Core Test for Concrete Structure
In Case of doubt regarding the grade of concrete used,either due to poor workmanship or based on results of cube strength tests,compressive strength test of concrete maybe carried out by core test.
Core drilling method is the most direct way of measuring the actual strength of concrete in the structure.It mostly involves proper selection of location and number of samples to be obtained.Core should be taken so to avoid the reinforcement.If avoidance of secondary reinforcement or surface reinforcement is inescaoable,strength of core can be taken as 10% less than measured strength.Cylindrical specimen of 100mm or 150mm diameter are common;other sizes may also be permitted but the least lateral dimension should not be less than 3 times the maximum size of the aggregates used.The core specimen to be tested should preferably have height of specimen as twice the diameter.
See More Core cutting Method
If there are difficulties of obtaining samples of such size,the length to diameter ratio is permitted to be lower,but in no case lower than 0.95.The samples are to be stored in water for two days prior to testing and are to be tested in moist coundition.The ends of specimens are trimmed and flatten and capped with molten sulphur or high alumina cement or some other permissible capping material to obtain a true flat surface.The specimen is then tested in compression.
Ebook to read Structural Engineer’s Book
Although drilling of cores and compressive strength test are quite simple (and are covered in IS: 1199 and IS:516),but the procedures and influencing factors are to be carfully understood as they affect the measured value and therefore the assessment of the quality of in place concrete.The provision of IS 456: 2000 vide clause 17.4.3 in this regard is given below;
Concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores in equal to at least 85% of the cube strength of the grade of concrete specified for the corresponding age and no individual core has strength less than 75 %.
Core test are carried out to ascertain segregation,honey-combing,thickness of concreting and the compressive strength of concrete in any concrete structure.
Shape of specimen – cylindrical
Specimen to be tested on the 28th day for comparison with casted cube.
Minimum no of cores to be tested – 3
Specimen to be submerged in water for 48 hours
The specimen is tested for compressive strength after surface – levelling and capping
Shape of specimen – Cylinder
Dia > 3 x (max. nominal size of C.A)
Length = 2 x dia
As per IS 516 – 1959 (CI 5.6.1)
EQUIVALENT CUBE STRENGHT OF CORE =1.25 X Cylinder Strength
Acceptance of Core Test ( IS 456 – 2000- CI. 17.4)
Equivalent cube strength of Individual core >
75% of cube strength of Individual Core
Average Equivalent cube strength of core >
85 % of cube strength of grade of concrete
Say Concrete Grade: 20 N/mm2 = 1) 13 2)14 3)15
Equivalent cube strength of cores = 1) 16.25 2) 17.50 3) 18.75
(= 1.25 x P)
Equivalent cube strength of Individual core (i.e) 1)16.25 2)17.50 3) 18.75
. > 75% cube strength of grade of concrete (0.75 x 20 = 15 N/mm2)
Average Equivalent cube strength of core = ( 16.25+17.5+18.75)/3 =17.50 N/mm2
85% of cube strength of grade of concrete = 0.85 x 20 = 17
Average Equivalent cube strength of core (17.50 N/mm2)
. > 85% of cube strength of grade of concrete (17 N/mm2)