Tuesday

Ready Mixed Concrete - Modern Technique of Transporting


A concrete whose constituents are weigh-batched at a central batching plant, mixed either at the plant itself or in a truck mixers, and then transported to the construction site and delivered in a condition ready to use is termed as Ready Mix Concrete (RMC).

Fig.1: RMC Plant
Fig.2: RMC Transporting Truck


The RMC is produced under factory conditions and permits a close control of all the operations of manufacture and transportation of fresh concrete. Due to its durability, low cost and the customization ability for different applications, RMC is one of the most versatile and popular building materials. RMC is ordered and supplied by volume in a freshly mixed and unhardened state.

Proportioning of RMC


The proportioning of an RMC aims at obtaining an economical and practical combination of materials to produce concrete with the properties desired for its intended use, such as the workability, strength, durability and appearance. The points to be considered for the proportioning of RMC are as follows:
  1. Concrete aggregates are required to meet appropriate specification and in general should be clean, strong and durable
  2. Fly ash or other supplementary cementitious materials which enhance concrete properties are normally added to RMC
  3. Admixtures are commonly used in relatively small quantities to improve the properties of fresh and hardened concrete such as the rate of setting and the strength development, especially during cold and the hot weather conditions

Advantages of RMC


The following are the advantages of RMC:

  1. Reduction in cement composition by 10 - 12% due to better handling and proper mixing. Further reduction is possible if the mineral admixture or the cementitious materials are used
  2. Since RMC use bulk cement instead of bagged cement dust pollution will be reduced and the cement will be saved
  3. Conservation of energy and resources because of saving of cement
  4. Environment pollution is reduced due to less production of cement
  5. With better durability of structure, their overall service life of increase and there is saving in the life cycle cost
  6. Quality assurance due to mechanical handling and uniformity of processes
  7. Eliminating or minimizing human error and reduction in dependency on labor
  8. General Benefits like speedy work stability of the structures etc.

Disadvantages of RMC

  1. Generation of additional traffic may affect the mix
  2. Concrete will limit the span between the mixing plant and the ddestination

Saturday

Elastic Properties of Concrete

Modulus of Elasticity of Concrete

The elasticity can be defined as that strain appears and disappears immediately on application and removal of stress. The modulus of elasticity of concrete is determined by subjecting a cube or cylinder specimen to uniaxial compression and measuring the deformations by means of dial gauges. The dial gauges reading divided by gauge length will give the strain and load applied divided by the area of the cross section will give stress. A series of readings are taken and stress-strain relationship is established.

Fig.1. Elastic Modulus Types From Stress Strain Curve For Concrete Mix
Fig.1. Elastic Modulus Types From Stress Strain Curve For Concrete Mix



The modulus of elasticity can also be determined by subjecting a concrete beam to bending and then using the deflection formulae and substituting the other parameters. The modulus of elasticity so found out from actual loading is called static modulus of elasticity. Concrete does not behave as an elastic material even under a short term loading. Stress-strain graph of concrete is not very much curved up to about 10 to 15% of the ultimate strength of concrete and hence the results are very accurate. For higher stresses the stress-strain relation will be greatly curved and the results are inaccurate.

Different Modulus of Elasticity


There are different types of modulus of elasticity to specify the elastic property of the concrete structure. They are explained below:

Initial Tangent Modulus

As shown in the figure-1, in concrete no part of the graph is straight. The modulus of elasticity is found with respect to the tangent that is drawn to the origin. The modulus of elasticity that is found from this tangent is referred as initial tangent modulus. For higher stress value it gives a misleading picture.

Tangent Modulus

The tangent can also be drawn at any other point on the stress-strain curve. The modulus of elasticity calculated with respect to the tangent is then called tangent modulus.

Secant Modulus

A line can be drawn connecting a specified point on the stress-strain to the origin of the curve. If the modulus of elasticity is calculated with respect to the slope of this line, the modulus of elasticity is referred as the secant modulus.
The secant modulus is the most commonly used elastic modulus. There is no standard method in order to determine the secant modulus. The value of the secant modulus decreases with the increase in stress.


Chord Modulus

The modulus of elasticity is found out with respect to the chord drwan between two specified points on the stress -strain curve is called chord modulus.

Relation between modulus of elasticity and strength


The stronger the concrete, higher is the elastic modulus. The modulus of elasticty of concrete increases approximately with the square root of the compressive strength, fck.
i.e. 



Where Ec is the static modulus of elasticity , As per IS:456-2000

Friday

Polymer Composites in Construction

Polymer Composites are produced using polymers with the help of cement, sand or aggregates.

Polymer composite in construction

These are also known as prepolymer cement concretes. The addition of polymer to concrete is to improve the following properties:

1. Compressive Strength
2. Resistance to wear and Tear
3. Fatigue Resistance
4. Impact Resistance
5. Impermeability
6. Durability
7. Chemical Resistance

Applications and Uses of Polymer Composites


1. Precast Products such as Kerb Stone
2. Bridge Ducts
3. Manhole Covers
4. Sewers
5. Tunnel Linings
6. Pipe carrying chemicals

Advantages of Polymer Composites


1. Cost Effective
2. Light Weight and thin
3. Superior to cement concrete

Polymer Concrete - Properties and Applications

Polymer Concrete

The polymer concrete is also called as resin concrete. It is the special type of concrete which can be manufactured by the addition of monomer or resin to the preheated aggregate consisting of coarse aggregate, fine aggregate, and other particle sizes. The commonly used binders are the styrene, methyl methacrylate, polyesters, and epoxies.

Fig.1. Polymer Concrete Products

In the pre-pack method graded dry aggregates are packed in molds and polymers is poured into the voids and if necessary, it is impregnated by means of vacuum. In the premix method, the polymer and the aggregate are mixed in the conventional mixers and the mix is transferred to the molds. This mix is vibrated in order to undergo compaction.


Properties of Polymer Concrete


1. Highly resistant to chemical attack
2. High Freezing and Thawing resistance
3. Zero permeability and absorption capacity

Comparison Between the Polymer Concrete and Plain Concrete




Applications of Polymer Concrete


1. Used to Manufacture pipe for carrying chemicals in industries
2. Floorings and for repairing works


Saturday

Prestressed Concrete - Basic Theory and Concept

What is Prestressing?


Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses that are arising from subsequent loads during its service period. This means that before the structure is subjected to any external loads, they will undergo some known stress.

The concept of prestressing existed before it has been applied in the concrete applications. The two such examples of methods are:




  • The force-fitting of the metal bands on the wooden barrels: Now the surrounding metal bands will induce initial hoop compression (C). The metal band is the element that promotes prestressing. The liquid that is filled inside the barrel will exert pressure which acts on the wall of the barrels. This is the tensional force (T). The force ' T ' will counteract the force ' C '.
Fig.1: Prestressing Action in Wooden Barrels

  • Pre-tensioning the spokes in a bicycle wheel: The spokes in the bicycle wheel are in tension. Because the spokes are tensioned for their arrangement. When the rider rides on the cycle, there is compression exerted on the spokes. As this compression is lesser than the tension on the spoke, easy resistance is carried out with spokes gaining a residual tension.


This principle of prestressing is applied in the prestressing of concrete. Hence the prestressed concrete can be defined as, concrete in which the effective internal stresses are induced ( usually by means of the tensioned steel) before the structure is loaded, in order to counteract the stresses resulting from the applied service loads.

Why Prestressing for concrete?



The tensile strength of concrete is only 8 to 14% of its compressive strength. So cracks are developed in the early stages of loading in the flexural members like beams and slabs. In order to prevent such cracks, compressive forces can be suitably applied in the longitudinal direction, either concentrically or eccentrically. This is called as Linear Prestressing.




This prestressing will enhance the bending, shear and torsional capacities of the flexural member. Other examples of prestressing are circular prestressing. Here the hoop tensile stresses are effectively counteracted by circular prestressing.


Friday

Classification of Aggregate - Size Based Classification

Fig.1.Fine Aggregate
The aggregates are generally classified based on size, as 

  1. Fine Aggregates
  2. Coarse Aggregates
  3. All-in-Aggregates
The Size of Aggregates used in concrete range from few centimetres or more down to a few microns



Fig.2: Coarse Aggregate



Fine Aggregate


It is the aggregate most of which passes through a 4.75mm IS sieve and contains only that much coarser materials as it is permitted by the specifications. Sand  is generally considered to have a lower size limit of about 0.07mm. The fine aggregate may be one of the following types:

Natural Sand : This are fine aggregate that is obtained from natural disintegration of rock and or that which has been deposited by stream and glacial agencies.

Crushed Stone Sand : Fine aggregates produced by crushing hard stone.

Crushed Gravel Sand: Produced by crushing natural gravel

The fine aggregates can be classified as coarse, medium and fine sands based on the fineness modulus 

  1. Fine Sand - Fineness Modulus of  2.2 to 2.6
  2. Medium Sand - Fineness Modulus of 2.67 to 2.9
  3. Coarse Sand - Fineness Modulus of 2.91 to 3.2

According to particle size distribution, IS: 383 -1970 has divided the fine aggregate in to 4 grading zones. The grading zone become progressively finer from grading zone 1 to grading zone IV.

Coarse Aggregate


The aggregate that is most retained on 4.75mm IS sieve . This contains only finer material as permitted by the specification. Based on the source, the coarse aggregate can be classified as:

  1. Uncrushed Gravel or Stone : It is obtained from the natural disintegration of rock
  2. Crushed Gravel or Stone : It results from crushing of gravel or hard stone
  3. Partially crushed gravel or stone : It is the product of blending of the above two aggregate

According to size, the coarse aggregate is described as graded aggregate of its nominal size .i.e 40mm, 20mm, 16mm and 12.5mm etc. For example a graded aggregate of nominal size 20mm means and aggregate most of which passes 20mm IS sieve. A coarse aggregate that has sizes of particulars mainly belonging to a single sieve size is known as single size aggregate. For example 20mm single size aggregate mean an aggregate most of which passes through 20mm IS sieve and its major portion of which is retained in 10mm IS sieve.

All in Aggregate

The all in aggregate composes of both fine aggregate and coarse aggregate. According to size , the all-in-aggregate is categorised as all in aggregates of its nominal size i.e. 40mm, 20mm etc.

For example all in aggregate of nominal size of 20mm means an aggregate in which most of them passes through the 20mm IS sieve and contains fine aggregates also.


Different Grades of Cement

Introduction

Grade of cement is the 28 day compressive strength when tested as per Indian standards under the standard conditions. The ordinary portland cement can be classified into 3 grades. viz. 33, 43 & 53 grades.

Fig:1. 53 Grade Cement 


It means that a cement with 33 grade will have strength that is equal to 33 Mpa (330 kg/cm2). Similarly 43 & 53 grades will have 43 and 53 Mpa strength respectively. The 33 grade is virtually phased out and has been replaced by 43 and 53 grades of cement

The details of different types of cement grade are explained below

33 Grade Ordinary Portland Cement ( IS: 269 - 1989)

The compressive strength after 28 days is 33N/mm2. It is used for general construction works in normal environmental conditions. It cannot be used where higher grades of concrete above M20 is required. The use of this cement has progressively decreased and virtually phased out.


43 Grade Ordinary Portland Cement (IS: 8112 - 1989)


Most widely used cement for general construction work. Minimum 28 days compressive strength 43N/mm2. It is used for construction of residential, commercial and industrial building, roads, bridges, flyovers, irrigation projects and other general civil construction works. Suitable for all types of applications -RCC -Plastering and Masonry. Rajashree is the premium OPC 43 brand in the market giving strength of around 65 MPa at 28 days.


53 Grade Ordinary Portland Cement (IS: 12269 - 1987 )


Introduced in 1991 by Grasim - Birla Super. Minimum 28 days compressive strength of 53N/mm2 gives 10-15% saving in the cement consumption and 5-8% saving in steel consumption provided higher grades of concrete say M30 and above are used. Useful for high rise buildings bridges, flyovers, chimneys and pre-stressed structures where high grade concrete is required. It gives better durability characteristic to the concrete. With the help of 53 grade cement high grade cement with water cement ratio can be created.

Difference Between 43 and 53 Grade Cement


Initial Strength: 53 Grade cement are used for fast paced construction were initial strength is to be achieved quickly. 53 Grade cement has fast setting compared to 43 grade cement. 53 Grade attains 27 mpa in 7days compared to 23 mpa by 43 grade cement.

Uses & Application:53 Grade OPC cement is Used in RCC and prestressed concrete of higher grades, cement grouts, instant plugging mortars etc. where initial higher strength is the criteria. 43 Grade OPC Cement are commonly used for plastering works, Non-RCC structures, pathways etc where initial setting time is not of importance.

Prices:53 grade cements are 2-3% costlier compared to 43 grade cement. 

Brands:Birla Cement, Ultratech Cement, ACC Cement, Zuari Cement, Coramandel Cement, Ramco Cement, Dalmia Cement are some of the well known brands in Southern India.





Thursday

Factors Affecting the Progress and Benefits of Accelerated Bridge Construction (ABC)


There are multiple factors that will affect the ABC costs and its progress. These will include the financial issues, ABC linked influential engineering operations, demolition, construction inventory like bridge lighting and traffic signs, mobilization and the site accessibility. Many construction activities in the conventional construction and the accelerated bridge construction are found to be similar. But certain minor differences between both the methods as well as in their management structure will make their completion rapid.

Fig.1. Bridge Construction Using ABC Method



Certain parameters that are highlighted in the ABC management structure are:

  • The unfamiliarity of the process: The modular design will undergo a test before the first use. This makes it more reliable. As per the saying " Practice makes perfect", which is best guided by technical experience.
  • The cost of training for the staffs: With the development of new technologies, it is necessary to train the staff to make the system more updated. The cost raised for the same is a good investment.
  • Fabrication Timeline: For new factory products, a realistic duration is necessary
  • Repetition of a continued work on similar projects by the same labors: For similar projects, no training is necessary. This would result in cost saving and timely completion.

Factors Affecting the Progress and Benefits of ABC



The factors affecting the progress of ABC construction are as follows:

1. Constraints Affecting The Timely Completion of ABC

2. Inventory and the Construction Sequence

3. Engineering Tasks that precedes the Accelerated Bridge Construction

4. Engineering Operations that are linked to ABC

5. The Construction Equipment used in ABC

Constraints Affecting Timely Completion


There are independently linked activities that are administrative and technical in nature , on any million-dollar project. These activities occur before starting the construction or during post-construction stage which tends to slow down ABC. These activities are summarized below:

  • Final Approval of Millions of Dollar: The demand for increased funding because of the rising costs, inflation, and the unforeseen circumstances is always the biggest problem. The highway trust fund and the state budget are the usual sources. There are chances for delaying the project even if it is compensated by ABC.
  • Preparing Bid Documents for a design -build contract: Accurate documents will contribute to the success of any project. This documentation will help in the proper calculation of the cost of the project. The contract documents are very important for the project implementation and their completion.
  • Contractor and the consultant team selection: It is not easy to change the core contractor or the consultant in the midstream of a project. So it is advised to acquire the right people for the right job.
  • Structural planning and design: Prefabricated, lightweight and durable structures are more preferred, in order to have speedy construction.
  • Feasibility of the construction schedule: The construction schedule is best controlled and monitored with the help of computer software like the program evaluation and review technique (PERT), CPM. These would identify the activities in the critical path.
  • Value Engineering: A detail study and comparison of merits of alternative solutions will help to identify expensive items for the design modifications before the project has been started. 
  • Environmental Permits and Approval: Any kind of undesirable or adverse effect on the environment like loss of fauna and flora have to be examined. The project completion should solve a single solution but must not create other problems.


Construction Sequence and Inventory


The factors relating to the inventory and the construction sequence are:

  • Relocation of utilities for water, gas and other pipes: In order to have utility pipes which are supported by cross beams under the bridge to provide water, gas, telephone and cable facilities it is necessary to have good coordination with the agencies during the course of construction.
  • Ease of Access to the site: The progress of the project is affected if there is a lack of easy accessibility to the construction site.
  • Preparation of the site: In the site preparation it is necessary to provide easy access for the labours and the equipment for the construction of foundation at the level below the bridge. This must be also provided at the deck level. Space must be arranged for the storage of materials. A yard has to be prepared for the assembling of the girders closer to the site.
  • Mobilization: A temporary office to carry out computing, telephone, parking and other necessary facilities are set up. It is used by 4 to 12 engineers and inspectors. This range is dependent on the size of the project. In order to monitor the daily progress, the quality and cost control full-time engineers will be resident. In order to support them, a backup team exists in the main office. Storage depots near to the bridge approaches is set up to facilitate mobilization.
  • Demolition: An existing superstructure can be removed in a single go or by stages. The demolition waste is recycled to meet the environmental regulations.
  • Entry and Exit Ramps: These ramps are located at a longer distance from the site.
  • Bridges Approach: The access to the site is either provided at the beginning of the bridge or at the ending of the bridge. For this, one or more lanes can be closed. In order to meet the completion milestone both day and night time will be required.
  • The Use of Grouting Methods or Soil Injections: This method helps in improving the foundation soil including the problems with liquefaction. This will help to protect the foundation in the salt water against corrosion, resistance to earthquakes, liquefaction, and erosion of soil under the footing. This will also help in fender damage from vessel collision.
  • Foundations: This is considered as a geotechnical discipline. In order to have soil reports, the borings must be made preferably at the approaches and behind the wing wall that is proposed. The geotechnical engineer having good expertise will recommend the soil report and the type of foundation. The ABC construction method will make superstructure to be lighter and hence reduce the weight on the foundation. Thus the ABC will make a foundation which is lighter.
  • Improving the performance of the bearing: In order to improve the performance of bearing during seismic effects multi-rotational and isolation bearings.
  • Staged Construction: During the replacement of projects, it is desirable to reconstruct one travel lane at a time so that the lane is cut and removed making the bridge usable. This will help in minimizing the traffic disruption.
  • Acceleration and Deceleration Lanes: During the planning of a new bridge, It becomes necessary to have an acceleration and a deceleration lane when the decks are placed closer to the exit or the entry ramps.
  • Bridge Lighting: For undergoing the night work, temporary generators will be required. An electrical engineer will ensure coordination with the utility company to permit long term deck and overhead lighting. The locations for having the overhead lighting poles near to the approaches and the deck have to be identified. For saving energy, the solar panels are encouraged. The electrical engineer will install the solar panel at the approaches for the bridge lighting and the signage.
  • Deck Drainages: For the rapid drainage of rain water, longitudinal and cross slopes will be provided. By planning for maximum intensity of rainfall, the size of the scuppers and inlet and the outlet pipes are computed. The scupper sizes and space is dictated by the maximum rainfall intensity. The deck drainage system will be connected to the public sewer system. There will be meeting with the utility company for obtaining advance approval.
  • Bio-retention ponds: The construction of bio-retention ponds will help to collect and filter runoff from the bridge deck.
  • The timely construction of approaches: The approaches are made using concrete slabs on grade with the retaining walls.

Engineering Tasks that precedes the Accelerated Bridge Construction


The acceleration construction is a method that is not successful and possible without demolition and mobilization. The Accelerated Bridge planning (ABP) and the staged construction(SC) are preceded by them for replacement. The replacement may be only for a bridge component, like the bearings or for the entire bridge. The requirements of the owner are made clear. Which includes:

  • Wider Decks
  • Reduced effect of earthquakes
  • Reduction in the schedule
  • Longer service life
  • Lower maintenance
  • Cost Saving
  • Increased bridge ratings

The accelerated bridge planning must promote the environmental or the preservation laws along with the durability factors. During the planning stage an evaluation matrix for the life cycle cost, the construction cost, constructability, social and environmental impact, future maintenance, inspection details and the aesthetic factors have to be prepared. The difference in accelerated bridge construction for small, medium as well as the long span is addressed in the planning stage. The constraints raised in meeting the accelerated construction target, like erection during extreme weather can be avoided or minimized by undergoing an efficient structural planning. The following factors are considered:


  • Improvement of aesthetics: The construction of an exterior arched beam can be carried out to hide the fascia girders. This would improve the appearance.
  • Installation of interactive touch screens: This would feature bridge information for motorists
  • Rider Comfort is increased: The deck cracking can be prevented by using a durable deck overlay. This is a protective system to keep the deck rid of cracks. This would also prevent post-tensioning that causes cracks in concrete. This helps in having a non-corrosive reinforcement. The layer would improve the crash worthiness of parapets and the barriers. One such application is the latex modified concrete.

Engineering Operations That are Linked to ABC


The important practical aspects related to this are mentioned below:

  • Employing prefabricated deck panels and the composite girders like Inverse.
  • The action of avoiding splices in long-span girders; Designation of highway route or the planning of the river navigation in order to make the delivery of heavy girders and the precast units from the yards where the fabrication is carried out at a rapid rate.
  • Rapid construction is boosted by the use of special construction cranes and the modern equipment.
  • The achievement of high strength concrete by improving the cold weather concrete methods
  • Reducing all possibilities of construction accidents by improving the safety throughout the project implementation.

The use of temporary signs is necessary during the construction. These involve the traffic signs, warning signs and variable message signs (VMS). When compared to fixed sign boards the electronic VMS signs are more preferred. The warning signs, as well as the exit and directional signs, must be displayed and has to be located conveniently.

In the case of location the site, it is necessary that it is in an area to where the delivery of materials and the transport is easily accessible. This must also allow for a lane closure and temporary ramp construction.

The Construction Equipment Used in ABC


The warranty provided by the manufacturer is desirable against the undesirable and inefficient performance of the construction equipment. In order to facilitate improved performance, it is necessary to have specialized equipment. The modern construction equipment will boost and help in speedy construction. Such efficient equipment is:
  • Mobile cranes
  • Truck-mounted cranes
  • Waste handlers
  • Excavators
  • Jib Cranes
  • Fork Lifts
  • Winches and Cable Pullers

For ABC, the modern construction equipment must be timely available and must have an experienced erection team also. For different structural elements like girders, box beams, arches, trusses, suspension cable bridges and cable-stayed ones different erection equipment is necessary.

An erection contractor must be equipped with the following requirements:

  • Lattice boom crawler cranes
  • Mobile Lattice boom cranes
  • Mobile Hydraulic Cranes
  • Tower Cranes
  • Lattice Ringer Cranes
The tower cranes that have a maximum lightweight pickup of 20tons and a height greater than 400 feet is necessary. The lattice ringer is used for heavy weight pick up to 1400 tons and 400 feet. The Mobile lattice boom cranes are used for lightweight to medium pickups up to 300 tons. The Mobile Hydraulic cranes are used for lightweight to medium with a pick up to 650 tons. The Lattice boom crawler cranes are used for lightweight to medium pickup to 400 feet.

Along with the above-mentioned accessories like separate foundations, temporary bents, hydraulic jacks, crane mats and temporary pier brackets are also necessary.





Wednesday

Alkali- Aggregate Reaction - Control Measures

The following precautions can be taken in order to control the alkali-aggregate reaction.

Fig.1. Alkali-Aggregate Reaction Effect on Concrete Structure

CONCRETE CANCER

Alkali-Aggregate Reaction is called as concrete cancer. The alkali-aggregate reaction is an important phenomenon that influences the strength of the concrete structure, how big or small the structure is.
Fig.1.Bridge girders and piers after alkali - aggregate reaction

Tuesday

Effect of Aggregates on the Properties of Hardened Concrete

The properties of aggregates and their influence on the properties of the hardened concrete are explained below:

Grading

It is found that the aggregate grading has no kind of influence on the properties of hardened concrete unlike fresh concrete. 


Bulk Density

This has no direct effect but has an indirect effect on the drying shrinkage . Bulk density is an indicator of packing capacity. This is a factor that influences water requirements, which in turn will affect the drying shrinkage of the hardened concrete. For a given particle specific gravity, the higher the bulk densities of sand and stone, the lower the water requirement and hence lower the drying shrinkage.




Sunday

Concrete Mix Design - Difference Between Nominal and Design Mix

What is Concrete Mix Design?


The process of selecting suitable ingredients of concrete and determining their relative amounts as economically as possible is termed as the Concrete Mix Design. The proportioning of the ingredients of concrete is governed by the required performance of concrete in the two states. i.e the







  • The Plastic State
  • The Hardened State
If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability is hence very important. The compressive strength of the hardened concrete which is generally considered to be an index of its properties mainly depends upon many factors. They are:

  • Quality and Quantity of Cement
  • Water and Aggregates
  • Batching and Mixing
  • Placing and Compaction
  • Curing

Objectives of Concrete Mix Design

The main objectives of concrete mix design are:
  • To ensure that the most optimum proportions of the constituent materials are obtained in order to fulfill the requirement of the structure built.
  • To achieve the desired and designed workability in the plastic state of concrete
  • To attain the minimum strength at its hardened stage
  • To achieve the desired durability in the given and considered environment conditions
  • To produce concrete as economical as possible

Cost Consideration in Concrete Mix Design


The cost of concrete is made up of the cost of materials, plant, and labor. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregates. Thus the main aim is to produce a lean mix as possible. From the technical point of view, the rich mixes may lead to high shrinkage and crack in the structural concrete. The rich mix also results in the evolution of high heat of hydration in the concrete mass, that will also result in the cracking.

Types of Mixes in Concrete Mix Design

The two main types of mixes in Concrete Mix design are:
  • Nominal Mix
  • Design Mix

Nominal Mix



The Nominal Mix gains a fixed cement-aggregate ratio that will ensure adequate strength. These offer simplicity. These also provide a margin of strength above that is specified under normal circumstances. However, due to the variability of the mix ingredients the nominal concrete for a given workability varies widely in strength. These mixes are hence used for less important works. The nominal mix will gain a higher cement content which makes them uneconomical. 

According to IS 456-2000, the nominal mix concrete may only be used for concrete with grade M20 or lower i.e. M5, M7.5, M10, M15, and M20. The proportion of the materials shall be in accordance with the following table as in IS: 456-2000.

The proportions of the materials for nominal mix design is given in the table-1.

Nominal Mix Proportion of Concrete Grade From M5 TO M 20
Table:1: Nominal Mix Proportion of Concrete Grade From M5 To M 20

Design Mix


In this method, the mix proportion is calculated by using standard design procedures. The mix design depends upon various factors like the strength, durability, workability and many other factors. This approach results in the production of concrete with the appropriate properties most economically.
Other factors like compaction equipment available, the curing method adopted, the quality of fine and coarse aggregates, the type of cement etc have to be kept in mind before arriving into the mix proportion.
The design mix is implemented for more variety of important structures, because of the better strength attained. This also shows reduced variability, leaner mix with consequent economy and assurance for the resultant quality.



Saturday

Rheology of Concrete - Representation of Rheological Behaviour

Rheology is the study of flow of matter primarily in liquid state or in a semisolid state. The flow of the liquid can be characterised based on Newton's law of viscosity.



Thursday

Effect of Aggregate on the Fresh Properties of Concrete

In the case of sands, it has been found that materials having a large deficiency or excess of any size fraction adversely affect the workability of the concrete. The most important size fraction in a concrete sand is those that passes through 0.30, 0.15 and 0.075 mm sieves. If these fractions are too low the concrete tends to be prone to segregation.

Effect of Aggregate on the Fresh Properties of  Concrete

Thumb rule for Optimum Percentage


The optimum percentage is 30%, 15% and 7.5% that is passing through 0.30mm, 0.15mm and 0.075 mm sieves. The percentage passing the 0.075mm sieve may be used as a first estimate of the concrete's tendency to bleed. The greater the fraction, lower is the tendency to bleed.


Effect of Aggregate on the Plastic Properties of Concrete


Certain specific properties of aggregate that will influence the fresh concrete properties are:

  • Particle Shape
  • Particle Surface Texture
  • Grading
  • Maximum Aggregate Size
  • Fines Content ( Minus 75 micrometer material) 

Effect of Particle Shape on Fresh Concrete


The shape of the particle influence greatly on the workability and the water requirement of a concrete mix. Among different aggregate shapes like rounded, flakey, irregular; Rounded or chunky ones roll or slide over each other's surface easily. This is not same in the case of angular or flaky aggregates.

Effect of Particle Shape on Fresh Concrete


The particle packing and interlocking are also influenced by the aggregate shape. This is a factor that finally determines the void content of the final structure. Use of different aggregates mainly different sands will bring difference in the standard water requirement (SWR) in excess of 50 litres per metre cube.

Effect of Particle Surface Texture on Fresh Concrete


The surface texture of the aggregates influence the surface area and the inter-particle friction of the aggregates. Hence the water requirement is affected thus the workability of the mix. Rough and angular aggregates have a great surface area compared to round aggregates. Higher the surface area more is the water requirement.


Effect of Grading of Aggregates on the Fresh Concrete


The grading of fine aggregates have more influence on the workability of the concrete mix than the grading of coarse aggregates. The total aggregate surface area (that is the area to be wetted) and the relative aggregate volume is influenced by the same. Conforming to the standard grading will serve the best workability. This practise would ensure that the voids of any one particle size are overfilled by the next smaller particle size. For those particles of minus 300 micron metre particular attention have to be provided. The finer fraction of minus 150 and minus 75 micron metres have a greater influence on the concrete mix cohesiveness and bleeding characteristics.

Effect of Grading of Aggregates on the Fresh Concrete


The quantities that are required in the mix will depend on the nature of the sand. In the case of high performance concrete (HPC), the grading of all constituents including the binder is considered more important.


Effect of Maximum Aggregate Size on the Properties of Fresh Concrete


It is observed that the increase in the size of aggregate up to a limit would lower the water requirement of the overall mix and also increases the concrete properties. We mainly employ smaller aggregates for High strength concrete (HSC). For aggregate size above 26.5mm, smaller sized aggregates are required for proper blending.

Fines Content ( Minus 75 micron metre material)


Special caution have to be taken in the case of nature of fines used. For example on the amount of clay that is present in the sand chosen. A strict control have to be carried out on the fines content of the natural sand. In the case of crusher fines, concrete properties can be improved by use of fine in excess of say 10%. But in such cases the factor of freeze - thaw resistance must be considered.


ALSO READ : Effect of Aggregates on the Properties of Hardened Concrete

Tuesday

Dangerous Construction


Dangerous Construction




The urge to construct new buildings will demand faster construction. Due to this course of speed, the safety on a construction site is neglected and given less importance. The safety in construction will ensure the safety of labor along with optimum utilization of resources. This measure will always help in the saving of project time and faster completion of the project with utmost success.

Some of the dangerous site construction that was sighted and seen by me through the web are listed below:









Monday

Concrete Sealers- Buying Tips for Concrete Sealers

What are Concrete Sealers??

Fig.1. Concrete Sealer Application in Driveways

The concrete sealers are a special mixture that is applied to the concrete surface in order to protect from surface damage, corrosion and staining problems. These undesirable effects would result in the  formation of pores that would result in:
  • Increase in porosity of the concrete surface
  • Water penetration-causing freezing and thawing
  • Crack Formation
  • Decrease in durability of the concrete surface
  • Subsurface failure due to water penetration, causing settlement


Why Use Sealer??


The concrete sealer will help in sealing the common root problem that is the pores. It covers the pores hence avoiding water penetration and related issues. This sealant helps in reduction of absorption of salt or any harmful chemical to which the structure is exposed to.

Fig.2. Water Penetration in Concrete floor sealed and unsealed

Major Concrete authorities have conducted research and concluded that the most common reason behind the concrete damage is the surface intrusion. The concrete damage through surface scaling due to rapid freezing and thawing process is found to be the most pervasive form. Other reasons for the same are:
  • Chemical Intrusion
  • Corrosion of the steel reinforcements
  • Improper design -formation of  voids

How to Choose The Right Sealer??


  1. What conditions will the sealer be exposed to?
  2. Is the sealer breathable?
  3. What is the drying time?
  4. What is the coverage rate?
  5. What type of finish and surface appearance do you want to achieve?
  6. What is the life expectancy of the sealer, and does the manufacturer provide a warranty?
  7. How easy is the sealer to apply?

Buying Tips for Concrete Sealers

Before deciding to buy any concrete sealer follow the steps:
  • Read the technical details provided in the product description/ or in the product websites
  • The required keywords are non-yellowing, dust proofing, water proofing, resistance to oil-grease and ash and finally breathability
  • The product should have detail description of the application procedures, further maintenance, and re-application
  • Ask the supplier for extra recommendations based on the purpose you are buying the product
  • Buy concrete sealer with good performance and review than buying a cheap one.

Types of Concrete Sealers

The high-quality concrete sealer will help to block almost 99% of the surface moisture to which it is exposed to. The concrete sealers can be categorized into two main types. They are:

Topical Sealers

Topical Sealers will facilitate visual enhancement as well as topical protection from stains and chemicals. In order to gain proper adhesion, they require a dry, clean surface during application. Topical sealers may alter the coefficient of friction which can make substrates slick when wet – a condition that can be remedied by adding anti-skid materials.The durability period is generally 1-5 years, although high-end epoxy/urethane systems can last significantly longer.

Penetrating Sealers

The application of the penetrating sealers are done over dry or damp surfaces and should be properly matched with substrate porosity in order to effectively penetrate the surface and react. The chemical reaction bonds active ingredients within the substrate blocking surface moisture. Penetrating sealers generally do not significantly modify substrate appearance or traction. The durability of penetrating sealers is generally 5 years or more.

Foundation in Construction - Types of Foundations

The foundation comprises the substructure of a building or a structure. It is the lowest part of the building that is constructed below the ground level. The main function of the foundation is to transmit the weight of the superstructure to the subsoil in a uniform manner. 



Sunday

What is Infrared Thermography - Infrared Camera Infrared Imaging


Infrared Thermography - Infrared Camera Infrared Imaging

  Infrared thermography method of inspection for the building to detect the moisture, plaster detachment, hidden structures and material deterioration

Fig.1.Infrared thermography method of inspection for the building to detect the moisture, plaster detachment, hidden structures and material deterioration



Infrared thermography (IRT) can be defined as a powerful tool which involves the science of acquisition and data analysis from a non contact thermal imaging device. This process of thermal imaging has simplified with time, incorporating high speed and efficient camera and data analyzers. The thermography literally means writing with heat.  

Saturday

Famous Structures & Time Taken For Construction

Hello everyone!! Today we will brush through the famous structural wonders around the world and the time taken for their construction. It's just amazing to know that the builders and designers had and have  great patience and hardworking nature without which beautiful structures we see today have not been present. We will go through each structures I have researched on one by one.


1. Leaning Tower of PISA, Italy

199 years of construction period. The construction started in 1172. The structure was designed to be straight. But the tower leaned at about 3.99 degrees, 297 steps lead to its top.

Fig.1

2. Great Pyramid of Giza

The construction time period is 20 years. It is the oldest of the Seven Wonders of the Ancient World, and the only one to remain largely intact.

Fig.2

3. ST. Peter's Basilica, Vatican City

The construction time period is 144 years. The construction commenced in 1506


Fig.3

4. ST. Basil's Cathedral, Moscow, Russia

The construction period is 123 years. It started in 1555. The full name is " The Cathedral of the Intercession of the Virgin by the Moat".

Fig.4

5. Buckingham Palace, London, England

The construction period is 23 years. The construction commenced on 1702.

Fig.5

6. Eiffel Tower, Paris, France

The construction time period is 2 years. The construction started in 2 years. This is the tallest structure in France

Fig.6


7. St. Paul Cathedral, London, England

The construction period is 36 years. The construction started in 1675.


Fig.7

8. Taj Mahal, Agra, India

21 years of the construction period. It started in 1632. 20,000 people where involved in the construction.

Fig.8

9. White House, Washington DC, USA

The construction time period was 13 years. The construction started in 1792.

Fig.9

10. Brandenburg Gate, Berlin, Germany

The construction Period was 3 years. The construction started in 1788.

Fig.10


11. Burj Khalifa, Dubai, UAE

The construction time period is 2 years. The construction started in 2004. It is the tallest building in the world.

Fig.11

12. Lotus Temple, New Delhi, India

The construction period is 6 years. The construction started in 1980.


Fig.12

13. Petronas Towers, Kuala Lumpur, Malaysia

The construction period is 3 years. It started in 1993.


Fig.13

14. Millennium Dome, London, England

4 years of construction period. Started in 1997

Fig.14

15. The Space Needle, Seattle, USA

1 year of construction period. Started in 1962.

Fig.15

16. CN Tower, Toronto , Canada

2 years of construction period. It started in 1973

Fig.16

17. Mount Rushmore, South Dakota, USA

The construction time is 14 years. It started on 1927.

Fig.17


18. Tower Bridge

8 years of construction period. The construction started in 1887. It is a movable bridge. The bridge rotates at 86 degrees to allow river traffic.


Fig.18

19. Palace of Westminster, London , England

30 years of construction period. The construction started in 1840. 

Fig.19


20.The Golden Gate Bridge

5 years of construction period. The construction started in 1933.The Golden Gate Bridge is a suspension bridge spanning the Golden Gate strait, the one-mile-wide (1.6 km), one-point-seven-mile-long (2.7 km) channel between San Francisco Bay and the Pacific Ocean. The structure links the American city of San Francisco, California – the northern tip of the San Francisco Peninsula – to Marin County, carrying both U.S. Route 101 and California State Route 1 across the strait.


Fig.20





About Us

This is a center to bring up and collect all bits of knowledge and technologies flourished and flourishing to reach out for those who wish to acquire knowledge in a broader way. I am a structural engineer by profession and would like to share all information I have acquired all these years till date. This site also aims in researching new topics, works on your questionnaires and fill it up with good facts and information.

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