CTVET Ghana Certificate II - Blocklaying & Concreting cheat sheet

Personal protective equipment (PPE) baseline on a blocklaying site: hard hat, safety boots, high-visibility vest, eye protection when cutting, dust mask when dry-cutting or mixing cement, gloves when handling blocks and rebar.

A hazard is any condition with potential to cause harm; a risk is the combination of likelihood and severity of that harm. Control hierarchy in order: eliminate, substitute, engineer controls, administrative controls, then PPE last.

Cement dust is alkaline (pH around 12–13 when wet) and can cause dermatitis and eye burns; wet cement burns may appear after hours with no immediate pain. Wash skin with clean water, not solvent.

Silica dust from cutting concrete blocks is a respiratory hazard; wet cutting or on-tool extraction with an appropriate respirator (minimum FFP2/FFP3 fitted) reduces exposure.

Manual handling rule of thumb: keep the load close to the body, bend the knees, avoid twisting; a 400 × 200 × 200 mm dense concrete block weighs roughly 17–20 kg, so two-person lifts apply for repeated handling.

Excavations deeper than 1.2 m generally require shoring or battering back; spoil and materials are kept at least 1 m from the trench edge, and ladders must protrude about 1 m above the access level.

Working platform rules: guardrails at about 1 m and 0.5 m with toe boards; scaffold inspection before first use and at intervals not exceeding 7 days, and after alteration or bad weather.

Site waste and environment: separate concrete washout from drains, cover stockpiles to control dust, bund fuel stores, and control noise hours; cement washout water is highly alkaline and must not enter watercourses.

Bricklayer's trowel sizes are commonly 250–300 mm blade length; a pointing trowel (smaller) is used for filling joints, a brick trowel for laying and buttering mortar.

Spirit level accuracy is expressed per metre or per 2 mm bubble; a 1.2 m level checks longer runs, a 600 mm level suits block faces and reveals. Check the level itself by reversing it on the same spot.

A club hammer (about 1–2 kg) splits blocks; a bolster chisel is used with it for cutting. A block splitter or masonry saw gives cleaner, more accurate cuts than a hammer and chisel.

Line and pins (or a line block) establish the coursing line; the line is stretched taut and checked against the block face to prevent 'belly' in long walls.

Concrete mixer drum capacity is rated by mixed output; a typical small tilt-drum mixer is 1:2:4-capable at about 0.1–0.15 m³ per batch. Never overload the drum; charge order is usually half the water, then aggregate, cement, sand, then remaining water.

Poker vibrator diameter should suit the section: typically 25–40 mm for thin walls and 50–75 mm for mass concrete; insert vertically at spacings of about 1.5 times the radius of action and withdraw slowly.

Material storage: cement bags stacked on pallets, off the floor, maximum about 10 bags high, used in rotation and within about 3 months; sand and aggregate on hard standing, separated by dividers.

Blocks should be stored on level hard standing and protected from ground moisture; many block types are laid within a specified moisture condition, so follow the manufacturer's guidance rather than soaking by habit.

The 3-4-5 method (or multiples such as 6-8-10) sets a right angle: measure 3 units along one line, 4 along the other, and adjust until the diagonal reads 5 units.

Setting out sequence: establish a datum and profile boards clear of the excavation, transfer the building line, square the corners, then check both diagonals - equal diagonals confirm a rectangle.

A dumpy level or automatic level with a levelling staff gives reduced levels; the height of collimation = backsight reading + known reduced level. Reduced level of a point = height of collimation − foresight (or intermediate sight).

Rise and fall method: difference between consecutive staff readings gives rise (later reading smaller) or fall (later reading larger); arithmetic checks are ΣBS − ΣFS = Σrise − Σfall = last RL − first RL.

Site strip and topsoil removal: topsoil is generally stripped to about 150–300 mm and stockpiled separately for reuse in landscaping; formation level is the excavated level on which foundations or hardcore sit.

Trench width for strip footings is normally the footing width plus working space each side (commonly 150–300 mm per side depending on depth and soil); trench bottoms must be firm, uniform and free of loose spoil.

Batter boards and profiles are set at a known offset from the building line (often 1–2 m) so that lines can be restrung after excavation without being disturbed.

Grid and coordinate control: establish at least two permanent reference points (offsets from boundaries or a site datum) and record them, so the setting out can be rechecked at any stage.

Standard mortar joint thickness for blockwork is 10 mm (typical acceptable range 8–12 mm); bed and perpend joints must be full and square, not 'furrowed'.

Common block face sizes in Ghana and the region: 400 × 200 mm (concrete block) and 450 × 225 mm or 440 × 215 mm (clay/ash bricks); wall thickness 100, 125, 150, 200 or 225 mm.

Nominal dimensions include the joint: a 400 × 200 mm block with 10 mm joints gives a nominal module of 410 × 210 mm; this governs coursing and setting out openings.

Bond rules: vertical joints in successive courses must be offset by at least one-quarter of the block length; running/ stretcher bond offsets by half a block, English and Flemish bonds by quarter.

Quoins are the corner blocks; they are laid first and levelled both ways so the line can be run between them. Racking back is limited - commonly to about 1.5 m in a day for stability, or per project specification.

Mortar mix by volume: 1:3 (cement:sand) for high-strength or exposed work, 1:4 for general blockwork, 1:5–1:6 for internal or low-stress work; lime or plasticiser may be added for workability, never to exceed manufacturer dosage.

Movement and control joints in long walls are typically at 6–8 m spacing for blockwork, or as designed; they must be built in, not cut afterwards.

Damp-proof course (DPC) is laid at least 150 mm above external ground level on a continuous bed, lapped at corners and joined to the damp-proof membrane; weep holes and cavity trays are provided above openings in cavity walls.

Nominal mixes by volume: 1:3:6 (about C10/C15), 1:2:4 (about C20/C25), 1:1.5:3 (about C30 and above); cement:sand:coarse aggregate. Water-cement ratio by mass typically 0.45–0.60; extra water lowers strength roughly 15–20% per 0.1 increase in w/c ratio.

Slump test (BS EN 12350-2): a standard cone 300 mm high, base 200 mm, top 100 mm; typical slumps are 25–75 mm for normal work, 75–150 mm for pumped or heavily reinforced sections.

Cube strength is measured on 150 mm cubes cured at 20 ± 2 °C and tested at 7 and 28 days; 7-day strength is typically about 65–75% of 28-day strength for ordinary Portland cement concrete.

Batching by volume must allow for bulking of damp sand (up to 20–30% for moist sand); gauge boxes of fixed volume are used, and one 50 kg bag of cement is roughly 0.035 m³ (about 33–35 litres).

Placing: free fall of concrete should not exceed about 1.5 m without a chute or drop pipe; layers are typically 300–500 mm thick and each layer must be compacted before the previous one sets.

Compaction by poker vibrator: insert at about 1.5 × radius of action, penetrate the previous layer by 100–150 mm, withdraw slowly; over-vibration causes segregation and bleeding.

Curing: keep concrete moist for at least 7 days for ordinary Portland cement (10–14 days in hot weather or for blended cements); curing compounds, wet hessian or polythene sheeting are common methods.

Hot-weather concreting: keep fresh concrete below about 32 °C, shade aggregates, use cool water, place early morning or evening, and start curing immediately after finishing.

Concrete cover to reinforcement (nominal, typical): 20 mm for internal slabs, 25 mm for beams/columns, 30–40 mm for foundations in contact with earth, 40–50 mm for exposed or marine conditions - always per the project specification.

Bar sizes are designated by diameter in mm (e.g. Y10, Y12, Y16, Y20); cross-sectional area of a bar = πd²/4, so a 12 mm bar has about 113 mm².

Lap length for tension bars is typically 40–50 times the bar diameter for grade 500 steel in normal concrete; laps should be staggered, not all in one section.

Formwork striking times (typical, ordinary Portland cement, moderate climate): sides of beams and columns 1–2 days, soffits of slabs 7–14 days, props left longer; always follow the specification.

Formwork must be rigid, tight and treated with release agent; a typical design load allowance includes concrete pressure plus construction live load (commonly 1.5 kN/m²) and impact.

Finishing sequence: screed and level, then float (wood or magnesium) to close the surface, then trowel for a smooth finish; power floating follows the initial set, and trowelling too early draws water to the surface and weakens it.

A brushed or tamped finish gives slip resistance on external paths; a power-trowelled finish is for internal floors. Curing must start as soon as the finish will not be damaged.

Construction joints are formed with stop ends and, in water-retaining or structural work, may need keys, water bars or roughened, cleaned surfaces before the next pour.

Common scales: 1:1, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500; a plan at 1:100 means 1 mm on paper equals 100 mm (0.1 m) on site.

First-angle projection: the object sits between the observer and the plane, so the view from the front is placed below the plan in the layout; third-angle projection places the front view above the plan.

Line conventions: continuous thick for visible outlines, continuous thin for dimension and projection lines, dashed for hidden edges, chain dotted for centre lines and cutting planes.

A dimension line carries the measurement with arrowheads or oblique strokes at each end; extension lines start about 1–2 mm clear of the object and extend about 2–3 mm beyond the dimension line.

Section drawings show cut material with hatching: concrete is often shown with dots/speckle or a standard hatch, brickwork with diagonal lines, hardcore with random stones, insulation with wavy lines.

Levels are shown on drawings as reduced levels in metres (e.g. +0.600, −0.150) with a datum marked as ±0.000; a finished floor level (FFL) and structural slab level (SSL) may differ by the floor build-up.

A bar bending schedule lists bar mark, diameter, shape code, number of bars, total length and bending dimensions; cutting length must include hooks and bends, which add to the straight length.

Drawing set completeness: site/location plan, floor plans, sections, elevations, details, schedules and specification; a revision cloud and revision table record changes, and only the latest revision is used for construction.

Blockwork is measured in square metres of wall face; deduct openings above a stated threshold (commonly over 0.5 m² or per the method of measurement used) and add for reveals and returns.

Number of blocks per m² = 1 / (nominal length × nominal height). For a 400 × 200 mm block with 10 mm joints, nominal 0.41 × 0.21 m = 0.0861 m², giving about 12.5 blocks per m² (commonly 12.5, or 10 blocks per m² for 450 × 225 mm).

Mortar volume for blockwork is roughly 0.02–0.03 m³ per m² of 200 mm wall (about 5–10% of wall volume); add for bedding, perpends and any filling of hollow blocks.

Concrete volume: length × width × depth, all in metres; add 5–10% waste for site-mixed concrete and round up to whole batches.

Reinforcement mass: mass per metre of a bar = d²/162 kg/m (d in mm), so 12 mm ≈ 0.888 kg/m, 16 mm ≈ 1.58 kg/m, 20 mm ≈ 2.47 kg/m; total mass = number of bars × length × mass/m, plus laps and waste.

Formwork is measured in square metres of contact area with the concrete, not the plan area of the element; for a beam, measure soffit plus both sides.

Excavation in cubic metres: length × width × depth; bulking of excavated soil (typically 20–30%) increases the volume to be carted away, while compaction reduces fill volume.

Labour and materials estimate: establish output rates (e.g. blocks laid per gang-day), then quantity ÷ output = gang-days; add allowances for weather, setting out and curing time.

A site instruction should be written, dated, numbered and acknowledged; verbal instructions are confirmed in writing the same day to avoid disputes about scope and cost.

A variation order changes scope, time or price; it must be agreed before work proceeds, with the effect on programme and cost recorded.

A take-off and quotation structure: measured quantities × unit rates, plus preliminaries (site setup, supervision, temporary works), overheads and profit margin, plus VAT/levies where applicable.

Break-even point in units = fixed costs ÷ (selling price per unit − variable cost per unit); contribution per unit is selling price minus variable cost.

Cash flow differs from profit: a job can be profitable but fail if payments arrive after wages and materials are due, so payment terms and retention (commonly 5–10% held until completion) must be planned.

Markup on cost vs margin on price: a 25% markup on cost equals a 20% margin on selling price; convert with margin = markup ÷ (1 + markup).

Record keeping for a small contractor: measurement sheets, delivery notes, wage sheets, plant hire records and a simple cash book; these support claims and tax returns.

Numeracy on site: percentages for waste and wastage, ratios for mortar and concrete mixes, and unit conversions (1 m³ = 1,000 litres; 1 bag cement ≈ 50 kg ≈ 0.035 m³).

Typical practical task sequence: interpret drawing, set out and level, prepare mortar/concrete, lay to line and level, check plumb, square and coursing, finish joints, cure, then clean down and self-check.

Assessment criteria commonly include dimensional tolerance (e.g. ±5 mm over a 1 m height, level within ±3 mm over 2 m), plumb, joint thickness consistency, bond correctness and finish quality.

Self-checking routine: check the first course is level and square before building up; check every course against the line; check plumb with a level on the face and end; check diagonals of openings.

Tools must be clean and in working order before assessment: trowel, level (tested by reversal), line and pins, club hammer, bolster, square, tape, jointer and float.

Time and material control: plan the batch size to the work available, mix only what can be used before initial set (commonly within about 1–2 hours for mortar in hot conditions), and clean tools and mixer at the end of the shift.

Finish and presentation: joints tooled to a consistent profile (flush, weathered or recessed), mortar struck off the face, and any mortar splashes on block faces removed before they set.

Safety during assessment: PPE worn, leads and mixer guarded, cutting done with eye protection and dust control, and the work area kept clear of trip hazards.

Records: keep a work diary with date, weather, quantities placed, labour used and any instruction received; this supports both assessment evidence and later claims.