Self Consolidation Concrete

Self-consolidating concrete or self-compacting concrete

(SCC) is characterized by a low yield stress, high deformability, and moderate viscosity necessary to ensure uniform suspension of solid particles during transportation, placement (without external compaction), and thereafter until the concrete sets.

Such concrete can be used for casting heavily reinforced sections, places where there can be no access to vibrators for compaction and in complex shapes of formwork which may otherwise be impossible to cast, giving a far superior surface than conventional concrete. SCC was conceptualized in 1986 by Prof. Okamura at Ouchi University, Japan.

The first generation of SCC used in North America was characterized by the use of relatively high content of binder as well as high dosages of chemicals admixtures, usually superplasticizer to enhance flowability and stability. Such high-performance concrete had been used mostly in repair applications and for casting concrete in restricted areas. The first generation of SCC was therefore characterized and specified for specialized applications.

The relatively high cost of material used in such concrete continues to hinder its widespread use in various segments of theconstruction industry, including commercial construction, however the productivity economics take over in achieving favorable performance benefits and works out to be economical in pre-cast industry. The incorporation of powder, including supplementary cementitious materials and filler, can increase the volume of the paste, hence enhancing deformability, and can also increase the cohesiveness of the paste and stability of the concrete. The reduction incement content and increase in packing density of materials finer than 80 µm, like fly ash etc. can reduce the water-cement ratio, and the high-range water reducer (HRWR) demand. The reduction in free water can reduce the concentration of viscosity-enhancing admixture (VEA) necessary to ensure proper stability during casting and thereafter until the onset of hardening. It has been demonstrated that a total sand content of about 50% of total aggregate is favorable in designing for SCC.

Electrical Tower Transmission

Electrical Transmission Tower Types and Design

The main supporting unit of overhead transmission line is transmission tower. Transmission towers have to carry the heavy transmission conductor at a sufficient safe height from ground. In addition to that all towers have to sustain all kinds of natural calamities. So transmission tower designing is an important engineering job where all three basic engineering concepts, civil, mechanical and electrical engineering concepts are equally applicable.

A power transmission tower consists of the following parts, 1) Peak of transmission tower 2) Cross arm of transmission tower 3) Boom of transmission tower 4) Cage of transmission tower 5) Transmission Tower Body 6) Leg of transmission tower 7) Stub/Anchor Bolt and Base plate assembly of transmission tower. The main parts among these are shown in the pictures.

Peak of Transmission Tower

The portion above the top cross arm is called peak of transmission tower. Generally earth shield wire connected to the tip of this peak.

Cross Arm of Transmission Tower

Cross arms of transmission tower hold the transmission conductor. The dimension of cross arm depends on the level of transmission voltage, configuration and minimum forming angle for stress distribution.

Cage of Transmission Tower

The portion between tower body and peak is known as cage of transmission tower. This portion of the tower holds the cross arms.

Transmission Tower Body

The portion from bottom cross arms up to the ground level is called transmission tower body. This portion of the tower plays a vital role for maintaining required ground clearance of the bottom conductor of the transmission line.

Design of Transmission Tower

During design of transmission tower the following points to be considered in mind, a) The minimum ground clearance of the lowest conductor point above the ground level. b) The length of the insulator string. c) The minimum clearance to be maintained between conductors and between conductor and tower. d) The location of ground wire with respect to outer most conductors. e) The mid span clearance required from considerations of the dynamic behavior of conductor and lightening protection of the line. To determine the actual transmission tower height by considering the above points, we have divided the total height of tower in four parts, 1. Minimum permissible ground clearance (H1) 2. Maximum sag of the conductor (H2) 3. Vertical spacing between top and bottom conductors (H3) 4. Vertical clearance between ground wire and top conductor (H4).

Types of Transmission Tower

According to different considerations, there are different types of transmission towers. The transmission line goes as per available corridors. Due to unavailability of shortest distance straight corridor transmission line has to deviate from its straight way when obstruction comes. In total length of a long transmission line there may be several deviation points. According to the angle of deviation there are four types of transmission tower- 1. A – type tower – angle of deviation 0^o to 2^o. 2. B – type tower – angle of deviation 2^o to 15^o. 3. C – type tower – angle of deviation 15^o to 30^o. 4. D – type tower – angle of deviation 30^o to 60^o.

As per the force applied by the conductor on the cross arms, the transmission towers can be categorized in another way- 1. Tangent suspension tower and it is generally A - type tower.

2. Angle tower or tension tower or sometime it is called section tower. All B, C and D types of transmission towers come under this category.

Apart from the above customized type of tower, the tower is designed to meet special usages listed below,

These are called special type tower

1. River crossing tower

2. Railway/ Highway crossing tower

3. Transposition tower

Based on numbers of circuits carried by a transmission tower, it can be classisfied as- 1. Single circuit tower

2. Double circuit tower

3. Multi circuit tower.

Intermediate Transition Zone (ITZ) in concrete

What is ITZ and how it is developed in concrete

ITZ in civil engineering terminology is known as interfacial transition zone or intermediate transition zone. This is the phase present in concrete where two different phases meet with each other. As you can see in the picture you can see clearly the layer of ITZ presence 

In concrete aggregate is considered as one phase and surrounding paste is considered as another. For concrete to bear load, it is imperative that there should be stress distribution between paste and aggregate.

When these two phases of concrete meet, there is always an ITZ layer present. This layer has properties which are neither of aggregate phase nor to the paste phase.

What happens is when that near aggregate slurry (water+cement) accumulates causing a layer which is shown in picture below. This layer is week because of its chemical composition.

Strength

In reality, ITZ has probably the lowest strength properties therefore under loadsstress in aggregate phase will be different than that of the paste for same strain level. This type of non-uniform stressdistribution will cause ineffective transmission of forces between paste and aggregate. If ITZ is not taken care of than this can lead to stress concentration which will ultimately cause cracking

Sources of Crack in Concrete

We all know cracking is one of the major problems in concrete and it should be considered while mix design, but for that, we have to know the sources which can cause cracking in concrete. Here is a list of sources of cracking in concrete

Use of low-grade materials:

One of the major sources is the use of lower quality materials which include both concrete and steel in reinforced concrete structures.

Shrinkage:

Shrinkage can cause cracking if not controlled during mix design and curing stage. Shrinkage can become critical in high strength concrete because of low water/cement ratio and also because of use of Mineral Admixtures.

Quality and Type Of Aggregate:

The quality of aggregate used in concrete determines the overall strength of concrete. If the aggregate is of poor quality it will not make a proper bond with cement.

Overloading of structure:

Overloading of structure especially at younger age is a common source of cracking. This can happen if formwork is removed before time or more construction load is present.

Mistakes at design stage in office:

If there are errors at design stage then it is obvious that problems will occur at site. concrete cracking is one of them.

Improper Curing:

Another major cause of concrete cracking. If curing is not done appropriately for given time span then one should expect cracking.

Early Formwork Removal:

If formwork is removed before concrete has achieved strength, there will be cracking.

Use of Congested Reinforcement in Lean Concrete:

If you use heavy reinforcement in average quality concrete then stress distribution between steel and concrete can become non-linear causing cracking.

Mistakes at Site or during erection:

Proper and trained labour and workmanship are necessary for any sitework. Lack of it during concreting can cause cracking.

 

Effects in concrete mix design

Concrete Mix design is complete science and it is based on a lot of research. Popular Mix Design method is highlighted in ACI 2011 committee report. It is a systematic approach in which user starts with defined slump and strength and some other parameters. End result gives quantities of different constituents.

Performance-based methods are alsoavailable in which engineer based on his experience selects initial proportions and modifies as needed. Both systematic and performance-based methods have certainadvantages and disadvantages which will be covered in later posts.

Though mix design is a comprehensive process still it is very useful for engineers to know certain mix design concrete thumb rules which will make his life easier. Here is the list,

By adding 1 litre of water in 1 cubic meter of concrete mix

Increase slump of about 25 mm is expected.It will decrease compressive strength of about 1.5 to 2.0 N/mm²Increase shrinkage potential of about 10%Waste as much as ¼ bag of cement

Effect of increasing concrete mix temperature by 1 celsius

About 4 liters of water per cubic metermaintains equal slumpAir content decreases about 1%Compressive strength decreases about 1.0 to 1.5 N/mm²

Effect of air content on concrete mix 

If air content increases 1%, it will result incompressive strength decrease of about 5%If air content decreases 1%, then it will cause yield to decrease about 0.03 cubic meter per 1 cubic meter of concrete mix.If air content decreases 1%, then slump willdecrease about 12.5 mmIf air content decreases 1%, it will result in durability decreases of about 10%.