SHEAR KEYS IN RETAINING WALL DESIGN

 

FUNCTION OF SHEAR KEYS IN THE DESIGN OF RETAINING WALLS

In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows: The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance. On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 0.8φ in the previous case and the sliding resistance can be enhanced.

Problems in Pumping Concrete

 MAJOR PROBLEMS IN USING PUMPING FOR CONCRETING WORKS 

In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure. The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to 

Dos and Donts in an Interview

Interview Do's…

• Do a practice run or mock interview before the real employer interview
• Do research the company: know who they are, what they do and how they do it
• Do dress for the job
• Do arrive 10-15 minutes prior to the interview time – get specific directions
• Do know the name of the person who is interviewing you
• Do know what job you are being interviewed for
• Do take copies of your resume, your letter and your reference page
• Do accentuate the positive – and BE POSITIVE!
• Do shake the employers' hand
• Do provide specific work examples and facts

Interview Don'ts…

• Don't be late – shows lack of pre-planning
• Don't chew gum, smoke, or wear strong perfumes or colognes
• Don't avoid eye contact with the interviewer
• Don't answer your questions with merely a “yes” or “no”
• Don't talk with your hands
• Don't argue with the interviewer
• Don't begin your answer with “I already told you that”…or “As I said earlier”
• Don't be quick to answer
• Don't take your cell phone in the interview – leave it in the car or at home
• Don't rely on your cover letter and resume to do the selling – you need to sell yourself!

 

Bleeding in Concrete

BLEEDING IN CONCRETE

What is Bleeding:

Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic.

The bleeding in concrete is not harmful if the rate of evaporation of water is equal to the rate of bleeding.

Causes of Bleeding:

The Prime factor for bleeding in concrete is the high dosage of Water cement ratio. Higher water-cement ratio weakens concrete and leads to excessive bleeding.

The cement type and fine aggregates can play a role in determining the bleed rate. The fewer fines you have in your mix, the more bleeding will occur.

Types of Bleeding:

• Normal bleeding refers to a uniform seepage of water over the entire surface of the structure.

• Channel bleeding refers to water rising through particular paths.

Effects of Bleeding:

• Concrete loses its homogeneity

• Due to bleeding, when the top surface is worked with trowel, the aggregate goes down and cement paste forms at the top surface forming Laitance which decreases the wearing capacity and decreases its life.

• This Laitance decreases the bond between successive concrete lifts

• Concrete becomes permeable

• Delays the surface finishing

• Decreases the pumping ability

Measures to reduce Bleeding:

• Reduce water content. Use lower slump mix

• Use finer cements

• Increase amount of fines in the sand

• Use supplementary cementitious materials

• Use air entraining admixtures

Segregation in Concrete

SEGREGATION OF CONCRETE

What is Segregation:

Segregation in concrete is a case of particle segregation in concrete applications. Common forms are

• Separation of Coarse aggregate from the concrete mixture,

• Separation of Cement pastes from the concrete during its plastic stage.

• Separation of water from the concrete mix (Bleeding in concrete)

Segregation in concrete is commonly thought of as separation of some size groups of aggregates from cement mortar in isolated locations with corresponding deficiencies of these materials in other locations.

Segregation also occurs due to over-vibration or compaction of concrete, in which cement paste comes to the top and aggregates settles at the bottom.

Internal Factors for Segregation:

Segregation could result from internal factors such as concrete that is not proportioned properly and not mixed adequately, or too workable a mix.

External Factors for Segregation:

Segregation from external factors include too much vibration, improper transportation, placement, or adverse weather conditions.

Segregation is also caused by dropping concrete from more than 1 m.

Measures to reduce Segregation:

• Concrete should not be dropped from more heights

• Concrete should be placed through temporary inclined chutes for heights more than 1 m.

• The angle of inclination may be kept between 1:3 and 1:2 for smooth travel of concrete.

• The delivery end of chute should be as close as possible to the point of deposit.

• Adding air entraining agents, admixtures and pozzolanic materials increase the viscosity, thereby reduce segregation.

• Segregation can be controlled by maintaining proper proportioning the mix.

• Water content should not be more than the desired amount.

• Formwork should not have any leakages.

Tests for Segregation:

• Random ultrasonic testing should be conducted for any presence of segregation.

• Segregation can be rectified by pressure grounding with special chemical compounds.

• After rectifying the defects by pressure grouting core test has to be performed to ensure that the strength of concrete has reached to the desired level.

Common Mistakes Pouring Concrete

COMMON MISTAKES IN SITE ENGINEERING

Pouring Concrete

• The common mistake usually seen in a site is the contractors pouring concrete using chutes.

• It is poured usually directly from Transit mixer and the same shall be avoided.

• According 456 -2000 the maximum height for pouring of concrete without segregation is 1.5 m and for pump able mix Concrete shall not be placed from a height more than 2.0 m

• Concrete shall be placed within 60 minutes after mixing of it. Strictly prohibit addition of extra water at batching plant, during transit or at site. Mix without proper slump or cohesion shall not be used.

• The amount of concrete requited shall be correctly estimated to avoid wastage

Site Preparation:

If the site is not level or doesn’t have the correct grade to it, it could cause water to run down.

Weather:

• Pouring concrete on a nice day is very different from pouring it in any other weather, especially if there’s going to be any type of precipitation or if it’s going to get very cold.

• During extremely hot weather, you’ll actually need to use sprinklers to keep the water in the concrete from evaporating too quickly.

• Checking the weather before concrete is important

Other Site Mistakes to avoid

• The construction materials like sand, bricks, aggregates, etc, are not washed and are full of deleterious material and dust

• Polythene bag and hand glows were found buried with concrete

• The cement-sand mix in the mortar and brick masonry is made quite early, prior to its use and in larger quantities than required

• Concrete stopper was not proper and few concrete get way during casting of plinth beam

• Shear key was not prepared on top of column

• The bottom strata of the foundation is not compacted properly.

• Honeycomb was observed in concrete

• In trapezoidal footings concrete is not vibrated properly

• Column concreting in usually not mechanically vibrated and machine mixed.

• Cover blocks are not provided to reinforcement in columns, beams and slab

• Proper care for uplift pressure in black cotton soil is not taken

• Rectification of misaligned columns at higher levels causing eccentricity

• Damp proofing course is not properly cast

• The plinth filling is not carried out in layers nor compacted

• Joints with RC surface is never roughened and hacking is not deone before plastering

• All joints in formwork shall be sufficiently tight to prevent leakage of grout.

• Mixing ratio of cement mortar is never measured and maintained

• Rebound materials are never handled instantly causing serious issues in removing them

• Corner reinforcement is not provided in two-way slabs leading to uplift of corners.

The following preventive/remedial measures are suggested for improvement in the quality of construction for low cost residential buildings.

• The total civil work is executed mainly by labor, masons and carpenters. A workshop or training schedules shall be arranged for training them. The training will also improve their approach towards quality work.

• Civil contractors who have undergone civil engineering education shall be preferred.

• Time and cost factor also affects the quality of construction. Proper coordination shall be ensured between different agencies

• Prior testing of construction materials like sand, yellow earth, bricks, aggregate, cement and reinforcement steel also helps to improve the quality of construction.

• Overall, a good supervision by the civil contractor is essential during the execution of construction.

PREVENTION IS BETTER THAN CURE

Grades of Concrete

GRADES OF CONCRETE

(Ref : IS 456: 2000 http://www.iitk.ac.in/ce/test/IS-codes/is.456.2000.pdf )

IS 456-2000 has designated the concrete mixes into a number of grades Say M10, M20 etc.,

In this designation the letter M refers to the mix and the number to the specified 28 day cube strength of 150mm size cube in N/mm²

Table 1: Grades of Concrete

Group	Grade Designation	Specified Characteristic Compressive Strength of 150mm cube at 28 days in (MPa) N/mm2
Ordinary Grades of Concrete	M10	10
	M15	15
	M20	20
Standard Concrete	M25	25
	M30	30
	M35	35
	M40	40
	M45	45
	M50	50
	M55	55
High Strength Concrete	M60	60
	M65	65
	M70	70
	M75	75
	M80	80

To concert Mpa to psi multiply by x145.038

1Mpa = 145.038 psi

Table 2: Nominal Mix Ratio

Grade Designation	Mix Ratio
M10	1:3:6
M15	1:2:4
M20	1:1.5:3
M25	1:1:2
M30 to M80	Design mix shall be carried out
Ratio of 1:3:6 refers to 1 Part of cement; 2 Parts of Sand and 3 Parts of aggregates
Calculation of materials for concrete can be obtained at https://civilengineeringdays.blogspot.com/2020/05/quantities-of-materials-for-concrete.html

Concrete of grades lower than those given in Table 1 may he used for plain concrete constructions.

Lean Concrete:

Lean concrete is one in which the amount of liquid in the strata is higher than the cement.

Other Grades

M5 – 1:5:10

M7.5 – 1:4:8

As per IS 456:2000, Grades less than M20 should be used for Reinforced Concrete

Properties of Concrete

1. Increase of Strength with Age

There is normally a gain of strength beyond 28 days. The quantum of increase depends upon the gradc and type of cement, curing and environmental conditions, etc.

The design should be based on 28 days charactcristic strength of concrete unless there is an evidence tto justify higher strengths for particular structure

For concrete of grade M 30 and above. the rate of increase of compressive strength with age shall be based on actual investigations.

2. Tensile Strength of Concrete

Tensile strength from the compressive strength, the following formula may be used:

Flexural strength, f_cr =0.7.(f_ck )^^0.5 N/mm²

where f is the characteristic cube compressive strength of concrete in N/mm²

3. Elastic Deformation

The modulus of elasticity of concrete can be assumed as follows:

E_c = 5000 (f_ck )^^0.5 N/mm²

E_c is the short term static modulus of elasticity in N/mm²

4. Shrinkage

The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions.

In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.0003

5. Creep of concrete

Creep of concrete depends, in addition to the factors listed in shrinkage, on the stress in the concrete. age at loading and the duration of leading

Age at loading	Creep coefficient
7 days	2.2
28 days	1.6
1 year	1.1

6. Thermal Expansion

The coefficient of thermal expansion depends on nature of cement, the aggregate., the cement content, the relative humidity and the size of sections.

Type of Aggreegate	Coefficient of Thermal Expansion for Concrete /oC
Quartzite	1.2 to 1.3 x 10-5
Sandstone	0.9 to 1.2 x 10-5
Granite	0.7 to 0.95 x 10-5
Basalt	0.8 to 0.95 x 10-5
Limestone	0.6 to 0.9 x 10-5

Types of Concrete

Types of Concrete

The various types of concrete used in the construction industry are:

1. PCC - Plain or Ordinary Concrete
2. RCC - Reinforced Cement Concrete
3. PSC - Prestressed Concrete
4. Precast Concrete
5. Light – Weight Concrete
6. Fibre Reinforced Concrete
7. High–Strength Concrete
8. High-Performance Concrete
9. High-Density Concrete
10. Air Entrained Concrete
11. Polymer Concrete
1. Polymer concrete
2. Polymer cement concrete
3. Polymer impregnated concrete
12. Self – Consolidated Concrete
13. Roller Compacted Concrete
14. Rapid Strength Concrete

Plain Cement Concrete

Plain Cement Concrete (PCC) is also called as Cement Concrete (CC) or Blinding Concrete. It consists of cement, sand and coarse aggregates mixed with water in the specified proportions

 

Reinforced Cement Concrete

Reinforced cement concrete is a composite material made up of cement concrete and reinforcement in which the concrete resists compression with reinforcement resisting the tension and shear

 

Prestressed Concrete

A prestressed concrete may thus be defined as a concrete in which stresses of suitable magnitude and distribution are introduced to counteract, to a desired degree, the stresses resulting from external loads. This phenomenon of prestressing will make the lower section of the concrete member to be stronger against the tension.

 

Precast Concrete

Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place ("tilt up"). In contrast, standard concrete is poured into site-specific forms and cured on site. Lightweight Concrete

 

Fibre Reinforced Concrete

Fiber-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers – each of which lend varying properties to the concrete. In addition, the character of fiber-reinforced concrete changes with varying concretes, fiber materials, geometries, distribution, orientation, and densities.

 

High-Strength Concrete

High-performance means that the concrete has one or more of the following properties: low shrinkage, low permeability, a high modulus of elasticity, or high strength. ... High-strength concrete is typically recognized as concrete with a 28-day cylinder compressive strength greater than 6000 psi or 42 Mpa.

 

High-Performance Concrete

High performance concrete is a concrete mixture, which possess high durability and high strength when compared to conventional concrete. This concrete contains one or more of cementious materials such as fly ash, Silica fume or ground granulated blast furnace slag and usually a super plasticizer.

 

High-Density Concrete

High density concrete is a concrete having a density in the range of 6000 to 6400 kg/cu. m. High density concrete is also known as Heavy weight concrete. High density concrete is mainly used for the purpose of radiation shielding, for counterweights and other uses where high density is required.

 

Air Entrained Concrete

Air-entrained Concrete. Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes.

 

Lightweight Concrete

Concrete is considered to be lightweight is the density is not more than 2200kg/m³ (the density of normal weight concrete is assumed to be between 2300kg/m³ and 2400kg/m³) and a proportion of the aggregate should have a density of less than 2000kg/m³. 

Lightweight aggregate concrete can be produced using a variety of lightweight aggregates. Lightweight aggregates originate from either:

• Natural materials, like volcanic pumice.
• The thermal treatment of natural raw materials like clay, slate or shale i.e. Leca.
• Manufacture from industrial by-products such as fly ash, i.e. Lytag.
• Processing of industrial by-products such as pelletised expanded slab, i.e. Pellite.

Polymer Concrete

Polymer concrete the aggregates will be bound with the polymer instead of cement. The production of polymer concrete will help in the reduction of volume of voids in the aggregate.

The available polymer concrete materials are polymer impregnated concrete (PIC), polymer cement concrete (PCC), polymer concrete (PC) and, partially impregnated and surface coated polymer concrete.

Self-consolidating concrete

Self-consolidating concrete or self-compacting concrete (commonly abbreviated to SCC)[1] is a concrete mix which has a low yield stress, high deformability, good segregation resistance (prevents separation of particles in the mix), and moderate viscosity (necessary to ensure uniform suspension of solid particles during transportation, placement (without external compaction), and thereafter until the concrete sets).

 

Pervious Concrete

Pervious concrete (also called porous concrete, permeable concrete, no fines concrete and porous pavement) is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass directly through, thereby reducing the runoff from a site ..

 

Roller Compacted Concrete

Roller-compacted concrete has the same basic ingredient as conventional concrete: cement, water, and aggregates, such as gravel or crushed stone. But unlike conventional concrete, it's a drier mix—stiff enough to be compacted by vibratory rollers. Typically, RCC is constructed without joints.

 

Rapid Strength Concrete

This type of concrete is able to develop high resistance within few hours after being manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process very quickly, repaired road surfaces that become fully operational in just a few hours. Ultimate strength and durability can vary from that of standard concrete, depending on compositional details.