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 

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.

Risk Management in Construction Projects

Risk Management In Project Execution

A Risk management plan shall be formulated for the successful implementation of any project. Risks shall be managed through sound construction practices and through careful preparation and review of the project contract documents. An effective and efficient risk management approach requires a proper and systematic methodology. Risk management in construction is designed to plan, monitor and control those measures needed to prevent exposure to risk. The following flow chart shows the elements of risk management.

Identifying Risks

Risk identification shall be carried in a way that is both forward-looking and in line with the progress of the project, since before the start of the project not all risks are completely recognizable and during the project implementation further risks may emerge.

Quality risks, Personnel risks, Cost risks, Set date / deadline risks, Risks of strategic decisions and External risks are some of the risks that occur in particular for a project. The identified risks are investigated with regard to the probability of their occurrence and the effect on the project. Criteria, on the basis of which individual risks can be assessed and compared with one another will be developed. Risks will be ordered according to the amount of damage or loss and the probability of their occurrence. Accordingly, the effects on the project and the need to take action are evaluated.

Assessing risks

The risk assessment comprises the qualitative assessment and quantitative measurement of individual risks including the interrelationship of their effects. With the help of the results of risk assessment for a detailed list of risks of a project are illustrated and compared with others. The procedure of quantitative risk determination primarily aims at estimating the probabilities of dangerous occurrences within a risk scenario. In the evaluation of the risk of major damage or loss, the maximum damage or loss possible or probable is determined.

Controlling risks

Controlling risk is the active influencing of the risks determined in the context of the risk analysis. Measures of dealing with risk can be differentiated between cause-related and effect-related measures. Cause-related measures are supposed to avoid or reduce risks, while effect-related measures serve to reduce or safeguard against the amount of damage or loss to be expected in the event of the damage or lossentailing event

Monitoring risks

Monitoring of risks is the continuous operative control of the effectiveness of the risk control measures. The goal of risk management is to ensure eliminate risks completely from the project.

Contract Risk Analysis

The main risks associated with this project are foreseen to be the following:

Contract Failure

Major Risks associated	Solutions for Risk Minimisation:
Breach of Contract by any contract party Unacceptable progress achievement Unacceptable quality of workmanship Force Majeure	Repairs to any breach that may occur Improve construction management Impose additional sub-contracting Forfeiture and reassignment

Loss of Budget Control

Major Risks associated	Solutions for Risk Minimisation:
Inaccurate measurement control and forecasting Failure to solve on-site construction problems Unforseen items not measured in the BOQ Inflation and contract price variation Contract claims and disruptions	Proactive approach between the supervisor and the contractor Installation of Management Information System (MIS) Weekly Technical meetings on site Accurate monthly measurement data Updated CPI application and forecasting procedures Immediate calculation and assessment of variations

Program Slippages and Delays

Major Risks associated	Solutions for Risk Minimisation:
Ineffectual construction management Lack of suitable construction resources Lack of sufficient cash flow funding Interference from external sources Inclement weather beyond the expected risks	Accurate progress monitoring and reporting Regular construction management assessment Resources efficiency monitoring and analysis Efficient payment application processing Adequate works protection

Inferior Construction

Major Risks associated	Solutions for Risk Minimisation:
Ignorance of Specifications Procurement of sub-standard materials Poor construction supervision on site Sub-standard testing procedures Abandoning works that are not complete	Interpretation and training in Contract Specifications Strict site monitoring, recording and reporting procedures Assessment of adequate supervision requirements Provision of Labour instruction and training Calibration and maintenance of testing equipment Definitive notices of works approval/rejection

Other risks

• Way leave access, expropriation and compensation

• Imported materials ordering, procurement and delivery

• Health and safety procedures

• Environmental control and impact minimisation

• Human resources management

• Administration procedures cooperation

• Public relations and awareness program

Concrete Mix Design as per IS Code

CONCRETE MIX DESIGN

(IS 10262 (2009): Guidelines for concrete mix design proportioning [CED 2: Cement and Concrete)

INTRODUCTION

Concrete has become an indispensable construction material. According to the present state-of-the-art, concrete has bypassed the stage of mere four component system, that is, cement, water, coarse aggregate and fine aggregate.

It can be a combination of far more number of ingredients for example, a judicious combination of ingredients from as many as ten materials. In the recent past, apart from the four ingredients mentioned above, fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, metakaoline and superplasticizer are six more ingredients which are generally used in concrete produced in practice as the situation demands.

The objective of proportioning concrete mixes is to arrive at the most economical and practical combinations of different ingredients to produce concrete that will satisfy the performance requirements underspecified conditions of usc. An intcgral part of concrete mix proportioning is the preparation of trial mixes and effect adjustments to

such trials to strike a balance between the requirements of placement, that is, workability and strength, concomitantly satisfying durability requirements.

Concrete has to be of satisfactory quality both in its fresh and hardened states. Mix proportioning is generally carried out for a particular compressive strength requirements ensuring that fresh concrete of the mix proportioned to possess adequate workability for placement without segregation and bleeding while attaining a dense state.

DATA FOR MIX PROPORTIONING

1.      Grade designation

2.     Type of cement

3.     Maximum nominal size of aggregate

4.    Minimum cement content

5.     Maximum water-cement ratio

6.    Workability

7.     Exposure conditions as per Table 4 and Table 5 of IS -456

8.    Maximum temperature of concrete at the time of placing

9.    Method of transporting and placing

10.   Early age strength requirements, if required

11.  Type of aggregate

12. Maximum cement content; and

13.Whether an ad mixture shall or shall not be used and the type of admixture and the condition of use.

 Target Strength for Mix Proportioning

The concrete mix has to be proportioned for higher target mean compressive strength (f_ck). The margin over characteristic strength is

f_t = f_ck + 1.65 s

f_t is the target mean compressive strength at 28days in N/mm²,

f_{ck is} characteristic compressive strength at 28days in N/mm²,

and s standard deviation  in N/mm²

 

Standard deviation can be taken as follows

Sl.No	Grade of concrete	Assumed Standard Deviation N/mm2
1	M10	3.5
2	M15	3.5
3	M20	4.0
4	M25	4.0
5	M30 to M55	5.0

Water-Content Ratio

The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form.

The table given below is used when only angular shaped aggregates are used in concrete as well as the slump should be 25 to 50mm.

Maximum Water Content per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate

Sl.No	Nominal Max Size of Aggregate (mm)	Maximum Water Content (kg)
1	10	208
2	20	186
3	40	165

Aggregate Air content

Air content in the concrete mix is determined by the nominal maximum size of aggregate used.

Sl.No	Nominal Max Size of Aggregate (mm)	Aggregate Air Content (% of volume of concrete)
1	10	5
2	20	2
3	40	1

 Cement Content Selection

Water – cement ratio is determined as described above and quantity of water is determined. The value obtained should satisfy the minimum conditions as given in the below table. The greater of the two values is decided as quantity of cement content.

Cement Content for Plain Cement Concrete

Exposure	Plain Cement Concrete (P.C.C)
	Minimum Cement Content Kg/m3	Max Free Water – Cement Ratio	Minimum Grade of Concrete
Mild	220	0.6	–
Moderate	240	0.6	M15
Severe	250	0.5	M20
Very severe	260	0.45	M20
Extreme	280	0.4	M25

Cement Content for Reinforced Concrete

Exposure	Reinforced Cement Concrete (RCC)
	Minimum Cement Content Kg/m3	Max Free Water –Cement Ratio	Minimum Grade of Concrete
Mild	300	0.55	M20
Moderate	300	0.5	M25
Severe	320	0.45	M30
Very severe	340	0.45	M35
Extreme	360	0.4	M40

Calculation of Aggregate Ratio

For the given nominal maximum size of aggregate, we can calculate the ratio of volumes of coarse aggregate and volume of total aggregates for different zones of fine aggregates from the below table.

Nominal maximum size of aggregate	Ratio of volume of coarse aggregate and volume of total aggregate for different zones of fine aggregate
	Zone – 1	Zone – 2	Zone – 3	Zone – 4
10mm	0.44	0.46	0.48	0.50
20mm	0.6	0.62	0.64	0.66
40mm	0.69	0.71	0.73	0.75

Calculation of Aggregate Content for Concrete

We already determine the coarse aggregate volume ratio in the total aggregate volume. So, it is very easy that, 1 – volume of coarse aggregate will give the volume of fine aggregate. Alternatively, there are some formulae to find the volume of fine and coarse aggregates as follows.

Mass of fine aggregate is calculated from below formula

Similarly, mass of coarse aggregate is calculated from below formula.

Where, V = volume of concrete

W = water content

C = cement content

Gc = sp. Gravity of cement

P = aggregate ration obtained in step6

F.A & C.A = masses of fine and coarse aggregates

Gf & Gca = sp. Gravities of fine and coarse aggregates.

Trial Mixes for Testing Concrete Mix Design Strength

Based on the values obtained above, conduct a trail test by making at least 3 cubes of 150mm size as per above standards. Test that cubes and verify whether the required strength is gained or not. If not, redesign the mix with proper adjustments until required strength of cube occurs.