The correlation between Compressive Strength and Tensile Splitting Strength

Word

What is the correlation between compressive strength and tensile splitting strength with reference to testing concrete paving blocks in compliance with SANS 1058:2012?  

One would imagine that if compressive strength of the concrete paver increased, then the tensile splitting strength would also increase. This is however not true.  

There is no correlation between compressive strength and tensile splitting strength. 

What is the influence of aggregate? 

It is a well-known fact that if the sand content in the concrete mix increases, the compressive strength of the unit will also increase. An increase in the sand content however influences the shear strength of the unit negatively, in other words the tensile splitting strength reduces. This can mainly be attributed to the homogeneous particle shape of sand and the way it binds with cement.    

Should the stone content in a concrete paver be increased (reduced sand) the tensile splitting strength will increase. The reason for this is that the inconsistent particle shape of the stone contributes to a less favourable shear path, thus increasing tensile splitting strength. 

It is general practice to increase the sand content in concrete pavers because it assists with the aesthetic appearance of the paving block. This is however a practice which manufacturers should apply with great caution.  

Specifying paving units 

The specification of concrete paving blocks in design, technical documents and tenders remains problematic. One would presume that the South African National Standard should and would be the ultimate specification.  

However, many architects, engineers, consultants, contractors, buyers and specifiers are not familiar with the requirements of the most current specifications.  For example, SANS 1058:2012 supersedes SANS 1058:2010 (edition 2) and the current version of the specification excludes compressive strength testing and only calls for tensile splitting strength.  

Concrete pavers seldom crumble under pressure (compressive strength). What is much more common is to see concrete pavers which have cracked under pressure (tensile splitting strength). When specifying concrete pavers in design, technical documents and tenders it should be in compliance with the South African National Standard. This is done by stipulating the class of the concrete paver. Although the classes still refer to compressive strength, the actual indicator of importance is tensile splitting strength.  

It is dangerous to specify concrete pavers only with regard to compressive strength. 

A concrete paver could have a high compressive strength but would fail as soon as a 

point load is applied to it. 

Cognisance should also be given to the two classes of concrete pavers mentioned in the standard. During testing, class 40/2.6 should perform above 90% of all testing parameters. Class 30/2.0 is not recommended for heavy traffic.   

The Concrete Manufacturers Association’s Producer Members manufacture precast concrete units of high quality and consistency because they all adhere to a certain level of quality management within the organisations. 

Precast concrete units certified to adhere to the South African National Standard specifications can be identified by the Concrete Manufacturers Association Certification Service’s mark of approval.  

About the CMA 

The Concrete Manufacturers Association (CMA) is the primary representative of the precast concrete industry.  Now in its 44^th year, it initiates standards in close cooperation with StanSA and collaborates with its members in developing new products and services. 

Precast concrete is a building material which slots very comfortably into the modern world of fast track and modular construction, in many instances leading the way with innovative technologies and applications.  

CMA’s promotional activities target architects, engineers, developers, contractors and property owners and the pooled knowledge and expertise of its members fosters an environment which encourages the development of innovative, environmentally and community-friendly products. 

The Association’s prime focus is to ensure that its members’ products are applied correctly. A CMA mark serves as a guarantee of quality and the CMA takes responsibility should a problem arise. 

Members are encouraged to hold accredited product certification such as the relevant SANS standard or to manufacture to specifications laid down by the CMA. Should a problem arise the CMA undertakes an investigation, and, if the product does not conform to the required standard, the member company is obliged to rectify the situation. 

A range of publications is available from CMA and courses and seminars form part of its offering to members and interested parties. 

More information from CMA, Tel: +27(0)11 805 6742 / email: marketing@cma.org.za / www.cma.org.za