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Tests


Materials


Structures

Internal Fracture Test

Introduction


It is sometimes necessary in the structural appraisal of existing construction to make tests on concrete in order to estimate its compressive strength.

This is normally achieved by taking a cored sample and crushing it in a compressive testing machine. However, on some members it would not be possible to safely extract a core of sufficient size to give an accurate assessment of the compressive strength. When this is the case, it is necessary to use a different technique to obtain the compressive strength. One such technique, devised by the Building Research Establishment is called the Internal Fracture Test.

Internal Fracture Test Procedure


The procedure for the internal fracture test is as follows:

A hole, 30-35mm deep, is drilled in the concrete with a rotohammer drill fitted with 6mm diameter hard tipped taper shank bit. A standard 6mm diameter wedge anchor bolt is tapped into the hole to a depth of 20mm.

The thrust pad and stool ring assembly is placed over the anchor bolt and secured finger-tight with the washer and nut, settling the thrust pad on its spherical seating. A dial reading wrench (torque meter) is applied to the nut on the anchor bolt and smoothly without jerking by half a turn at a time, approximately 10 seconds to complete each half turn.

At each turn the torque is recorded and the procedure is continued until the meter readings reach a maximum and begin to fall or a maximum reading of 10 Nm is reached.

When making a test the following details of procedure are to be followed:

The hole must be between 30-35mm deep and the dust particles removed.

The bolt should be a push fit in the drilled hole which should be checked by inserting a used bolt stem by hand.

The depth marker, e.g. tape on the bolt, must be positioned 20mm above the lower edge of the wedge sleeve. Alternatively a purpose made placing tool may be used to determine the correct depth.

The threads and the nut should be checked to ensure that the nut runs freely and then lightly greases. A new nut must be used each time.

There is a possibility that the bolt will shear off or the torque will be very high if, during testing, the nut reaches the end of the threads and bears on the shaft of the bolt. This can be avoided by using washers under the nut so that only 3mm of the bolt projects above the washers before the nut is placed on the bolt.

If more than six revolutions of the nut are made after the initial tightening, the test should be discontinued and repeated.

The equipment is designed so that it can be aligned with the hole in the concrete. As the nut is screwed into contact the thrust pad should be aligned by gently sliding the top plate appropriately in the stool ring.

Location of Tests


In choosing the locations of points for test on a concrete component, the following considerations arise:

There may be considerable variation in the mean strength of the concrete depending on location; e.g. weaker concrete may be present at the tops of columns. Where the concrete is thought to be of uniform quality throughout, six convenient locations, along the length of the component may be appropriate. If the presence of weak zones of concrete is suspected, then it may be most useful to make six tests in these areas.

The strength of the concrete in the most highly stressed zones will often be of the greatest interest. Locating tests in these critical zones may be desirable where significant variation in concrete strength within the components is suspected internal fracture tests be located in regions where compressive stresses are low. Only in exceptional circumstances should tests be made in highly stressed zones; in these cases the effect of precompression should be considered.

Locations of testing points should be planned bearing in mind the concrete surface may be visibly damaged during the test, and that, internally, fracture of the concrete will be induced. Holes should not be drilled at any point on the surface close to an arris or other discontinuity. Testing points should be more than 100mm apart and holes should not be made near to reinforcement. A cover meter should be used to determine the pattern of reinforcement before testing. For preference, testing should be located on surfaces that are not visible in normal circumstances. Since the concrete is fractured close to the surface by the test it is probable that the surface will be more susceptible to local frost damage on completion of testing (for external members). If other considerations allow, it would be preferable therefore, to locate tests in regions where the concrete is least likely to be subjected to frost.

Number of Tests


Normally a set of four tests should be carried out at a single location.If the range of results exceeds 20% of the lowest value then further tests should be carried out until:

a) A group of four results are obtained within 20% of the lowest of the group or

b) Eight tests have been carried out at the same location.If more than six full turns are made without significant increase in load, a further test should be carried out.

Interpretation


The type of failure should be noted under every test. N/F indicating normal failure. The other most likely modes of failure are slippage of the bolt in the hole or fracture of the bolt, which should be noted clearly on the record sheet.

References


BRE Information paper IP22/80.

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