Construction Technology Flashcards
What are the differences between cohesive soils and non-cohesive soils?
Cohesive soils (clay, silt) have smaller/smoother particles, attract positive water charge due to a small negative electrical charge, are easily moulded under load due to water present giving them plasticity, are affected by seasonal changes in water content causing them to swell in winter/shrink in the summer, are subject to frost heave and are affected by tree roots drawing up water from the soil.
Non-cohesive soils (sands/gravels) are coarse grained, irregular in shape, exhibit high frictional resistance when compressed under load, high load bearing capacity, do not bond together if not consolidated, which leads to low loadbearing capacity in saturated state caused by high groundwater levels separating particles. Excavation sites need a lot support.
What is the difference between displacement piles and replacement piles?
- Displacement piles (driven) are driven or jacked into the ground, displacing the soil around the pile shaft outwards and downwards. Used in offshore applications.
- Replacement piles (bored auger) remove the soil to form a hole and concrete is poured in situ. Popular in urban areas as there is minimal vibration.
In a congested urban building site, what type of pile would you recommend using and why?
Replacement or bored auger piles, as there is minimal vibration. These piles are also quick to construct, no prefabricated elements are required except steel reinforcement and they can be used where there is limited headroom.
How are piles/pile caps linked together?
A reinforced concrete beam supports the loadbearing wall over the piles and links them all together.
What effect would removing a large tree have on foundations?
Large trees can affect soils and foundations to a depth of 4m. When trees are removed, moisture in cohesive soils builds up and the soil expands, causing heave. Large broad -leaved trees of high water demand are notorious for causing damage. Ground displacement is likely to only be 150mm or less, but foundations can move and basement walls and floors crack and lift, potentially causing serious structural damage.
How would a crack taper following the removal of a large tree? (What effect would removing a large tree have on foundations? How would the subsequent crack taper?)
Generally cracking to brickwork and windows. The cracking is more likely to be vertical (subsidence more likely to be diagonal). The crack would be wider at a lower level, tapering as it rises. Cracks generally occur at right angles to the diagonal and upward movement.
How deep should a foundation be to avoid frost heave?
In frost susceptible soils like chalk the depth to the underside of the foundation should be at least 450mm, but in clay soils subject to volume change, strip foundations should be taken to a depth of 750mm on low clay shrinkage soils, 900mm on medium clay shrinkage soils and 1m on high shrinkage soils according to Approved Doc A.
Name 2 types of underpinning.
- Mass concrete underpinning – one of the most common methods. It is conducted in successive sections, leaving the greater proportion of the existing foundations supported throughout. Each section is generally between 900mm and 1.5 m.
- Beam and pier underpinning – comprises a reinforced concrete beam, inserted either directly above or below the existing foundation, supported by mass concrete piers constructed at 2.5-3m centres.
- Pile and needle underpinning – comprises reinforced concrete needles inserted horizontally through the existing wall above foundation level and supported at each end by small-diameter piles that transmit the building’s loads to a deeper, firmer stratum. The needles are inserted at approximately 1.5m centres along the length of the wall being underpinned. The piles are taken down to a suitable loadbearing strata.
- Cantilever ring beam underpinning – comprises horizontal steel I-section cantilever needles inserted into the wall and supported on a reinforced concrete ring beam and mini piles. The needles transmit the loads from the wall to the beam and thence piles to a deeper bearing stratum.
- Double angle mini-pile underpinning involves the installation of small-diameter piles in pairs formed at an angle through the existing foundation at between 1-1.5m intervals.
Under what conditions would you expect a raft foundation to be used?
Raft foundations are used for lightly loaded buildings on poor soils where the top 450-600mm of soil is overlaying a poor quality strata. It is normally the same size or larger than the plan area of the building and it effectively floats on the ground by spreading the load over a larger area.
There are 3 types –
- Nominally reinforced rafts (lightly loaded low-rise instances) – poor soils, light loads
- Designed reinforced concrete slab rafts (heavily loaded or point loaded situations)
- Reinforced concrete beam and slab rafts
What is the purpose of a foundation?
- To safely sustain and transmit to the ground combined dead, imposed and wind loads to prevent movement which would impair the stability of the building. The safe transmission of the load to the ground should take place without differential settlement or any settlement that could adversely affect the neighbouring or surrounding buildings and structures.
- Foundations are designed to sustain and transfer dead and live loads
- Resist the movement of soils so that swelling, shrinkage of cohesive soils or expansion due to freezing of water in clays does not affect the stability of the building.
- Remain durable, resisting attacks from salts and chemicals in the ground.
- Can be designed to resist the passage of heat transfer.
What is the difference between live, dead and imposed loads?
- Dead loads refer to the structure’s self-weight and generally remains constant throughout the structure’s life.
- Live loads are also known as imposed loads, and consist of all loads temporarily placed on the structure, like loads of people, furniture, machines, traffic, etc. This keeps changing and depends on the use of the building.
Define subsidence.
Subsidence occurs when the soil beneath a building is unstable and sinks downward, usually associated with volumetric changes of subsoil. It is caused by a change in ground conditions caused by prolonged dry spells, especially when coupled with shrinkable clay soils. Leaking drains (washing away soil around foundations), broken culverts, effects of trees and shrubs can all absorb significant volumes of water from the soil, collapsed mine workings or decomposing organic fill. Differential subsidence is problematic.
What factors would you consider in determining the foundation type?
- Loads from the building
- Types of soils
- Soil bearing capacity
- Ease of construction
- Water table level
- Adjoining structures, water bodies, slopes
- History of natural disasters or extreme weather
- Economics if a number of options available
Why would you use a piled foundation?
- They are typically used to transfer the load from superstructures through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads.
- They are typically used for large structures and in situations where soil is not suitable to prevent excessive settlement.
What is the disadvantage of a raft foundation?
- advantages:
o constructed for shallow depth and requires less excavation
o suitable in soils of low bearing capacity
o loads coming from the superstructure are distributed over a larger area
o differential settlement of soil can be reduced - disadvantages
o In some cases a large amount of reinforcement is required, which increases the cost of the project
o Special attention is required in the case of concentrated loads
o If they are not treated properly, there is a chance of edge erosion
o Skilled workers are required to construct the raft foundations.
What is settlement?
- Settlement is the downward foundation movement as a result of soil being compressed by the weight of a building within ten years of construction or poorly compacted made ground.
- Occurs shortly after construction
- Usually small movements and non-recurring
- Differential settlement can cause serious problems
How would you investigate whether a building was subsiding?
- Inspect the cracking (diagonal cracks at 45° subject close to windows and doors) with tapered cracks becoming larger with height. Crack displacement both vertically and horizontally, extend below the DPC and ground level, are at least 3mm wide and both internal and external.
- Inspect the surrounding areas for the presence of trees, bushes or hedges
- Consider if there are any drainage issues to the area.
- Consider the weather conditions – drought?
- Check the soil type for the area and if there is any history of mining in the area.
- Recommend that a structural engineer inspects, and monitor the cracking.
Name four different types of foundation solutions normally found in construction?
- Pad foundations
- Raft foundations
- Strip foundations
- Piles
What is CBR?
- California Bearing Ratio - a test performed on construction materials laboratories to evaluate the strength of soil subgrades and base course materials.
How are earthworks supported and why?
- They are supported by shuttering to stop movement and the collapse of the earth.
Under what conditions would you expect a raft foundation to be used for a substructure?
- Used on soft or loose soils, such as sand where the load needs to be spread.
What are the different types of piling and when would you expect each of them to be used?
- Friction piles – develop most of the pile bearing capacity by shear stresses along the sides of the pile and are suitable where harder layers are too deep to reach economically. The pile transmits the load to the surrounding soil by adhesion or friction between the surface of the pile and the soil.
- Continuous flight auger piles (CFA) – a cast in situ method of piling first used in the 1960s in the UK and is now one of the most common. Due to its low level of vibration and sound it is particularly suited to environmentally sensitive sites and soft/water-bearing strata where deep casings would otherwise be necessary. The auger screws into the ground and when reaching the design depth, concrete is pumped through the hollow stem of the auger, whilst it is slowly extracted, removing the ground material. Advantages – low sound/vibration, installed quickly and economically, high load bearing, sheer and movement capacities and can be adapted to operate in conditions with low headroom/confined spaces.
- Impact driven piles – (also known as displacement piles) are commonly used to provide support for structures, transferring their load to layers of soil or rock that have sufficient load bearing capacity and suitable settlement characteristics. Can be the most cost effective. It is a long slender column made of pre-formed material and is installed by impact hammering, vibrating or pushing it into the ground to a design depth/resistance. Can be installed to accommodate compression, tension and lateral loads. Can be steel, precast concrete, timber or composite. Advantages – pre-fabricated off site, displace and compact the soil, increasing the load bearing capacity of the pile and no soil and little spoil needs to be removed, cost effective, superior structural strength. Disadvantages – planning required as well as heavy equipment, adequate reinforcement needed for pre-cast/pre-stressed concrete piles, length may not be exact and splicing may be required, may not be suitable for ground with poor drainage or compact sites where vibrations may impact neighbouring properties, noisy to install.
Describe the component of a piled substructure and what their function is.
- Pile foundations are deep foundations that are formed by long, slender columnar elements typically made of steel or reinforced concrete and sometimes timber. It is defined as piled when its depth is more than three times its breadth.
- Principally used to transfer loads from superstructures through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads. Typically used for large structures and in situations where soil is not suitable to prevent excessive settlement.
Describe three different ways of waterproofing a new build basement area.
- Type A (Barrier) system – Tanking – objective is to provide a continuous waterproof membrane which is applied to the base of the slab and walls – it can be applied internally or externally according to site circumstances. Alternatives to mastic asphalt or polythene sheeting are bituminous compounds – epoxy resin compounds and bitumen laminates. External mastic asphalt tanking preferred as it prevents the ingress of water and protects the structure from aggressive sulphates in the soil and ground water. Internal mastic asphalt tanking should only be adopted if external tanking is not possible as it will not protect the main structure and may be forced away from the walls/floor by hydrostatic pressure. To be effective the horizontal and vertical mastic must be continuous.
- Type B (Structurally Integral) system – Dense monolithic concrete – forms a watertight basement using dense, high quality or prestressed concrete by a combination of good materials, workmanship, detailing and construction methods. Not always vapour proof and may also need a coating/tanking. Watertightness depends on the water/cement ratio and degree of compaction.
- Type C (Drained) system – drained cavity system – accepts a small amount of water seepage through a monolithic concrete wall and collects moisture and drains it away. Achieved by building an inner non-load bearing wall to form a cavity, which is joined to a floor composed of triangular tiles laid to falls, which enables the water to drain away to a sump. The inner wall should be vapour tight or the cavity should be ventilated.
Name 3 types of materials for rooflights.
- Glasses reinforced plastic (GRP)
- Polycarbonate
- Georgian wired glazing
What are the two common finishes for roof sheets?
- PVC Plastisol
- Polyester paint coated
How do you distinguish between Plastisol and Polyester Coated Finish roof sheets?
- Plastisol has an embossed leather grain texture with good corrosion resistance and excellent abrasion resistance.
- Polyester coated finish is cheaper and offers a smooth finish, but does not have such a long lifespan.
What are the three types of portal frame?
- Pitched roof symmetric portal frame.
- Tied portal frame
- Mono pitch portal frame
- Propped portal frame
- Mansard portal frame
What are composite panels?
- Typically used in the external cladding of buildings.
- Can be bent, curved and joined together in an almost unlimited range of configurations
- First emerged in the 1960s.
- Two metal skins are bonded to an insulating core, forming a composite sandwich panel.
- The metal component can be aluminium, zinc, stainless steel etc.
- Available in a wide variety of colours, finishes and profiles
- It can be manufactured from an insulating material such as polyethylene, or from a fire retardant material.
- Advantages: Weather resistance, lightweight, acoustic insulation, thermal insulation, a consistency of finish that requires little maintenance, they do not wrinkle as the external skins are bonded to the core under tension.
- They can be more cost effective and can be installed faster than precast panels and have reduced structural support requirements because of their lighter weight.
What are the issues surrounding composite panels?
- Flammable cores that are difficult to extinguish and can result in a total loss situation.
- The fire can spread rapidly within the panels and it is shielded from extinguishing water. Sprinkler protection, gas extinguishing systems and water mist systems would be unable to control or extinguish the fire.
From a technology perspective, what is the difference between steel and concrete frames? (i.e. fire resistance, light weight, etc.)
- Steel – flexibility (endless design applications), lightweight (60% lighter and may allow for a less expensive foundation system), building design may be easier to modify, high lead in time, fast erection on site, inherently non-combustible, but strength reduces when heated to extreme heat so needs fire protection (thin intumescent coatings can be applied offsite), reduced labour costs through dryness of form, fluctuating prices of steel, structurally strong, stiff, tough and ductile, long life span and can be reused or adapted, it may corrode if it comes into contact with water.
- Concrete – waste materials can be included within the mix (ground granulated blast-furnace slag and pulverised fuel ash), inherent fire protection, precast concrete can reduce build time, heavier, ongoing maintenance and repairs required, consistent pricing, high strength in compression, but lacks tensile strength and must be reinforced by steel rebar to increase tensile capacity, ductile strength and elasticity, can be moulded into many different shapes, but there are limitations for floor-to-floor construction heights and long open spans, reinforced concrete is water resistant and will not corrode with proper construction and care.
- Both have high embodied energy in their manufacture. Steel is nearly 100% recyclable (90% used today created from recycled steel). Concrete contains natural products – can be crushed and reused in future admixtures.
What would you consider in the design of a concrete floor in a warehouse?
- Floor loadings
- Subsoil types – subgrade soils could be a problem when they are highly expansive or compressible (silts, clays) and do not provide uniform support.
- Structural design of the concrete floor slab – ground bearing slab or a pile supported suspended slab. If a consolidation of plastic soils is determined to be a potential problem a suspended slab may be the only effective solution in which the floor slab is built on piles or between ground beams.
- Joint design requirements – proper design of the concrete mix, use of concrete reinforcement, satisfactory curing and appropriate joint spacing contribute to crack prevention. A specialised joint system must be specified that will accommodate the movement and support the traffic without creating a discontinuity in the surface level.
- Surface characteristics – ability to resist wear and dusting – depends on the composition of the concrete ad hardness and toughness of the topping material, including finish coatings. Chemical resistance (floor should be protected with chemically resistant material and coating that resists any aggressive substances)
- Whether to decorate/apply an epoxy resin.
How would you remove redundant racking bolts from a concrete floor slab?
- Core drill out the redundant fixing bolts, clean the hole to remove all dust and lose material and inject an epoxy resin into the hole.
How often are movement joints required in a brickwork wall?
- Clay brickwork – 10-12m
- Lightweight concrete block/brick with lightweight aggregates – 6m
- Dense concrete block and brick (dense aggregate) – 7.5-9m
- Calcium silicate brick – 7.5-9m
- Masonry in a parapet wall – half the above spacings and 1.5m from corners
- Movement joint widths for clay bricks not less than 1.3mm/m and not less than 10mm/m for other masonry.
How often are wall ties required in cavity wall construction?
- Every 450mm vertically and every 900mm horizontally
What are ACMs?
- Aluminium composite material
- Commonly used for cladding buildings, typically as a form of rainscreen (drained and ventilated or pressure equalised facade) is part of a double wall construction.
- Rainscreen simply prevents significant amounts of water from penetrating into the wall construction. Thermal insulation, airtightness and structural stability are provided by the second, inner part of the wall construction.
- Consists of 2 skins of aluminium bonded to either side of a lightweight core of materials such as polyethylene (PE), polyurethane (PUR) or a mineral core.
- Popular due to its flatness, variety of surface finishes and colours, lightweight and formability.
- During a fire, the panels can delaminate, exposing the core material.
- Grenfell – with a polyethylene core (PE) contributed to the fire. Reynobond PE ACM cladding used was a cheaper, more flammable version of the Reynobond FR, which had a fire retardant core, or Reynobond A2 which has a non-combustible core.
- Types – Non-combustible mineral fibre (fibreglass/mineral wool)
- Polyisocyanurate foam (PIR)
- Phenolic foam (PF)
- Polyurethane foam (PUR)
- Expanded (XPS) or extruded polystyrene (EPS)
What is the purpose of the Building Regulations?
- Establish requirements for specific aspects of building design and construction.
- Approved Documents then provide guidance for satisfying those requirements in common building situations.
What is the LPCB?
- Loss Prevention Certification Board
- Part of BRE Global – sets the standards needed to ensure that fire and security products and services perform effectively.
- Offer 3rd party certification
What is the Fire Resistance standard: Euroclass system?
- One of the main standards used in Europe to classify the fire reaction of construction and partition products. There are 7 levels of categorisation from A1 to F (A1 best) The additional categorisation for smoke is S1 to S3, with S1 the best. For flaming droplets they range from d0 to d2, with d0 the best.
- A1 – Non-combustible
- A2 – Limited combustible (No flashover)
- B – Combustible (No flashover)
- C – Combustible (flashover after 10 minutes)
- D – Combustible (flashover before 10 minutes)
- E – Combustible (flashover before 2 minutes)
- F – Combustible (No performance determined)
(flashover is a near simultaneous ignition of most of the directly exposed combustible materials in an enclosed area). - A1 or A2 with an S1 sub-rating are equivalent to non-combustible. A ratings are only likely to apply to mineral fibre insulated panels
- Best for expanded plastic insulated panels is category B. Panels with s1 or s2 sub-rating and d0 sub-rating are considered as a construction with limited combustibility, but not fully equivalent to non-combustible.
If a client requested replacement of a natural roof slate covering with cement tiles, what would you advise and what would be the considerations?
- I would try and understand their motivations for this alteration.
- If cost is the key driver, I would advise that although natural roof slate is more expensive, cement tiles would weigh considerably more than the slates, and a structural engineer would need to be involved to ensure that sufficient structural support is installed to accommodate this, bringing up the cost.
- Aesthetically, I would also highlight that these may affect the appearance of the property if this is out of keeping with the type of property, and they tend to weather poorly.
- There is also a greater environmental cost to the production of cement.
Name three different types of pitched roof truss.
- King post
- Queen post
- Hammerbeam
What is the difference between a hipped and pitched roof?
- A hipped roof is a type of roof where all sides slope downwards to the walls, usually with a fairly gentle slope. It has no gables or other vertical sides to the roof. It is self-bracing, requiring less diagonal bracing than a gable roof. Gable roofs only have 2 triangle-shaped slopes that extend from the bottom of the roof’s eaves to the peak of its ridge.
- A pitched roof is a roof that slopes downwards, typically in 2 parts at an angle from the central ridge.
What are the advantages of constructing a warm flat roof over a cold flat roof?
The cold roof has a vapour control layer beneath the insulation to reduce the risk of condensation to the underside of the weatherproof layer, but the difficulty of ensuring the integrity of the VCL leads to the need to ventilate the roof space and reduce condensation risk. The warm flat roof is a simpler construction as ventilation is not required, is cheaper and has a reduced probability of thermal bridging and condensation through the roof joists and hangers, as the insulation is continuous, rather than placed between the joists in cold roofs. It is the best option for the UK climate due to high levels of humidity, as there is likely to be insufficient ventilation to the cold roof.
Name four different types of covering for a flat roof.
- Mastic asphalt
- Bitumen felt
- Singly ply membranes
- EDPM (Ethylene Propylene Diene Monomer) rubber roofing
- Glass reinforced plastic
- Liquid applied coating.
Name two ways by which a single ply membrane can be fixed.
- Mechanical fastenings. These secure the single ply membrane over the top of the insulation and vapour control later, and are fastened in place to the roof deck.
- Adhesive bonding. In the past it was necessary to secure roof membranes with hot melt adhesive, but now it can be fully adhered by applying cold adhesive.
- Full and partial bonding.
How are slates fixed to a pitched roof?
They are normally fixed with 2 nails per slate, centre nailed, and a further fixing is used to secure the bottom edge of each slate to prevent it lifting in high winds or due to thermal expansion.
What is a counter-batten and when it is used?
They are wooden strips that are installed vertically over pitched roofs to which horizontal tiling battens are attached. They are used to allow rainwater to pass off the tiles and away from the roof.
How would you repair a leaking parapet gutter on an industrial unit? Brand/manufactures/cost:
- HD Sharmans’ Plygene gutterline system - £86/lm.
- Apply a new silicone coating - £55/lm (HD Sharmans’ seamsil)
Where does the vapour control layer sit in a warm roof construction?
Beneath the insulation, but above the decking.
What is the disadvantage of single ply membranes?
- Over time single ply membranes can shrink, become brittle and lose elasticity, resulting in seam failure.
- They are subject to damage from wind uplift, which can tear off sections of the membrane.
- Blisters can result from localised loss of adhesion with water getting trapped.
- It shrinks over time, placing stress on flashings and resulting in tears/cracking.
- It is easily punctured, and the thin layers are not very durable, so gravel/loose screws can rip through them.
- It is vulnerable to leaks, although they are generally sealed pretty tightly with adhesives, tape or mechanical fasteners.
- It is sensitive to UV degradation and can degrade quickly, opening the roof up to damage, tears and leaks.
