Civil Engineering Reference
In-Depth Information
With these limitations in mind, the maturity concept has
gained greater acceptance for representing the compres-
sive strength of the concrete for removal of shoring or
opening a pavement to traffic; but it is no substitute for
quality control and proper concreting practices (
Malhotra
1974
and
ACI Committee 347
).
should be left in place. Before shores and forms are
removed, fully stressed structural concrete should be
tested to determine if in-place strengths are adequate,
rather than waiting an arbitrary time period. In-place
strengths can be monitored using one of the following:
(1) field-cured cylinders (ASTM C 31 or AASHTO T 23);
(2) probe penetration tests (ASTM C 803); (3) cast-in-place
cylinders (ASTM C 873); (4) pullout testing (ASTM C 900);
or (5) maturity testing (ASTM C 1074). Many of these tests
are indirect methods of measuring compressive strength;
they require correlation in advance with standard cylin-
ders before estimates of in-place strengths can be made.
If in-place compressive strengths are not documented,
Table 14-3B lists conservative time periods in days to
achieve various percentages of the standard laboratory-
cured 28-day strength. The engineer issuing project draw-
ings and specifications in cooperation with the formwork
contractor must determine what percentage of the design
strength is required (see
ACI Committee 306
). Side forms
can be removed sooner than shoring and temporary false-
work (
ACI Committee 347
).
To monitor the strength development of concrete in
place using the maturity concept, the following informa-
tion must be available:
1. The strength-maturity relationship of the concrete used
in the structure. The results of compressive strength
tests at various ages on a series of cylinders made of
a concrete similar to that used in the structure; this
must be done to develop a strength-maturity curve.
These cylinders are cured in a laboratory at 23°C ± 2°C
(73°F ± 3°F).
2. A time-temperature record of the concrete in place.
Temperature readings are obtained by placing expend-
able thermistors or thermocouples at varying depths in
the concrete. The location giving the lowest values pro-
vides the series of temperature readings to be used in
the computation (Fig. 14-26).
See
ACI 306R-88
for sample calculations using the
maturity concept.
MATURITY CONCEPT
The maturity concept is based on the principle that
strength gain in concrete is a function of curing time and
temperature. The maturity concept, as described in
ACI
306R-88
and ASTM C 1074 can be used to evaluate
strength development when the prescribed curing
temperatures have not been maintained for the required
time or when curing temperatures have fluctuated. The
concept is expressed by the equation:
Metric:
M
=
∑
(
C
+ 10)
∆
t
Inch-Pound Units:
M
=
∑
(F - 14)
∆
t
where
M
= maturity factor
∑
= summation
C
= concrete temperature, degrees Celsius
F
= concrete temperature, degrees Fahrenheit
∆
t
= duration of curing at temperature C (F), usually
in hours
The equation is based on the premise that concrete
gains strength (that is, cement continues to hydrate) at
temperatures as low as -10°C (14°F).
Before construction begins, a calibration curve is
drawn plotting the relationship between compressive
strength and the maturity factor for a series of test cylin-
ders (of the particular concrete mixture proportions) cured
in a laboratory and tested for strength at successive ages.
The maturity concept is not precise and has some
limitations. But, the concept is useful in checking the
curing of concrete and estimating strength in relation to
time and temperature. It presumes that all other factors
affecting concrete strength have been properly controlled.
