Item Property to be checked Test Frequency Notes
2. Materials
Material acceptance
Bulk density and voids SANS 5845 When changes noted or Information used in design content every 50 to 150 m3 of mixes or calculations of
yield of mixes. Voids content used as a control check on quality of blended aggregates
Bulking of sand SANS 5856 When moisture content of Required for adjusting sand changes significantly volumes of sand in batching Pigments Type Visual check on containers When delivered
and/or delivery note Batching
General That correct materials and Visual check materials. Daily and when new mix correct mix proportions used Check mix proportions proportions specified
Cement By mass With 50 kg bags no test Daily A split bag of 25 kg parts required. Check load scale can be measured by equal
accuracy volumes
By volume Check container volume When container first used Note: Different types of that allowance made for or when type of cement cement bulk differently. bulking changed Volume batching of cement
is poor practice Aggregates By mass Check moisture content of Daily
sand and adjust mass
By volume Check container size. Check When first used. When Establish by experiment a allowance for bulking significant moisture changes sand bulking versus
occur moisture content curve for each sand.
Admixture By volume Check dispensing apparatus When first used. set correctly Daily
Pigments By mass Check correct mass Daily Normally pigments pre- weighed and placed in container for subsequent introduction into the mix By volume Check container size When first used
Low density aggre- Soundness (pop-outs) Refer to SANS 794 and When pop-outs noted and Test required when clinker, gates such as clinker SAN 1215 then twice per month ash and similar waste
products used Sulphur trioxide Refer to SANS 794 Once per month or when
problems noted
Loss on ignition Refer to SANS 794 Once per month or when problems noted
Water addition Water content Visual assessment by Each mix made Various aids can be used by operator of concrete operator in visually judging workability and amount of water required, i.e. hand water required and bar tests
Reinforcement Type, size and not Measurement and visual When used Check dimensions if contaminated examination reinforcement bent, i.e.
Table 7: Recommended quality control checks – (continued)
Item Property to be checked Test Frequency Notes
2. Materials
Mixing
Materials to be used Visual check that all Daily ingredients of correct type and mixer not overloaded
Uniformity of mix Visual Frequently Particularly important with hand mixing
Cleanliness of mixer Visual Daily or when break in With hand mixing ensure mixing surface on which ingredients
mixed is clean
Adjustment of scrapers, etc. By measurement Daily Important in pan mixers to in mixer (if present) for prevent segregation of wear and correct adjustment ingredients
Order of introduction of Visual Daily Order of batching to be
mix ingredients determined by experience.
Pre-mixing absorbent aggregates such as clinker with water to be considered Mix consistence Visual Each mix If consistence viz workability,
plasticity, etc. not correct immediately investigate reason
Duration of mixing Check time of mixing from Frequently Minimum and maximum time all water and cement mixing times to be given to are in contact, to discharge mixer operator
Re-tempering Visual When mixing time extended, Operator to be advised on loss of workability and extra maximum amount of water water added. that can be added
Discharge Visual to ensure complete After each mix Ensure no build-up of mortar discharge or concrete in the mixer Precast concrete products (refer also to general comments)
Mould Correct type, dimensions, Visual and by measurement At beginning of casting and Check moulds for wear and accuracy and undamaged afterwards at frequent damage
intervals
Preparation for casting Visual Frequently Check use of mould release oil. Rubber moulds do not require a release agent Reinforcement Correct type, diameter and Measurement Frequently Check against drawings
bending dimensions
Table 7: Recommended quality control checks – (continued)
Item Property to be checked Test Frequency Notes
2. Materials
Concrete masonry units (refer also to general comments)
Mould Correct type, dimensions, Visual and by measurement Regular checks on growth Dimensional tolerances to accuracy and undamaged of unit sizes SANS 1215
Relationship between plan Visual and by measurement When excessive amount of Also applies to small dimensions of moulds and concrete extruded between machines hand operated tamper head tamper head and mould
Machine operation Functioning of machine By observation of how When problems noted With single block moulds equipment operates ensure proper compaction
of concrete in mould Distribution of concrete into Measure heights and mass On introduction of new Non-uniform distribution of mould box of fresh units over total moulds, and/or concrete concrete in moulds requires
mould area mix, at frequent intervals special attention
Primary and secondary Check duration of vibration Frequent intervals The degree of compaction
vibration times of units by vibration
significantly affects strength of units and productivity. With hand compaction ensure unit well tamped to required height
Surface appearance on Visual Frequently Check presence of water extrusion from mould that sheen on face of unit, units not damaged or concrete sticking to tamper
distorted head on extrusion,
uniformity of texture, colour and compaction
Height of unit By measurement Frequently Weighing unit at same time as height measured gives an assessment of the
distribution of concrete in moulds and uniformity of compaction
Splitting of units to Visual appearance Visual together with When significant variations For consistent surface give a ‘rock face’ compressive strength tests in surface texture noted texture compressive
strength of units should be in an agreed range General comments: precast concrete and concrete masonry units
Moulds, pallets and Condition of moulds, pallets Visual Frequently Maintain moulds and pallets. working area and concrete surface on Keep working surfaces free
which operations take place from debris, etc. Handling Damage, if any, in de- Visual Frequently Products should be stored
moulding, or depalletising, and stacked on hard, plane transporting and stacking and unyielding surfaces
which drain.
Curing Protection of fresh products/ Visual Ongoing Extent and types of curing to units and curing (duration suit local requirements and and temperature) products / units being made Storage Marking of stacks for Acceptance and control Ongoing Allow for products / units
subsequent identification, testing not complying with required testing and clearance for standards to be stacked
delivery separately for subsequent
management decision on their disposal
3. Testing
Ideally all products made should be tested regularly for compliance with requirements such as strength, etc. and the production process monitored to ensure that products are not over or under strength, i.e. that the manufacturing process is not wasteful, viz. too much cement being used mainly or products condemned for say inadequate strength. There should be a continuous process of assessing the quality of products made and modifying the process to give best and most economical production.
If testing is impractical or unaffordable, products should be continually assessed for dimensional accuracy, surface finishes and whether corners and edges damaged on de-moulding or when transported, stacked and delivered to site. Reports back from site on satisfactory or
Table 8 provides some insights into the employment potential of precasting operations of a typical medium-sized manufacturer.