Monitoring
early strength gain of concrete in the field is crucial task for the success of
some construction works like starting critical shoring and reshoring steps
which is required during multi-storey building construction and removal of
formwork.
There are several methods that can be used to monitor or measure the early strength gain of concrete in the field. For instance, compressive test for field curried cylinders, concrete maturity, field-cured cylinder attached to forms, penetration resistance, and pullout test.
Each of these methods or combination thereof produce compressive strength data required in the field to control the timing of shoring/reshoring and form removal.
1. Field-Cured Cylinders
In this method,
cylinders are casted in the field based on ASTM C31/C31 M standard, and cured
under the same conditions (temperature and moisture) as the concrete they
represent. These cylinders are tested in lab according to ASTM C39/C39M
standard at the specified age to evaluate concrete compressive strength.
2. Field-Cured Cylinders Attached to Forms
Another method
of early concrete strength measurement is the use of special cylinder molds
which is attached to the forms in the field. This test is carried out in
accordance with Standard Test Method for Compressive Strength of Concrete
Cylinders Cast in Place in Cylindrical Molds (ASTM C873/C873M) which is limited
to use in slabs where the depth of concrete is from 125 to 300 mm.
The height of
the mold is the same as slab thickness. Specimens are casted and cured as the
same conditions as concrete slab. The samples are taken out of their molds at
designated time and then tested to estimate concrete compressive strength. This
technique determine slab bearing capacity and hence time for removal of strips
and shores.
3. Concrete Maturity
The results of
concrete maturity test can be correlated to evaluate concrete compressive
strength. This correlation should be established initially for the specific
concrete mixture planned for use on the project by testing compressive strength
cylinders at various stages of maturity.
So, if maturity
is known and a correlation curve for the project-specific mixture is
established, then strength of the concrete can be determined. The maturity test
is conducted based on the specification of ASTM C 1074.
4. Pullout Tests
The results of
pullout tests can be used to evaluate concrete compressive strength in the
field. But the correlation between compressive strength and the
pullout values
for the specific concrete mixture planned for use on the project by testing cylinders
and pullout specimens at various ages need to be developed at first. Pullout
test can be conducted according to ASTM C 900.
5. Penetration Resistance
Similar to concrete maturity and pullout test, penetration resistance test of hardened concrete can be used to estimate concrete compressive strength in the field. In order to be able to use this technique, development of the correlation between compressive strength of laboratory-tested specimens and the penetration values at various ages for all concrete mixtures planned for use on the project is a must.
Then, penetration resistance results can be converted to concrete strength easily. Penetration resistance measured in accordance with ASTM C803/C803M.