Betontechnologie / seit 1996

# A fair-faced wall
is decided before
anybody pours it.

Mix design, formwork strategy, trial panels and curing. We are the people who say no in week four so nobody has to say it in week forty.

SB 4, board-marked, tie grid at 1,200 mm. Cast in November.

SB 1 to 4
  Classes advised
214
  Trial panels witnessed
7 d
  Curing, minimum
0
  Walls we have agreed to render

## Four classes

1. Class Typical use Formwork Tolerance Trial
2. SB 1 Basements, plant rooms Any Joints ± 20 mm None
3. SB 2 Stair cores, back of house Sheet, unspecified Joints ± 15 mm None
4. SB 3 Visible interiors Specified panel, sequenced Joints ± 10 mm 1 panel
5. SB 4 Facades, exposed halls Board-marked, numbered Joints ± 5 mm 2 panels

## Four things that decide the wall

1. 01

   ### Mix before form

   A mix is chosen for colour, workability and bleed before anyone draws a panel layout. Doing it the other way round is how walls come out mottled.

2. 02

   ### One cement, one quarry

   A single delivery source for the whole visible surface. Change the quarry mid-pour and no amount of curing will hide it.

3. 03

   ### Trial panels, full height

   Two panels, cast on site, in the weather the real pour will get. Signed off by the architect before the first real lift.

4. 04

   ### Curing is the finish

   The wall is decided in the first seven days. We write the curing regime into the contract and inspect it, because nobody else will.

Panels numbered and sequenced. Reuse is capped at four.
Cover checked twice: once by the steelfixer, once by us.

## Trial panels

Two panels, full height, cast on the site by the crew who will do the real pour, in whatever weather turns up. Then everybody stands in front of them and agrees, in writing, what acceptable means.

A photograph is not a trial panel. Neither is a sample from another job, another supplier or another season. Concrete is a local material and the panel is the only honest specification.

The signed panel stays on site until practical completion. When a dispute happens, and it does, the wall is compared to the panel and not to anybody’s memory of the panel.

Cubes at 7 and 28 days. Colour is judged on the panel, not on these.

## Engaging us

Office
  Hardturmstrasse 260
  8005 Zürich
Write
  beton@sichtbeton.example
When
  At concept design. Later is possible and more expensive.
Site work
  Trial panel witnessing, first lift, and a weekly walk.

## Three things that show on a finished wall

Invisible in the specification, unmissable in daylight, permanent either way.

01

### The joint

Day joints and panel joints are the drawing. Set them out with the architect at 1:20 and they read as intent rather than as an accident.

02

### The pour

Rate, vibration and lift height. Most blowholes and most colour variation are decided in the ninety minutes the concrete is moving.

03

### The cure

Seven days minimum, and the wall is finished in the first three. Strip early and you get a lighter, chalkier surface nobody can fix later.

## Typical mix, SB 4

One example, published because clients ask what a fair-faced specification actually contains.

1. Constituent Quantity Source Note
2. CEM I 42,5 N 340 kg/m³ Single works No change mid-project
3. Aggregate, 0/16 1 810 kg/m³ Single quarry Washed, graded
4. Water 153 l/m³ Mains w/c 0.45
5. Plasticiser 1.2 %One supplier No retarder
6. Air content 2.0 %Measured Every load
7. Slump class F4 Measured Every load

## Questions from contractors

Can we reuse the formwork more than four times?
  Not on a visible face. Panel wear shows as a progressive lightening across a facade and there is no remedy short of replacement.
What if the trial panel is rejected?
  We cast another. Two panels are in the price precisely so that rejecting the first is a normal outcome and not an argument.
Do you accept a photographic sample?
  No. Concrete is a local material; a photograph of somebody else’s wall tells us nothing about your aggregate or your crew.
Can defects be repaired?
  Small blowholes, sometimes, by a specialist, on a sample first. Colour variation across a lift, no. That one is permanent.