Drop Test Breakage Index for Rocks.
Scope.
The drop test is used on heavy grading to reproduce
degradation related to construction. The result are expressed in terms of the
breakage number(BN) or drop test breakage index(Id50).
Sample size.
The sample shall contain at least 50 pieces, using sampling
in accordance with BS EN 13383-2 from masses above the ELL (extreme lower
limit)of the grading class.
Procedure.
Determine the individual weights of each armourstone and the
initial mass distribution using the standard method of EN 13383. Prepare the
bed of rocks on a 0.5 +/- 0.05 m thick layer of crushed rock aggregates.
Subject each block in the test sample in turn of a drop of fall height 3 m +/-
0.1 m on to the rock bed. After each drop, weight and record the mass of each
stone piece heavier than the ELL. Record the result of each drop, noting the
number and type of visible flaws in blocks and number and type of broken pieces
resulting.
Plot the cumulative weight distribution curves for the
sample before drop testing and after drop testing for all the pieces heavier
than ELL. Determine the median weight before drop testing (Mi50)and
after drop testing(MfEEL50).
Note: A lifting equipment(e.g., grab or similar)shall be
used for lifting rock pieces to the required height. During the lifting and
dropping process, the technician shall move to a safer distance to avoid any
injury that may cause by the broken pieces of rock.
Calculation and expression of results.
Calculate the loss as a percentage by the difference between
the median weight of before and after drop test.
Drop test breakage index, Id50 = (Mi50
– MfEEL50) / Mi50