Rebound Number of Hardened Concrete
Scope:
This test is also known as the Schmidt hammer or impact hammer, and is a non-destructive method of testing concrete. The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges.
The calibration of the Schmidt hammer
Apparatus:
1- Rebound hammer.
2- Abrasive stone: consisting of medium-grain texture silicon carbide or equivalent material.
Selection of Test Surface:
Concrete members to be tested shall be at least 100 mm thick and fixed within a structure. Smaller specimens must be rigidly supported. Areas exhibiting honeycombing, scaling, rough texture, or high porosity should be avoided.Concretes should be approximately the same age and moisture condition in order to be compared. Dry concretes give higher rebound numbers than wet concrete, and the surface layer of concrete may be carbonated, yielding higher rebound numbers.
Preparation of test surface:
A test area shall be at least 150 mm in diameter. Heavily textured, soft, or surfaces with loose mortar shall be ground smooth with the abrasive stone. Smoothformed or toweled surface shall be tested without grinding. Concretes over 6 months old may require grinding to a depth of 5 mm if they are to be compared to younger concretes. Grinding to this depth is not feasible without power equipment.
Testing concrete using the Schmidt hammer.
Rebound Number of Hardened Concrete Procedure
1- Firmly hold the instrument in a position that allows the plunger to strike perpendicularly to the surface tested. Gradually increase the pressure on the plunger until the hammer impacts.
2- After impact, record the rebound number to two significant figures.
3- Take ten readings from each test area. No two impact tests shall be closer together than 25 mm.
Calculation:
Discard readings differing from the average of 10 readings by more than 5 units and determine the average of the remaining readings. If more than 2 readings differ from the average by 7 units, discard the entire set of readings.
