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In-Situ Crushing Resistance of Screeds

The BRE Drop hammer Test is a screed testing method developed by the Building Research Station in the 1970’, and is now included in the BS Code of Practices for measuring the ‘soundness’ or ‘in-situ crushing resistance’ of screeds.

Soundness of screed is generally defined as the ability of the screed to withstand imposed loads and traffic in service without crushing. The soundness of a screed is usually influenced by:

  • The level of compaction of the screed mix
  • The binder content of the screed
  • Aggregate grading
  • Water content
  • Curing

The method of soundness assessment varies with the screed material and the method of construction. For solidly supported screeds, i.e. bonded and unbonded cement/sand or fine concrete screeds, soundness is assessed using the BRE Test method described in BS8204-1:2003&A1:2009 Annex D, while floating screeds are tested using the method specified in Annex E.

BRE Drop Hammer Test for bonded and unbonded screeds

The test is carried out by subjecting the screed to four repeated blows by dropping an annular weight of 4kg down a cylindrical guide rod, onto a hardened steel anvil placed in contact with the screed. The depth of indentation on the screed after four consecutive blows is measured with the depth measuring device to assess the soundness of the screed.

Sampling

The method of sampling usually varies depending upon the size and shape of the screeded area and the nature of any base, such as positions of joints. The general sampling stipulations are:

  • The screed should be at least 14 days old ( well compacted screeds form an exception and are found to be capable of taking the test in 3-4 days)
  • A minimum of three tests should be conducted in areas of 20 sq.m or less
  • A minimum of three tests should be conducted in every 20-25 sq.m for areas larger than 20 sq.m
  • Test corridors at intervals of 3-5 m
  • Select random positions for testing, but ensure areas vulnerable to traffic damage such as doorways are included
  • Avoid testing within 300mm of joints and cracks as the test can result in further cracking
  • ​Conduct additional tests where non-complying results are indicated

Acceptance Limits

Indicative values for depth of indentation after four blows on various types of bonded and unbonded screeds are shown in the table below.

Interpretation of Results and Rectification Measures

Results are plotted on the floor plan and colour coded to indicate compliance or non-compliance with the specified categories listed on the table above.

  • Areas of screed outside the compliance limit should ideally be cut out and replaced
  • Alternatively, the unsound screed areas can be upgraded by impregnating with ultra-low viscosity resin or a fibre-reinforced flow applied cementitious overlay after seeking specialist advice
  • For screed areas of marginal quality, ensure further tests are carried out and expert advice sought

BRE Test for floating screeds

The regular BRE screed test can be inappropriate for floating screeds laid on insulation or quilt, as the impact can result I punching a section of screed through into the insulation. The modified method is described in Annex E of BS8204-1:2003&A1:2009.

The testing method is the same as that of bonded and unbonded screeds for screed Categories A and B (screeds subjected to heavy traffic and public areas), for screeds with a  minimum depth of 75mm.

For screed category C (screeds subjected to light floor traffic), with a minimum thickness of 65mm, a 2kg annular weight is used in place of the standard 4kg weight, and ​the proposed indentation is limited to 2.5 mm instead of 5mm.

Interpretation of Results

If a fracture occurs during the test, it is to be recorded as the test result. According to the British Standards code of practice, a fracture leads to a truncated cone of material, which can be knocked out from the underside of the screed.

  • A fracture of the screed can be determined by the change in note produced when the weight hits the anvil. When a normal blow would produce a ringing sound, a fracture is usually indicated by a dull thud, accompanied by a sudden increase in the depth of the indentation.
  • If a truncated cone fracture is recorded during the first three drops the screed is to be considered unsatisfactory. However, a screed can be considered satisfactory if the fracture occurs only on the fourth drop and the indentation is within the appropriate limit.
  • Any portion of the screed where the depth of indentation is greater than the relevant limit should not necessarily be rejected. Such screeds should be assessed by other means to ascertain whether they are fit for purpose. This can require the removal of samples for the determination of cement content, strength, thickness, etc.
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