A Schmidt hammer, also known as a Swiss hammer or a rebound hammer, is a device to measure the elastic properties or strength of concrete or rock, mainly surface hardness and penetration resistance.
The calibration of the Schmidt hammer
1- Rebound hammer.
2- Abrasive stone: consisting of medium-grain texture silicon carbide or equivalent material.
Selection of Test Surface:
Concrete members for testing should be as a minimum 100 mm thick and fixed inside a structure. Smaller specimens need tobe rigidly supported. areas showing honeycombing, scaling, rough texture, or excessive porosity must be avoided.Concretes have to be about the identical age and moisture condition with a purpose to be compared. Dry concretes provide betterrebound numbers than wet concrete, and the surface layer of concrete may be carbonated, yielding higher rebound numbers.
Preparation of test surface:
A test surface shall be at least 150 mm in diameter. Heavily textured, tender, or surfaces with loose mortar will be ground smooth with the abrasive stone. Smoothformed or toweled surface will be examined without grinding. Concretes over 6 months old may additionally require grinding to a depth of 5 mm if they’re to be compared to younger concretes. Grinding to this depth isn’t always feasible with out power equipment.
Testing concrete using the Schmidt hammer.
1- Firmly hold the device in a position that permits the plunger to strike perpendicularly to the surface tested. step by stepincrease the pressure on the plunger till the hammer strikes.
2- After impact, record the rebound number to two significant figures.
3– Take ten readings from every test area. No impact tests shall be closer together than 25 mm.
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.