Construction World April 2017

CONSTRUCTION WORLD

APRIL

2017

CEMENT AND CONCRETE TECHNOLOGY

Brenton Brouard, technical manager:

concrete at CHRYSO Southern Africa,

explains that an acceptable standard mix

design demands the use of ingredients that

are of an acceptable quality.

“The choice of materials is vital,” he

says. “Use good cement; one with the

South African Bureau of Standards (SABS)

mark that complies with the SANS 50197

standard. It is probably most practical to

use cement in the 32 N and the 42,5 N

(preferred choice when making plaster)

strength class.”

One should also use clean, well-graded

sand. This, Brouard explains, is because

washed plaster sand will ensure that clay

levels have been reduced. “Sand with too

much clay is not good for plastering as the

clay greatly increases the water demand of

the mix and this increases the potential for

shrinkage and cracking. He says that even

water quality is important and says clean,

potable water should be used.

This should give a good quality mortar

and plaster that is soft and plastic;

cohesive without being sticky and can

be placed easily. Using sub-standard

materials in plasters may result in cracking,

dusting, poor cohesion and crazing – those

narrow, shallow, interconnected cracks that

appear on plastered surfaces within a few

days of casting.

Brouard says one of the biggest problems

on site is the mixing of quantities of mortar

and plaster which are too large to be

accommodated by the rate of application.

This results in the rapid loss of workability

due to evaporation (caused by sunlight,

heat and wind).

On many sites, the solution is to increase

the workability to the original level by re-

tempering with additional water. “In almost

all cases, this both reduces the strength

and increases the potential for shrinkage

cracking of the mix,” he says.

Brouard says mixed batches should

be protected by immediately placing the

mortar or plaster into watertight containers

with lids and only quantities that will be

applied relatively quickly should be removed

from these containers.

This will reduce the risk of evaporation

from the mix and as a result, increase the

pot life of the mix.

Chemical solutions company CHRYSO

Southern Africa recommends the use of

CHRYSO

Stab rather than water in order

to retain workability of plaster or mortar.

CHRYSO

Stab acts as a retarder and an air

entrainer, imparting micro-bubbles into the

plaster or mortar, significantly reducing the

amount of water in the mix. This reduces

the risk of shrinkage and the potential

cracking resulting from it.

The key to

MAKING QUALITY

mortar and plaster

With so many reports of poor workmanship in the construction of

houses and other buildings, it is worth going back to basic truths

about how to achieve a high standard of mortar and plaster.

CHRYSO

Stab also makes the mix light and

fluffy and for an optimum workability, with

sufficient entrained air, a targeted slump of

120 to 150 mm is recommended.

In conjunction with the appropriate

protection, typical dosages of CHRYSO

Stab will produce brick laying mortars

with pot lives of eight to 36 hours and

plasters with pot lives of four to six hours.

Brouard cautions that it is important to

remember that extended pot lives are

not recommended for plasters because

additional bleeding from large exposed

surface areas will increase the risk of

shrinkage cracking.

In addition to CHRYSO

Stab, using

CHRYSO

Fibrin Fibres will inhibit surface

cracking. Once the plaster has been applied,

it is also important to cure the surface using

fog sprayers as well as a curing agent from

The Vodacom Data Centre where a waterproof plaster was used to prevent tiles from

falling off the façade.

Brenton Brouard, technical manager:

concrete at CHRYSO Southern Africa,

explains that an acceptable standard mix

design demands the use of ingredients that

are an acceptable quality.

CHRYSO

Stab will produce plasters with pot

lives of four to six hours.