Important thing to remember in Casting and testing of Cubes to be followed in construction site.For the assessment of compliance of ready-mixed concrete or site mixed concrete, the point and time of sampling shall be at discharge from the producer’s delivery vehicle to pump or site or discharge from mixer respectively.
The following steps should be observed:
(a) Check if moulds are cleaned & lightly oiled.
(b) In assembling the mould for use, the joints between the sections of mould shall be thinly coated with mould oil and a similar coating of mould oil shall be applied between the contact surfaces of the bottom of the mould and the base plate in order to ensure that no water escapes during the filling.
(c) Ensure angle between adjacent internal faces & between internal faces shall be 90O+ 0.5OC
(d) Thoroughly remix the sample shovelling into a heap
(e) Fill the mould with concrete in 50 mm layers
(f) 35 tamps for each of the three layers in 150 mm mould or vibrating table
(g) After filling each layer tamp sides with a wooden mallet to expel out any entrapped air
(h) The workability of concrete at time of filling cubes should be between 75 -100 mm.
(i) Remove surplus concrete & number the moulds for identification & record details.
(k) Cover each mould with damp cloth / Hessian cloth immediately after initial set.
(l) Store at a place free from vibration. Protect the cube moulds at all times from high / low temperatures & driving winds.
(m) Complete Sampling & cube making certificates or records.
(n) After demoulding the cubes shall be immediately submerged in clean, fresh water or saturated lime solution and kept there until taken out just prior to test (Temperature 24 to 30oC).
(o) The specimens shall not be allowed to become dry at any time until they have been tested.
(p) The testing machine may be of any reliable type, of sufficient capacity for the tests and capable of applying the load at the rate of 14 N/sq. mm/minute. The permissible error shall be not greater than ± 2 percent of the maximum load.
(q) The bearing surfaces of the testing machine shall be wiped clean and any loose sand or other material removed from the surfaces of the specimen which are to be in contact with the compression platens.
(r) The specimen shall be placed in the machine in such a manner that the load shall be applied to opposite sides of the cubes as cast, that is, not to the top and bottom.
(s) Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ± 15 percent of the average.
(u) If failure is observed during the testing of the first cube, stop testing the balance two cubes, immediately inform the senior authority of site engg and the supplier of the concrete and test the balance two cubes in presence of the supplier’s authorized representative and have a joint record.
(v) The following information shall be included in the report on each test specimen:
a) identification mark,
b) date of test,
c) age of specimen,
d) date of casting ,
e) weight of specimen,
f) dimensions of specimen,
g) cross-sectional area,
h) maximum load,
j) compressive strength, and
k) appearance of fractured faces of concrete and type of fracture, if these are unusual.
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