Penetration resistance test
This is outlined in ASTM C803, Standard Test Method for Penetration Resistance of Hardened Concrete. To complete a penetration resistance test, a device drives a small pin or probe into the surface of the concrete. The force used to penetrate the surface, and the depth of the hole, is correlated to the strength of the in-place concrete.
Pros
It is relatively easy to use and can be performed directly onsite.
Cons
Data is significantly affected by surface conditions, as well as the type of form and the aggregates used. It requires pre-calibration using multiple concrete samples for accurate strength measurements.
Ultrasonic pulse velocity
This is covered in ASTM C597, Standard Test Method for Pulse Velocity Through Concrete. This technique determines the velocity of a pulse of vibrational energy through a slab. The ease at which this energy makes its way through the slab provides measurements regarding the concrete’s elasticity, resistance to deformation or stress, and density. This data is then correlated to the slab’s strength.
Pros
This is a non-destructive testing technique which can also be used to detect flaws within the concrete such as cracks and honeycombing.
Cons
This technique is very highly influenced by the presence of reinforcements, aggregates, and the moisture in the concrete element. It also requires calibration with multiple samples for accurate testing.
Pullout test
The main principle behind this test, as per ASTM C900, Standard Test Method for Pullout Strength of Hardened Concrete, is to pull the concrete using a metal rod that is cast-in-place or post-installed in the concrete. The pulled conical shape, in combination with the force required to pull the concrete, is correlated to compressive strength.
Pros
It is easy to use and can be performed on both new and old structures.
Cons
This test involves crushing or damaging the concrete. A large number of test samples are needed at different locations of the slab for accurate results.
Cast-in-place cylinders
This is covered in ASTM C873, Standard Test Method for Compressive Strength of Concrete Cylinders Cast in Place in Cylindrical Molds. Cylinder moulds are placed in the location of the pour. Fresh concrete is poured into these moulds which remain in the slab. Once hardened, these specimens are removed and compressed for strength.
Pros
It is considered more accurate than field-cured specimens because the concrete is subject to the same curing conditions of the in-place slab.
Cons
This is a destructive technique that requires damaging the structural integrity of the slab. The locations of the holes need to be repaired afterward. A lab must be used to obtain strength data.
Drilled core
This is outlined in ASTM C42, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. A core drill is used to extract hardened concrete from the slab. These samples are then compressed in a machine to monitor the strength of the in-situ concrete.
Pros
These samples are considered more accurate than field-cured specimens because the concrete that is tested for strength has been subjected to the actual thermal history and curing conditions of the in-place slab.
Cons
This is also a destructive technique that requires damaging the structural integrity of the slab. The results can be influenced by the direction of the extraction (e.g. columns or slabs). The locations of the cores need to be repaired afterward. A lab must be used to obtain strength data.
Tests like the rebound hammer and penetration resistance technique, while easy to perform, are considered less accurate than other testing methods, as they do not examine the centre of the concrete element, only the curing conditions directly below the surface of the slab. This is especially important during cold weather as temperature differentials can affect the durability of the structure. Practices such as the ultrasonic pulse velocity method and the pullout test are more difficult to perform as their calibration process is lengthy, requiring a large number of sample specimens to obtain accurate data.
