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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.