20 Feb 2019

Factors Affecting Concrete Strength

Concrete

Factors Affecting Concrete Strength

In addition to Mixing, Pouring and Compaction, the concrete strength mainly depends on the following factors:
Factors Affecting Concrete Strength

(1) Water-Cement Ratio (W/C)


                 To decrease in W/C  will increase the strength of concrete. Normally water-cement ratio is 0.45-0.55 for normal concrete.
W/C Effects on Concrete

(2) Aggregate-Cement Ratio

                    To decrease in Aggregate-Cement Ratio increase the strength of concrete upto numerical value of 2, further decrease of Agg/Cement may cause decrease in strength on concrete.

(3) Aggregate

                    The concrete strength is affected by 
  • Aggregate Strength
  • Surface Texture
  • Grading
  • Maximum size of aggregate in concrete

(4) Curing

                 Prolonged moist curing results in getting highest concrete strength.

Rate Of Strength Gain by Concrete

                 The ACI code proposes the rate of strength gain for concrete in which type-I concrete was used at 70F° by the following Equation


fc'₍t ₎= fc'₍₂₈₎⎨t ⁄ 4+0.85t ⎬

In the above Equation 
t= Time in days
fc'= Compressive strength at age t in days

Variation in Strength of Concrete

The strength variation of finished concrete depends on
  1. Property of concrete component 
  2. Methods of transportation of concrete
  3. Compaction of concrete
  4. Discripenses in the test of concrete
Compressive Test on Concrete



14 Feb 2019

Structural Systems of Slabs


Structural System


Structural Systems of Slabs and Its Types, Purpose

Structural System

Structural system in building construction, the particular method of assembling and constructing structural elements of a building so that they support and transmit applied loads safely to the ground without exceeding the allowable stresses in the members.

Types and Purpose of Structural System

There are various types of structural system used in design of Civil Engineering Structures:

(1) Flat Plate System

A flat plate is a slab system in which the slab of uniform thickness is supported directly on columns. This system is economical for short and medium span (15' - 25') and for normal loads. In this slab punching shear is the failure criteria.

(2) Flat Slab System

It is a beam less slab system with drop panel or column capitals or both is known as Flat Slab System. This system is economical with span ranging from (20' to 30'). The drop panel is thick part of slab around the column, while column capitals is head of increased size.

(3) Beam Supported Slab System

This system consists of Reinforced Concrete beams in one or both direction cast monolethically with slab. This system is suitable for long spans normally 30' and for intermediate to heavy loads.
Slabs Systems

(4) Beams and Girder Slab System 

It consist of series of parallel beams supported at the ends by girder which is then connected to columns. This system can be adopted for any type of loads and spans.

(5) One Way Joists Slab

This consists of monolethic combinations of regular placed ribs and a top slab in one direction. This system is economical for the (30' - 50') span.
Slabs System

(6) Two Way Joist Slab

It is also called "waffle Slab System" which consist of evenly spaced joist in both direction with a top slab.
Two Way Joist or Waffle Slab

(7) Composite Slab System

In this system columns, beams and girders consists of structural steel while the floor RCC slabs. The spacing of beam is normal 6' - 8'. Steel ratio with respect to concrete increase in beams, increased upto 50% that's why it is so called composite slab system.

(8) Prestressed Slab System

It can be a pretension or post-tension slab system in which the hollow conduits are provided in slab through which steel  tendons are placed. The tendon are tension after the concrete has gained sufficient strength or before the concreting. In this type of slab the column and beams an normally RCC member.

Post-tension Slab System


Concrete and Its Properties

Normal Concrete

Concrete

Concrete is the mixture of Coarse aggregate, Fine aggregate, Water, Cement, Air (entraped) and sometimes admixtures.

Proportions of the Materials

The normal composition of concrete include
  • Cement→ 7% to 15%
  • Aggregate (Fine and Coarse Aggregate)→ 60% to 75%
  • Water→ 14% to 21%
  • Air (entraped)→ 4% to 8%

Properties of Concrete

Compressive Strength 

The uniaxial compressive strength is normally measured by compressive test of a standard cylinder, normally tested after 28 days of curing. Using the testing method of ASTM standard "C31" and 'C39". The compressive strength of concrete is normally represented by strain-stress curve. Which is used to check concrete strength for quality control. The normal rang for standard concrete strength is:

fc'= 3000 psi to fc'= 5000 psi

fc'= Compressive strength
Concrete Compressive Test 

Tensile Strength

This strength normally ranges from 8% to 15% of compressive strength (fc'). The type of test used to determine tensile strength is 
  1. Flexure Test
  2. Split Cylinder Test
Concrete Tensile Test

Relationship b/w Compressive & Tensile Strength

The concrete tensile strength increases with increase in compressive strength. So these both properties are directly proportional to each other:

Tensile Strength ∝ √fc'
Concrete

8 Feb 2019

Flexible Pavements and Components

Components of flexible Pavements

Flexible Pavements

It is one of the most convenient,safe and cost effective method of road construction throughout the world. It consists of HMA ( Hot Mix Asphalt) layers on the top surface of the road which is flexible due to contain of bitumen content and it is black in color, that's why it is called flexible pavement or black top pavement.

Components of Flexible Pavements

Flexible pavements are consists of the following Components:

1) Subgrade

It is the natural soil which acts like foundation for the pavement. The stability of pavement depends upon Subgrade. In order to stabilize the road, the subgrade is compacted to achieve its maximum density. Through compaction all the voids are removed from the soil particle. To achieve required compaction, various laboratory test has to be conducted e.g FDT ( Field Density Test), Moisture Content etc.

2) Subbase

It is the layer of coarse aggregate of the size 0.5" to 1". It is an optional layer and provide a base and level surface for its subsequent layer. It can increase the load carrying capacity of the road.

3) Base Course

It consists of broken rocks or coarse aggregate of size 2.5". Its is compact by roller to thoroughly compact the layer. Most of the strength provides by this layer of the pavement. To fill all the voids among the aggregates a thin layer of stone dust is provided on it and then water is sprinkle on it and again compact it. Then by brushes the surface dust is removed.

4) HMA (Hot Mix Asphalt)

i) Binder Course & Tack Coat

Binder is mixture of Bitumen and Lubricating oil. It is sprayed on the Base Course layer and then traffic is on for 3 to 4 months to thoroughly compact the pavements components and any rutting that can cause in future. After that another thin layer of bitumen called tack coat is sprayed on it in order to make a perfect bond between Base course and subsequent layer.

ii) Asphalt Wearing Course and Seal Coat

After Binder Course and Tack Coat, Asphalt Wearing Course is laid on it and it is the final layer of the flexible pavement. Its normal thickness is almost 4" to 6" and then compact it by roller. Its thickness can be increase according the traffic capacity. After that seal coat is sprayed on it which consists of Bitumen and emulsion in order to fully seal the wearing course. Th e rain water cannot percolate through Seal Coat.
Components of Flexible Pavements


5 Feb 2019

Column and Types


Column Failure Mechanism

Definition

A long vertical slender member subjected to an axial compressive load is called Column.

Types of Column

There are three types of Columns, based on their nature of failure:

1) Short Column

The length of this type of column is less than 8 times the least lateral dimension. The type of failure in this type of column is due to direct crushing only. Bending or buckling plays much less important role.

2) Medium Column

The length of this type of column varies from 8  to 10 times the least lateral dimension. In this type of column, failure may occur partly due to crushing and partly due to buckling.

3) Long Column

The length of this type of column varies from 30 times their least lateral dimension. In this type of column, failure is due to buckling.
Slender long Column


1 Feb 2019

Properties and Shapes of Steel Members

Structural Steel


Mechanical properties of steel 

Material standard for structural steel usually follow American Society for Testing and Materials ( ASTM ) standards.
  1. The ASTM specification A6 generally requirement of structural steel.
  2. The ASTM specification A370 provides steel testing procedures.
  3. The ASTM A36 is called mild steel (low carbon steel).

Shapes of Steel Members

Various types of steel shapes for Tension members, Compression members and Flexural members are discussed below:

1) Steel Shapes for Tension Members 

  • Round
  • Rectangular
  • W-Section
  • Single Angle
  • Double Angle
  • T-Section
  • Built-up Section

2) Steel Shapes for Compression Members

  • T-Section
  • W-Section
  • S-shape
  • Double Angle
  • Tube-Section
  • Built-up Section

3) Steel Shapes for Flexural Members

  • W-Section 
  • Double Angle Section
  • Channel Section
  • Built-up Section
  • Composite Steel-Concrete Section
Shapes of Structural Steel

Steel Structure

Steel Structure


Introduction

Steel Structure is an important material of construction and its a modified form of iron. The iron is known to be the pioneer construction material and its name is also mention in our Holy Quran six times. In which Allah almighty says (Sura-Al-Hadeed) " We sent down iron in which there is a strong power and benefits for the people".

The steel is manufacture in steel industries in which a chemical reaction is executed to produce steel billets. There is a term twinching used in which the steel is heated upto critical temperature and the rapid coating in a desirable medium such as water and oil to obtain desired properties. Another term tampering is refer to heating the steel above critical temperature and then cooling it to freezing temperature followed by reheating it, to intermediate temperature require to give harden shape.

Carbon Content in Steel and Classification

The carbon content has major effect on steel properties such that steel hardens and tensile strength increases. By increase the carbon content upto 0.8%. Based on the %age of carbons the steel is classified is
  1.  Mild Steel (Containing upto 0.25% of carbon)
  2. Medium Carbon Steel (Containing upto 0.45% of carbon)
  3. High Carbon Steel (0.5% - 1.5% carbon)
  4. If small %age of others metal such as nickel, chromium and tungsten is added to steel that it produce alloy steel.
Steel Structure