NVQ Diploma in Bricklaying (Construction) Repair, Maintenance and Cladding Systems Syllabus

What is the principle behind installing remedial (retro-fit) wall ties, and name two common types.

Remedial ties mechanically re-pin the two leaves together to restore composite/structural action after the originals fail. Common types: mechanical expansion ties and helical (screw-in) stainless steel ties driven into pre-drilled holes, and resin-bonded ties grouted into a sleeve. They are usually installed in a staggered grid and replace corroded ties.

Explain the helical bar crack-stitching technique for repairing cracked masonry.

Bed joints are raked out (about $25$ to $35\,\text{mm}$ deep) crossing the crack, typically every $4$ to $6$ courses; a bead of thixotropic grout/resin is gunned in, a stainless helical bar is pushed in, more grout covers it, then the joint is repointed. The bars bond into the masonry on both sides to redistribute tensile stress and stitch the crack.

How far should crack-stitching helical bars extend beyond each side of a crack, and why staggered courses?

Each bar should embed a minimum of around $500\,\text{mm}$ beyond the crack on both sides to develop adequate bond into sound masonry. Bars are placed in staggered courses (not all in one vertical line) to avoid creating a new line of weakness and to spread the tensile reinforcement over the height of the cracked zone.

Describe how a retro-fit chemical (injection) damp-proof course is installed.

A line of holes is drilled into a mortar bed joint (or brick) at close centres just above ground level/below the lowest timber; a water-repellent silane/siloxane cream or fluid is injected or gun-applied to saturate the masonry, where it lines the pores and blocks capillary rise. Plaster is then hacked off, allowed to dry and replaced with a salt-resistant render.

Name three methods of retro-fitting a damp-proof course other than chemical injection.

(1) Physical DPC insertion — cutting a continuous slot along a bed joint and inserting a sheet membrane (e.g. by mechanical saw or in short lengths). (2) Electro-osmotic DPC — installing electrodes to reverse the capillary potential. (3) Porous/atmospheric tubes inserted to aid evaporation. Chemical injection is the most common; physical insertion is the most positive barrier.

State the fundamental principle of underpinning and the most common traditional method.

Underpinning extends/strengthens a building's foundation by transferring its load to deeper, more stable strata or by widening the base. The most common traditional method is mass-concrete (pin) underpinning, excavating and concreting beneath the existing footing in short sections.

Explain the 'hit-and-miss' (rule of three/five) sequence in traditional underpinning and why it is used.

The foundation is divided into bays; only alternate, non-adjacent bays (typically no more than one in three or one in five, and usually $\leq 1\,\text{m}$ wide) are excavated and concreted at any one time. This keeps the unexcavated length supporting the wall, preventing collapse, before the intervening bays are done in sequence.

What is 'dry-packing' in underpinning and why is it critical?

Dry-packing is ramming a stiff, almost dry sand-cement mortar into the gap between the top of the new underpinning concrete (left short by about $75\,\text{mm}$) and the underside of the existing foundation, after the concrete has cured. It ensures positive, non-shrinking load transfer so the building bears solidly on the new foundation.

Name two forms of temporary support used to stabilise a structure during repair or underpinning.

Raking shores (inclined timbers/props bracing a wall externally), flying shores (horizontal struts between two buildings), dead shores (vertical props with needles passing through the wall to carry load while the wall below is removed), and acrow/strongboy props internally. Needles and props relieve the wall while works proceed.

What is thin-joint blockwork and what mortar joint thickness defines it?

Thin-joint masonry uses precision (close-tolerance) aircrete blocks laid with a thin-bed adhesive cement mortar applied with a toothed scoop/sledge, giving joints of about $2$ to $3\,\text{mm}$ instead of the conventional $10\,\text{mm}$. It speeds construction, gives rapid early strength and improved thermal performance with fewer mortar cold bridges.

Compare a thin-joint mortar joint with a conventional joint in thickness and setting behaviour.

Conventional joints are about $10\,\text{mm}$ thick using sand-cement (or cement-lime) mortar with a relatively slow set. Thin-joint adhesive mortar joints are about $2$ to $3\,\text{mm}$, gain handling strength in roughly $1$ to $2$ hours, allowing several courses or full storey heights to be built rapidly in one operation.

What is aircrete (autoclaved aerated concrete) and what gives it its properties?

Aircrete is a lightweight blockwork made from sand/PFA, cement, lime and aluminium powder; the aluminium reacts to release hydrogen, creating millions of tiny air cells, then it is steam-cured (autoclaved). The entrained air gives low density, good thermal insulation (low $\lambda$), easy cutting, and reasonable compressive strength for its weight.

Give two key advantages and one limitation of aircrete blocks in rapid-build construction.

Advantages: excellent thermal insulation (low thermal conductivity, helping meet U-value targets) and light weight with large block sizes for fast laying; also easily cut/chased. Limitation: relatively low compressive strength and point-load/fixing capacity compared with dense concrete blocks, so special fixings are needed and high loads may require denser units.

Why are movement joints needed in modern clay-brick and concrete-block masonry, and how do the two materials differ?

They accommodate dimensional changes to prevent cracking. Clay bricks expand irreversibly with moisture over time (moisture expansion), needing compression-absorbing expansion joints. Concrete/calcium-silicate/aircrete shrink as they dry, needing contraction (shrinkage) joints. Joints are filled with a compressible filler and sealant.

State typical maximum spacings for movement joints in clay brickwork versus concrete blockwork.

Clay brickwork: vertical movement (expansion) joints at roughly $10$ to $12\,\text{m}$ centres (about $15$ to $16\,\text{mm}$ wide, first joint near $\tfrac{1}{2}$ spacing from a corner). Concrete/calcium-silicate blockwork: contraction joints at about $6\,\text{m}$ centres (often $\leq 7.5\,\text{m}$ unless reinforced). Bed-joint reinforcement can increase spacing.

How is the width of a clay-brickwork expansion joint estimated relative to panel length?

As a rule of thumb the joint width is taken as roughly the panel length multiplied by the expansion allowance, commonly about $30\%$ of the panel's expected movement; in practice designers allow about $1\,\text{mm}$ of joint width per metre of brickwork plus a margin, giving the typical $16\,\text{mm}$ joint at $\sim 12\,\text{m}$ spacing.

What is a brick-slip cladding system and where is it commonly used?

Brick slips are thin facing slices of brick (about $15$ to $25\,\text{mm}$ thick) bonded or mechanically fixed to a backing/carrier board or insulation to give the appearance of solid brickwork without the weight or thickness. Common on steel/timber-frame buildings, soffits of lintels, balconies and rainscreen/external-wall-insulation systems.

Distinguish adhesive-bonded brick slips from a mechanically fixed brick-slip cladding system.

Adhesive-bonded slips are stuck to a board/render carrier with polymer adhesive and pointed. Mechanically fixed systems hang/clip the slips onto rails or a track (or use slips with extruded keys engaging carrier rails) so each slip is positively held; mechanical fixing is preferred for taller buildings and where fire/robustness demands non-reliance on adhesive.

What is the role of a brick-slip support angle (shelf/support angle) in a framed building, and why?

A stainless-steel support (shelf) angle is fixed back to the structural frame/slab to carry the weight of a panel of brick slips or brickwork above it, dividing the cladding into storey-height lifts. This stops the full self-weight of the cladding bearing on the brick below and accommodates frame movement at each floor via a soft joint beneath the angle.

Why are masonry support angles, brackets and restraint fixings specified in austenitic stainless steel?

They are built into damp, sometimes chloride-rich masonry and must last the life of the building without corroding (unlike the galvanised ties that failed historically). Austenitic stainless steel (grade 1.4301/304, or 1.4401/316 in aggressive/marine/coastal or swimming-pool environments) gives the required durability and avoids staining.

Differentiate a load-bearing support fixing from a restraint fixing in masonry cladding.

A support (shelf-angle/bracket) fixing carries the gravity (vertical dead) load of the masonry and transfers it to the frame. A restraint fixing (e.g. tie, wind-post connector, head restraint) resists horizontal wind and out-of-plane forces and provides lateral stability but is not designed to carry the panel's self-weight.

What is a 'soft joint' (compression joint) beneath a support angle and what does it accommodate?

A soft joint is a horizontal compressible gap (filled with closed-cell foam and sealant) left immediately under each support angle. It accommodates downward frame deflection/creep and upward brickwork expansion so the cladding above does not bear on, and crush, the brickwork below; it is typically $10$ to $15\,\text{mm}$ or more depending on calculated movement.

What is the function of a sealant in a movement or perimeter joint, and what property must it have?

A sealant fills the joint to make it weathertight (air and water resistant) while still allowing the joint to expand and contract. It must be elastic/flexible with adequate movement accommodation factor (MAF) and good adhesion to the joint faces, applied over a backer rod/bond-breaker so it bonds on only two sides.

Distinguish mastic, silicone and polyurethane sealants by typical use.

Oil-based mastic: low-movement, paintable, gap-filling, short life. Silicone: high movement accommodation and weathering, excellent for glazing and around frames but generally not paintable and not always over-paintable. Polyurethane: good movement and abrasion resistance, paintable, used in masonry movement joints and floor joints. Polysulphide is used for high-movement structural/below-water joints.

What is the purpose of a backer rod in a sealed movement joint, and why avoid three-sided adhesion?

A closed-cell backer rod sets the sealant depth and acts as a bond-breaker so the sealant adheres only to the two opposing joint faces. Three-sided adhesion (the sealant also stuck to the joint back) restrains the sealant and causes it to tear or peel as the joint moves, so two-sided bonding lets it stretch and recover freely.

What is the recommended width-to-depth ratio for a sealant bead in a typical masonry movement joint?

About $2:1$ width to depth for elastic sealants in joints over $\sim 12\,\text{mm}$ wide (e.g. a $20\,\text{mm}$ joint sealed about $10\,\text{mm}$ deep), with a practical minimum depth of around $6\,\text{mm}$. The correct hourglass bead shape (thinner middle) maximises movement capacity and prevents over-stressing the bond lines.

How does cavity-wall construction itself act as a weatherproofing/damp-defence detail?

The clear cavity (typically $50\,\text{mm}$ minimum, kept clean of mortar droppings) breaks the path of penetrating damp: water crossing the outer leaf runs down the cavity face and is discharged outward via DPCs, cavity trays and weep holes rather than reaching the inner leaf. Wall ties have a drip and are laid with any twist/drip pointing down to shed water.

What is the function of a cavity tray and weep holes, and where are they required?

A cavity tray is a stepped DPC that bridges the cavity to collect water and direct it to the outer face; weep holes (open perpends, about every $450\,\text{mm}$) let that water drain out. They are required wherever the cavity is bridged or interrupted — above lintels/openings, at abutments, roof junctions, and where an external ground level or floor slab crosses the cavity.

In a repair survey, what evidence distinguishes active (ongoing) cracking from historic (stable) cracking?

Active cracks are clean, sharp-edged, may have fresh debris, and tell-tales/markers placed across them show continuing movement over time. Historic/dormant cracks are dirty, painted-over, weathered, previously filled, and crack monitors show no further movement. Tell-tales (e.g. calibrated glass or proprietary gauges) are fitted and monitored over several months to confirm.