Civil Engineering Reference
In-Depth Information
depends on inspection requirements.
The individual inspection mainly applies to independent structures such as cast-in-place
shell structures, chimneys, water towers, tunnels and consecutive walls, and individual mem-
bers such as columns, beams, roof trusses, plates and foundation in which quality and
strength of concrete are suspect, and some structures or members are evidently of inferior
quality.
The sampling inspection mainly applies to concrete structures or members that are of
identical strength grade, basically in the same material, mixing proportion, age, and tech-
nical producing conditions. Tested samples should be randomly selected and account for
at least 30% of the same kind of structures or members and the total number of test areas
should not be less than 100. The detailed sampling method should be specified by agreement
of the owner, the construction unit and the inspection unit.
(2) Inspection Method
After learning the condition of concrete structures or members to be tested, the test areas
should be chosen and located. The test areas denote every sample to be inspected, which
is equivalent to a set of test blocks of the same condition from the sample. Following are
some stipulations from the professional standard
Technical Specification for Inspection of
Concrete Compressive Strength by Rebound Method.
The concrete from a structure or a
member is the minimum element for concrete strength evaluation, and at least 10 test areas
should be set. For structures or members shorter than 3 m or lower than 0.6 m, however,
the number of test areas can be reduced correspondingly, but under no circumstances be
less than 5. The proper size of test area should accommodate 16 rebound test points. The
surface of test area should be clean, smooth and dry without seams, finishing coat, stucco
layer, laitance, oil crust, voids or pores. Grinding wheels can be used in cleaning work if
necessary. Test areas should be evenly distributed on the inspection surface of structures or
members without oversize spacing between each other. The spacing between areas could be
increased up to a maximum of 2 m if the concrete is of good uniformity and quality. Stress
parts and places within structures or members, such as joints of beams and columns, which
are prone to generate defects, should be allocated for test. Test areas are preferably located
on the sides of concrete perpendicular to casting direction. If unavailable, they should be on
the top or bottom surfaces. Test areas should avoid the reinforced bars or embedded pieces
set in the vicinity of concrete cover layer. For members with small volume and low stiffness
or members thinner than 100 mm at test areas, braces should be installed to support them
and prevent them from cracking during rebounding.
With samples selected and test areas laid according to the methods mentioned above,
the rebound value is measured first. When testing, the rebound apparatus should be kept
perpendicular to the testing surface and should not contact pores or exposed aggregate.
Each of the two sides of every test area should be hit with rebound apparatus on 8 test
points. In the case that the test area has only one side, it should be hit on 16 points. Every
test point receives only one hit with the reading precision of one grade. Test points should
be distributed evenly on test area with net space between each other generally not less than
20 mm. The distance between test points and member edges, exposed bars or embedded
pieces should be no less than 30 mm.
The carbonization depth should be measured right after rebounding. Proper tool is em-
ployed to make a hole into the test area surface with a diameter of 15 mm in which the
powder and chipping should be removed (Caution: No liquid can be used to flush the hole.)
And immediately, several drips of 1% phenolphthalein alcohol solution are dropped on the
inner wall edge of the hole, and the perpendicular distance from the test surface to the edge
of the in-hole place where the color does not change into pink is the carbonization depth of
this area, which should be measured one to two times, with a precision of 0.5 mm.
