LONG-AWAITED STANDARD ON CONCRETE RETAINING BLOCKS (CRB) NOW OFFICIAL:

Concrete retaining blocks are widely used in the construction of earth-retaining walls and are manufactured either by dry or wet casting, the former being the most prevalent and more modern method. SANS 508 is applicable to both.

CMA president elect and vice president of its CRB Division, Silvio Ferraris, says compressive strength is a critical feature of the Standard, as a deficiency in this standard could result in the failure of a wall.

“The quality of materials used in the production of a block such as cement, aggregates, water and pigments, are also integral to the Standard as are other quality-related factors such as dimensional accuracy, shape, appearance and colour,” notes Ferraris.
 
SANS 508 gives clear directions as to the quality of cement used in the manufacture of a block and each delivery batch must be accompanied by a certificate from the manufacturer in compliance with SANS 50197-1. In addition, if extenders are used, they should comply with the requirements of SANS1491-1, SANS 1491-2 and SANS 1401-3.

Aggregates are also subject to a standard, in this instance SANS 1083. Every sand delivery must be checked for moisture content so that the quantity of water used in the concrete mix can be accurately determined. Furthermore, the size of the sand granules must also comply with a specified grading envelope.

SANS 508 further stipulates that the water used in the mixing of the concrete should be free of any impurities which might impair the strength or durability of the concrete. Moreover, any pigmentation must comply with the requirements of BS 1014.

Specific details on dimensional accuracy and compressive strength are covered in Tables 1 and 2 respectively in the Standard document, and testing frequencies are covered in Table 3. Besides dimensional accuracy and compressive strength, SANS 508 also includes the testing of shape, appearance and colour and mass.

The Standard requires that a minimum of five units per machine per day be tested for dimensional accuracy. This test applies to dry-cast units only, as it is very difficult to control the slump and instant de-moulding which occurs after wet-casting. Allowable tolerances on length, width and height for dry-cast products is + plus or - minus three millimetres.

The testing frequencies on compressive strength and mass for dry-cast blocks is as follows:  where production is less than 2 000 units three blocks per production batch must be tested; in instances where production is greater than 2 000 units, six units must be tested. The compressive strength testing frequencies for wet-cast cubes are exactly double that of dry-cast units.

The Standard stipulates that the average compressive strength rating of a dry-cast unit should be 13MPa whereas that of wet-cast blocks should be 20MPa. The minimum permissible strength rating is 11MPA for dry-cast units and 16MPa for wet-cast units.

Compressive strength must be tested by a purpose-built crushing machine, which applies hydraulic pressure at a rate of 15MPa per minute until a block or cube is crushed. In the case of dry casting, the actual blocks are tested by a “full platen test”. With wet casting, standard moulded cubes are tested. In dry-cast testing the actual bearing surfaces of the blocks are tested whereas with wet-cast blocks the bearing surfaces are not sufficiently even nor flat for accurate testing. Tolerances can vary by as much as 30mm, and this is why compressive strength testing on wet-cast blocks can only be tested with specially prepared cubes.

Although they don’t form part of the Standard, the basic principles of the design for a particular type of retaining block are also covered in Annex C as these offer the designer additional performance guidelines, such as the coefficient for block-on-block friction, nib-shear strength and the crushing strengths of blocks as they apply to specific retaining block products. Details on block-on-block friction, nib-shear stress, back-line load and front-line load testing are included in the Annex. Line-load testing is generally undertaken when blocks are installed with open spacing.

Ferraris says, although not stipulated by the Standard as a requirement, it would be well nigh impossible to achieve the SANS 508 mark without operating to a quality management system such as ISO-based SANS 9001 and the Standard touches on this aspect in Annex B. 

“A quality management system doesn’t have to be ISO-based but setting up a system normally requires the services of a consultant. Traceability is all important. If a problem occurs, test records must be available to determine possible causes.

“As yet there is no official standard on the actual installation of blocks, although the CMA has published an installation manual which can be used as a standard guideline. An installation CD is also obtainable from the CMA as a training aid. 

The standard for the design and construction of reinforced soils and fills, SANS 207-2006,  is  available from Standards South Africa. It defines reinforced slopes as any structure with a face angle less than 700 whereas walls are defined as those structures steeper than 700. Moreover, two publications, the “Code of Practice for Gravity Walls” and “Design of Reinforced CRB Walls”, are available from the CMA to assist engineers with structural designing,” advises Ferraris.

Ends

Captions-1/2/3
A concrete retaining block undergoes a compressive strength test in a purpose-built crushing machine. The machine applies hydraulic pressure at a rate of 15MPa per minute until the block is crushed.

Ends

DATE                            November, 2008
ISSUED BY                  David Beer Communication Consultants
FOR                              Concrete Manufacturers Association 
ENQUIRIES                  Silvio Ferraris 011 393-5504 or 082 851 9512
David Beer (011) 478 0239 or 082 880 6726.  Email: bigsky@ibi.co.za