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Impact studies on Fibre reinforced high performance concrete
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Posted By: imthiyas arafath s Member Level: Gold Points/Cash: 5
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| The status of this Project is Deleted. |
Impact Resistance:
Impact Resistance (dynamic energy absorption as well as strength) is one of the important attributes of FRC. Several types of tests have been used to measure the impact resistance of FRC. These can be classified broadly, depending upon the impacting mechanism and parameters monitored during impact, into the following types tests.
a) Weighted pendulum Charpy-type impact test
b) Drop-weight test(single or repeated impact)
c) Constant strain-rate test
d) Projectile impact test
e) Split-Hopkinson bar test
f) Explosive test
g) Instrumented pendulum impact test
Conventionally, impact resistance has been characterized by a measure of the energy consumed to the fracture notched beam specimen (computed from the residual energy stored in the pendulum after impact); the number of blows in the “repeated impact” test to achieve a prescribed level of distress; and the size of damage ( crater/performance/ scab) or the size and velocity of the spall after the specimen is struck with a projectile or after the specimen is subjected to a surface blast loading
Results from such tests are use full for ascertaining the relative merits of the different mixtures as well as for providing answer to specific practical problems. How-ever, they depend on the specimen geometry, test system compliance, loading configuration, loading rate, and the prescribed failure criterion. The simplest of the conventional test is the “repeated impact”, drop-weight test described in the next subsection.
More recently, instrumented impact tests have been developed that provide reliable and continuous time histories of the various parameters of the interest during the impact-load, deflection, and strain. These provide basic material properties at the various strain rates for the calculation of the flexural/tensile strength, energy absorption capacity, stiffness, and load-deformation characteristics. These types tests are described in the instrumented impact test subsection
Impact Strength:
The impact strength tests were carried out on concrete specimens of size 152 mm diameter and 62.5 mm thickness at the age of 28 days curing using drop weight testing device. Drop weight test recommended by ACI committee 544. 2R-89 which was originally devised by Schrader yields the number of blows necessary to cause prescribed level of distress in the test specimen. The number serves as a quantitative estimate of the energy absorbed by the specimen at that distress level.
The specimen was placed at the center of the base plate and it is free to move horizontally, 2.8 mm among the four positioned lugs at the periphery of base plate. A bracket with cylindrical sleeve was fixed in place and the hardened steel ball was placed on the top of the specimen within the bracket. The steel ball is free to move vertically within the sleeve. Elastometer pads were placed between the specimen and the positioned lugs to restrict movement of the specimen during testing till the first crack was visible. A drop hammer weighing 45 N was then placed with it on the steel ball and held vertically. The drop hammer was dropped repeatedly through a height of 457 mm and the number of blows required to cause the first visible crack and to cause ultimate failure were recorded. Ultimate failure is defined in terms of the number of blows required to open the cracks in the specimen sufficiently to enable the fractured pieces to touch three or four positioning lugs on the base plate. Each blow of the hammer represents 20.2 Nm of energy. The stages of ultimate failure were clearly recognized by the fractured specimen butting against the lugs of the base plate. The view of Schrader drop weight impact test set up is shown in Figures 6.2 and 6.3 shows some of the failed specimens due to impact load.
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