NABTEB GCE 2025 ADVANCED BRICKLAYING/BLOCKLAYING & CONCRETING ANSWERS (ESSAY – TYPE A)

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ADVANCED BRICKLAYING/BLOCK LAYING & CONCRETING ANSWERS (TYPE A)

SECTION A: ANSWER ALL QUESTIONS

(1a)
Concrete coping is required on walls to provide a protective covering that prevents water penetration into the wall structure, thereby reducing the risk of water damage and enhancing durability. It acts as a barrier to stop rainwater from seeping into the wall, which can cause weakening or deterioration over time.

(1b)
(i) Frost damage (freeze-thaw cracking).
(ii) Efflorescence on the wall surface.
(iii) Weakening and crumbling of mortar joints.
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(2a)
(Draw the diagram)

(2b)
(Draw the diagram)

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(3a)
Mottle is a defect in stones characterized by uneven coloration or blotchy, patchy appearances on the stone surface. This discoloration is primarily aesthetic but can be seen as a sign of uneven mineral distribution or impurities within the stone. Mottle does not usually affect the stone’s strength but can reduce its visual appeal, making it less desirable for walling where uniform appearance is required.

(3b)
Creep in stones refers to the gradual deformation or movement of stone materials over time under sustained load or stress. In walling, creep can cause the stone to slowly shift or sag, leading to misalignment, cracks, and structural instability. This defect compromises the integrity and durability of the stone wall and may require repair or reinforcement to prevent failure.
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(4a)
(i) To allow natural light to pass through: They transmit daylight into interior spaces while still maintaining privacy.
(ii) To provide privacy while admitting light: Their patterned or textured surfaces prevent clear visibility through the wall.
(iii) To improve aesthetics: Glass blocks create attractive decorative wall panels and add visual appeal to buildings.
(iv) To provide sound insulation: Because they are hollow or thick-walled units, they reduce external noise.
(v) To give partial transparency with security: They let in light but are strong enough to act as a wall element.

(4b)
(i) Bathrooms and toilets (for privacy lighting).
(ii) Stairwells and corridors (to brighten dark areas).
(iii) Partition walls inside buildings.
(iv) Entrance lobbies for decorative lighting.
(v) Kitchen walls or high-level openings.
(vi) Basements to bring in daylight while keeping security.
(vii) External walls where light is needed but clear visibility is not desired.
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(5a)
(Draw the diagram)

(5b)
(Draw the diagram)

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(6a)
Loss of strength occurs when cement is no longer able to produce concrete with its normal compressive strength. This often results from aging, moisture absorption, or improper storage.

=Effect on Cement Used for Concrete=
(i) Weak concrete: When cement loses strength, the resulting concrete becomes weak and cannot carry the designed loads, leading to structural failure.
(ii) Poor binding with aggregates: The cement loses its ability to form strong bonds with sand and gravel, reducing the overall durability of the concrete.
(iii) Increased porosity: Concrete becomes more porous, allowing water to penetrate easily, which reduces long-term durability.
(iv) Cracking and disintegration: Weak concrete is more likely to crack under stress, shrink excessively, or crumble during service.

(6b)
Loss of adequate setting time means the cement either sets too quickly (flash set) or too slowly. This happens due to improper composition, expired cement, or exposure to moisture before use.

=Effect on Cement Used for Concrete=
(i) Difficult to work with (poor workability): If cement sets too quickly, workers cannot properly place, compact, or finish the concrete before it becomes stiff.
(ii) Poor finishing and surface quality; Loss of proper setting time leads to rough surfaces, honeycombs, or poor alignment of concrete members.
(iii) Weak or unstable concrete: If cement sets too slowly, the concrete takes too long to harden and may be damaged by vibration, movement, or early loading.
(iv) Compromised strength development: Incorrect setting time affects hydration and results in concrete that may never achieve its intended strength.
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(7a)
(i) Designate proper storage areas: Allocate separate, well-drained areas for sand, cement, aggregates, steel, and bricks to prevent contamination or mixing.
(ii) Protect materials from weather: Cover cement, sand, and other moisture-sensitive materials with tarpaulin, waterproof sheets, or store in sheds to prevent dampness and deterioration.
(iii) Use stacking and labeling methods: Stack materials in uniform layers with supports to avoid collapsing. Clearly label materials to track inventory and usage.
(iv) Follow FIFO (First-In-First-Out): Ensure older materials are used before newer ones, reducing wastage and deterioration, especially for cement.
(v) Prevent theft or unauthorized access: Use fenced storage areas and secure locks to safeguard materials.

(7b)
(i) Measure materials accurately: Use measuring boxes, weighing scales, or calibrated equipment to ensure correct quantities of cement, sand, aggregates, and water.
(ii) Follow mix design strictly: Ensure the concrete mix complies with the engineer’s specification (e.g., 1:2:4 ratio) for strength and workability.
(iii) Supervise manual or machine batching: Check that materials are added in the correct sequence and mixed thoroughly to avoid weak spots.
(iv) Maintain records: Keep daily logs of materials used and batch sizes to track consistency and minimize wastage.
(v) Check water content: Control water addition to prevent overly wet or dry concrete, which affects curing and strength.
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(8)
(i) Ensure formwork is strong, stable, and properly supported.
(ii) Provide safe access and working platforms with guardrails.
(iii) Handle and place concrete carefully to avoid segregation.
(iv) Protect and cure the concrete immediately after casting.
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(9a)
Cantilever Slab: A cantilever slab is supported at one end only.

Reinforcement arrangement:
(i) Top reinforcement: Placed near the top surface at the fixed end to resist tension caused by bending.
(ii) Bottom reinforcement: Runs along the length of the slab, mainly near the free end where tension is lower.
(iii) Distribution steel: Light reinforcement across the slab width to control cracking

(9b)
Road Slab: A road slab (pavement slab) is generally simply supported or continuous over the subgrade.

Reinforcement arrangement:
(i) Top reinforcement: Placed at the top near supports to resist negative bending moments (tension on top).
(ii) Bottom reinforcement: Placed near the bottom at mid-span to resist positive bending moments (tension at the bottom).
(iii) Distribution steel: Provided across the width to resist temperature and shrinkage stresses.
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(10)
(i) Durability: Ability of the material to withstand wear, weather, and environmental conditions.
(ii) Aesthetic appeal: Colour, texture, and overall appearance suitable for the building design.
(iii) Cost: Affordability and overall budget for procurement and maintenance.
(iv) Maintenance requirements: Ease of cleaning, repair, and long-term upkeep
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SECTION B: ANSWER FIVE QUESTIONS ONLY.

(11)
(Draw the diagram)

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(12a)
(Draw the diagram)

The sketch shows the trench walls cut back at a sloping angle instead of being vertical. By slanting the sides outward, the weight of the surrounding soil is spread out and reduced against the trench, which helps keep the walls stable and prevents them from collapsing inward.

(12b)
(i) It requires a much wider excavation area, which may not be possible on restricted or congested sites.
(ii) It increases the volume of soil to be removed, leading to higher labour, equipment, and disposal costs.

(12c)
(i) Soil type and stability: The nature of the soil, whether it is soft, loose, sandy, clayey, or firm, affects how deep a trench can be safely dug. Unstable soils require shallower depths or additional support to avoid collapse.
(ii) Groundwater level: High water tables weaken trench walls and make deep excavation unsafe. Dewatering may be required, and trench depth must consider the risk of water seepage and wall failure.
(iii) Purpose of the trench and required foundation level: The depth depends on what the trench is meant for such as foundations, pipes, cables, or drainage. For foundations, it must reach a stable stratum below topsoil and vegetation, and must also consider frost depth, load requirements, and structural design levels.
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(14a)
(i) Flat roofs require less material and are economical to construct.
(ii) They provide usable space for services such as water tanks, solar panels, or recreational areas.
(iii) They are easier and safer to access for inspection, maintenance, and repairs.

(14b)
(i) Hessian-based bitumen felt
(ii) Fibre-based bitumen felt
(iii) Sand-faced bitumen felt

(14c)
To lay the hessian-based bitumen felt, the roof surface is first cleaned and made dry to ensure proper adhesion. A primer coat of hot or cold bitumen is then applied uniformly over the prepared surface. The felt is rolled out carefully, ensuring correct alignment and removing air pockets by pressing it down with a roller. Overlaps are made along the edges and joints, after which hot bitumen is applied to seal the overlaps securely. Finally, the entire surface is pressed firmly to achieve a watertight bond.
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(15a)
(i) Rising damp: Moisture from the ground moves upward through the wall by capillary action due to the absence or failure of a proper damp proof course.
(ii) Rain penetration: Water enters the wall from the external surface because of cracks, defective rendering, or poorly finished joints.
(iii) Condensation: Moisture forms inside the building when warm, humid air comes into contact with cold wall surfaces.
(iv) Plumbing leaks: Water from damaged or poorly installed pipes seeps into the wall structure, causing damp patches over time.

(15b)
(i) Flexible D.P.C membrane: It is a damp proof material that can bend, fold, or adapt to irregular surfaces without cracking, making it suitable for locations where movement or structural vibration may occur.
(ii) Non-flexible D.P.C membrane: It is a rigid material that cannot bend or adapt to uneven surfaces and is mainly used where the structure is stable and requires a firm, solid damp proof layer.

(15c)
(i) Example of flexible D.P.C: Polythene sheet.
(ii) Example of non-flexible D.P.C: Slate.
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(16i)
Porosity: This refers to the presence of tiny voids or air spaces within a sandcrete block. These voids are created during mixing, compaction, and curing. The higher the number of pores, the more porous the block becomes. Porosity influences the block’s strength, durability, weight, and its ability to retain or absorb moisture.

(16ii)
Permeability: This is the ability of a sandcrete block to allow water or moisture to pass through it. A highly permeable block easily permits water movement from one side to the other due to interconnected pores. Permeability affects the block’s resistance to dampness and determines how well it can withstand prolonged exposure to moisture.

(16iii)
Buckling: This refers to the bending, bulging, or sideways deformation of a sandcrete block wall when it is subjected to excessive compressive load without adequate lateral support. Buckling occurs when the wall or column becomes unstable, often due to slenderness, poor-quality blocks, inadequate reinforcement, or foundation settlement.

(16iv)
Absorbent capacity: This is the ability of a sandcrete block to take in and retain water within its pores. It depends on the block’s porosity, density, and compaction. A block with high absorbent capacity will soak up more water, which can weaken its structural performance and increase the likelihood of dampness problems in a building.
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(17a)
(i) Strength
(ii) Durability
(iii) Flexibility
(iv) Resistance to wear
(v) Proper size and uniformity
(vi) Non-reactivity with other building materials

(17b)
Bricks are manufactured by first selecting suitable clay, which is then crushed and ground into a fine powder. Water is added to the clay to make it pliable, after which the mixture is molded into the desired brick shape, either manually or using machines. The molded bricks are dried to remove excess moisture and then fired in a kiln at high temperatures to harden them and achieve strength and durability. Finally, the bricks are cooled and sorted for use in building construction.
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