Civil Engineering Reference
In-Depth Information
Strength Specimens
Specimens molded for strength tests should be made and
cured in accordance with ASTM C 31 or AASHTO T 23
(field-molded specimens) or ASTM C 192 or AASHTO T
126 (laboratory-molded specimens). Molding of strength
specimens should be started within 15 minutes after the
composite sample is obtained.
The standard test specimen for compressive strength
of concrete with a maximum aggregate size of 50 mm
(2 in.) or smaller is a cylinder 150 mm (6 in.) in diameter
by 300 mm (12 in.) high (Fig. 16-7). For larger aggregates,
the diameter of the cylinder should be at least three times
the maximum-size aggregate, and the height should be
twice the diameter. While rigid metal molds are preferred,
parafined cardboard, plastic, or other types of disposable
molds conforming to ASTM C 470 can be used. They
should be placed on a smooth, level, rigid surface and
filled carefully to avoid distortion of their shape.
A 100-mm (4-in.) diameter by 200-mm (8-in.) high
cylinder mold has been commonly used with high strength
concrete containing up to 19 mm (
3
⁄
4
in.) maximum-size
aggregate (
Burg and Ost 1994
,
Forstie and Schnormeier
1981
, and
Date and Schnormeier 1984
). The 100 x 200-mm
cylinder is easier to cast, requires less sample, weighs con-
siderably less than a 150 x 300-mm (6 x 12-in.) concrete
cylinder; it is therefore easier to handle and requires less
moist-curing storage space. In addition, the smaller cross-
sectional area allows higher compressive strengths to be
reached by a testing machine that has a smaller load capac-
ity. The difference in indicated strength between the two
cylinder sizes is insignificant as illustrated in Fig. 16-8. The
standard deviation and coefficient of variation of 100-mm
cylinders is slightly higher or
similar to that for 150-mm
cylinders (
Burg and others 1999
and
Pistilli and Willems 1993
).
The predominant size used in
Canada is the 100-mm diameter
cylinder. Consult job specifica-
tions for allowable cylinder sizes.
Beams for the flexural
strength test should be 150 x 150
mm (6 x 6 in.) in cross section for
aggregates up to 50 mm (2 in.).
For larger aggregates, the mini-
mum cross-sectional dimension
should be not less than three
times the maximum size of
aggregate. The length of beams
should be at least three times
the depth of the beam plus 50
mm (2 in.), or a total length of
not less than 500 mm (20 in.) for
a 6 x 6-in. beam.
Test cylinders to be rodded
(slump of 25 mm [1 in.] or more)
should be filled in three approx-
imately equal layers with each
Fig. 16-7. Preparing standard test specimens for
compressive strength of concrete. (69790)
6x12-in. cylinder strength, psi
0
5,000
10,000
15,000
20,000
25,000
25,000
160
20,000
120
15,000
80
10,000
Moist cured
Air cured
Best fit line
40
5,000
0
0
0
40
80
120
160
150x300 mm cylinder strength, MPa
Fig. 16-8. Comparison of 100 x 200-mm (4 x 8-in.) and 150 x 300-mm (6 x 12-in.) cylinder
strengths (
Burg and Ost 1994
).
