ELASTOMERIC BEARINGS
TABLE 5.3 DIMENSIONAL TOLERANCES
SN ITEMS TOLERANCES
1. Overall plan dimensions -0, + 6 mm 2. Total bearing thickness -0, + 5%
3. Parallelism
A. Of top surface of bearing with repect to the bottom surface
as datum 1 in 200
B. Of one side surface with respect
to the other as datum 1 in 100 4. A. Thickness of individual internal + 20%
layer of elastomer (max. of 2mm)
B. Thickness of individual outer layer -0, +1 mm 5. A. Plan dimensions of laminates -3 mm, +0
B. Thickness of laminate + 10%
C. Parallelism of laminate with respect 1 in 100 to bearing base as datum
5.7 INSPECTION AND TESTING
The inspection and tests on elastomer and the finished bearings are very important aspects of ensuring a satisfactory performance of the bearing. IRC:83 has laid down detailed testing plan and acceptance criteria for elastomeric bearings. The important aspects are highlighted in the following paragraphs.
5.7.1 Lot size : Testing and acceptance of elastomeric bearings should be done lot-wise. A ‘lot’ of bearings shall comprise of all bearings of equal or near equal size produced under identical conditions of manufacture to be supplied for a particular project. For the purpose of grading the
levels of acceptance test, the lots are classified as under :
1. A lot size of 24 or larger number of bearings is defined as large lot.
2. A lot size with less than 24 bearings is defined as small lot.
When the number of bearings for a bridge project is large and phased production is permitted, bearings supplied in any one phase will be considered as a large lot. The levels of acceptance test applicable will be as under : Large lot - ‘Level 1’ Acceptance testing Small lot - ‘Level 2’ Acceptance testing.
5.7.2 Level 1 Accpetance Testing : This will include the following tests:
A. General inspection
B. Test on specially moulded test piece C. Test on complete bearings.
A. General inspection
1. All bearings of the lot shall be visually inspected for absence of any defects in surface finish, shape or any discernible superficial defects.
2. All bearings of the lot shall be checked for dimensional tolerances.
3. All bearings shall be subjected to an axial load corresponding to normal pressure of 15 MPa applied in stages and held constant while visual examination is made for:
a. Misalignment of reinforcing plates b. Poor bond at interface
c. Variation in elastomer thickness d. Surface defects
e. Low stiffness.
The deflection under loads between 5 MPa and 15 MPa should be measured with sufficient accuracy. Variation in stiffness of any individual bearing from the mean of all such bearings should not be more than 20% of the mean value.
B. Test on specially moulded test piece The test piece shall be moulded by the manufacturer with identical compound and under identical vulcanizing conditions as used in manufacture of the bearings. The test pieces should be suitably identified and certified. The test pieces will be subjected to the following tests:
• Test for chemical composition, specific gravity, ash content etc.
• Test for physical properties such as (i) Hardness
(ii) Ultimate tensile strength (iii) Elongation at break (iv) Accelerated aging test (v) Compression set test (vi) Ozone test
The details of these tests are given in Table 5.2.
C. Test on complete bearings
Two bearings should be selected at random from the ‘lot’. Various tests should be conducted on these ‘test bearings’. These bearings should be excluded from the accepted lot because all the tests given below except ‘Shear Modulus test’ are destructive tests.
1. Shear Modulus 2. Elastic Modulus 3. Adhesion strength
4. Ultimate compressive strength.
In addition to above tests, the ash content (%) and specific gravity of elastomer of test pieces from test bearing shall be compared with those of corresponding specially moulded test pieces and maximum acceptable variation will be as given below:
Ash content + 0.5%
Specific gravity + 0.2
The test specifications and acceptance criteria should be as per Appendix II of IRC 83 Part II.
The excerpts are given below:
General about tests
1. All testing shall be done at room temperature.
2. No bearing shall be tested earlier than a week after vulcanization.
3. Bearing sections shall be cut from test bearings without overheating the rubber and with smooth cut square edges.
4. Test for determination of ‘E’ may precede that of ‘G’ when both tests are conducted on the same pair of test bearings.
Test for determination of Shear Modulus
• Conditioning load : Bearings shall be preloaded with maximum horizontal load Htest (with Ntest or vertical load held constant) and unloaded before test loading.
• Rate of loading : Ntest corresponding to σm = 5 MPa shall be held constant during test and the horizontal loading H shall be gradually increased to yield a shear stress rate of approximately 0.05 to 0.1 MPa per minute.
• Maximum test loading Ntest: The horizontal loading H shall be increased upto a maximum Htest which corresponds to horizontal deflection equal to ‘h’ (total elastomer thickness).
• Measurement : Load and deflection measurements shall be calculated at approximately equal intervals not less than 5.
Evaluation
A shear stress strain curve shall be plotted and the value of shear modulus G determined as shown in Fig. 5.7.
The test result shall be deemed satisfactory if value of G is within + 20% of 1 MPa and provided there is no evidence of instability, defect or damage detected by close inspection during the test.
2 H
RIGID CONCRETE SLAB ( FIXED )
RIGID CONCRETE SLAB (DEFLECTED UNDER 'H')
Ntest
RIGID CONCRETE SLAB (FIXED)
SHEAR
SHEAR STRAIN
0.0 0.2 0.4 0.6 0.8 10.0
STRESS
a) TEST ASSEMBLY
b) SHEAR STRESS STRAIN CURVE