We recommend the use of 25mpa concrete for all walls. They can be hand-filled in stages or pumped up to 2.5m high in one application.
We suggest that the first two courses of blocks (500mm) be hand-filled. This is the time to get them level and straight - if you get this level correct it will eliminate troubles for the rest of the walls.
Pumping the 250mm blocks requires a drier mix, as concrete when too wet can cause blowouts. Filling the 125mm blocks requires a wetter mix because of the narrower openings in these blocks. A line pump with a 50mm hose outlet is best for pumping; ‘slow & steady’ is the key to successful pumping.
If you’re building in an area prone to rising damp, then a damp course may be desired. If 35mpa concrete is used in the first two courses it’s considered to be waterproof. As these two courses are frequently hand-filled, this is the ideal time to add a liquid damp course into the concrete as it arrives on the mini-mix truck.
Using a vibrator on the walls is neither required nor recommended. Just patting the walls with your hands is sufficient to ensure that the cavities are completely filled.
Windows and doorways can be placed in position at this stage and braced back. Blocks can then be placed around these items and then filled. Builders often prefer to leave the windows out until the walls are complete and the roof is on, in which case they should leave appropriate openings in the walls.
NOTE: 'Fire-rating’ walls
No unmodified polystyrene blocks can be fire-rated, because of the polystyrene bridges that secure one side of the block to the other. To achieve a fire-rating (required when, for instance, Thermacell blocks are used in a party-wall between units), we either remove the bridges and fill the holes with mortar to produce a solid concrete core, or rely on 16mm fire-rated plasterboard, fixed to both sides of the wall. Manufacturers of 'fire-rated’ plasterboards should be consulted as to the duration of the fire-rating.