Control Tests for Concrete - Design and Control of Concrete Mixtures - page 285

pH Testing Methods

Each method has limitations and caution should be exer-
cised against acceptance of nondestructive test results as
having a constant correlation to the traditional compres-
sion test; for example, empirical correlations must be
developed prior to use ( Malhotra 1976 , NRMCA 1979 ,
Malhotra 1984 , Clifton 198 5, Malhotra and Carino 1991) .
An NDT program may be undertaken for a variety of
purposes regarding the strength or condition of hardened
concrete, including:

There are three practical methods for measuring the pH of
hardened concrete in the field. The first uses litmus paper
designed for the alkaline range of pH readings. Place a few
drops of distilled water on the concrete, wait 60 ± 5 sec-
onds and immerse an indicator strip (litmus paper) in the
water for 2 to 3 seconds. After removing the strip, compare
it to the standard pH color scale supplied with the indi-
cator strips. A second method uses a pH “pencil.” The
pencil is used to make a 25 mm (1 in.) long mark after
which 2 to 3 drops of distilled water are placed on the
mark. After waiting 20 seconds, the color is compared to a
standard color chart to judge the pH of the concrete.
Finally, the third method uses a wide-range liquid pH indi-
cator on a freshly fractured surface of the concrete or a core
obtained from the concrete. After several minutes, the
resulting color is compared to a color chart to determine
the pH of the concrete. This method is also effective for
measuring the depth of carbonation present on the
concrete surface. See PCA (2000) for more information.

• Determination of in-place concrete strength
• Monitoring rate of concrete strength gain
• Location of nonhomogeneity, such as voids or honey-
combing in concrete
• Determination of relative strength of comparable
members
• Evaluation of concrete cracking and delaminations
• Evaluation of damage from mechanical or chemical
forces
• Steel reinforcement location, size, and corrosion activity
• Member dimensions

Irrespective of the type of NDT test used, adequate
and reliable correlation data with standard 28-day com-
pressive strength data is usually necessary to evaluate the
accuracy of the NDT method. In addition, correlation to
in-place compressive strengths using drilled cores from
one or two locations can provide guidance in interpreting
NDT test results; these can then be used to survey larger
portions of the structure. Care should be taken to consider
the influence that varying sizes and locations of structural
elements can have on the NDT test being used.
Rebound Hammer Tests. The Schmidt rebound hammer
(Fig. 16-20) is essentially a surface-hardness tester that
provides a quick, simple means of checking concrete
uniformity. It measures the rebound of a spring-loaded

Permeability

Both direct and indirect methods of determining perme-
ability are used. Table 16-2 shows typical concrete perme-
abilities. Resistance to chloride-ion penetration, for
example, can be determined by ponding chloride solution
on a concrete surface and, at a later age, determining the
chloride content of the concrete at particular depths
(AASHTO T 259). The rapid chloride permeability test
(RCPT) (ASTM C 1202 and AASHTO T 277), also called
the Coulomb or electrical resistance test, is often specified
for concrete bridge decks. Various absorption methods,
including ASTM C 642, are used. Direct water perme-
ability data can be obtained by using the Army Corp of
Engineers CRC C 163-92 test method for water perme-
ability of concrete using a triaxial cell. A test method rec-
ommended by the American Petroleum Institute for
determining the permeability of rock is also available.
ASTM is in the process of developing a standard method
for hydraulic permeability of concrete. All the above test
methods have limitations. For more information, see
American Petroleum Institute (1956) , Tyler and Erlin
(1961) , Whiting (1981) , Pfeifer and Scali (1981) , and
Whiting (1988) .

Nondestructive Test Methods

Nondestructive tests (NDT) can be used to evaluate the
relative strength and other properties of hardened con-
crete. The most widely used are the rebound hammer,
penetration, pullout, and dynamic or vibration tests.
Other techniques for testing the strength and other prop-
erties of hardened concrete include X-rays, gamma radi-
ography, neutron moisture gages, magnetic cover meters,
electricity, microwave absorption, and acoustic emissions.

Fig. 16-20. The rebound hammer gives an indication of the
compressive strength of concrete. (69782)