7 FACTORS INFLUENCING REBOUND HAMMER TEST OF CONCRETE

REBOUND HAMMER TEST 

The Schmidt rebound hammer is principally a surface hardness tester with the little apparent theoretical relationship between the strength of concrete and therefore the rebound number of the hammer. 

However, within limits, empirical correlations are established between strength properties and therefore the rebound number. Further, a scientist has attempted to determine a correlation between the hammer rebound number and therefore the hardness as measured by the Brinell method.

 The rebound hammer method might be used for: 

1. Assessing the likely compressive strength of concrete with the assistance of suitable correlations between rebound index and compressive strength
2.  Assessing the uniformity of concrete,
3.  Assessing the standard of the concrete in reference to standard requirements, and
4. Assessing the standard of 1 element of concrete in reference to another.

                                                            Rebound hammer test

FACTORS INFLUENCING REBOUND HAMMER TEST

 

The rebound numbers are influenced by a variety of things like sorts of cement and aggregate, surface condition and moisture content, age of concrete and extent of carbonation of concrete. 

1. sort of CEMENT

 Influence of sort of Cement Concretes made with high alumina cement can give strengths one hundred pc above that with ordinary hydraulic cement . Concretes made with super sulfated cement can give 50 percent lower strength than that with ordinary hydraulic cement. 

2. sort of AGGREGATE 

The influence of a sort of Aggregate differing types of aggregate utilized in concrete give different correlations between compressive strength and rebound numbers. Normal aggregates like gravels and gravel aggregates give similar correlations, but concrete made with lightweight aggregates require special calibration.

 3. SURFACE CONDITION & MOISTURE CONTENT 

Influence of Surface Condition and Moisture Content of Concrete The rebound hammer method is suitable just for close texture concrete. Open texture concrete typical of masonry blocks, honeycombed concrete or no-fines concrete are unsuitable for this test. All correlations assume full compaction because the strength of partially compacted concrete bears no unique relationship to the rebound numbers. Trowelled and floated surfaces are harder than molded surfaces and have a tendency to overestimate the strength of concrete. A wet surface will produce to underestimation of the strength of concrete calibrated under dry conditions. In structural concrete, this will be about 20 percent less than in the same dry concrete.

 4. CURING AND AGE OF CONCRETE

 Influence of Curing and Age of Concrete the connection between hardness and strength varies as a function of your time. Variations in the initial rate of hardening, subsequent curing and conditions of exposure also influence the connection. The effect aged are often ignored for concrete above 14 days old.

 5. CARBONATION OF CONCRETE 

Influence of Carbonation of Concrete Surface The influence of carbonation of concrete surface on the rebound number is extremely significant. Carbonated concrete gives an overestimate of strength which in extreme cases is often up to 50 percent. The carbonation depth shall be checked in cases where the age of concrete is quite six months. it's possible to determine correction factors by removing the carbonated layer and testing the concrete with the rebound hammer on the uncarbonated concrete.

 6. TEST POSITION 

Influence of vertical distance from rock bottom of concrete placement. The influence of vertical distance from the rock bottom of concrete placement on the rebound number is extremely significant. Generally, a better rebound number is observed near rock bottom of concrete placement as during compaction concentration of aggregates are going to be higher at rock bottom.

 7. STRENGTH CORRELATIONS 

Influence of surface conditions utilized in the development of a correlation between compressive strength and Rebound Number. The positive correlation between rebound numbers and strength of wet cured and wet tested cubes isn't recommended. it's necessary to determine a correlation between the strength of wet tested cubes and therefore the strength of dry tested cubes on which rebound readings are taken. The ratio of a dry tested cube to a wet tested cube in compression is usually about 1.05.

Er.SP.ASWINPALANIAPPAN., M.E.,(Strut/.,)

Structural Engineer

Madras Terrace Architectural Works

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