ICOLD Bulletin: The Physical Properties of Hardened Conventional Concrete in Dams
Section 2 (Strength properties)
As submitted for ICOLD review, march 2008 Section 2-12
Another factor contributing to the difference between core and laboratory strength is the
relief of the multiaxial stress condition after coring. This can be particularly influential in
Alkali-Aggregate Reaction (AAR)-damaged concrete, where a damaged matrix can
expand and thus reduce core strength.
The above dichotomy leads recommending to core dam concrete as a rule and not only
if unsatisfactory laboratory tests are obtained, at least for large, highly stressed dams.
Using cored strength values in a back-analysis would provide more reliable safety
margins. However, it should be mentioned that checking the compressive strength from
drilled dam cores is only successful if drilling is done with much care, if curing is done
properly, and if pre-test conditions are representative and realistically interpreted.
Otherwise the core strength tends to be unrealistically low.
ASTM C 42 [2.12] and EN 12504 [2.13] can be used as a reference for drilled cores. So
can ACI 318, which stipulates that field-cured test cylinders should achieve at least
85% of the strength from laboratory-cured cylinders [2.14]. ACI 318 defines two field
curing conditions depending on the expected condition in the structure: one is air
drying, the other soaking of the specimens. For dam concrete, soaking is more
appropriate for a saturated concrete mass and it should be long enough to eliminate
moisture gradients in the sample (curbing the influence of pore water pressure). If the
concrete to be tested is assumed to be in a steady-state dry condition then the cores
should not be wet-cured. This is important because the difference in compressive core
strength between the 2 curing methods can be 20 to 35%, with the air dried being the
larger of the two values [2.15].
Other comparative test results for structural concrete (28 days) demonstrate 90% to
76% for core as compared to laboratory strength [2.16]. Percentages depend on stress
level: it decreases from close to 100% for low strength (∼20 MPa) to 70% for high
strength concrete (∼60 MPa) [2.17].
2.2.3 Nominal age of concrete strength
In large hydraulic structures, structural elements are rarely required to withstand
substantial stress at early age. Unless unusual circumstances prevail, the Committee
recommends an age in excess of 180 days as basis for evaluating the characteristic
strength. This last one is defined in the following section 2.2.5.
It is utterly conservative to use 28 or even 90 days for assessing strength safety
margins, as occasionally specified for dam concrete. For most dams at least 180 days
seem reasonable. ACI 207 reports that design strength is on ages between 90 days
and sometimes up to two years [2.18]. In California 1 year is accepted too [2.19].
Accounting for the continued strength development beyond 90 days, especially where
pozzolans are applied, the correlation factors at 1 year may range as shown on Tab.
2.5 and Fig. 2.3 from [2.20].