Cert III Concreting cheat sheet

Concrete mix ratio by volume for general work: 1 part cement : 2 parts sand : 3 parts aggregate; 1:1.5:3 gives higher strength for structural elements; 1:3:6 suits mass or non-structural fill.

Water-cement ratio governs strength and durability: lower w/c ratio increases compressive strength and reduces permeability; excess water causes bleeding, shrinkage and dusting.

Nominal concrete grades: N20 = 20 MPa characteristic strength at 28 days, N25 = 25 MPa, N32 = 32 MPa, N40 = 40 MPa; characteristic strength is the value exceeded by 95% of test results.

Slump test (AS 1012.3) measures workability: typical structural slump 80-100 mm; a true slump indicates cohesive mix, a collapsed or shear slump indicates poor mix or excess water.

Compaction by vibrator: insert vertically at 1.5 times the vibrator radius spacing, withdraw slowly, do not drag horizontally, and avoid over-vibration which causes segregation.

Curing: keep concrete moist for at least 7 days (3 days for accelerated/heat-cured, 14 days for aggressive exposure); curing compounds, wet hessian, plastic sheeting and ponding are accepted methods.

Formwork stripping times: sides of beams and columns typically 24-48 hours; soffits of slabs require props left until concrete reaches specified strength, commonly 7 days for slabs and 14 days for beams.

Personal protective equipment and site safety: high-visibility clothing, hard hat, steel-capped boots, gloves, safety glasses and hearing protection near plant; silica dust controls required when cutting or grinding concrete.

Setting out and levels: use dumpy level or laser for reduced levels, string lines and profiles for edges; check diagonals for square, and confirm fall (typically 1:100 to 1:80) for drainage before pouring.

Reinforcement cover requirements: minimum cover depends on exposure classification; typical internal slabs 20 mm, external exposed 30-40 mm, marine or aggressive soils up to 50 mm; cover achieved with bar chairs and spacers.

Bar laps and anchorage: tension lap length typically 40 times bar diameter for deformed bars in tension (grade-dependent), and must be staggered; never lap bars in high-stress zones without design approval.

Saw-cut contraction joints: cut to one-quarter to one-third of slab depth, spacing generally 24-36 times slab thickness, cut within 6-24 hours of finishing while concrete is green.

Slab thickness and reinforcement placement: mesh must be supported at the correct level (top third for suspended slabs, mid-depth for ground slabs); chairs at 1 m centres prevent mesh displacement during pouring.

Concrete placement in hot weather: limit concrete temperature at placement to 35 degrees Celsius maximum, use chilled water or ice, shade aggregates, and cure immediately to prevent plastic shrinkage cracking.

Cold weather concreting: do not place concrete on frozen ground; maintain concrete above 5 degrees Celsius until it reaches 5 MPa strength; use insulated formwork and heated enclosures as needed.

Pumping and placing: prime the pump line with grout or slurry, keep a continuous supply to avoid cold joints, and never add water at the pump to increase slump without approval.

Surface finishes: trowelling too early causes dusting and craze cracking; wait for bleed water to disappear, then float and trowel in stages; broom finish provides slip resistance for external paths.

Testing and sampling: take slump and compressive strength samples from the discharge point or pump line; cylinders cured at 20 plus or minus 2 degrees Celsius and tested at 7 and 28 days.

Work health and safety duties under model WHS legislation: PCBU must ensure health and safety so far as reasonably practicable; workers must follow reasonable instructions and report hazards.

SWMS (Safe Work Method Statement) is required for high-risk construction work such as work at heights above 2 m, excavation deeper than 1.5 m, and work near live services; it must be reviewed and signed before work starts.

Manual handling risk controls follow the hierarchy: eliminate, substitute, engineer (hoists, trolleys), then administrative controls; team lifting is a last resort and does not remove the hazard.

Excavation and trenching: shoring or benching required for trenches deeper than 1.5 m; spoil and materials kept at least 1 m from trench edge; ladders within 9 m travel for access and egress.

Environmental controls: sediment fences, silt traps and washout bays prevent concrete slurry entering stormwater; concrete washout water is alkaline (pH up to 12) and must be contained and disposed of off-site.

Waste management: segregate construction and demolition waste into timber, metal, concrete and general; concrete rubble can be crushed and reused as base course or fill, reducing landfill.

Reading plans and specifications: identify grid lines, datum, reduced levels, section details, reinforcement schedules and hold points; hold points require inspection sign-off before proceeding.

Quality and conformance: maintain inspection and test plans (ITPs), record batch dockets, delivery dockets and test results, and raise non-conformance reports for out-of-specification work.

Communication and teamwork: use clear two-way radio or hand signals with crane and pump operators, confirm instructions in writing where possible, and follow site-specific emergency evacuation procedures.