How to test compressive strength of concrete by cube test method

 Importance

Define Compressive Strength of Concrete:

Compressive strength of concrete is defined load carrying capacity of concrete in compression. This parameter denotes the quality of concrete as concrete strength is main in compression only. Compressive strength is main criteria to design a structure using concrete in it. Compressive strength determined from lab test is also recognized as characteristic compressive strength.

Compressive Strength Test Machine

 

Objective of the test

Making, Curing and testing of Concrete cube for determination of Compressive strength of Concrete in the Laboratory

Equipments & Apparatus

6 nos. of Cube Moulds [size @150X150X150mm]

Steel tamping rod [dia 16 mm & 600 mm long and bullet pointed lower end ]

Compression testing machine [The percentage of error shall not exceed ±1.0 percent of the indicated load]

Mixing tray / small concrete mixer machine Trowel

Graduated cylinder [500 ml capacity]

Compacting vibrator (if compacting by vibrator) etc.

Preparation of specimens for test

Sample of cement should be taken a little portion from each number of bag present on the site. Test sample of aggregates shall be taken from larger lots by dividing it into parts. 

All materials shall be took to room temperature, preferably 27°± 3°C before performing the test.

The cement samples when  arrived  at the laboratory, should be comprehensively mixed dry either by hand or in a suitable mixer in such a manner as to warrant the greatest possible blending and uniformity in the material. Care should be taken to evade the imposition of foreign matter. Samples of aggregates for each and every batch of concrete shall be of the wanted grading and shall be in an air -dried condition.

In general, the aggregate shall be divided into fine and coarse sections and recombined for each concrete batch in such a manner as to make the wanted grading. The proportions of the materials, including water, in concrete mixes used for determining the appropriateness of the materials available, shall be similar in all respects to those to be employed in the work.

Weighing of quantities

The quantities of cement, each size of aggregate, and water for each batch will be determined by weight, to an accuracy of 0·1 percent of the total weight of the batch.

Mixing Concrete

The concrete will be mixed by hand, or preferably, in a laboratory batch mixer, in such a way as to evade loss of water or other materials.

Each batch of concrete shall be of such a quantity as to remain about 10 % excess after forming the desired numbers of test specimens.

Description of test moulds

6  nos cubes (3 for 7-days testing and 3 for 28-days testing but depending upon the quantity of concrete to be used it may varies as per codal specification ) of cubical in size of  150 150 150 mm.

If the largest nominal size of the aggregate not exceeding 20mm nominal size, cylindrical test specimens shall be used with specimen length equal to twice the diameter of it. Standard Specimens  should be 150 mm in diameter and 300 mm long.

The height of the mould and the distance between opposite smooth faces shall be the specified size with maximum tolerance of  ± 0·2 mm.

The angle in between neighboring internal faces and between internal faces with top and bottom planes of the mould shall be 90° with tolerance limit of ± 0.5°.

Casting Cubes

 

Casting of cubes

The cube mould plates should be detached, properly cleaned reassembled and all the bolts should be completely close-fitted.

A thin layer of lubricant or oil then shall be applied on all the faces of the mould. It is to be ensured  that cube side faces must be parallel to each other.

After taking concrete samples and mixing them properly, the cubes shall be cast as soon as possible. The concrete sample shall be filled into the cube moulds in layers approximately each layer of 50 mm deep .

At the time of  placing of concrete in the mould with help of a scoop, it shall be moved around the top edge of the mould as the concrete slides from it, in order to ensure a balanced distribution of the concrete with in the mould.  

Each layer shall be compacted either by hand or by the vibration as described below.

Compaction by hand

Each layer of the concrete filled in the mould shall be tempered by not less than 35 blows by tamping bar (or 25 blows each layer for 100 mm cubes).

The blows shall be penetrate into the underlying layer and the bottom layer shall be entered throughout its depth.

Where voids are left by the tamping bar the sides of the mould shall be stroked to meet up  the voids.

Compaction by vibration

Compaction can be done by vibrating each layer by means of an electric or pneumatic hammer or vibrator or a suitable vibrating table until the specified condition is achieved .

Vibrating Table Machine

 

Curing 

The casted cubes shall be stored under shed at a place free from the ant type of vibration at a conserved temperature 27°± 2°C for 24 hours ± 1/2 hours covered with wet straw or gunny sacking.

The cube shall be taken out from the moulds carefully at the end of 24 hours and immersed in clean water at a temperature 25°C to 29°C till the 7 or 28-days age of testing.

For the correct picture of actual strength of concrete in the structure, extra cubes shall be cast, stored and conserved as per the identical conditions of that structure, and tested at required age.

For  making and curing of samples in the field, the temperature of the place of storage shall be maintained within the range of 22° to 32°C.

After the period of 24 hours, these should be marked for further identification, taken out from the moulds and unless required for testing within 24 hours, stored in clean water at a temperature of 24° to 30°C until these are taken to the testing laboratory.

Testing of cube mould samples 

At least three specimens, ideally from different batches, shall be made for testing at each selected age. The cubes will be always  tested in the saturated and surface dry condition.

The dimensions of the specimen to the nearest  of ±0.2 mm and their weight shall be noted before testing. The bearing surfaces of the testing machine shall be cleared and any loose materials or others should be removed from the surfaces of the specimen which are to be in contact with the compression platens.

In the case of cubical shaped specimen, the specimen shall be positioned in the machine in such a  that the load shall be applied to opposite sides of the cubes as cast, in such a manner that  the top and bottom of the axis of the specimen shall be aligned with the center of thrust of platen.

 No packing shall be permitted between the faces of the test specimen .

The load should be applied without vibration and Increased continuously at a rate of approximately 140 kg/cm2/min until the specimen resistance to the increasing load breaks and no further load can be continued. The maximum load applied to the specimen shall then be noted and the appearance of the concrete and any infrequent features in the type of failures shall be recorded.

Calculations 

The compressive strength of concrete shall be calculated from: Maximum

load/Cross-Sectional area of cube & to be reported to the nearest ±0.5N/mm2

Maximum load applied = P1 tones = P N

Compressive strength = (Load in N/ Area in mm2)=(P N)/ (A mm2)

=(P/A) N/mm2

Average of three values shall be taken as the representative of the batch provided the Individual variation is not more than ± 15 % of the average. Otherwise repeat tests shall be made.

Report 

a) Identification mark ……

b) Date of test ……

c) Age of specimen …..

d) Curing conditions, including date of manufacture of specimen ….

f) Appearance of fractured faces of concrete and the type of fracture if they are unusual

Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)

Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)

Specifications as per MoRT&H & IS code for Acceptance criteria 

Ref. Clause No. 1717.5, page No. 559, Section 600 (as well as ref. clause No. 17.4.3 of IS: 456-2000)

Test Specimen and Sample Strength:

Three (3) test specimens shall be made from each sample for testing at 28 days.

The test strength of the sample shall be the average of the strength of 3 cubes. The individual variation should not be more than ±15 % of the average.

If variation is more,the test results of the sample are invalid and gets discarded.

Acceptance Criteria for Compressive Strength of Concrete as per IS: 456 –2000 

[ Ref. Clause No. 16.1, page No. 29 & table 11, page 30]

1. The concrete shall be deemed to comply with the strength requirements when both the following condition are met:

a) The mean strength determined from any group of 4 consecutive test results compiles with the appropriate limits:

for M15 grade conc. ≥ fck + 0.825 X established Standard Deviation (rounded off to neatest 0.5 N/mm2)

or fck +3 N/mm2 , whichever is greater

for M20 or above grade conc. ≥ fck + 0.825 X established Standard Deviation (rounded off to neatest 0.5 N/mm2)

or fck +4 N/mm2 , whichever is greater

 

b) Any individual test result complies with the appropriate limits in col 3 of Table 11.

for M15 grade conc. ≥ fck -3 N/mm2

for M20 or above grade conc. ≥ fck – 4 N/mm2

2. Standard Deviation (S.D.)can be taken from table 8, page 23,

Grade of Conc. M10, M15 , SD= 3.5 N/mm2 ;

Grade of Conc. M20, M25 , SD= 4.0 N/mm2 ;

Grade of Conc. M30, M35 , M40, M45 , M50, SD= 5.0 N/mm2 ;

Frequency of sampling of concrete of each group as per IS: 456 – 2000 

[ Ref. Clause No. 15.2, page No. 29]

A random sampling procedure shall be adopted to ensure that each concrete batch shall have a reasonable chance of being tested that is, the sampling should be spread over the entire period of concreting and cover all mixing units. The minimum frequency of sampling of concrete of each grade shall be in accordance with the following:-

Note: At least one sample shall be captured from each Shift, where concrete is produced at continuous production unit, such as ready mixed concrete plant, frequency of sampling may be agreed upon mutually by suppliers and purchasers.

 

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