The cardinal difference between Working state method (WSM) and Limit State method (LSM) is: WSM is an elastic design method whereas LSM is a plastic design method.
In elastic design, i.e. WSM, the design strength is calculated such that the stress in material is restrained to its yield limit, under which the material follows Hooke’s law, and hence the term “elastic” is used. This method yields to uneconomical design of simple beam, or other structural elements where the design governing criteria is stress (static).
However, in case of shift of governing criteria to other factors such as fatigue stress, both the methods will give similar design. Also, WSM substantially reduces the calculation efforts.
Now coming to plastic design, i.e. LSM, as the name suggests, the stress in material is allowed to go beyond the yield limit and enter into the plastic zone to reach ultimate strength. Hence the “moment-rotation” capacity of beam, for example, is utilized making the design more economical. However, due to the utilization of the non-linear zone, this method involves arduous calculation.
All other differences are mostly derived from the above stated fundamental difference along with few general differences. Some of these differences are stated below:
1) Serviceability check in case of LSM is required because after the elastic region strain is higher, which results in more deformation, hence a check is necessary.
2) LSM is strain based method whereas WSM is stress based method.
3) LSM is non-deterministic method whereas WSM is deterministic approach.
4) The partial safety factor is used in LSM whereas Safety factor is used in WSM.
5) Characteristic values (derived from probabilistic approach) are used in case of LSM whereas Average or statistic values are used in WSM.
HOW TO CALCULATE CUTTING LENGTH OF BENT UP BARS IN SLAB
CUTTING LENGTH OF BENT UP BARS IN SLAB:
As a site engineer, you need to calculate the cutting length of bars according to the slab dimensions and give instructions to the bar benders.
For small area of construction, you can hand over the reinforcement detailing to the bar benders. They will take care of cutting length. But beware, that must not be accurate. Because they do not give importance to the bends and cranks. They may give some extra inches to the bars for the bends which are totally wrong. So it is always recommended that as a site engineer calculate the cutting length yourself. In this article, we will discuss how to the calculate length for reinforcement bars of slab. Let’s start with an example.
Diameter of the bar = 12 mm
Clear Cover = 25 mm
Clear Span (L) = 8000
Slab Thickness = 200 mm
Development Length(Ld) = 40d
Cutting Length = Clear Span of Slab + (2 x Development Length) + (2 x inclined length) – (45° bend x 4) – (90° bend x 2)
Inclined length = D/(sin 45°) – dD/ (tan 45°) = (D/0.7071) – (D/1)= (1D – 0.7071D)/0.7071= 0.42 D
As you can see there are four 45°bends at the inner side (1,2,3 & 4) and two 90° bends ( a,b ).
45° = 1d ; 90° = 2d
Cutting Length = Clear Span of Slab + (2 X Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2) [BBS Shape Codes]
d = Diameter of the bar.
Ld = Development length of bar.
D = Height of the bend bar.
In the above formula, all values are known except ‘D’.
So we need to find out the value of “D”.
D = Slab Thickness – (2 x clear cover) – (diameter of bar)
= 200 – (2 × 25) – 12
= 138 mm
Now, putting all values in the formula
Cutting Length = Clear Span of Slab + (2 x Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2)
= 8000 + (2 x 40 x 12) +(2 x 0.42 x 138) – (1 x 12 x 4) – (2 x 12 x 2)
∴ Cutting Length = 8980 mm or 8.98 m.
So for the above dimension, you need to cut the main bars 8.98 m in length.
WHAT IS THE BEST CONCRETE MIX DESIGN?
The cost of concrete determines the characteristic strength, quality control, workability of mix (cost of labor) which includes the high degree of compactions. The best concrete mix design is the one which satisfies all the aspects for which it was designed.
The strength of 95% cube cast after 28 days of curing should be greater than the characteristic strength for which concrete has been designed.
WORKABILITY & PLACING:
As working condition changes so the properties desired from concrete also changes, the concrete which can be easily placed without segregation and with least compaction required.
WATER CEMENT RATIO:
Water should be maximum (0.45 – 0.65).
The concrete must be durable enough to face harsh conditions of atmosphere for which it has been designed.
These are the main properties considered while designing concrete and the designed concrete satisfying such conditions can be called as best concrete mix design.
There are three type of mixes,
1. Nominal mixes
2. Standard mixes
3. Designed mixes
1. NOMINAL MIXES:
The mixes which have fixed cement aggregate ratio but the nominal mix concrete for a given workability varies widely In strength.
2. STANDARD MIXES:
1. It is designated by code book of IS456:2000
2. The minimum compressive strength have included
3. May result in under and over rich mixes.
3. DESIGNED MIXES:
1. The mix proportion is designed by producer of concrete
2. The concrete is specified by the designers
3. It does not guarantee the concrete mix proportion for the prescribed performance.
As we normally use the standard mixes which are safe and economic, as per standard code book.
TYPES OF JOINTS IN CONCRETE
Except in small jobs, it is not possible to place concrete in one continuous operation. Joints are also required for functional consideration of the structure. Concrete joints can be classified under following categories:
1. Construction Joints.
2. Expansion Joints.
3. Contraction Joints.
4. Warping Joints.
1. CONSTRUCTION JOINTS:
These joints are provided where there is a break in construction programme. Concreting operation should be so planned that the work is completed in one operation. If, however, it has to be stopped before completion of entire work, construction joints are provided. Location of construction joints should be such that it interferes minimum with the functional characteristics of the structure. Best locations for construction joints are as following:
i) Beam: Joint may be located at mid-span or over the center of the column in direction at right angles to the length of the beam.
ii) Columns: Joints should be located a few cm below its junction with the beam.
iii) Slab: Joints may be placed at mid span or directly over the center of the beams, at right angles to the slab.
Formwork for construction joint should be placed at the end of each day’s work.
Before new concreting is started, the concrete surface of hardened concrete should be cleaned, roughened, saturated with water, and applied cement grout. This will ensure proper bond between old and new concrete works. New concreting is started before the applied grout on old surface attains initial set.
2. EXPANSION JOINTS:
These joints are provided to allow for expansion of the concrete, due to rise in temperature above the temperature during construction. Expansion joints also permit the contraction of the element. Expansion joints in India are provided at an interval of 18 to 21 m. A typical expansion joint is shown in Fig 1. The open gap of this joint varies between 2 cm and 2.5 cm. Sometimes, to transfer load from one slab to the adjacent slab, dowel bars are also used at suitable intervals at these joints.
3. CONTRACTION JOINTS:
These joints are provided to permit contraction of the concrete. These joints are spaced closer than expansion joints. These joints do not require any load transfer device as it can be achieved by the interlocking of aggregates. However, some agencies recommend use to dowel bars fully bonded in concrete.
Warping joints are provided to relieve stresses induced due to warping effect. These joints are also known as hinged joints.
DIFFERENCE BETWEEN ONE WAY SLAB AND TWO WAY SLAB
ONE WAY SLAB:
One way slab is a slab which is supported by beams on the two opposite sides to carry the load along one direction. In one way slab, the ratio of longer span (l) to shorter span (b) is equal or greater than 2,
i.e Longer span (l)/Shorter span (b) ≥ 2
Verandah slab is a type of one way slab, where the slab is spanning in the shorter direction with main reinforcement and the distribution of reinforcement in the transverse direction.
TWO WAY SLAB:
When a reinforced concrete slab is supported by beams on all the four sides and the loads are carried by the supports along both directions, it is known as two way slab. In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2.
i.e Longer span (l)/Shorter span (b) < 2
This types of slabs are mostly used in the floor of multi-storey buildings.
DIFFERENCE BETWEEN ONE WAY SLABS AND TWO WAY SLABS:
1. In one way slab, the slabs are supported by the beams on the two opposite sides.
In two way slab, the slabs are supported on all the four sides.
2. In one way slab, the loads are carried along one direction.
In two way slab, the loads are carried along both directions.
3. In one way slab, the ratio of Longer span to shorter span is equal or greater than 2. (i.e l/b ≥ 2).
In two way slab, the ratio of l/b is less than 2 (i.e l/b < 2).
SLUMP TEST OF CONCRETE
CONCRETE SLUMP TEST:
The concrete slump test is an empirical test that measures the workability of fresh concrete. The test is performed to check the consistency of freshly mixed concrete in a specific batch. Consistency refers to the ease and homogeneity with which the concrete can be mixed, placed, compacted and finished. This test is most widely used due to the simplicity of apparatus and simple test procedure.
The slump test gives satisfactory results for the concrete mix of medium to high workability and unfortunately, it does not give the correct indication of low workability, which may give zero slumps. This test is also known as slump cone test.
APPARATUS FOR CONCRETE SLUMP TEST:
1. Mould or slump cone with a height of 300 mm, bottom diameter 200 mm, and top diameter 100 mm.
2. Standard tamping rod.
3. Non-porous base plate.
4. Measuring scale.
PROCEDURE OF TEST:
First, clean the inner surface of the empty mould and then apply oil to it.
Set the mould on a horizontal non-porous and non-absorbent base plate.
Fill the mould fully by pouring freshly mixed concrete in three equal layers.
Stroke each layer 25 times with the standard tamping rod over the cross section.
After stroking 25 times the top layer is struck off level, now lift the mould slowly in the vertical direction without disturbing the concrete cone.
Use the measuring scale to measure the difference level between the height of the mould and the concrete sample.
The subsidence of concrete is known as the slump and the value of slump is measured in mm.
TYPES OF SLUMP:
True Slump: The concrete mass after the test when slumps evenly all around without disintegration is called the true slump.
Shear Slump: When one-half of the concrete mass slide down the other is called the shear slump. This type of slump is obtained in a lean concrete mix.
Collapse Slump: When the sample is collapsed due to adding excessive water, it is known as collapse slump.
Zero Slumps: For very stiff or dry mixes it does not show any changes of the slump after removing the slump cone.
ADVANTAGES OF CONCRETE SLUMP TEST:
The procedure of slump test is simple and easy than any other workability test.
Inexpensive and portable apparatus are required for this test.
Slump test can be performed at the construction site as well as in the laboratory
LIMITATIONS OF CONCRETE SLUMP TEST:
The slump test is limited to concretes with the maximum size of aggregate less than 38 mm.
The test is suitable only for concretes of medium or high workabilities (i.e having slump values of 25 mm to 125 mm).
For very stiff mixes having zero slumps, the slump test does not show any difference in concretes of different workabilities.
Recommended Values Of Slumps For Different Concrete Mixes:
CONCRETE MIX DESIGN : DESIGN MIX CONCRETE & NOMINAL MIX CONCRETE
CONCRETE MIX DESIGN:
Mix design is a method which determines the proportions of cement, water, fine aggregates and coarse aggregates to produce the concrete of required strength, workability and durability with minimum cost.
Mix design can be divided into following two categories
1. Design mix concrete and
2. Nominal mix concrete.
1. DESIGN MIX CONCRETE:
When the proportions of concrete ingredients are decided by adopting certain established relationships ( based on assumptions from a lot of experiments) to produce the concrete it is known as design mix concrete.
In design mix concrete, it is assumed that compressive strength of concrete is totally dependent on
the water-cement ratio.
2. NOMINAL MIX CONCRETE:
When the concrete is produced by taking standard arbitrary proportions of concrete ingredients, it is known as nominal mix concrete. This method is generally used when the quality control requirement for design mixes are difficult to execute. As we have explained for normal work, nominal mix concrete can be designed by taking cement, fine aggregate and coarse aggregate in the ratio of 1 : n : 2n.