Pullout Test

Pullout testing of concrete
Cast-in-place device enables concrete to be evaluated in the structure with only minor damage
BY JOHN A. BICKLEY TROW LTD., CONSULTING ENGINEERS

T

esting of field-cured cylinders to determine whether concrete strength is satisfactory for either removal of forms or application of post-tensioning has been required by many codes and specifications. Howe ve r, itis difficult to be sure that field-cured cylinders have received the same protection or curing conditions that were provided for parts of the structure they are supposed to represent. Therefore the interest in in-place test methods, including pullout tests, has been growing. A pullout test measures the force required to pull a specially shaped steel rod or disc out of the hardened concrete intowhich it has been cast. Because of its shape, the steel rod is pulled out with a cone of concrete whose surface slope is approximately 45 degrees to the vertical. A hollow tension ram bearing on the concrete surface exerts the necessary pull on the steel rod, with power supplied by a hand-operated hydraulic pump. The force required for pullout is then related to the com-

Figure 2. Load ishydraulically applied by turning the handle of the testing machine. Note pulled out cone of concrete and cavities from which cones have been pulled.

Figure 1. Compact, lightweight pullout testing equipment is small enough to be carried in a briefcase and weighs about 10 pounds.

pressive strength of the concrete. The principal parts— ram and pump—are commercially available, and peripheral parts suchas the rods, washers and sleeves can be manufactured locally, or all of the components may be purchased as one proprietary system. Pullout testing is not a recent development. As early as 1938, the American Concrete Institute published a report of in-place concrete testing in the Soviet Union, pullout tests were prominent among them. It was not until the 1970s howe ve r, following work by Richardsand Malhotra, that this approach to testing began to be regarded as a practical site method.

deck forms. After the concrete has hardened, the bolt holding the stem to the formwork is removed as is the plug in the formwork. A special wrench is then used to remove the stem from the pullout disc. At the time of the test a pull-bolt is threaded into the disc from the surface and attached by acoupling to the testing machine. The counterpressure ring of the testing machine is placed against the surface of the concrete and load is hydraulically applied by turning the handle of the testing machine. The machine incorporates a special valve so that within a wide range of speeds of turning the handle the load is applied at a uniform and constant rate. Figure 3. Removable shaft of the pulloutdevice may be attached through a circular hardboard plate nailed to the form (left); by means of a screw which passes through the form (center); or with a flotation cup (right) which rests on the unformed concrete surface.

Test procedure
Load can be applied up to a required proof load and then released, in which case there is no failure of the concrete around the insert. Alternatively, load canbe applied until failure just occurs, in which case little damage occurs to the surface of the concrete, and the cone of concrete fractured by the test does not come out of the mass of concrete. If this procedure is followed, all that shows on the surface of the concrete is a slightly raised ring, the size of the inside of the counterpressure ring. Finally, if required, loading can be continuedpast failure until the cone of concrete and the pullout insert are removed from the concrete. In this case, the small hole made by this procedure may subsequently have to be repaired. Each test takes approximately 2 minutes and it is recommended that about 10 inserts be used for each 100 cubic yards of concrete placed. Where very large pours are involved, the number of inserts per unit volume may...