• Radiographic Tomography - The absorption and scatter of radiation 

 is used to produce a visual image of the concrete cross section at 

 the point of inspection, indicating the thickness and density. 



• Surface Hardness - The compressive strength of the concrete is 

 correlated with the size of an indentation produced by a mass 

 impacting the surface. 



• Penetration Techniques - The compressive strength of concrete is 

 correlated with the depth of penetration of a hardened probe that 

 is explosively fired into the concrete surface. 



• Pullout Testing Techniques - The compressive strength of concrete 

 is correlated with the force required to pullout an anchor rod 

 embedded in the surface of the concrete. (This is a destructive 

 test and not desirable for underwater inspections because of the 

 probability of exposing rebar.) 



• Coring - This is the standard technique for determining the quality 

 and strength of concrete. Underwater coring equipment has been 

 developed and is available. Generally, coring should only be 

 considered when other inspection techniques indicate that a serious 

 problem exists. 



The first four of these techniques were identified as offering the 

 greatest potential to improve the Navy's ability to inspect concrete 

 structures underwater (Ref 4) . This report presents the results of 

 laboratory and field evaluations of selected equipment that use the 

 first three techniques: 



Technique Commercial Name 



Magnetic Rebar Location R-Meter 



Rebound Method Schmidt Hammer 



Ultrasonic Testing V-Meter 



The fourth technique, radiographic tomography, is not included in 

 the report. However, a feasibility analysis and conceptual design of a 

 tomography system to inspect concrete and timber have been completed and 

 are described in References 5, 6, and 7. 



CONCRETE DETERIORATION AND INSPECTION REQUIREMENTS 



The most common damage resulting in the premature deterioration of 

 concrete structures in or near seawater is cracking and loss of material 

 or cross section. Softening of the concrete due to chemical action is 

 another form of damage less common than cracking. The damage to concrete 

 is generally most severe in the splash and tidal zones. 



Damage from corrosion of the steel reinforcement occurs when the 

 corrosion products cause the volume of the structural element to expand, 

 resulting in tensile stresses and cracking. As corrosion progresses, 

 the corrosion products continue to expand, causing more cracking. 



