Housing in Southern Africa

oncrete plays an important

role in housing foundations

and the type of cement, how

it is stored, used and mixed all play

a role in the final strength of the

foundation.

THE CONCRETE FACTS

Cement in bags must be stored and

protected from elements, which

cause deterioration. Where it is neces-

sary to store in the open, bags should

be stacked on a wooden platform set

on supports of at least 100mmabove

the ground. Cover with a tarpaulin or

plastic sheeting to ensure that water

runs off without wetting the bags.

Bags should be stacked not ex-

ceeding 12 bags and arranged so

that consignments can be used in the

order of delivery, as cement which is

stored in bags lose about 30% of its

potential strength after about four to

six weeks. If ‘old cement’ is used, then

the mix must have a richer cement

content. Cement that has lumps in it

that cannot be easily crumbled with

a finger should not be used.

Aggregates for concrete should

be stored separately so as to prevent

contamination.

Cement and lime should be mea-

sured by volume. A bag of common

cement has a mass of 50 kg and a

volume of approximately 33 litres

when packed under air pressure at

the factory.

Cement, however, fluffs up when

poured into a container, and 50 kg

cannot readily be contained in a

box of 33 litres. The volume of loose

(bulked) cement, depending on its

compaction, can be up to 20% more

than in a bag. For this reason it is

always preferable to use whole bags

of cement when volume batching.

Concrete mixed on site using com-

mon cements has a strength class of

32,5 MPa or more.

Concrete left standing for a limited

period must be covered with plastic

sheets or wet stacks to prevent it

from drying out. Wet concrete needs

to be remixed before being placed,

in the event that stone particles have

settled at the bottom of wheelbar-

rows during transportation.

Concrete should be deposited ver-

tically into its final position to avoid

segregation of aggregates.

Concrete must be compacted by

mechanical vibrations or tamping,

spading, rodding or forking so that

the concrete is thoroughly worked

against the formwork and around the

reinforcement, and other embedded

items without displacing them. This

will ensure that the concrete is free

from honeycombing and planes of

weakness.

Inadequate compaction can

seriously affect the quality of the

concrete. The concrete may be as-

sumed to be fully compacted when

the air bubbles cease to rise to the

surface of the concrete in being

vibrated. Contact between the vibra-

tor and formwork can damage the

surface ‘off-shutter’ concrete.

Where possible concrete should be

placed in a continuous process. If this

is not possible because of weather

conditions, construction joints at the

point of stoppage may be formed by

sloping the edge of the poured con-

crete at 45 degrees to the horizontal.

The surface of the joint should,

prior to continuing with concreting,

be brushed with a steel wire brush,

swept clean with a light brush, wet

and covered with a 10mm thick layer

of mortar, (composed of one part ce-

ment to three parts of concrete sand)

before placing new concrete.

After the initial set, the concrete

needs to be protected from con-

tamination and loss of moisture by

covering with waterproof sheeting.

Back

Basics

The fully comprehensive NHBRC Home Building Manual is a

valuable resource of information on every aspect of home

building. The state regulatory body continues to raise the bar

on housing and assists the sector. Here are some of the tips

from the NHBRC Home Building Manual.

Inspectors Zondi Doro and Sinfingatwa Mantsha