What are G5, G6 and G7 Materials

What do G5, G6 and G7 mean?

The “G” classification system is used to classify materials suitable for different pavement layers according to their engineering properties.

• G5 and G6 are gravel-based pavement materials that may be natural, modified or processed.
• G7 falls into the gravel-soil classification and is generally used in lower pavement layers than G5 and G6.
• The classification is determined by a combination of material properties rather than particle size alone.

The COTO Standard Specifications for Road and Bridge Works classify G4 to G6 as natural, modified or processed gravel materials, while G7 to G9 are classified as gravel-soil materials.

Why does grading matter

Grading affects how the particles fit together within the pavement layer. A suitably graded material contains a distribution of larger particles, intermediate particles and fines. This allows the material to pack together effectively and achieve the required characteristics when placed and compacted. For a crushing and screening contractor, this is where plant configuration becomes important. Crusher settings, screen selection, screen apertures, recirculation and the number of stages in the circuit can all influence the resulting particle-size distribution. The objective is not simply to make the material smaller. It is to produce the required product within the specified grading envelope.

COTO vs COLTO

You will often see both COTO and COLTO referenced in South African road-construction documentation. The COLTO Standard Specifications for Road and Bridge Works for State Road Authorities were published in 1998. The COTO Standard Specifications were subsequently developed to replace the COLTO specifications. The Department of Transport states that the COTO 2020 Standard Specifications were approved by COTO in August 2020 and became mandatory for procurement documents advertised from 1 March 2021, while existing contracts and tenders already based on COLTO remained unaffected. This means that the applicable project specification should always be checked. For a crushing and screening subcontractor, this distinction is important because the required product specification should be understood before selecting the crushing and screening configuration.

Is crushing and screening enough

Crushing and screening primarily control particle size and grading; it does not automatically determine all of the engineering properties of the source material. Depending on the material classification and project requirements, specifications can also include properties such as:

• CBR
• Plasticity Index (PI)
• Liquid Limit (LL)
• Linear Shrinkage (LS)
• Grading Modulus (GM)
• Swell
• Flakiness
• Durability

This is why the quality of the source material matters just as much as the crushing process. A crushing and screening train can be configured to produce the required particle-size distribution, but laboratory testing is still required to establish whether the resulting material meets the complete specification for its intended use.

Where does Eco-Match fit into this process

For a mobile crushing and screening subcontractor, the starting point should be the required end product rather than simply identifying available equipment. Before a crushing train is selected, important information needs to be determined, including the source and type of raw material, maximum feed size, expected production volumes, required final products and grading envelope, applicable project specifications and existing laboratory results. After obtaining this information, the correct and most efficient crushing and screening train can be configured on site.