What is this blog post about?
Dear readers, this blog post covers requirements of the machine used for testing of compressive strength of concrete cube, mortar cube or similar materials. The machine is called CTM or compression testing machine and used for concrete cube and mortar cube test. The Indian standard code IS: 14858: 2000 covers the specifications of compression testing machine for concrete cube and mortar test.
A Short Interoduction to Compression Test
The compressive strength is an important and useful property of cement mortars, concrete, bricks, paver blocks and similar materials. It tells how the quality of the material is, especially about strength. The test is performed using compression testing machine or universal testing machine.
The compressive strength of the specimen (mortar, concrete cube, brick or pavement) is calculated by dividing the maximum load applied (before failure) to the specimen during the test by the cross-sectional area.
Specifications of Compression Testing Machine for Concrete Cube and Mortar Test
Digital Compression Testing Machine makes compressive strength test easy. Place the mortar, cube, brick or pavement, whatever you want to know the compressive strength of, in the appropriate place in the testing machine and turn on the machine. Now note down the maximum load applied before the specimen breaks. Then divide the maximum load by the cross-sectional area of the specimen. Do this experiment with three different samples. Finally find the average of the quotient obtained from all the three experiments. It gives the value of compressive strength of the specimen.
IS Code for Compression Testing Machine
IS: 14858: 2000 is the IS code for specifications of compression testing machine.
Construction of Digital Compression Testing Machine (in short)
- The compression testing machine may be of any reliable type, of sufficient capacity for the tests and capable of applying the load at the rate specified in test procedure.
- The permissible error shall be not greater than ±2 percent of the maximum load.
- The testing machine shall be equipped with two steel bearing platens with hardened faces.
- One of the platens (preferably the one that normally will bear on the upper surface of the specimen) shall be fitted with a ball seating in the form of a portion of a sphere, the centre of which coincides with the central point of the face of the platen.
- The other compression platen shall be plain rigid
- bearing block.
- The bearing faces of both platens shall be at least as large as, and preferably larger than the nominal size of the specimen to which the load is applied.
- The bearing surface of the platens, when new, shall not depart from a plane by more than 0·01 mm at any point, and they shall be maintained with a permissible variation limit of 0·02 mm.
- The movable portion of the spherically seated compression platen shall be held on the spherical seat, but the design shall be such that the bearing face can be rotated freely and tilted through small angles in any direction.
Design of Compression Testing Machine
The machine shall be power operated and shall apply the load continuously rather than intermittently, and without shock.
The space provided for test specimens shall be large enough to accommodate, in the readable position, an elastic calibration device which is of sufficient capacity to cover the potential loading range of the testing machine.
Loading
Load Control
The machine shall be capable of applying the load at the specified rate, uniformly, without shock, using manual or automatic control.
Load Pacers
- If the machine is not equipped with an automatic load control, a load pacer shall be fitted to enable the operator to manipulate the machine controls to maintain the specified rate.
- If the pacer has a scale, this scale shall be basically linear such that 1 mm represents not more than 100 N/s. Over the operating range of the scale the accuracy shall be within ±5 percent.
- If the pacer is fitted with a variable speed control or has preset speeds, then once the variable speed control has been set, or a preset speed has been chosen the pacer speed shall remain within ±5 percent of the specified speed over the operating range.
Load Scale Indicators or Digital Displays
The machine shall be provided with the following:
A.) Either easily read dials or scales or electrical load indicators, with a visual display.
Note – The visual display may be supplemented by recording devices, that are calibrated to the same accuracy as the display.
B.) A resettable device which registers the maximum load sustained by the specimen.
C.) The width of the needles shall be less than the width of the graduation.
Accuracy
- The accuracy of the testing machine shall be such that the percentage of error for the loads within the proposed range of use of the testing machine shall not exceed ±1.0 percent of the indicated load.
- The indicated load of a testing machine shall not be corrected either by calculation or by the use of calibration diagram to obtain values within the required permissible variation.
Means of Applying the Load
The means of applying the load shall provide for the load to be applied either with the specimen in direct contact with the machine platens. or spacing blocks, or with auxiliary platens interposed between each machine platen, or spacing block, and the specimen.
Rate of Loading
For testing machines of the screw type, the moving head shall travel at a rate of approximately 1.3 mm/min when the machine is running idle. For hydraulically operated machines, the load shall be applied at a rate of movement corresponding to a loading rate on the specimen within the range of 0.14 or 0.324 MPa/s.
Machine Platens
The testing machine shall be equipped with two steel bearing blocks with hardened faces (Vickers hardness not less than 550), one of which is a spherically seated block that will bear on the upper surface of the specimen, and the other a solid block on which the specimen shall rest. Bearing faces of the blocks shall have a minimum dimension at least 3 percent greater than the dimension of the specimen to be tested. Except for the marking described below, the bearing faces shall not depart from a pIane by more than 0.025 mm in any 152 mm of blocks in diameter or larger, or by more than 0.0225 mm in the diameter of any smaller block; and new blocks shall be manufactured within one half of this tolerance. When the dimensions of the bearing face of the spherically seated block exceeds the dimension of the specimen by more than 13 mm, markings not more than 0.8 mm deep and not more than 1.2 mm wide shall be inscribed to facilitate proper centering.
Bottom Bearing Blocks
Bearing block shall conform to the given requirements.
The bottom bearing block is specified for the purpose of providing a readily machinable surface for maintenance of the specified surface condition The top and bottom surfaces shall be parallel to each other. The block may be fastened to the platen of the testing machine. Its least horizontal dimension shall be at least 3 percent greater than the dimension of the specimen to be tested.
The bottom bearing block shall be at least 25 mm thick when new, and at least 22.5 mm thick after any resurfacing operations.
Spherically Seated Bearing Block
- The maximum diameter of the bearing face of the suspended spherically seated block shall not exceed the values given below:
| Diameter of Test Specimen, mm |
Maximum Diameter of Bearing Face, mm |
|---|---|
| 51 | 105 |
| 76 | 127 |
| 102 | 165 |
| 152 | 254 |
| 203 | 279 |
Note – Square beming faces are permitted provided the diameter of the largest possible inscribed circle does not exceed the diameter.
- The centre of the sphere shall coincide with surface of the bearing face within a tolerance of ±5 percent of the radius of the sphere. The diameter of the sphere shall be at least 75 percent of the diameter of the specimen to be tested.
- The ball and the socket shall be so designed by the manufacturer that the steel in the contact area does not permanently deform under repeated use, with loads up to 82.7 MPa on the test specimen.
- The movable portion of the bearing block shall be held closely in the spherical seat, but the design shall be such that the bearing face can be rotated freeIy and tilted at least 4.0 in any direction.
Auxiliary Platens
- The auxiliary platens shall be made of a material which, when tested in accordance with IS 1501, shall have a hardness value of at least 550. Also the material shall not deform irreversibly when the machine is used.
- The distance between either pair of opposite edges of a square auxiliary platen, or the diameter of a circular platen, shall be the nominal size of the specimen (100 or 150 mm) ±0.2 mm; the distance between their contact faces of the platen shall be at least 23 mm.
- The flatness tolerance for each contact face of the platens shall be 0.03 mm wide.
- The squareness tolerance for each edge of the auxiliary platens with respect to the adjacent edge as datum shall be 0.06 mm wide.
- The parallelism tolerance for one contact face of the auxiliary platen with respect to the other contact face as datum shall be 0.06 mm wide.
- The Ra value for the surface texture of the contact faces of the auxiliary platen shall be between 0.4μ
- and 3.2μ.
Spacing Blocks
If it is required to reduce the space between the machine platens, up to four spacing blocks shall be located either beneath or on the lower machine platen.
Spacing blocks used on the lower machine platen shall be made of a material, which when tested in accordance with IS 1501, shall have a hardness value of at least 550. Also the material shall not deform irreversibly when the machine is used.
All spacing blocks shall comply with the flatness and parallelism tolerances required for auxiliary platens.
Marking
The following information shall be clearly and
indelibly marked on the machine:
a) Indication of the source of manufacture,
b) Date of manufacture, and
c) Serial number.
BIS Certification Marking
Each machine should be marked with the Standard Mark.
Calibration
- After an elapsed interval not exceeding 12 months from the previous verification.
- On original installation or relocation or subjected to major repairs or adjustments.
- Whenever there is reason to doubt the accuracy of the results, without regard to the time interval since the last verification.
- The accuracy of the testing machine shall be verified by applying five test loads in four approximately equal increments in ascending order. The difference between any two successive loads shall not exceed one third of the difference between the maximum and minimum test loads.
The load as indicated by the testing machine and the applied load computed from the readings of the verification device shall be recorded at each test point.
Calculate the error, E and the percentage of error, Ep for each point from these data as follows:
E = A – B
Ep= E/B × 100
where,
A = Load in N; indicated by the machine being verified, and
B = Applied load in N; as determined by the calibrating device.
Download – IS 14858: 2000.pdf
File size: 1.85 MB
Total Page: 12 pages
