Instrucciones

1.0 INTRODUCTION
The James HR-Rebar Locator is a rugged, precision instrument for inspection of reinforcing bars in concrete. The Rebar Locator generates a magnetic field and measures any change due to the presence of magnetic material. The magnitude of this change is indicated on the instrument meter, which has been calibrated to read directly in bar size and distance from the base of the instrument.

The use of the instrument is not restricted to reinforcing bars; it can be used for detection of any magnetic material.

Typical applications include:
Location of tendons, tendon splices, and broken ends, etc. in prestressed concrete.

Location of pipes conduit, flues, ducts, and manhole covers.

Since this unit measures something that can not be generally seen or otherwise conveniently measured, it is strongly recommended that practice in the use of the HRRebar locator be done on simulated or known reinforced structures before using the instrument on site. The location and measurement of individual, reasonable spaced rebars is straightforward. However, when two or more parallel rebars lie within the field of the instrument, interpretation of the readings becomes more difficult. Depending on the geometry of the rebar system in extreme cases it may be possible only to locate the center lines of the rebars. Then, an estimate of depth or size can be made. This type of structure is often encountered in columns and beams of relatively small cross-section with heavy reinforcements. Practice on a simulated structure of this sort is very helpful.

2.0 TURN ON PROCEDURE
1. Remove the instrument from its carrying case.
2. Turn on the instrument by switching the on-off switch; allow about one minute for the battery output to stabilize.
3. Check battery voltage by depressing the battery check push button. The meter should read at or above the battery OK index on the meter. If it does not, refer to Section 3.0 Battery Charging.
4. Hold the instrument in a position so that it is at least 18” to 24” (45 to 60 cm) away from the magnetic material. If meter does not read zero, turn the zero adjustment control. If the meter is set even slightly below zero, substantial errors in readings and an apparent insensitivity in meter indication will be introduced.
5. When zeroing the instrument, hold it always at the same attitude (i.e. angle to the earths surface) that it will be used in practice.

3.0 BATTERY CHARGING
The rechargeable battery system in the HR-Rebar Locator should not be totally discharged. “Battery check OK” mark on the linear scale is the minimum voltage that the system should be allowed to reach. Turn off the HR Rebar Locator when not in use.

The battery system has three months of shelf life at 20 degrees C. Store in a cool dry place; make sure to fully charge again before use. Batteries that have been over-charged or totally discharged and damaged are not covered by the manufacturer’s warranty. The Rebar Locator is powered by a twelve volt, Nicad rechargeable battery. It will provide approximately 8 hours of operation between charges. The Rebar Locator incorporates an external charger unit. This charging system will provide a full charge from a full discharge in 16 hours (overnight). To charge, plug in the cord with phone plug on the charger to the phone jack on the Rebar Locator first, then plug in the charger to the appropriate AC power source. Model C-4990 charger is for 117 volt, 50/60 Hz power source.

During charging, the charger becomes warm. This is normal and is not a cause for concern.

Overnight charging from full discharge is required to recover to full capacity.

Overcharge should be avoided to get the maximum life of the battery and for safety reasons.

4.0 LOCATING THE POSITION OF REINFORCING BARS
The location of primary and secondary reinforcing bars is accomplished by moving the instrument along the surface of the concrete. The meter needle will indicate a maximum deflection when the axis of the instrument is parallel to and directly over the axis of a reinforcing bar or a group of bars.

Curves I, II, and III in Figure 2 indicate the readings obtained on the meter with various distances between reinforcing bars when the instrument is parallel to the bars. This results in maximum meter deflection. As the number of bars is increased and the distance between them is decreased, the variation in meter readings decrease. By observing the needle movement, the operator may determine whether there is a single bar or several in the location.

5.0 MEASUREMENT OF CONCRETE COVER OVER
REINFORCEMENT BARS
To perform cover measurements with maximum accuracy, meter zero must be set accurately and checked frequently.
1. Place the instrument on the concrete at the chosen location. Align the instrument so the axis of it is parallel to and directly over the reinforcing bar. This is indicated by the maximum deflection of the meter needle.
2. Read the cover directly from the meter needle using the scale corresponding to the reinforcement bar diameter.

This value corresponds to the distance between the bottom surfaces of the instrument and the top of the reinforcing bar which is closest to the surface (see Fig. 3). If the exact diameter of the bar is unknown, a value may be assumed, and the error will be small unless bar size is grossly in error.

For given depth, the meter deflection will increase with increasing bar cross sectional area and with decreasing steel strength. Variations in reading because of steel temper or strength are of the order of 5 percent for #5 and larger bars. The meter scale calibration for the Rebar Locator is based on a “Best Fit” average for different bar strengths.

When examining a structure, the needle may reach full scale (100 on the liner scale), or go above full scale. This indicates that the concrete cover over the reinforcing bar is too thin to be read directly on the meter scale, or one pole is touching the tie rod. When this happens, a measurement can be made by interposing a non-magnetic shim of known thickness between the concrete surface and the instrument. The thickness of the shim is subtracted form the value read on the meter.

Before attempting to measure the amount of concrete covering reinforcing steel, it is advisable to determine whether the concrete itself contains any magnetic material. To do this, the instrument is turned on and the battery is allowed to stabilize. The instrument is placed on the concrete, at least ten inches away from any reinforcing bars or other magnetic object. Note whether or not there is a deflection of the meter pointer. If there is deflection, the presence of magnetic materials in the concrete is indicated. Paragraph 8.0 must be read before proceeding with measurements.

5.1 THE EFFECT OF CLOSE PARALLEL BARS
Curves I, II, and III in Figure 2 illustrate the effects of bars parallel to each other and some distance below the surface of the concrete. It is apparent that moving the instrument parallel to the reinforcing bar axis will cause the meter needle to fall as the instrument passes over the space between two bars, the amount depending upon their combined influence on the instrument.

In theory, the combined influence of two bars may be neglected only in the case where their axes are separated by at least three times the thickness of the concrete cover. If the meter needle drops to 1 or less, on the linear scale when the instrument is between the two bars, their effect can be neglected.

When two bars lay one above the other in a plane perpendicular to the concrete surface with their axes parallel to the surface, the meter indication will be substantially higher as indicated in Curve I, of Fig. 2. It is not possible to make a direct determination of the effect of the more distant bar. This phenomena helps to find the end of the splice. Whenever reinforcing bars are close together, the depth of cover indicated by the meter is slightly less than that indicated for a single bar (see Curves II and III, Fig. 2)

Before making any concrete cover measurement, it is necessary to locate carefully the position of each rebar in the area under test. For determining the correction factors, use the following procedure:
1. Locate the exact position of each rebar in the area under test.
2. Consider each bar parallel to the concrete surface and spaced more than four inches from the outside of te nearest adjacent bar, to be isolated and read the cover depth from the scale corresponding to the bar size.
3. When two bars are closely adjacent to one another in a plane parallel to the concrete surface (Fig. 4), read the depth of cover from the scale corresponding to the sum of the diameters of the two bars.
4. When two bars are closely adjacent to one another in a plane perpendicular to the concrete surface (Fig. 5), read the depth of cover from the scale corresponding to three fourths of the sum of their diameters.

This procedure gives good estimates. If very precise measurements are required, the comparison methods described in paragraph 6.0 should be used.

5.2 THE EFFECT OF PERPENDICULAR ADJACENT BARS
Always locate all of the elements of the reinforcing bar system before starting measurement of concrete cover. When the instrument is placed above a bar, the presence of bars perpendicular to the axis of the instrument have less effect on the measurement of the cover of the concrete than that of parallel bars. The measured values will contain a small error. It is sufficient to compensate by reading the depth of concrete cover to be slightly more than the meter indicates.

The accuracy of the measurements can be improved as follows:
If the perpendicular bars are underneath the bar under test, the effect is practically negligible. There are no effects if the instrument is not directly over the axis of the perpendicular bars.

If the perpendicular bars are above the bar under test, three cases must be considered: (a) Bars spaced 14” or more between axes: Carry out the measurement without any correction by placing the instrument equidistant between the perpendicular bars (fig. 6, position A).
(b) Bars spaced 7” to 14” between axes: Carry out the measurement without any correction by placing the instrument straddling one of the perpendicular bars (Fig. 6, position B).
(c) Bars spaced less than 7” between axes: Calculate the cover over the lower bar by adding the diameter of the reinforcing bar above it.
(d) Hooks and hoops: When operating near hooks, or other secondary elements which are smaller in diameter than the main reinforcing bars, their effect is negligible if they are under the bars. If they are placed above the main bars, eliminate their effect by placing the instrument above the main elements and as far as possible from the secondary elements.

Finally, if the spacing is such that it is impossible to avoid placing the instrument close to secondary elements, use the comparison method in paragraph 6.0.

6.0 COMPARISON METHOD
This method consists of making an exact, full-scale model of the reinforcing structure. The instrument is first placed upon the work at the desired spot and the maximum deflection of the needle is noted on the linear scale. A second measurement is then made on the model in the same manner, using shims until the meter reading is identical to the first reading. When that position is found, the thickness of the concrete over the reinforcing bar is equal to the thickness of the shims, provided that zero adjustment has not been changed. Zero adjustment must be checked before and after each measurement. This measurement does not interfere with the calibration of the instrument. It should be used whenever measurements are being made on bars for which the instrument has not been calibrated.

When other materials are to be measured for which the instrument has not been calibrated, a similar procedure may be used. For example, to measure the thickness of a refractory furnace lining, it is necessary to make a calibration chart. This can be done using a piece of steel plate or sheet of the sort used for the exterior of the furnace, about 18” square. Making sure that there is no other magnetic material near the instrument, space the instrument ½ inch from the center of the plate and note the meter reading on the linear scale. Increase the spacing in ½ inch increments, noting the reading for each increment.

For meter readings that are below 50 on the linear scale, zero should be checked and reset if necessary for each new increment. The resulting chart can be used to determine lining thickness form meter reading on a furnace. This procedure can be used, for example, for iron pipe, using an 18” length of pipe , etc.

7.0 SIMPLIFICATIONS
The shape of some concrete structures allows measurements to be made more easily than previously indicated. Consider the structure of figure 7.

Simply by locating the bar axis upon the two faces of the concrete, the exact position of the bar is determined. Instrument adjustment in this case is not a factor.

8.0 CORRECTION FOR MAGNETIC CONCRETE
When a concrete contains magnetic materials such as pozzolans, fly ash, etc., the meter needle will deflect when the instrument is placed upon the concrete, even though no reinforcing steel is used. Metallic debris on the concrete surface has the same effect and should be removed. This will obviously affect the measurement of the concrete cover and the following corrections should be applied:
After zero adjusting the instrument, place it on the concrete at a place where there is no reinforcing bar (reinforcement is more than 10” away) and note the position of the needle on the 100 division linear scale, N1; carry out the measurement of concrete cover as upon an ordinary non-magnetic concrete, and note the new position of the needle, N2.

The correct position, N, of the meter needle will be: N = N2 – N1. If the bar diameter is known, then the concrete cover can be determined from the value N.

This method will compensate for magnetic concrete as long as the deflection due to the magnetic concrete is not too great. The value N1 may be reduced by interposing one or more shims between the instrument and the concrete. After making measurements as described above, deduct the thickness of the shims from the thickness of the concrete cover which has been measured.

9.0 ESTIMATION OF UNKNOWN BAR DIAMETER
When neither the bar diameter, nor the depth of the bar is known, one may use figure 8, to estimate this information. This method requires two successive measurements, one of which is made at the surface of the concrete. The second measurement is then made at a distance away from the concrete surface, by interposing shims of known thickness. The readings taken are from 0 – 100 linear scale on the meter. This method is illustrated in the examples below.
EXAMPLE I
The meter reads 22 on the linear scale when the instrument is against the concrete. A shim of .75 is introduced between the instrument and the structure, and the meter now reads, 11. Determine the bar size as follows:

9.0 ALTERNATE METHOD
After locating the exact position of the rebar, drill a small diameter hole to the rebar surface; measure the cover and from the instruments meter scale, read the bar diameter.

10.0 APPLICATIONS
The instrument can be additionally used for wire mesh and pre-stressed cables. The same basic procedure described in this manual applies to these elements.

11.0 COMPARISON OF REINFORCEMNT BAR SIZES IN METRIC AND
US/ENGLISH SYSTEM
The table below provides a comparison of the common U.S. rebar sizes (English system) and rebar sizes (metric) as used in other countries. The following table compares “inches of cover” with “millimeters of cover”. inches mm inches mm
0.5 12.7 3.5 88.9
1.0 25.4 4.0 101.6
1.5 38.1 4.5 114.3
2.0 50.8 5.0 127.0
2.5 63.5 5.5 139.7
3.0 76.2 6.0 152.4

12.0 CARE OF THE INSTRUMENT
1. Keep the instrument clean.
2. Avoid shock damage.
3. Avoid getting the instrument wet.
4. Keep the battery charged.
5. If the instrument malfunctions, return it to James Instruments where it will be repaired promptly – normally within seven to twelve working days. Note: please check the battery first.

WARRANTY STATEMENT
The HR-Rebar Locator, except the battery, is guaranteed for a period of one year from the date of purchase against faulty workmanship and/or material.