MORTAR TECHNICAL BULLETIN CHEMICAL TESTING VS SCRATCH TESTING-THE FACTS

Chemical Analysis:

Recommended test. 

Chemical analysis of Mortar is used to confirm the mix proportions of a mortar and can be conducted both during the laying of brickwork and up to many years after the fact.

It separates the Cement, Lime and Sand contents, and identifies whether plasticizers or thickening agents have been used and the types of Cement and Sand used in the mortar. It will also identify any unwanted or contaminating materials such as excessive brick cleaning acid. The presence or absence of waterproofing chemical additives can also be confirmed.

It also allows a check on the mix purported to have been used by the bricklayer in cases of dispute where poor workmanship or unsatisfactory performance has occurred.

The results of testing are commonly provided as a Nominal Mix ratio which can be expressed as Parts by weight or Parts by volume and in general terms an experienced person can assess the likely or predicted performance of a mix using the data generated from an analysis.

Mortar mixing on site is technically done on a volume basis, and the AS 3700 and BCA guidelines identify deemed to satisfy mixes by volume.

A 1: 1 : 6 mix means 1 part Cement: 1 Part Lime and 6 parts Sand for example.

This is currently the most commonly used mortar mix for general purpose masonrywork.

Australian Standards and Building Codes have provided guidance as to the acceptable mix ratios of Cement, Lime and Sand for building applications and thus the data generated by a chemical analysis can be matched against those criteria to resolve disputes or verify conformance of the mix supplied.

With recent innovations, the chemical analysis method provides a means of accurately analyzing the composition of mortar.

The greatest advantage of a chemical analysis is that it can be conducted during bricklaying on wet mortar from mixer, boards, barrel, or freshly laid work. This allows any anomalies or problems with the mix to be picked up before completion of the work. Changes to technique, materials, or mix rates can then be applied immediately.

Scratch Test:

Not Recommended.

- The scratch test unit has not been designed for use with uneven bricks which are frequently encountered on residential sites.

- The unit has not been designed for rolled or raked joints - Many bricklayers strike the final surface of the mortar which compresses the surface material and can give a firm surface, hiding soft mortar below. The unit is designed primarily for flush joint mortar and flat faced brick or blockwork.

- Low test results are often found with this technique.

- The unit is extremely operator dependent- measurements made by four different operators on the same piece of brickwork were found to vary dramatically, providing 4 different data sets.

- The scratch tester shows a significant bias towards giving LOW readings with many zero readings being achieved due to inadequate seating of the pins of the test unit, which would falsely pass the mortar on an M2 or M3 basis.

- Mortar with compositions as poor as 1:15 have been found to PASS the scratch test, yet this mix ratio is well outside any acceptable tolerances for any type of brickwork and could lead to collapse of the brickwork.

- The scratch tester cannot be used until at least 7 days after construction and preferably 28 days after completion of brickwork- this limits the application for use in quality control applications. By then it may be too late to rectify faulty workmanship.

- The test cannot be relied on after 12 months from construction date -AS 3700 states that results generated after 12 months are to be considered as “guidance” value only.

- In ideal circumstances it can be used to confirm or validate an analysis result