MAGNETIC REBAR LOCATION - R-METER 

 General Description and Operation 



Reinforcing bar location devices detect the disturbance in a magnetic 

 flux field caused by the presence of magnetic material. The magnitude 

 of this disturbance is used to determine the location and orientation of 

 steel reinforcing bars in concrete and to measure the depth of concrete 

 cover over the rebar. These instruments are commercially available for 

 measuring the depth of concrete cover in dry environments. 



Rebar locator devices typically consist of a U-shaped magnetic core 

 upon which two coils are mounted. A magnetic field is produced by apply- 

 ing an alternating current to one coil and measuring the current induced 

 in the other coil. The magnitude of the induced current is affected by 

 both the diameter of the rebar and its distance from the coils. There- 

 fore, if either of these parameters is known, the other can be determined. 

 By scanning with the probe until a peak reading is obtained, the location 

 of the rebar can also be determined. A maximum deflection of the meter 

 needle will occur when the axes of the probe poles are parallel to and 

 directly over the axis of a reinforcing bar, thus indicating orientation. 



An R-meter rebar locator (Model C-4956) was purchased and modified 

 for underwater use and is shown in Figure 1. This instrument is powered 

 by a rechargeable 12-volt, 4.5-amp-hour storage battery and will operate 

 for about 10 hours between charges. To fully recharge the battery from 

 a completely discharged state requires about 16 hours. A detailed opera- 

 tions manual is supplied with the unit (Ref 8). 



The R-Meter is calibrated for rebar that varies from No. 3 to No. 16 

 in size. (Appendix A defines the nominal dimensions of reinforcing steel.) 

 The R-Meter can be used to measure the depth of concrete cover over rebar 

 in the range of 1/4 to 8 inches, or conversely, it can measure the diam- 

 eter of the rebar. The actual value of concrete cover measured corre- 

 sponds to the distance between the tips of the probe and the top of the 

 reinforcing bar as illustrated in Figure 2. The best accuracy (±10%) is 

 obtained for concrete cover less than 4 inches thick. 



To obtain maximum accuracy for concrete cover measurements when 

 using the R-Meter, the meter zero must be set accurately and rechecked 

 frequently. The meter zero will drift with the battery charge level and 

 temperature variations. Figure 3 shows the effect of zero setting error 

 on the measurement of concrete cover (Ref 8) . Curve A represents the 

 condition of thick concrete cover and smaller diameter rebar. For this 

 situation, small zero offsets introduce significant errors in the 

 measurement. Curve D represents the condition of very thin concrete 

 cover and larger diameter rebar. For this situation, zero offset does 

 not introduce any significant error in the measurement. Curves B and C 

 represent other measurement conditions and indicate that this effect is 

 more pronounced for increased concrete cover and smaller diameter rebar. 



The primary limitation that effects the operation of the R-Meter is 

 the presence of other metallic objects in the vicinity of the rebar where 

 the measurement is being made. For example, in heavily reinforced struc- 

 tures, the effect of nearby rebar cannot be eliminated and accurate depth 

 readings are difficult or impossible. The effects of parallel 1-inch 

 diameter rebar, located 2 inches below the surface of the concrete, is 



