Concrete and Masonry NDT
Many factors are considered in developing a non-destructive testing program for concrete and masonry. Some of the factors include the thickness of the test material, access to one or both sides, and objective of the testing program. Our NDT professionals understand the strengths and weaknesses of various NDT methods, and typically develop a program of multiple NDT methods to answer our client’s questions. It is common to perform limited destructive testing to calibrate and confirm the interpretation of non-destructive test data.
Some applications of NDT to existing construction include:
- Determine concrete slab-on-grade thickness.
- Determine the size and spacing of concrete reinforcement.
- Determine the continuity of CMU wall grout fill.
- Determine if concrete honeycombing is present.
- Determine if a slab is in contact with the subgrade.
Some information on basic NDT techniques:
Ground Penetrating Radar (GPR)
GPR sends a signal and measures the reflected signal. It is used to identify steel reinforcement and concrete cover. It can be used to determine relative concrete slab thickness. If a known slab thickness is used to correlate the signal strength, the slab thickness may be approximated.
Infrared Thermal Imaging (IR)
IR measures minor differences in surface temperature. It is used to locate grouted cells within a concrete masonry wall which change temperature at a different rate than non-grouted cells.
Ultrasonic Pulse Velocity (UVP)
UPV is used to estimate the depth of surface cracks, estimate concrete quality over large thicknesses (greater than 12 inches), estimate the extent of honeycombing, and compare the wave speed of cylinders versus field concrete for estimation of compressive strength.
Radiography is used to detect, evaluate, and readily distinguish between embedded targets such as rebar and conduit with a high degree of accuracy. Access to both sides are required so the radiographic point source and film may be placed on the opposite sides. Because radiation is from a point source, the actual size/depth of the target must be back calculated.
Ultrasonic Tomography combines the benefits of one-side access of Impact Echo with the more sensitive ultrasonic shear waves and adds a phased array of 48 transducers. The system will create 2-D and 3-D images of concrete using sound waves. MIRA is used to locate honeycombing, delamination, and voids within concrete.
Slab Impulse Response (SIR)
SIR is used to locate voids beneath concrete slabs and quickly screen concrete structures for internal defects. SIR measures the mobility of the structure using an instrumented hammer and a velocity transducer.
Supplemental Test Methods
Non-destructive methods do not provide definitive strength values which are frequently necessary for design. The following test methods may be used to determine the inplace strength of existing concrete or the bond strength between an overlay and substrate.
Cut and Pull Out (CAPO)
A hole is drilled in the concrete surface and under reamed. A test device is inserted into the hole and the force required to break the concrete for its removal measured. The measured force is correlated to concrete tensile strength which is correlated to concrete compressive strength. CAPO is used to estimate the in-place compressive strength of concrete.
A steel dolly is adhered to the test substrate with epoxy glue. The circumference of the dolly is cut below the bond depth. The dolly is pulled and the force at failure used to calculate the bond strength. Bond testing is used to determine the in-place direct tensile strength of concrete and bond strength of epoxy or asphalt overlays.
Shear Strength by Torque Method
Torque system allows the estimation of the shear strength of asphalt and concrete overlays.