How to get defect-free backfill

How to get defect-free backfill

In this series of ToolBox Tips, we will be looking at a product from our Magma range of reinstatement & surfacing materials, the HardMaster W610 & W615.

Both of these mortars are used as backfill products. The HardMaster W610 is a trowelable mortar that can be used as haunching. Typical applications include the backfill of carriageway ironwork, post and barrier erection.

The HardMaster W615 is a flowable mortar used for backfilling around manhole frames and gullies, which can also be used for surface repairs. Both are 90 minute rapid set with 50 Newton-metre strength.

It is essential to have the right tools for this type of application such as a gorilla bucket, water carrier, measuring jug, paddle mixer with power source, steel float and bucket trowel.


Before starting the application, it's essential to ensure that the substrates are sound, clean, and free from dirt, oil and other contaminants. Proper surface preparation is vital to ensure the successful application and durable performance. All substrates should be pre-wetted with water prior to the concrete application and, any standing water should be removed.

 HardMaster w610 and w615 preparation



These can be mixed mechanically or by hand. If by hand, ensure that the mixing is vigorous for best results. HardMaster should be mixed using a heavy duty mixing whisk with a 10mm Ox pro rubber blade mixing paddle. Mix each 25kg unit with 2.3 – 2.8 litres of clean water. Pre-measure the required volume of water into a clean mixing vessel and steadily add the powder to the water. The product should be mixed for 2-3 minutes until a smooth, homogenous concrete consistency is achieved. Due to the rapid setting nature of these mortars, only mix a quantity of material that can be used and placed into position within 5 minutes of the end of mixing.

Note: This product must not be over-watered. The addition of excess water will result in slower strength gain, reduced final compressive strength and the possibility of shrinkage cracking.

 HardMaster w610 and w615 mixing process



HardMaster W615 Flowable should be poured into place 60mm below the required surface (if required lightly trowel to ensure good contact with edges). A single pass can achieve thicknesses of 30mm - 250mm. If thicker sections are required, use the layer-on-layer method. The concrete should be placed on the prewetted application area without delay after mixing. Due to the flowing nature of the product, you should quickly achieve a good compaction. This flowable concrete should be used to encase the flange of any ironwork. The concrete should then be brought to a height of 40mm below the required surface of the carriageway of the footpath. Once the product has reached the initial set, the reinstatement process can continue. Clean any mixing equipment with water immediately after use. W615 should not be used when temperatures are below 0°C or above 30°C.  

The application of the HardMaster W610 trowelable should be placed on the pre-wetted application area without delay after mixing, applied with an OX Pro Carbon Steel Trowel 120x280mm below the required surface, then compacted to ensure no voids are evident. The final surface should be rough floated to achieve a level surface. Once the product has reached the initial set, the reinstatement process can continue.

 HardMaster w610 and w615 application

Here are some of the key features & benefits of the W610 & W615

  • Early strength gain, ready to receive traffic after just 90 minutes
  • Easy to use, pre-blended, one component product - just add water and mix
  • Flowable and self-compacting
  • Excellent bond strength
  • Shrinkage compensated
  • High final strength

Take a look at the Training Academy video for yourself:

If you want to know more about our HardMaster W610 & W615 you can find additional technical data, within our technical datasheet for the W610 & W615 here. If you are interested in using these products please don't hesitate to contact us by calling 023 9220 606 or emailing us at