Facing Brickwork in a Frost Forecast with Frozen Sand
It is mid-November and you are on the facing brick outer leaf of a two-storey house extension, 60 m² of work, coloured sand-faced brick in a 1:1:6 mortar with a plasticiser. At 08:00 the air temperature is 2 °C and falling, with a clear night forecast to −3 °C. The sand is delivered loose and stored under a sheet at the edge of the site; the top 200 mm of the pile has frozen into lumps. Bricks are on pallets, dry except for the top two courses, which were rained on yesterday. There is no screening or heating on site. The general foreman wants at least the front elevation to two-course gauge built today because the scaffold is booked to come down on Friday, and he has suggested adding a shovel of calcium chloride to the mix to 'keep it going'. The spec calls up an F2 frost-resistant facing brick and an S2 designation mortar but does not mention cold-weather working.
1. Using the conditions on site, decide whether bricklaying should proceed today. Justify your decision with the specific evidence.
Read the worked answer
On these conditions, the correct decision is not to lay to the line today; at most, preparatory work and protected work in a sheltered/enclosed area.
The controlling facts are that the air temperature is 2 °C and falling, the sand pile contains frozen lumps, and there is no cover or heat. Mortar does not gain strength by drying - it gains strength by cement hydration, and hydration effectively stops near freezing. A mix that freezes before setting is permanently weakened, even after it thaws, and the ice lenses leave a friable, porous mortar with poor bond to the brick. Frozen sand must never be used: the ice lumps melt after the work is built, leaving voids and shrinkage, and they stop the mix from being gauged consistently.
Wet bricks are a second problem: saturated units with a frozen surface form an ice film that kills adhesion, and if they freeze while saturated they can spall. The F2 designation of the brick tells me it is frost-resistant as a unit and lets me use it in exposed positions - it does not make wet, freezing brickwork acceptable.
The work can be restarted when the temperature is safely above the cold-weather threshold and rising, or when the work area is enclosed, the sand is thawed and protected, mixing water is warmed, and finished work is covered overnight.
2. The foreman's suggestion is to add calcium chloride to the mortar. Explain why you would refuse, and set out what measures would actually make cold-weather bricklaying acceptable.
Read the worked answer
Calcium chloride is an accelerator, but in masonry it is the wrong tool. Chloride ions migrate to the steel and destroy the passive oxide layer. This wall has galvanised or stainless wall ties, and probably stainless brick ties and reinforcement at the openings; chloride in the mortar puts them in a chloride-contaminated, permanently damp cavity - exactly the conditions that drive corrosion. Corrosion produces expansive product that cracks the mortar bed and eventually lifts the brickwork. On top of that, chlorides increase efflorescence and can affect the colour of pigmented mortar. The specification does not permit it, so it is a variation, not a site decision.
The legitimate measures are:
- Do not lay when the air temperature is at or below the cold-weather threshold and falling, or when the brick or block is frozen.
- Thaw and protect the sand; store it on a hard standing under a weighted cover so it stays workable and gauged consistently.
- Use warmed mixing water - never hot enough to flash-set the cement - and mix smaller batches so nothing stands.
- Use an approved, non-chloride accelerator or a factory-produced mortar with declared cold-weather performance, only with the designer's approval.
- Cover the finished work overnight with insulating blankets, and protect the top of the wall and the cavity.
- Keep a max/min thermometer on the wall and record it, so there is a defensible record of what the mortar was actually exposed to.