry. However, the current state of the city in multiple perspectives does not allow for this development to be on par with the international requirements of code authorities such as the ASTM or ACI. Therefore the city is a perfect example of a Challenging Environment and will be used as a case study.
1.6 Scope and Significance
It is clear from the project statement what the problem is and what this project is intended to provide as a result. However, this section will further clarify the need for the project and its scope.
1.6.1 Significance
It was previously pointed out that increasing the material or concrete strength will directly provide a short term solution to the risk problem in our construction industry. However, how and why does increasing the material strength allow for safe construction in Peshawar.
Case-I, No Engineer Involved
In the construction of 2-6 story buildings, as discussed, there is little or no intervention of an Engineer. Therefore these structures are a great threat from any potential hazard for their residents’ lives and the community’s economy in general. This is due to the clients increasing the number of stories while the masons’ using the same conventional methods and design numbers learned decades ago in projects designed by Engineers in the same challenging environment. Their use is illogical and misplaced, not only does this increase the cost many times, but it also increases the dead weight of the structural elements.
Another serious problem with these structures is the use of brick masonry and bricks in the slab/roof of these structures. Masonry has been used for thousands of years in the subcontinent and many parts of the world, however, their design philosophies are relatively new. Reinforced masonry concrete is a relatively new field of study which evolved much after the 1906 San-Francisco earthquake in the US (Taly, 2010). Even then, these design methods have not yet been completely implemented in the US, let alone in the Indian Subcontinent.
If and when the use of concrete in slabs is ensured, high strength concrete implementation can provide for reduced element sizes. However we cannot expect illiterate masons to understand these concepts, they will express distrust in the reduction of element sizes simply because of a material change. But even if they don’t and keep up with the use of those old misplaced designs, the structures would become much safer for its inhabitants to live in as the design specifications would be achieved ( at least). Also, once the use of High Strength Concrete ensues in high rise buildings in a city, the masons will to quickly pick up the dimensions as thumb rules for use in lesser structures.
Case-II, Engineer Involved
In the construction of structures with 5 and above stories, the clients tend to be cautious and hire engineers since a great deal of money, time and reputation are at risk. Therefore in these structures, the nature of the problem tends to be different.
The problem of resident risk tends to be minimal due to the presence of an engineer and serious contractor supervision. However it still persists due to the non-standard methods used by masons, one such example is the carelessness in producing concrete, masons generally tend to use measurement devices that are not too accurate, and one example is using the bag of cement for calculating all dry component volumes. Similarly, when any design result; whether it be an element dimension, a concrete proportion or structural detail tends to become something the masons are unfamiliar with. They most likely construct it the wrong way. Therefore the engineer has to take these factors into consideration when carrying out a design and tries to lean towards the safe side, making it more expensive and bulky, so less space.
The client and stakeholders in these type of project value space more than money, the use of High Strength Concrete would prove beneficial to them in the true means by not only decreasing the cost but also providing extra space. However for words to become reality we need the masons to either become acquainted with bizarre numbers or we could bring these numbers to something they’re familiar with. The outcomes of this project will include providing methods that the masons will feel comfortable with. This will also help Case-I since the masons tend to use these methods in lesser structures as well.
1.6.2 Scope
The following list provides in chronological order the scope of this project;
- This project provides methods and guidelines for the production and use of High Strength Concrete in Challenging areas.
- The definition of Challenging Environment projects to a very wide horizon. The complete analysis of all of the problems that would give rise to a challenging environment is not within the scope of this project.
- This project will work explicitly on buildings, however, the work may be extended to any type of structure by simply repeating the methodology.
- This Project will be using Peshawar as a case study, any conditions; environmental, technical, legal and social that do not apply to the city are not within the scope of this project.
- The results of this project can be used as a role model and the procedure expanded to any challenging area by repeating the process.
- It is not within the scope of this project to introduce High Strength Concrete in the Building Code of Pakistan.
1.7 Approach
The project will use the following approach;
1. Setting the borderline strength that defines High Strength Concrete in Peshawar
2. Identify the Challenges in Peshawar that will most likely make standard procedures impractical
3. Propose solutions for the challenges
4. Characterization of builders of any structure based on a set of criteria
5. Assigning a set of methods (finalized) for different categories to be explicitly used by them
6. Conclusion with an approach for authoritative bodies to follow for implementing High Strength Concrete.
Chapter 2. Literature Review
This chapter will provide a basis for the understanding of the reader and the discussions that will be carried out in the coming chapters.
2.1 Concrete
2Before proceeding, the reader will be provided with enough information as to what is concrete and the assumptions that will be used in this paper regarding its mechanics and composition. This section will provide a very brief introduction to concrete, its composition, making and other important properties (Neville, 2004)
2.1.1 Concrete
Concrete in civil engineering refers to a mixture of water, aggregates and a hydraulic binding material.
- Hydraulic Binding material refers to any material that upon coming in contact with water will become hard and entrap any other component that would be present inside it at the time.
- Aggregates refer to inexpensive rocks, sand or any material that would provide for the bulk of the concrete while being inert to any other component and locally available at the same time.
- Water acts as the final material that initiates the hardening by reacting with the hydraulic binder.
2.1.2 Portland cement as Hydraulic Binder
A hydraulic binder can be anything that meets the criteria, however, the most commonly used binder in the world is Portland cement or most commonly known as cement. Cement is a synthetic material made from the grinding of clinker resulting from the mixing of calcareous materials such as limestone or chalk, and from alumina and silica found as clay or shale.
2.1.3 Aggregates
Aggregates are inert materials added to concrete so as to provide stability and strength to concrete that cannot be achieved from cement alone. Its primary purpose was to act as an inert material dispersed throughout the cement paste largely for economic reasons.
2.1.4 Cement Hydration and Concrete formation
It was previously stated that cement acts as a binder and hardens on contact with water, forming concrete. What actually happens during this time? This section will make that clear.
- Cement consists of mainly the following compounds, which largely define its properties
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Table 1- Compounds Present in Portland cement, (Neville, 2004)
- Along with other minor components like alkalis and the like that have certain effects in concrete. These include Alkali-aggregate reaction and Loss of ignition. That will be revisited when required.
- Each compound in the cement particle reacts with water as it would have reacted independently (Hydration of Portland Cement Compounds, 1934)
- C3S and C2S upon hydration form Calcium silicate Hydrate and calcium hydroxide.
For C3S:
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For C2S:
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- The physical properties of Calcium Silicates Hydrates resemble those of crystal lattices in clay particles, like montmorillonite and halloysite. Therefore the lattice is easily able to accommodate the Ca(OH)2 generated in the process.
- Calcium silicate Hydrates are commonly referred to as C-S-H by concrete experts.
- The CSH lattice along with the water makes a complex skeletal structure of solid CSH lattice and a gel, which consists of CSH and water.
- Both of these give strength to the concrete and surround any aggregate added to the concrete.
- Fine aggregate 3 which is sand is considered part of the skeleton as a solid medium due to heavy confinement by the skeleton.
- Course aggregates 4 which are normally rocks are considered an alien medium that the skeleton (Now consisting of fine aggregate as well) interacts with.
- As the CSH gathers on the surface of the aggregate, the Ca(OH)2 that would have been accommodated in an opposite CSH lattice gets accumulated on the surface of the aggregate.
- This creates a zone where there is an excess of Ca(OH)2. Which is weaker than CSH, therefore a weak zone is created known as the transition zone. (234, 1996)
- The CSH skeleton is commonly referred to as mortar and will be further called as such.
- The other two components of concrete do not have a significant effect on the mortar properties except for C3A which affects early age strength in the absence of gypsum. Therefore they will not be visited hereafter.
2.1.5 Fresh Concrete and its transport, placement problems
The term fresh concrete is referred to the concrete right after it is mixed and before it hardens to shape. Important properties in case of fresh concrete known as workability and slump are described below.
Workability
The ease with which fresh concrete can be handled and placed is known as workability.
Slump
A measure of the fluidity of the concrete. Visit (Neville, 2004) for details.
During this period it is extremely sensitive to
- Segregation: The separation of constituents of the concrete so that it is no longer constituent.
- Bleeding: A form of segregation in which the mix water tends to rise to the surface.
- Strength Loss: Any sort of segregation, physical or chemical change can reduce the strength of the concrete.
- Formwork time: Loss of strength gain means more time for the formworks to holds the concrete in place. Hence increasing cost and losing time.
Therefore care should be taken with its transport and placement so as not to cause segregation in the concrete. There are a number of standards addressing the procedures for transporting and placement. For example, the ACI 304R-97 is one such code providing details (304, 1997).
2.1.6 Concrete Strength and Failure Mechanism
It is common knowledge amongst Civil Engineers and concrete experts that the tensile strength of concrete is far too less compared to its strength in compression5, therefore its tensile strength is usually neglected in design, and the strength of concrete is that in compression alone. Concrete strength depends upon a large number of parameters, the strength of the mortar being only one of them. As a matter of fact, the strength of the mortar is not tested by the load only until later when we reach 50% of the total load bearing capacity of the concrete. Concrete strength is tested according to different standards, the ones commonly used are the ASTM cylinder test (C39, 2015) and the BSI cube test (BSI, 1983). The BSI cube test uses cubes rather than cylinders, therefore the aspect ratio (being less) causes the cubes to report more strength than the cylinders. Nonetheless, we will be using the ASTM C39 during our report.
The failure mechanism of concrete can be summarized as follows6, (Wight, et al., 2012)
1. During hydration, a decrease in volume occurs due to the formation of the lattice, which is restrained by aggregate. This stresses the transition zone even when no load is present.
2. Later when the load is applied, at approximately 30-40% of the compressive strength, the transition zone stars to yield and develops cracks. Known as bond cracks, this stage is referred to as stable crack initiation stage.
3. As the load is increased to 50-60%, cracks begin to develop in the mortar due to decreasing stress transfer to the aggregate because of cracks in the transition zone. Leading to stress accumulation in the mortar. Discontinuity limit and stable crack propagation stage.
4. At 75-80% of the ultimate load, the transition zone has completely failed and the mortar cracks begin to form patterns of what is referred to as micro-cracking. This stress is known as critical stress after which the stress-strain behavior no longer remains linear.
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Figure 2- Failure Mechanism in Concrete (Wight, et al., 2012, Figure 3.1)
2.1.7 Factors affecting the strength of concrete
This section will provide the reader with a brief description of the factors upon which concrete strength depends (Wight, et al., 2012)
a. Water/Cement Ratio
The strength of concrete is governed in large part by the ratio of the weight of the water to the weight of the cement for a given volume of concrete. Decreasing this ratio will decrease the overall amount of water available after complete hydration and hence decrease the non-solid products of hydration. Reducing the porosity of the hardened concrete and thus increases the number of interlocking solids.
b. Type of Cement
Traditionally, five basic types of Portland cement have been produced:
Normal, Type I: used in ordinary construction, where special properties are not required.
Modified, Type II: lower heat of hydration than Type I; used where moderate exposure to sulfate attack exists or where moderate heat of hydration is desirable.
High early strength, Type III: used when high early strength is desired; has a considerably higher heat of hydration than Type I.
Low heat, Type IV: developed for use in mass concrete dams and other structures where the heat of hydration is dissipated slowly. In recent years, very little Type IV cement has been produced. It has been replaced with a combination of Types I and II
cement with fly ash.
Sulfate resisting, Type V: used in footings, basement walls, sewers, and so on that are exposed to soils containing sulfates.
Each of the above cement has been made for a purpose, which may or may not be a strength, however, if their use if warranted for a project. The Engineer will have no choice but to design while keeping their properties in mind. Therefore the type of cement will have a direct effect on the ultimate strength of the concrete, even if the strength gain is not the same.
c. Supplementary Cementitious Materials
At times the addition of cement may or may not have beneficial effects, in cases where its addition in an amount causes adverse effects7. Another kind of binder may be added blended with the cement, these binders are known as Supplementary Cementitious Materials or SCMs for short. Not only do they counter those effects but decrease the cost since they tend to be cheaper than cement as a whole. There are many types of SCMs, the most commonly used ones are pozzolans and slag cement
Pozzolans: Materials that have little or no binding qualities alone but will exhibit such qualities in the presence of Ca(OH)2.
Slag Cement: Cement made from intimate mixing of Portland cement and ground granulated blast furnace slag. This slag is a waste product in the manufacture of pig iron.
Their role in increasing strength is dependent upon other parameters like porosity or bond strength, we shell revisit them later when required.
d. Aggregate
The strength of concrete is affected by the strength of the aggregate, its surface texture, its grading, and, to a lesser extent, by the maximum size of the aggregate. Strong aggregates, such as felsite, trap rock, or quartzite, are needed to make very-high strength concretes. Weak aggregates include sandstone, marble, and some metamorphic rocks, while limestone and granite aggregates have intermediate strength. Normal-strength concrete made with high-strength aggregates fails due to mortar cracking, with very little aggregate failure.
e. Moisture Conditions during curing
Water is not added only during the mixing of concrete, it is added all the way to the formworks’ removal. This is required to ensure that there is enough water present at all times at the surface to stop the outer surface from running dry and generating a suction to draw out even more water, reducing the water initially provided for hydration. This supply of water after the mixing stage is called curing and the strength depends upon it heavily.
2.1.8 Reinforced Concrete
As we have previously discussed, concrete is weak in tension. Therefore to overcome this weakness during its use in construction. Steel is incorporated in areas of the concrete elements where it would be under tension. Therefore utilizing its compressive capabilities while ignoring its tensile ones. Such type of concrete is known as reinforced cement concrete or RCC for short. In almost any construction RCC is used instead of plain cement concrete (or PCC for short).
2.1.9 Importance
Concrete has been used for about 200 years, since first being introduced by Joseph Aspdin in 1824. Its suitability, ease of access and cheap construction cost are indisputable. Over the 20th century, many codes and standards have been made based on the use of concrete. Therefore the construction of any modern building is simply incomplete without the use of concrete.
2.2 High Strength Concrete (363, 2010)
The main theme of this project is to achieve high strength concrete, therefore this section will introduce the reader to High Strength Concrete, the standards involved and provide the basis for the coming sections.
2.2.1 Definition
According to ACI 363-10, High Strength Concrete is defined as
Concrete that has a specified compressive strength for design of 8000 psi (55 MPa) or greater
The net value of 8000 psi (55 MPa) was selected because it represented a strength level at which special care is required for production and testing of the concrete and at which special structural design requirements may be needed. As technology progresses and the use of concrete with even higher compressive strength evolves, it is likely that the definition of high strength concrete will continue to be revised.
Although 8000 psi (55 MPa) was selected as the lower limit, it is not intended to imply that there is a drastic change in material properties or in production techniques that occur at this compressive strength. In reality, all changes that take place above 8000 psi (55 MPa) represent a process that starts with the lower-strength concretes and continues into higher strength concretes.
The committee also recognized that the definition of HSC8 varies on a geographical basis. In regions where concrete with a compressive strength of 9000 psi (62 MPa) is already being produced commercially, HSC might range from 12,000 to 15,000 psi (83 to 103 MPa) compressive strength. In regions where the upper limit on commercially available material is currently 5000 psi (34 MPa) concrete, 9000 psi (62 MPa) concrete is considered high strength.
2.2.2 Materials for Production
Increasing the strength of concrete is not a matter as simple as increasing cement to increase the solid products of hydration. An engineer has many things to take care of even if they have not been explicitly mentioned. These include workability of the concrete during mixing and placement, durability, abrasion resistance, etc. To make sure that each of these requirements is fulfilled along with increasing strength, the materials used in high strength concrete are different from those used in normal strength concrete. They are described as follows
a. Binder
The hydraulic binder or the cementitious materials are the most important component of HSC, the type of binder defines the properties of HSC as a whole. However, as we previously discussed, cement alone is not the only binder available or commonly used. Supplementary Cementitious materials or SCMs are as equally important and their use is often more than less necessary. A brief discussion of different kinds of cementitious materials is presented below.
Portland cement: The different types of cement have already been discussed, please Type of Cement in Factors affecting the strength of concrete.
Pozzolans: Pozzolans have been discussed in Type of Cement as well. However, there are two types of pozzolans that need special emphasis.
Fly Ash: Fly ash is a by-product of the coal industry and is normally produced from burning anthracite or bituminous coal and has strong pozzolanic properties but little or no hydraulic properties.
Silica Fume9: Silica fume is a by-product of the manufacture of silicon and ferrosilicon alloys from high-purity quartz and coal in a submerged-arc electric furnace. The escaping gaseous SiO oxidized and condenses in the form of extremely fine spherical particles of amorphous silica (SiO2); hence the name silica fume.
b. Admixtures
As previously stated, there is more to HSC then increasing strength alone. A lot of implicit requirements need to be full filled while increasing strength as well. To take care of this certain chemicals are added to HSC, these chemicals are known as admixtures. There is a long list of these admixtures however the ones related to the project will be discussed.
Retarding Admixtures: HSC mixtures incorporate a higher percentage of cementitious material content than normal strength concrete. Retarding chemical admixtures are highly beneficial in controlling early hydration, particularly as it is related to strength. Chemicals with retarder behavior are usually sugar, carbohydrate derivatives, soluble zinc salts, etc. (Neville, 2004). Apart from their need in terms of strength, they are also used when there is a need for long distance transport and delaying the hardening of concrete is required.
High Range Water Reducing Admixture (HRWR): HSC uses cement and pozzolanic materials such as silica fumes which require a lot more water for hydration and surface lubrication compared to normal strength concrete. This leaves little or no water for lubrication of the concrete during mixing, making it almost impossible to arrive at a homogeneous mixture. To cope with this we add HRWR to concrete, which imparts a negative charge to concrete and silica fume particles, forcing them to separate from each other and free the water that would be trapped between the particles sticking to one another. It also increases the rate of hydration reaction since we now have more surface of the binder available, hence care must be taken during proportioning so as not to render the retarding admixture useless. HRWR are usually Sulphonated naphthalene and/or melamine formaldehyde condensates.
Air Entraining Admixture: In cases when HSC or even normal concrete is being used in cold climatic zones, the temperature might fluctuate between below and above water freezing temperatures. This might cause the water inside the mortar to freeze at night and thaw in the morning, as ice has a more volume compared to water. This causes stress reversals in the lattice which can cause cracking of spalling in the concrete. To avoid this problem chemicals that produce air pockets when added to the concrete are usually added so that the air pockets will compress accommodating the volume increase. The use of air-entraining admixtures has been discussed, however since Peshawar does not have a climate justifying the use of Air-Entraining admixtures. Therefore they will not be used in any experimental work.
c. Sand and Aggregates
Both sand and coarse aggregate requirements will be discussed in an upcoming section of specifications. However here explicit requirements of sand and aggregates will be discussed.
Sand: In the context of the project, specifying any extra details to sand will have adverse rather than beneficial effects. Also, the implementation of any such change is very hard because of the lack of confidence of locals in such measures.
Aggregate: Aggregates, however, is a different story, according to (Caldarone, 2009) use of aggregates of smaller size will have a beneficial effect. For the following reasons.
- Increasing the maximum size of the coarse aggregate decreases the w/c ratio required and hence gives way to increase the strength.
- However increasing the aggregate size too much would decrease the strength, the reason being the difference in the poison ratio and the elastic moduli of the two particles (aggregate and mortar) and the sudden change in the medium for the stresses.
- The concrete transfers the stresses from the mortar (Fine aggregate and cement considered as one material) to the aggregate via the transition zone. To transfer large stresses of High Strength Concrete the zone has to be densified in order to transfer the said stresses.
- Keeping the aggregate size small would result in a relatively lesser amount of stress transfer via the transition zone since the number of aggregate particles would increase.
- Leading to higher critical stress.
- Hence the size of the aggregates needs to be kept below a maximum limit during mixing, usually 9mm or 3/8 in.
2.2.3 Proportioning
Concrete proportioning is a science and art in which trail batches are made with different proportions of the main constituents to achieve a set of desired properties for the concrete. HSC requiring both high strength and other properties needs extra care and precaution during this stage.
a. Strength Required (318, 2011)
As with most structural concretes, HSC is usually specified in terms of its compressive strength. In mixture proportioning the strength used for making the mix is more than that specified for the HSC in the contract. The reason for this is the variation in strength in different batches of concrete made for the same proportioning design. These variations tend to be due to different causes, including placement and transportation non-uniformities. Nevertheless, the target strength during proportioning is kept more than the required strength. The specified strength of HSC is denoted by fc’ whereas that used in proportioning is known as required average strength denoted by fcr’. The fcr’ is computed using the following equation, selecting the larger of the two.
Abbildung in dieser Leseprobe nicht enthalten
The term ss is the standard deviation of concrete strengths reported from testing concrete specimens of the same proportions and the same batch. If however such data is not available ACI 318 allows for finding fcr’ using the following equation
Abbildung in dieser Leseprobe nicht enthalten10
b. Test age
Selection of mixture proportions can be influenced by the testing age or early age strength requirements. Testing age depends upon construction requirements, for example in Pre-tensioned concrete operations where high strengths are required within 12-24 hours. Different test age strengths and their proportioning requirements are given below
Early Age: Sometimes there is a need for very early strength, as in the case of Pre-tensioned concrete, pavement traffic lanes or special applications for early use of machinery foundations. To achieve this early age strength development, the engineer will be using materials of a nature that will increase the rate of hydration i.e. Type-III cement, Admixtures countering the ill effects from using Type-III cement.
Twenty Eight Days: A common test age for compressive strength of normal concrete is 28 days. Therefore even in the use of HSC a 28-day test age will not warrant any extra specification details.
Later Age: Concrete gains strength with time, therefore if there is a need for higher strength in certain elements, for example, the sheer walls or ground floor columns of a high rise building. The proportioning will be such so as to guarantee higher strength even if it means more time for development. Normally these durations are 56 or 90-day test strengths.
c. w/c ratio
Water to cement ratio as discussed previously dictates strength directly and therefore is the first to be selected in the process. Its selection has a limit though, decreasing the water to cement ratio increases the strength by decreasing the porosity. But there needs to be enough water for complete hydration of the cement, otherwise, the paste density will decrease rather than increasing since the dry cement would have a greater volume.
d. Aggregate ratios (Caldarone, 2009)
Usually, HSC is paste rich or have higher contents of cement in them. Therefore the workability of high-strength concretes can be maintained using coarser sands, that’s why if the methods of normal concrete proportioning were to be used, it would result in an over-sanded mixture. Hence, instead of doing so it is advised to increase the aggregate ratio in advance since this would not only avoid the given scenario but would allow for better strength estimates as well.
e. SCMs
Supplementary cementitious materials used in HSC are usually blended/mixed with cement before application in a percentage of the mass or volume of cementitious material. Since we will be using Silica fume, therefore its usage details will be presented.
Silica Fume: Is normally used at 5-15% by mass of the total cementitious materials. In our case, we shall be using a fixed value of 6% from Dr. Qaiser on the basis of his experience in the field. (Ali, 2019)
f. HRWR
The use of HRWR in concrete is a trial and error process. The dosage is changed based on workability achieved and this is repeated until is fixed value is reached.
2.2.4 Fresh HSC
In the US, UK and other developed countries the construction process is significantly different from what it is like here. A project of any scale will require hiring a local builder, surveyor, and engineer registered with the local government. The builder, referred to here as the contractor may have to provide the material either by contacting a producer or assemble a plant by himself depending upon the nature of the project. This section provides details as to what extra precautions need to be carried out in different life stages of construction from ordering the concrete to Quality control procedures. General details of all these stages are present in (304, 1997).
a. Ordering
In developed countries, concrete is usually ordered from a production company that provides the concrete in batches. It is advised to order HSC in equally sized bates to help ensure consistency and uniformity. For example, if 10 yd[3] of concrete per batch is required and the delivery equipment has a maximum output of 7 yd[3] each, it would be better to order the batches in two of 5 yd[3] and 5 yd[3] rather than 9 yd[3] and 1 yd[3].
b. Batching
Abbildung in dieser Leseprobe nicht enthalten
Figure 3- Concrete Batching Plant, with the Silo’s, (Concrete Plant, Digital Image, Wikipedia, and 22nd April, 2019)
Batching is normally carried out in batch plants that have silos for storage and release of aggregates, cement, water, and admixtures if any. This is an efficient system and is used in all parts of the world including Pakistan. In the case of HSC, the moisture of aggregates should be uniform throughout all of the batches, other such types of care should be exercised.
[...]
1 Pictures available in Appendix
2 Explanation of Stress, Strength or any such terminologies are not within the scope of this text, the reader is advised to first study (Hibbler, 2011) and then continue from this point onward.
3 Hereafter Fine aggregate will be referred to as sand
4 Hereafter Course aggregate will be referred to as aggregate
5 Its tensile strength lies between 8-15% of its compressive strength (Wight, et al., 2012, Chapter 3)
6 This section will be revisited in detail at a later stage
7 Excessive heat generation, excessive shrinkage etc.
8 High Strength Concrete
9 Silica fume and Type-I Portland cement will be the focus of this research hereafter. Therefore other binders will not be discussed
10 Valid for fc’ less than 5000 psi
- Citation du texte
- Omair Shafiq (Auteur), Abdul Jabbar (Auteur), Ghufran Ullah (Auteur), 2019, Achieving High Strength Concrete in challenging areas, Munich, GRIN Verlag, https://www.grin.com/document/496075
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