CTVET Ghana Certificate II - Blocklaying & Concreting flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Hazard versus risk

A hazard is a condition or substance with potential to cause harm; risk is the likelihood and severity of that harm occurring. Controls follow the hierarchy, with PPE as the last resort.

Wet cement burns

Wet cement is strongly alkaline and abrasive. Prolonged skin contact causes dermatitis and deep burns that may appear hours later without immediate pain, so wash skin with clean water promptly.

Silica dust control

Cutting or grinding concrete blocks releases respirable crystalline silica. Use wet cutting or on-tool extraction and a fitted respirator to reduce exposure below harmful levels.

Excavation spoil clearance

Spoil and materials are kept at least one metre back from a trench edge to reduce collapse risk. Deeper trenches generally need shoring or safe battering back.

Brick trowel versus pointing trowel

A brick trowel, typically 250 to 300 millimetres long, lays and buttering mortar. A smaller pointing trowel fills and finishes joints. Both must be kept clean and free of set mortar.

Testing a spirit level

Place the level on a flat surface, note the bubble position, then reverse it end for end. If the bubble reads the same both ways, the level is accurate; a shift shows error.

Cement storage limits

Stack cement bags on pallets off the floor, no more than about ten bags high, and use in rotation. Bagged cement is normally used within about three months of delivery.

Poker vibrator spacing

Insert the poker vertically at spacings of about 1.5 times its radius of action, penetrate the previous layer by 100 to 150 millimetres, and withdraw slowly to avoid leaving voids.

3-4-5 right angle method

Measure 3 units along one line and 4 along the other from a corner. Adjust until the diagonal reads exactly 5 units; the corner is then square. Multiples also work.

Height of collimation

Height of collimation equals the known reduced level of a point plus its backsight staff reading. Any reduced level then equals height of collimation minus that point's staff reading.

Rise and fall arithmetic check

In the rise and fall method, the sum of backsights minus the sum of foresights equals the sum of rises minus falls, and also equals the last reduced level minus the first.

Equal diagonals check

After setting out a rectangular building, measure both diagonals. Equal diagonals confirm the corners are square; unequal diagonals mean a corner must be adjusted before excavation.

Standard blockwork joint thickness

Mortar joints in blockwork are nominally 10 millimetres thick, usually accepted between 8 and 12 millimetres. Bed and perpend joints must be full and square.

Nominal block module

Nominal dimensions include the mortar joint. A 400 by 200 millimetre block with 10 millimetre joints gives a nominal module of 410 by 210 millimetres, which governs coursing and openings.

Bond offset rule

Vertical joints in successive courses must not align. They are offset by at least one-quarter of the block length; stretcher bond offsets by half a block, English and Flemish bonds by a quarter.

Damp-proof course position

The damp-proof course is laid on a continuous mortar bed at least 150 millimetres above external ground level, lapped at corners and joined to the damp-proof membrane below the floor.

Nominal concrete mix ratios

Common nominal mixes by volume are 1:3:6 for lower grades, 1:2:4 for general structural work and 1:1.5:3 for higher strength. The proportions are cement to fine to coarse aggregate.

Slump test cone dimensions

The standard slump cone is 300 millimetres high, 200 millimetres across the base and 100 millimetres across the top. The measured drop of the concrete indicates workability.

Curing duration

Ordinary Portland cement concrete is kept moist for at least seven days; hot weather or blended cements may need ten to fourteen days. Curing starts as soon as the surface is safe.

Bulking of damp sand

Moist sand occupies more volume than dry sand, sometimes 20 to 30 percent more. Batching by volume must allow for bulking, otherwise mixes end up short of sand and harsh.

Nominal cover to reinforcement

Typical nominal cover values are 20 millimetres for internal slabs, 25 for beams and columns, and 30 to 40 for foundations in contact with earth. Always follow the project specification.

Bar cross-sectional area

Area of a round bar equals pi times diameter squared divided by four. A 12 millimetre bar has about 113 square millimetres; a 16 millimetre bar about 201 square millimetres.

Formwork striking times

Typical striking times are one to two days for beam and column sides and seven to fourteen days for slab soffits, with props left longer. The project specification always governs.

Trowelling too early

Trowelling a concrete surface before initial set draws bleed water up and weakens the top layer. Wait until the surface water has gone and the concrete can carry the trowel.

First-angle projection layout

In first-angle projection the object lies between the observer and the projection plane, so the front view is placed below the plan and the left view to the right.

Drawing line conventions

Continuous thick lines show visible outlines, continuous thin lines show dimensions and projections, dashed lines show hidden edges, and chain dotted lines mark centre lines and cutting planes.

Reduced level notation

Levels are written in metres on drawings, for example plus 0.600 or minus 0.150, against a datum marked plus or minus 0.000. Finished floor and structural slab levels may differ.

Blocks per square metre

Blocks per square metre equal one divided by nominal length times nominal height in metres. A 400 by 200 millimetre block with 10 millimetre joints gives about 12.5 blocks per square metre.

Rebar mass per metre

Mass per metre of a steel bar equals diameter squared divided by 162, with diameter in millimetres. So a 12 millimetre bar is about 0.888 kilograms per metre.

Measuring formwork quantities

Formwork is measured in square metres of contact area with concrete, not plan area. For a beam, measure the soffit plus both sides, and deduct large openings.

Break-even point formula

Break-even units equal fixed costs divided by selling price per unit minus variable cost per unit. The denominator is the contribution each unit makes towards fixed costs.

Markup versus margin

Markup is profit divided by cost; margin is profit divided by selling price. A 25 percent markup on cost equals a 20 percent margin on price.

Practical assessment tolerances

Typical trade assessment tolerances are plus or minus 5 millimetres over a one metre height and level within 3 millimetres over two metres, together with consistent joints and correct bond.