HKIA Exam Paper 4 Quiz 76

Seawater if used in cement will cause surface dampness, efflorescence and a moderate reduction in strength (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).

Tolerance is also set up for reinforcement regarding clear distance to formed surfaces, minimum spacing between bars, top bars in slabs, or beams at ±6mm.
(Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Tolerances).

A cavity wall is a wall constructed in two leaves with space or cavity between them. The two leaves are constructed of solid bricks or building blocks, each not less than 100mm thick with the intervening cavity not less than 50mm and not more than 75mm wide. Galvanized iron ties not less than 20 x 3mm in cross-section are used to unite the two leaves but not to transmit moisture. Such ties are built into the horizontal bed joints at distances not exceeding 900mm horizontally and 450mm vertically apart.
Limitations
Every external wall of a building shall be constructed of
i. masonry not less than 225mm thick
ii. plain concrete or reinforced concrete not less than 100mm thick (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.5 – Brickwork and Block Work – Cavity Wall).

Depth of caisson can go to 40 m. Diameter depends on the design and may range from 1.5 m to 7 m where excavating machines can be used for digging (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – Deep Foundations – Caissons).

Shoring against adjacent properties or roads will be in accordance with the approved shoring plans. Shoring systems such as Flying Shore, Raking Shore, and Dead Shore may be used (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – General Obligations).

For the test, concrete test cubes are made with the water and the appropriate test cement and the average compressive strength of the concrete test cubes must not be less than 90% of the average strength of the control test cubes (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).

According to the depth of the bearing strata, foundations can be classified as shallow or deep. Shallow foundation includes spread footing, strip footing, raft foundation, and grillage foundation. Deep foundation includes piling, caissons and bored piles (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – Foundation Types).

Cement-lime mortar consists of cement, lime putty, and sand 1:1:6 for external wall and 1:2:9 for internal walls as well as for vermiculite blocks (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.5 – Brickwork and Block Work – Mortars).

Whilst as coarse aggregate, not more than 10 percent by mass will pass a 5mm mesh screen and at least 95 percent by mass will pass a mesh of a size 6mm less than the minimum lateral distance between reinforcing bars, or 6mm less than the minimum cover, whichever is the smaller, or in case of solid slabs, the standard is reached when those can pass a 20mm mesh screen (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Aggregate).

Cement is the setting agent of concrete and can be ordinary Portland cement, rapid hardening Portland cement or Portland blast-furnace cement. These are made up of chalk or limestone and clay. High alumina cement which hardens fast will not be used in general structural concrete works (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Cement).

Nominal size of clay bricks in 225 x 105 x 70mm. Concrete block is constructed of mixes as 1 part Portland cement, 2 parts clean washed sand, 3 parts granite fines plus 5 parts 10mm granite aggregate. Standard block size is 390mm long by 190mm deep with varying thickness to suit wall construction (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.5 – Brickwork and Block Work – Materials).

Rubble walling consists of stones finished in a rough or uneven surface presenting a natural appearance. Patterns of stones include random rubble, squared rubble, and square coursed rubble. Thickness of stone is 75mm minimum and mortar joints are 5 to 15mm wide (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.6 – Masonry and Granite/Marble Works – Traditional Masonry).

Water is tested by the initial setting time test and compressive strength test (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).

Shrinkage cracks, voids, and honeycomb are among the common defects in concrete causing water leakage and even structural failure in worse situations.

Remedial measures which include cement grouting, epoxy grouting, or guniting should be recommended and supervised by the Registered Structural Engineer (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Common defects).

According to the depth of the bearing strata, foundations can be classified as shallow or deep, Generally, shallow foundation goes to approximately 5m and support a load of 50 to 300 KPa. (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – Foundation Types).

Concrete is composed of aggregate mixed with cement and water. The proportion of each material controls the strength and quality of the resultant concrete.
(Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work).

For plain concrete, aggregate consists of sand, well-burnt brick, well-burnt tile, well-burnt clinker, stone or other approved by Building Authority (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Aggregate).

For block work not exceeding 337.5 x 225 x 112.5mm, the average crushing of blocks will not be less than 7.0N/sq mm. For larger size units, the average crushing strength of 10 blocks will not be less than 2.8N/sq mm (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.5 – Brickwork and Block Work – Strength and Shrinkage of Concrete Blocks).

The design and choice of foundation will make reference to:
i. Soil bearing capacity of the ground which is dependent on the type of soil as determined by site investigation. Data from adjoining sites can be used as reference. The level of bearing strata thus established determines the level of the foundation and if deep or shallow foundation is desired.
ii. The probable differential settlement of the building and the probable total settlement of the building. In general cases, an estimated resulting angular distortion in excess of 1:300 in any part of the structure will not result.
iii. The surrounding environment overloading the foundations of adjacent buildings or the ground supporting such foundations needed to be avoided either by dewatering or vibration or direct loading distribution. Interference with drins, nullahs, sewers, or other services needed to be prevented. There must be no unstable conditions created in adjacent slopes or roads (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – General Foundation Design).

Water for concreting must be clean and uncontaminated portable water from Government’s main supply. The amount of water used for making concrete should not exceed that required the concrete to be uniformly mixed and workable (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).

Taps shall be:
a. Sensor tap shall conform to BS EN 816. Operating pressure between 0.3 – 10 bar. Flow rate not more than 6 litres per min. Max. water temp. 80 degree. Aerator. Self-cleaning. Turn-off control pre-set 0 – 3sec.
b. Sensor mixer or cold.
c. Self – closing tap for time delay function.
d. Bib tap (General for Building 2017 Edition – Section 19 – Sanitary Appliances – Taps – 19.07).

Concrete mixes are expressed as proportions of cement, fine aggregates and coarse aggregate, e.g. 1-2-4 nominal mix = 1 part cement, 2 parts fine aggregate and 4 parts coarse aggregate (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Concrete Mixes).

Steel piling commonly used for foundation can be as deep as 40m.

Common piles are of 305mm x 305mm serial size and 110 to 223kg per metre
(Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – Deep Foundations – Piling).

As guidance, for concrete surfaces, variation from vertical plumb can be 9mm for each storey. Variation from the horizontal is set to a maximum 6 mm in 6000 mm. For columns and beans, the tolerance is minus 6mm and plus 12mm (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Tolerances).

The setting time for cement is not less than 45 minutes initially and not more than 10 hours at the final stage (Building Materials and Technology in Hong Kong- Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Cement).

Certain impurities in water are permissible. Chlorides must not exceed 50mg per litre. Sulphates shall not exceed 1000mg of sulphur trioxide per litre. Alkali carbonates and bicarbonates shall not exceed 1000mg per litre (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).

Steel reinforcement is added to acquire the strength to stand tension (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work).

The slump test aims to determine the consistency of fresh concrete. A galvanized iron mould in form of frustum of a cone with bottom diameter 200mm, top diameter 100mm and height 300mm is used. The mould is to be filled with concrete in four approximately equal layers. Each layer is tampeed with 25 strokes of a tamping rod (16mm diameter, 600mm long with round and for tamping) (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Slump Test).

Spread footing is a reinforced concrete mass supporting point load from columns and can be classified as rock or soil footing.
Strip footings are also reinforced concrete mass supporting load-bearing walls
Raft foundation is a continuous slab of spread footings used on poor soils to acquire a big base to spread out the loading of the superstructure
Grillage foundation makes use of a grid system of H-section steel beams to spread out the load. Concrete is then used to cover the steel beams (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.4 – An Outline of Foundation System in Hong Kong – Shallow Foundation).

The water/cement ratio in terms of weight is usually between 0.4 and 0.7 (Building Materials and Technology in Hong Kong – Chapter 2 – Trades of Materials and Technology – 2.3 – Concrete Work – Water).