Prefabrication

Prefabrication Structures

Definition:

Prefabrication is the practice of assembling the components of a structure in a factory or manufacturing site and transporting complete assembly to the construction site where the structure is to be located. It is the new technique and is desirable for large scale housing practices.

Principle of Prefabrication:

1.       To improve quality of work, as it is manufactured under improved and controlled conditions

2.       To speed up the construction, since no curing is necessary

3.       To achieve a good quality of construction with minimal cost

4.       To use materials which possess innate characteristics such as light weight, sound insulation, thermal conductivity etc…,

5.       To use locally available materials with required characteristics

6.       Use of reusable materials in construction

7.       Design for flexibility and adaptability

8.       Simplify and standardize building systems

9.       To provide safety working platform during deconstruction

10.   To minimize complexities in building design

Advantages ofprefabrication construction

Disadvantages ofprefabrication construction

Disadvantages of prefabrication construction

1.       Careful handling of prefabricated panels are required

2.       If the joints are not properly sealed there may be problem of leakages

3.       Cost of transportation may be high

4.       Transport of high voluminous materials requires larger road width

5.       Traffic congestion is the big issue during mass transportation

6.       Large prefabricated structure requires heavy duty crane, high precision measurement and proper placing of components

7.       Attention has to be paid towards strength and corrosion resistance of sections while joining in order to avoid failures

8.       Local jobs may be lost

Principle of prefabrication

Advantages of prefabrication

Advantages of prefabrication construction

1.       Self supporting readymade components are used, hence use of scaffolding and shuttering is greatly reduced

2.       Speeder construction than normal one

3.       Quality control can be easier in factory than in construction site

4.       Quality obtained in prefabrication process is higher than normal one

5.       Prefabricated construction is independent of bad weather conditions in the construction site

6.       Onsite construction and congestion is eliminated

7.       Provides safer working platform for labourers

8.       There is minimization of noise and dust in the construction site

9.       Prefabrication mode is economical than conventional methods

10.   Lesser expansion joints are required

Principle of prefabrication

Disadvantages of prefabrication

Objectives of research

The main objectives of the research is as follows

1. To gain familiarity with the phenomena and achieve new insights in it.
2. To represent accurately the characteristics of particular,individual,group or situation.
3. To determine the frequency with which something occurs.
4. To test the hypothesis of causal relation between variables.
5. To distuinguish truth and fact.
6. To understand social life and gain control over social behaviour.
7. To find new conclusions with old data.
8. To put forward entirely a new theory.
9. To explain unexplained horizon of knowledge.
10. To study and resolve contradiction in the area of research.

problems on concrete mix design

Examples of concrete mix proportioning 

1 Problems 1 - M40 pumpable concrete 

A-1 Design stipulations for proportioning 

a) Grade designation : M40 

b) Type of cement : OPC 43 grade confirming to IS 8112 

c) Maximum nominal size of aggregates : 20 mm 

d) Minimum cement content : 320 kg/m3 

e) Maximum water cement ratio : 0.45 

f) Workability : 100 mm (slump)

g) Exposure condition : Severe (for reinforced concrete) 

h) Method of concrete placing : Pumping 

i) Degree of supervision : Good 

j) Type of aggregate : Crushed angular aggregate 

k) Maximum cement content : 450 kg/m3

 l) Chemical admixture type : Superplsticiser 

A-2 TEST DATA FOR MATERIALS 

a) Cement used : OPC 43 grade confirming to IS 8112 

b) Specific gravity of cement : 3.15 

c) Chemical admixture : Superplasticiser conforming to IS 9103 

d) Specific gravity of Coarse aggregate : 2.74 Fine aggregate : 2.74 

e) Water absorption Coarse aggregate : 0.5 percent Fine aggregate : 1.0 percent 

f) Free (surface) moisture Coarse aggregate : Nil (absorbed moisture also nil) Fine aggregate : Nil 

g) Sieve analysis Coarse aggregate : Conforming to Table 2 of IS: 383 Fine aggregate : Conforming to Zone I of IS: 383 

A-3 TARGET STRENGTH FOR MIX PROPORTIONING

 f‟ck = fck + 1.65 s Where f‟ck = Target average compressive strength at 28 days, 

fck = Characteristic compressive strength at 28 days, s= Standard deviation From Table 1 standard deviation, s = 5 N/mm2 Therefore target strength = 40 + 1.65 x 5 = 48.25 N/mm2 

A-4 SELECTION OF WATER CEMENT RATIO 

From Table 5 of IS:456-2000, maximum water cement ratio = 0.45 Based on experience adopt water cement ratio as 0.40 0.4 < 0.45, hence ok 

A-5 SELECTION OF WATER CONTENT 

From Table-2, maximum water content = 186 liters (for 25mm – 50mm slump range and for 20 mm aggregates)

 Estimated water content for 100 mm slump = 186 + 6/100 x186 = 197 liters 

As superplsticiser is used, the water content can be reduced up to 20 percent and above 

Based on trials with SP water content reduction of 29 percent has been achieved. Hence the water content arrived = 19 x 0.71 =140 liters

 A-6 CALCULATION OF CEMENT CONTENT 

Water cement ratio = 0.40 Cement content = 140/0.40 = 350 kg/m3 From Table 5 of IS: 456, minimum cement content for severe exposure condition = 320 kg/m3 350 kg/m3 > 320 kg/m3, hence OK

 A-7 PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT 

From Table 3, volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone I) for water-cement ratio of 0.50 =0.60 

In the present case w/c= 0.40. 

The volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As w/c ratio is lower by 0.10, increase the coarse aggregate volume by 0.02 ( at the rate of -/+ 0.01 for every +/- 0.05 change in water cement ratio). 

Therefore corrected volume of coarse aggregate for w/c of 0.40 =0.62 

Note: In case the coarse aggregate is not angular, then also the volume of CA may be required to be increased suitably based on experience. 

For pumpable concrete these values should be reduced by 10 percent Therefore volume of coarse aggregate = 0.62 x 0.9 = 0.56 Volume of fine aggregate content = 1- 0.56 = 0.44

 A-8 MIX CALCULATIONS The mix calculations per unit volume of concrete shall be as follows 

a) Volume of concrete = 1 m3 

b) Volume of cement = = [350/3.15] x [1/1000] = 0.111 m3 

c) Volume of water = [140/1] x [1/1000] = 0.140 m3 

d) Volume of chemical admixture = [7/1.145] x [1/1000] = 0.006 m3 ( SP 2%by mass of cement) e) Volume of all in aggregates 

(e) =a – (b + c + d) = 1 – (0.111 + 0.140 + 0.006) = 0.743 m3 

f) Volume of coarse aggregates = e x Volume of CA x specific gravity of CA = 0.743 x 0.56 x 2.74 x 1000 = 11140 kg 

g) Volume of fine aggregates = e x Volume of FA x specific gravity of FA = 0.743 x 0.44 x 2.74 x 1000 = 896 kg

 A-9 MIX PROPORTIONS FOR TRIAL NUMBER 

Cement = 350 kg/m3 

Water = 140 kg/m3 

Fine aggregate = 896 kg/m3 

Coarse aggregates = 1140 kg/m3 

Chemical admixture = 7 kg/m3 

Water cement ratio = 0.40 

Aggregates are assumed to be in SSD.

Otherwise corrections are to be applied while calculating the water content. Necessary corrections are also required to be made in mass of aggregates. 

CONCRETE MIX DESIGN

CONCRETE MIX DESIGN 

1. Introduction

The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking. The actual cost of concrete is related to the cost of materials required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete. The extent of quality control is often an economic compromise, and depends on the size and type of job. The cost of labour depends on the workability of mix, e.g., a concrete mix of inadequate workability may result in a high cost of labour to obtain a degree of compaction with available equipment. 

2. Requirements of concrete mix design 

The requirements which form the basis of selection and proportioning of mix ingredients are :

 a ) The minimum compressive strength required from structural consideration 

b) The adequate workability necessary for full compaction with the compacting equipment available. c) Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions 

d) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete. 

2.1 Types of Mixes 

i. Nominal Mixes In the past the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. These offer simplicity and under normal circumstances, have a margin of strength above that specified. However, due to the variability of mix ingredients the nominal concrete for a given workability varies widely in strength. 

ii. Standard mixes The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in strength and may result in under- or over-rich mixes. For this reason, the minimum compressive strength has been included in many specifications. These mixes are termed standard mixes. IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively. 

iii. Designed Mixes

In these mixes the performance of the concrete is specified by the designer but the mix proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. This is most rational approach to the selection of mix proportions with specific materials in mind possessing more or less unique characteristics. The approach results in the production of concrete with the appropriate properties most economically. However, the designed mix does not serve as a guide since this does not guarantee the correct mix proportions for the prescribed performance. For the concrete with undemanding performance nominal or standard mixes (prescribed in the codes by quantities of dry ingredients per cubic meter and by slump) may be used only for very small jobs, when the 28-day strength of concrete does not exceed 30 N/mm2. No control testing is necessary reliance being placed on the masses of the ingredients. 

3. Factors affecting the choice of mix proportions 

The various factors affecting the mix design are: 

3.1. Compressive strength It is one of the most important properties of concrete and influences many other describable properties of the hardened concrete. The mean compressive strength required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix. The other factor affecting the strength of concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to Abraham‟s law the strength of fully compacted concrete is inversely proportional to the water-cement ratio. 

3.2. Workability The degree of workability required depends on three factors. These are the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For the narrow and complicated section with numerous corners or inaccessible parts, the concrete must have a high workability so that full compaction can be achieved with a reasonable amount of effort. This also applies to the embedded steel sections. The desired workability depends on the compacting equipment available at the site. 

3.3. Durability The durability of concrete is its resistance to the aggressive environmental conditions. High strength concrete is generally more durable than low strength concrete. In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the water-cement ratio to be used. 

3.4. Maximum nominal size of aggregate In general, larger the maximum size of aggregate, smaller is the cement requirement for a particular water-cement ratio, because the workability of concrete increases with increase in maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in size of aggregate. IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate should be as large as possible.

 3.5. Grading and type of aggregate The grading of aggregate influences the mix proportions for a specified workability and watercement ratio. Coarser the grading leaner will be mix which can be used. Very lean mix is not desirable since it does not contain enough finer material to make the concrete cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which can be achieved by mixing different size fractions. 

3.6. Quality Control The degree of control can be estimated statistically by the variations in test results. The variation in strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control. 

4 Mix Proportion designations 

The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass. 

4.1 Factors to be considered for mix design • The grade designation giving the characteristic strength requirement of concrete. • The type of cement influences the rate of development of compressive strength of concrete. • Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000. • The cement content is to be limited from shrinkage, cracking and creep. • The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction. 

Necessity of defining the research problem.

1. The problem to be investigated must be defined unambiguously to discriminate relevant data from irrelevant ones.
2. Proper definition of the research problem will give a clear picture to the researcher as how to carry out the research work without failure.
3. It gives clarity to the readers.
4. Makes the study feasible.
5. The definition helps the researcher to control subjectivity or biases of the researcher.
6. It helps to select variables required for study.
7. It helps to derive objectives accurately.
8. It helps to arrive at proper methodology for the research work.

Selecting the research problem

The following are the various factors to be considered while selecting a research problem

1. Subjects/Topics which are overdone should not be chosen ,since it is difficult to arrive at new ideas in such case.
2. Controversial subject should not be the choice of average researcher.
3. Too narrow subject or too Vague problems should be avoided.
4. The researcher should have great interest and should be familiar with the selected research problem.
5. The research problem which is selected should be made sure that it has adequate amount of data available.
6. Importance of subject, qualification and training of researcher,cost and time involved should also be considered while selecting a problem.
7. The selection of problem must be preceded by a preliminary study.
8. The researcher should preliminary check the availability of the required resources before selecting a problem.
9. Identifying gaps through literature surveys throw up new problems.
10. Writing should be started at the very beginning in order to encourage critical thinking, to understand sources better and to draft more effectively.
11. Interactions should be made with experts and practitioners.

Characteristics /criteria of good research.

1. The purpose of research should be clearly defined.
2. The research  procedure should be described sufficiently in detail to permit another researcher to work for the further advancement,keeping continuity of what he has already attained.
3. The procedural design of the research should be carefully planned to yield results that
   are as objective as possible.
4. The researcher should report with complete frankness, flaws in procedural design and
   estimate their effects upon the findings.
5. The analysis of data should be sufficiently adequate to reveal its significance. The
   methods of analysis used should be appropriate.
6. Conclusions should be confined to those justified by the data of the research and
   limited to those for which the data.
7. Greater confidence in research is warranted if the researcher is experienced.
8. Good research should be always systematic ,empirical ,logical and replicable.

Types of Research

The basic types of research are as follows

1. Descriptive vs Analytical Research 
2. Fundamental vs Applied Research
3. Quantitative vs Qualitative Research
4. Conceptual vs Empirical Research

Types of Research(Fundamental vs Applied Research)

FUNDAMENTAL RESEARCH	APPLIED RESEARCH
It is always theoretical and exploratory in nature.	It is practical and descriptive in nature.
Aims in generalization and formulation of theory.	Aims in finding solution for the practical problem.
It is less associated with technology	It is associated with advancement of technology.
It does not have direct commercial objectives	It have direct commercial objectives
Often several disciplines work together for solving the problem.	Problems are analyzed from the point of one discipline.
Less connected with economy	More connected with economy
It has wider scope	It has more specific scope
This research is always curiosity driven.	It is always client driven.
Less often appears in academic publications.	More often appears in academic publications.
Reports are compiled in a common language.	Reports are compiled in a language of technical language of discipline.

Types of Research(Qualitative vs Quantitative Research)

QUALITATIVE RESEARCH	QUANTITATIVE RESEARCH
It is based quality or nature of the proposed work.	Quantitative research is based on the measurement of quantity or amount.
Nature of this research is holistic.	Nature of this research is particularistic.
Approach is subjective.	Approach is objective.
Type of research is Exploratory.	Type of research is conclusive.
Sampling is purposive.	Sampling is Random.
Data here is verbal.	Data here is measurable.
Inquiry is process oriented.	Result oriented.
Hypothesis here is generated.	Hypothesis here is tested.
Elements of analysis are images and objects.	Elements of analysis are numerical data.
Methods of research are always non structured such as interviews, group discussions etc….	Methods of research are always structured such as questioners, observations etc….
Results develop initial understanding.	Results recommends final course of action.