STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-18

5.4.3.4 Calculation

If the fracture initiates in the tension surface within the middle third of the span length, we can calculate the modulus of rupture as follows:

                                         R=Pl/bd²

where

R= modulus of rupture, psi

P= maximum applied load indicated by the testing machine, lbf

l = span length, inch

b= average width of specimen, inch

d= average depth of specimen, inch

5.4.4 Diagrams for shear strength

Fig No. 5.3: Shear force and Bending moment diagram.

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-17

5.4.3.2 Test specimen

The test specimen shall confirm to all applicable requirements. The specimen shall have a test span within 2% of being three times its depth as tested. The sides of the specimen shall be at right angles with the top and bottom. All surfaces in contact with load-applying and support blocks shall be smooth and free of scars, indentations, holes or inscribed identifications.

5.4.3.3 Procedure

First we turned the test specimen on its side with respect to its position as molded and center on the bearing blocks. The loading system is centered in relation to the applied force. Then we brought the load applying blocks in contact with the surface of the specimen at the third points between the supports. The load may be applied rapidly, up to approximately 50% of the breaking load. Therefore the load is applied continuously at a rate which constantly increases the extreme fiber stress, until the rupture occurs.

Fig No. 5.1: Beam specimen for flexure test.

Fig No. 5.2: The failure pattern of beam after rupture. 

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-16

5.4.2 Cylinder test for tensile strength

The test is carried out by placing a cylindrical specimen horizontally between the loading surfaces of a compression testing machine and the load is applied until failure of the cylinder, along the vertical diameter.

The loading condition produces a high compressive stress immediately below the two generators to which the load is applied. But the larger portion corresponding to depth is subjected to a uniform tensile stress acting horizontally. It is estimated that the compressive stress is acting for about 1/6 depth and remaining 5/6 depth is subjected to tension.

In order to reduce the magnitude of the high compressive stresses near the points of application of the load, narrow packing strips of suitable material such as plywood are placed between the specimen and loading plates of the testing machine.

5.4.3 Beam test for flexural strength

5.4.3.1 Apparatus

The testing machine shall confirm to the requirements of the sections on basis of verification, correction and time interval. Motorized pumps or hand-operated positive displacement pumps having sufficient volume in one continuous stroke to complete a test without requiring replenishment are permitted and shall be capable of applying loads at a uniform rate without shock or interruption. The third-point loading method shall be used in making flexure tests of concrete employing bearing blocks which will be perpendicular to the face of the specimen and applied without eccentricity.

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-15

Care has to be taken to ensure that the sulfur compound is not over-heated, as it will not then develop the required compressive strength. Sulfur caps are allowed to harden for at least 2 hours before applying the load. 

5.4 Testing of specimen 

5.4.1 Cylinder test for compressive strength 

5.4.1.1 Apparatus 

The testing machine used to test the compressive strength of concrete cylinder was a hydraulically operated testing machine of 3000 KN capacity. The rate of loading for this test was controlled at the rate of 10 KN/sec. 

5.4.1.2 Test procedure

Compressive strength of moist cured specimens was made after removal from the curing basin. For the uniform load distribution over the bearing area of specimen, sulfur capping was used on both the bearing surfaces of each specimen. 

5.4.1.3 Placing of specimen 

The lower bearing plate was placed with its hardened face up on the table of the testing machine directly under the upper bearing plate [6]. The bearing faces of the test specimen were wiped clear and the test specimen was placed on the lower bearing plate. The axis of the specimen was carefully aligned with the centre of the upper bearing plate. 

5.4.1.4 Crushing of the specimen 

The load was applied continuously without shock. It was applied at a constant rate of 10 KN/sec. No adjustment was made in the controls of the testing machine while a specimen was yielding rapidly, immediately before failure. The compressive strength of concrete was calculated, dividing the maximum load carried by the specimen by the average cross sectional area of the specimen.

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-14

5.2 Curing of the specimen

Curing is the name given to the procedure used for promoting the hydration of cement and consists of control of temperature and of the moisture movement from and into the concrete. Immediately after finishing, precautions shall be taken to prevent evaporation and loss of water from the specimens.

   

Upon completion of initial curing and within 30 min after removing the molds, store specimens in moist conditions with free water maintained on their surfaces at all times at a temperature of 73.4 ± 3 ⁰F. Temperature between 68 and 86 ⁰F are permitted for a period not to exceed 3 hrs immediately prior to test, if moisture is maintained on the surfaces of the specimen at all times, except when capping with sulfur.

 

Fresh concrete grains strength most rapidly during the first days and weeks. Structural design is generally based on 28 days strength and 70 percent of which is reached at the end of the first week after placing.

5.3 Capping of cylindrical specimen

Capping is applicable to cylindrical specimens ^[1]. A cap shall be at least as strong as the concrete. The ends of all cylindrical test specimens that are not plane within 0.05 mm are capped. The capped surfaces are not departed from a plane by more than 0.05 mm and shall be nearly at right angle to the axis of the specimens. The planeness of the cap is required to be checked by means of a straight edge and feeler gauge. Caps are made as this as practicable and care should be taken so that flow or fracture does not take place, when specimen is tested. Capping can be done on completion of casting or a few hours prior to testing to specimen.

              

Just prior to testing, the cylindrical specimen are capped with a sulfur mixture consisting of 1 part of sulfur to 2 or 3 parts of inner filler, such as fire-clay. The specimens are securely held in a special jig so that the caps formed have a true plane surface.

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-13

CHAPTER − 5 

TEST PROCEDURE 

Cylinder and beam specimens were used for the test program. The following works were performed for the investigation purpose: 

1. Preparation of the specimen 

2. Curing of the specimen 

3. Capping of the specimen 

4. Testing of the specimen

Concrete Cylindrical Specimen Under
Test Arrangement

5.1 Preparation of the specimen [6]

1. At first, sand and coarse aggregate were screened by the sieves respectively.

2. Ingredients (cement, sand and coarse aggregate) were measured separately by weight according to required proportions.


3. The coarse aggregate were soaked thoroughly in water 24 hours before use.

4. Sand and cement were first mixed dry on a clean platform.

5. The coarse aggregate was then added and mixed thoroughly.

6. The mould and frame were oiled by lubricating oil before placing of concrete in the mould and frame.

7. The mixture was placed in the mould within an hour.

8. The concrete was compacted by tamping rod till the water appeared on the top and no coarse aggregate was then left uncovered by mortar.

9. After 24 hours the moulds were removed and the specimens were cured in water for 28 days to attain its proper strength.

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-11

4.2 Details of mix design

The mix ratio is 1:2:4

Diameter of the cylinder = 6 inch.

Height of the cylinder = 12 inch.

Volume of the cylinder = Π/4X6²X12   in³          

                                         = 0.196 ft³           

Wet volume = (0.196 X 1.5) = 0.294 ft³  

Quantity of cement = 0.294 X 1/7 X 50 X 1/1.25

                                  = 1.68 kg

Quantity of sand = 0.294 X2/7 X100 X 1/2.2

                             = 3.82 kg

Quantity of brick chips = 0.294 X4/7 X 68.5 X 1/ 2.2

                                         = 5.23 kg

Quantity of stone chips = 0.294 X 4/7 X 95X 1/ 2.2

                                         = 7.25 kg

Table no. 4.1

Determination of fineness modulus of sand.

Sieve size	Wt. retained in gm	Cumulative wt retained in gm	Cumulative percentage retained in gm
1.18 mm	0.5	0.5	0.1
600 micron	1.5	2	0.4
300 micron	116	118	23.6
150 micron	350	468	93.6
Filler	32	500	-
Total	500		117.7

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-10

(d) Placing: The concrete must be placed in a systematic manner to yield optimum result. The placing and compacting operations are, perhaps, the most critical field activities. 

(e) Compaction: It is the process adopted for expelling the entrapped air from the concrete. The different processes of compaction are - 

(1) Hand compaction 

# Rodding 

# Ramming 

# Tamping  (2) Compaction by vibrator  (3) Compaction by pressure or jolting  (4) Compaction by spinning  (f) Curing: It is the process of maintaining satisfactory moisture content and a favorable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed.   Heat Curing Methods: (1) Water curing  # Immersing

* Membrane curing  Heat Curing by Powerblanket

# Ponding 

# Spraying 

# Water covering 

(2) Membrane curing 

(1) Application of heat 

(2) Miscellaneous

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-9

Water/cement ratio expresses the dilution of the paste – cement content varies directly with the amount of paste. Gradation of aggregate is controlled by varying the amount of given fine and coarse aggregate. Consistency is established by practical requirements of placing. 

4.1 Process of manufacture of concrete 

The various stages are the following: 

(a) Batching: The measurement of materials for mixing is known as batching. It can be done by two ways – (1) Volume batching (2) Weight batching  (b) Mixing: The mixing should ensure that the mass becomes homogeneous, uniform in color and consistency. Mixing can be done by two ways -  Multiple Shaft and End Seal Protector Concrete Mixture (1) Hand mixing  (2) Machine mixing  (c) Transporting: The precautions to be taken while transporting concrete is that the homogeneity obtained at the time of mixing should be maintained.  Methods:  (1) Mortar pan  (2) Wheelbarrows  (3) Bucket and rope way  (4) Truck mixers and dumpers  (5) Belt conveyors  (6) Chute  (7) Displacement pumps

Mobile Concrete Mixer Machine from MACONS EQUIPMENTS Truck Mixture and Chute Arrangement For Concrete Transportation Belt conveyors for Concrete Transportation

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-8

CHAPTER − 4 

CONCRETE MIX DESIGN 

The design of concrete mix is not a simple task on account of the widely varying properties of the constituent materials, the conditions that prevail at the site of work and the conditions that are demanded for a particular work for which the mix is designed. Design of concrete mix requires complete knowledge of the various properties of these constituent materials, the implications in case of change on these conditions at the site, the impact of the properties of plastic concrete on the hardened concrete and the complicated inter-relationship between the variables. All these make the task of mix design more complex and difficult. 

Mix design can be defined as the process of selecting suitable ingredients of concrete and determining their relative proportions with the object of producing concrete of certain minimum strength and durability as economically as possible. The purpose of designing as can be seen from the above definitions is two-fold. The first object is to achieve the stipulated minimum strength. The second object is to make the concrete in the most economical manner.   With the given materials, the four variable factors to be considered, in

A Trail Batch for Mix Design

connection with specifying concrete mixes are:

(1) Water-cement ratio 

(2) Cement content or cement-aggregate ratio 

(3) Gradation of the aggregates 

(4) Consistency 

In general all four of these inter-related variables can not be chosen or manipulated arbitrarily. Usually two or three factors are specified, and the others are adjusted to give minimum workability and economy. 

STUDY ON STRENGTH OF CONCRETE USING VARIOUS COARSE AGGREGATES, PART-7

In which σ = stress in the fiber farthest from the neutral axis, psi 

M = bending moment at the section, lb-in. 

I = moment of inertia of the cross section, in. 

c = distance from neutral axis to farthest fiber, in.

Since the assumptions on which the flexure formula are based do not hold true at high stresses approaching failure, modulus of rupture is a fictitious value; however, it is convenient for purposes of evaluation and is commonly used. The modulus of rupture ranges from 11 to 23 percent of the compressive strength. The use of rough-textured or angular aggregates results in relatively high flexural strength as compared to compressive strength.

An approximate relationship between flexural and compressive strength is

f¢_c X (R/K)²

in which  f¢c = compressive strength, psi  R = modulus of rupture, psi K = a constant, usually between 8 and 10 3.4 Shear strength The following formula is adopted for the shear strength of concrete– SS = VQ/(Ib)     In which, SS = Strength in psi V = Vertical shear obtained from SFD.

Shear strength Calculation for Beam Section

Q = Static moment about the neutral axis of the shaded area          

    

             
I = Moment of inertia.

b = Width of the beam.