PLAIN AND REINFORCEMENT CONCRETE

MIXING OF CONCRETE:

There are different methods of mixing concrete. The best mixing technique for a particular job depends upon following factors

· Location of construction site.(distance from batching plant)

· The amount of concrete needed.

· Construction schedule.(volume of concrete required per hour)

· Cost.

Hand mixing:

· Hand mixing of concrete is used for mixing of small volume of concrete.

· This practice is not good because mixing is not proper.

· Now portable concrete mixers have replaced hand mixing.

Mixers:

There are two main categories of mixers

· Batch mixers(one batch at a time)

· Continuous mixers(concrete production at a constant rate)

Portable mixer

Batch Mixers:

There are two types of batch mixers based on orientation of rotating axis.

· Horizontal or inclined mixers(drum mixers)

· Vertical(pan mixer)

Continuous Mixers:

· The materials are continuously fed into mixer at the rate same as rate of discharge.

· They are usually non-tilting drums with screw-type blades rotating in the middle of the drum. The drum is tilted downward toward the discharge opening. The mixing time is determined by the slope of the drum (usually about 15˚).

· The methods chosen for placing and compacting the concrete will depend on the type of construction, the total volume to be placed, the required rate of placing and the preferences and expertise of the construction companies involved.

· There are, however, several basic rules which should be followed to ensure that the concrete is properly placed and compacted into a uniform, void free mass once it has been delivered to the formwork in a satisfactory state.

· The concrete should be discharged as close as possible to its final position, preferably straight into the formwork.

· A substantial free-fall distance will encourage segregation and should therefore be avoided.

· With deep pours, the rate of placing should be such that the layer of concrete below that being placed should not have set, this will ensure full continuity between layers and avoid cold joints and planes of weakness in the hardened concrete.

· Once the concrete is in place, vibration, either internal or external, should be used to mould the concrete around embedment's e.g. reinforcement, and to eliminate pockets of entrapped air, but the vibration should not be used to move the concrete into place.

· High-workability mixes should not be over vibrated – this may cause segregation.

Standard Test Method for The Slump Of Hydraulic Cement Concrete.

Code: ASTM C-143/C-143 M-03

Scope & significance:

This test method is used in lab and in field for finding out the slump (decrease in the height of concrete when we lift up the mould). This test is used extensively in site works all over the world. The slump test does not measure the workability of concrete directly but it co-relates the workability with some physical measurement.

The main significance of this test is as follows;

· This test method is used to determine the slump of plastic hydraulic cement concrete. Slump<15mm (Non-Plastic) Slump>15 (Plastic)

· This test method is applicable to plastic concrete having coarse aggregate up to 37.5mm in size. If the coarse aggregate is larger than the 37.5mm then this test method is not applicable.

· This test method is not applicable to non-plastic and non-cohesive concrete (due to larger amount of water presence).

Apparatus:

· Metal mould, thickness is 1.15mm, it is in cone form with the base 200mm diameter and 300mm height with the top diameter 100mm. the top and base of cylindrical mould is open and parallel to each other. The mould is provided with foot pieces and handles.

· Temping rod, 16mm diameter and 600mm in length having temping ends.

Results:

Slump Value = 1.5 inches

Test Method for The Flexural Strength of Concrete Using Simple Beam With two-Point Loading.

Code: ASTM C 78 – 02

Scope & significance:

· This test method is used to determine the flexural strength of specimens prepared and cured in

· accordance with the specifications. Results are calculated and reported as the modulus of

· rupture.

· The strength determined will vary where there are differences in specimen size, preparation,

· moisture condition, curing, or where the beam has been molded or swayed to size.

· The results of this test method may be used to determine compliance with specifications or as a

· basis for proportioning, mixing and placement operations. It is used in testing concrete for the

· construction of slabs and pavements.

· The modulus of rupture is also used as an indirect measure of the tensile strength of concrete.

FLEXURE THEORY:

Theory of the deformation of a prismatic beam having a length at least 10 times its depth and consisting of a material obeying Hooke's law, in response to stresses within the elastic limit.

Design and Analysis:

The main task of a structural engineer is the analysis and design of structures. The two approaches of design and analysis will be used in this chapter:

i) Design of a Section.

This implies that the external ultimate moment is known, and it is required to compute the dimensions of an adequate concrete section and the amount of steel reinforcement. Concrete strength and yield of steel used are given.

ii) Analysis of a Section.

This implies that the dimensions and steel used in the section (in addition to concrete and steel yield strengths) are given, and it is required to calculate the internal ultimate moment capacity of the section so that it can be compared with the applied external ultimate moment.

Basic Assumptions in Flexure Theory

Five basic assumptions are made.

1) Plane sections before bending remain plane after bending.

2) Strain in concrete is the same as in reinforcing bars at the same level, provided that the bond between the steel and concrete is sufficient to keep them acting together under the different load stages i.e., no slip can occur between the two materials.

3) The stress-strain curves for the steel and concrete are known.

4) The tensile strength of concrete may be neglected.

5) At ultimate strength, the maximum strain at the extreme compression fiber is assumed equal to 0.003, by the Egyptian Code. The assumption of plane sections remaining plane (Bernoulli's principle) means that strains above and below the neutral axis NA are proportional to the distance from the neutral axis, Fig. 1.1. Tests on reinforced concrete

members have indicated that this assumption is very nearly correct at all stages of loading up to flexural failure, provided good bond exists between the concrete and steel. This assumption, however, does not hold for deep beams or in regions of high shear.

Single reinforced beam section with strain distribution

Behavior of a Reinforced Concrete Beam Section Loaded to Failure

To study the behavior of a reinforced concrete beam section under increasing moment. Let us examine how strains and stresses progress at different stages of loading:

Non-cracked (Linear Stage):

In this stage the entire concrete section is effective, with the steel bars at the tension side sustaining a strain equal to that of the surrounding concrete ( ) but the stress in the steel bars is equal to that in the adjacent concrete multiplied by the modular ratio n. Utilizing the Transformed Area Concept, in which the steel is transformed into an equivalent concrete area , the conventional elastic theory may be used to analyze the "all concrete" area.

Transformed section for flexure before cracking

Cracked, Linear Stage:

When the moment is increased beyond Mcr, the tensile stresses in concrete at the tension zone increased until they were greater than the modulus of rupture fctr, and cracks will develop. The neutral axis shifts upward, and cracks extend close to the level of the shifted neutral axis. Cracked concrete below the neutral axis is assumed to be not effective and the steel bars resist the entire tensile force. The stress-strain curve for concrete is approximately linear up to 0.40 fcu; hence if the concrete stress does not exceed this value, the elastic (straight line) theory formula M/Z may be used to analyze the "all concrete" area

Transformed section for flexure somewhat after cracking

Cracked or Nonlinear Stage:

For moments greater than these producing stage 2, the maximum compressive stress in concrete exceeds 0.40. However, concrete in compression has not crushed. Although strains are assumed to remain proportional to the distance from the neutral axis, stresses are not and, therefore, the flexural formula M/Z of the conventional elastic theory cannot

be used to compute the flexural strength of the section. The Internal Couple Approach, instead, will be used to compute the section strength. This approach allows two equations for equilibrium, for the analysis and design of structural members, that are valid for any load and any section. As Fig. 1.5

indicates, the compressive force C should be equal to the tensile force T, otherwise the section will have a linear displacement plus rotation. Thus,

C = T

The internal moment is equal to either the tensile force T multiplied by its arm yct or the

compressive force C multiplied by the same lever arm. Thus,

Transformed section for flexure after cracking

Ultimate Strength Stage:

For the given section, when the moment is further increased, strains increased rapidly until the maximum carrying capacity of the beam was reached at ultimate moment Mu. The section will reach its ultimate flexural strength when the concrete reaches an extreme fiber compression strain Xcu of 0.003 and the tensile steel strain Xs cloud have any value higher or lower than the yield strain.

Single reinforced beam section with flexure at ultimate

Preparation of sample specimen and testing

Apparatus:

· Universal Testing Machine

· Supporting Beam and Roller/hinge supports

· Two point loading arrangement

Procedure:

· .Preparation: Make the specimens in accordance with the concrete batch procedure. Test the concrete for slump and air content. Fill the beam forms with three lifts of concrete, tamping each lift 25 times with the 16 mm (5/8 in.) tamping rod or fill the form in one lift and consolidate the concrete with a mechanical vibrating table. Be careful not to over vibrate since that would cause segregation.

Results:

1 Beam-I

Fc ‘ = 3000psi

As = 4 # 3 bars

# 2 @ 12” c/c

Ram`s X- sectional area = 17.53 in2

Load = 450*17.53 = 7.9 k

Deflection

Pressure

0

0

0.2

100

0.8

200

1.6

300

2.4

400

3.6

450

6.4

450

7

450

10.6

400

o Curing: Allow the specimens to remain in the steel forms with the top properly covered for about 24 hours at normal room temperature. Strip the forms and place the specimens in the curing facility until ready for testing.

· Testing: Remove the specimens from the curing facility and mark the beam where it will be in contact with the supports and at the opposite side where it will be in contact with the third-point loading. Remember that none of these contact points should be on the top or hand-finished surface of the specimen. In other words, the beam should be tested 90° to its casting position.

· Record the ultimate load, the exact location of fracture, and the type of failure.

Ram`s X- sectional area = 17.53 in2

Load = 450*17.53 = 7.9 k

2 Beam-II

Fc ‘ = 3000psi

As = 4 # 3 bars

# 2 @ 12” c/c

Deflection

Pressure

0

0

0.3

200

0.8

250

1.7

300

2

400

2.6

450

3.2

500

3.8

550

4.6

500

Ram`s X- sectional area = 17.53 in2

Load = 550*17.53 = 9.6 k

Test Method for The Compressive Strength of Cylindrical & Cubical Concrete Specimens.

Code: ASTM C 39 (Only for cylinders

Scope & significance:

The purpose is to determine the compressive strength of cylindrical specimens, either molded or drilled cores. The method is limited to concrete having a density of at least 800 kg/m3 (50 lb. /ft3).

The 28-day compressive strength (f’c) of molded cylinders is normally used in design.

Apparatus:

· Universal Testing Machine

· Cylindrical Concrete Specimens

· Cubical Concrete Specimens

Procedure:

· Preparation of cylindrical specimens.

· Prepare and cure the specimens in accordance with ASTM Designation: C 192. Perform air content, slump, and penetration tests on the fresh concrete prior to casting the specimens in accordance with ASTM Designations: C 143, C 231, and C 360. Fill the cylinders with three lifts of freshly mixed concrete, tamping each lift 25 times with the tamping rod. Also tap each lift lightly with a mallet 10 to 15 times. Strike off the excess concrete with the tamping rod and finish to a smooth surface with a steel trowel. It is recommended that specimens be prepared and tested in groups of three Curing of the concrete specimens

· Allow the specimens to set for about 24 hours at normal room temperature, with the top surface covered to prevent loss of moisture. Strip the mold from the specimens and place in the curing facility until ready for testing.

· Compression testing procedure

Remove the specimen from the curing facility just prior to testing. Specimens shall be tested while still in a moist condition. Measure the diameter of the specimen, determined at right angles to each other about mid-height of the specimen. Average the two values to the nearest 0.25 mm (0.01 in.). Center the capped specimens in the testing machine and load them. Load to failure. Record the ultimate load, the angle of fracture, and any other pertinent aspects of failure such as voids.

Use the same procedure for testing cube

Results:

Result of cylinder test at different time period is attached separately.

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