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Concrete Technology & Sustainable Materials Pt 2 - Coggle Diagram
Concrete Technology & Sustainable Materials
Pt 2
Quality Control
Mixing
Purpose
To coat the surfaces of all aggregates with cement paste.
To blend all the ingredients of concrete to give a uniform consistency.
Types
Central-mixed concrete (mixing @ central plant)
Truck-mixed concrete (batched in a central plant but are mixed in a mixer truck).
Properties of fresh concrete
Concrete in the fresh state is subjected to mixing, handling, transporting, placing and compaction to required shape or form.
Important element of fresh concrete workability, setting time and stability to prevent segregation.
Workability Test
Workability is the ease of a concrete mix that can be handled from the mixer to its finally compacted shape.
Measurement method: Slump test.
Relevant Standards: Testing fresh concrete – BS EN 12350;
Types of slump
True Slump
Shear Slump
Collapse Slump
Other important Properties of fresh Concrete
Bleeding
Is due to rise of water in the mix to the surface because of the inability of the solid particles in the mix to hold all the mixing water when they settle downwards.
Caused by
Too much water
Coarse sand grading
Not enough of fines
Effects
Bleeding leaves long chains of pores and weaken concrete surface.
Segregation
Has to be avoided and repair works need to kick in when segregation happens to the concrete structure.
This increases the porosity of the concrete and loss in strength
there is often a tendency of the heavier particles to separate from the lighter ones, which leads to a state of non-uniformity of the mix.
Hardened Concrete
Main properties of hardened concrete
Strength
Strength is defined as the maximum load or stress it can carry at day 28.
Strength is considered as the most important property and the quality of concrete is often judged by its strength.
Durability
As long as the specified strength is achieved with proper concrete cover and proper compaction has been carried out during concreting (avoid segregation), the durability of the concrete will be good.
Relevant Standards
Testing concrete – BS 1881; SS 78.
Testing hardened concrete – BS EN 12390.
Factors Affecting Strength of Hardened Concrete
Factors affecting the compressive strength of concrete
Water/Cement Ratio
Water / cement ratio (W/C) is one of the most important properties in concrete.
As the water to cement ratio increases, the strength of a concrete decreases.
Reduction in w/c ratio, however, will affect it's workability.
Gel/Space Ratio
Define as ratio of volume of cement gel to the sum of the volume of cement gel and capillary pores.
Capillary porosity will increase with increased w/c ratio, hence resulting in lower strength of concrete.
Reduced in w/c ratio, however, will affect it's workability.
Partially compacted mix will contain a large percentage of voids and the concrete strength will drop.
Time and Temperature
Time
Strength of concrete increases with age.
Typically, at normal curing condition, concrete is able to achieve around 40%, 65%, 90% and 99% of the strength at day 3, 7, 14 and 28 days respectively.
Therefore, strength of concrete is usually defined as compressive strength of concrete at day 28.
Testing of strength at early age is necessary to interpret the final strength at day 28 in a construction site.
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Temperature
The temperature at which concrete is cured has an important bearing on the development of its strength with time.
The rate of gain in strength of concrete increases with increase in curing temperature at early ages but not at the later ages.
Maximum Size of Aggregate
Concrete strength decreases as the maximum size of aggregate is increased in a given mix proportion.
Optimum maximum aggregate size varies with the richness of the mix, being smaller for richer mixes, and generally lies between 10 and 50 mm.
In Singapore, the maximum size of aggregate is generally specified to be < 20mm due to minimum concrete cover of 25mm for structural element under mild environmental exposure.
Shape and Surface Texture of Aggregate
Aggregate shape and surface texture affect the tensile strength more than the compressive strength.
A smooth rounded aggregate will result in a weaker bond between the aggregate and matrix than aggregate with a rough surface texture or angular or irregular aggregate.
Tests
Destructive Tests
To measure strength of concrete
Mortar Cube Test
Concrete Cube Test
Cubes are cast in 100mm steel cube moulds.
Demoulded after curing for 24hr in humid conditions at room temp.
Stored in water until tested in a compression machine on the 1st, 3rd, 7th, 28th day.
Cylinder Test
May be used for compressive or tensile strength test.
Concrete Strength
Concrete Grade : G20, G25, G30, G35, G40, G45, G50
G20 refers to Grade 20 concrete that can achieve a minimum compressive strength of 20N/mm2 at day 28.
Testing ages : 1, 3, 7 & 28 days.
Compressive strength – the most important property to judge the hardened concrete quality.
Tensile strength – for design of concrete roads and pavements – concrete is weak in tensile (about 1/10 of compressive strength).
Non-Destructive Tests (NDT)
NDT are techniques which are used on existing structures to obtain in situ concrete material parameters with minimal damage to the structure.
Examples
Rebound Hammer
Ultra-sonic Tester
Penetration Resistance
CAPO
Pull-off Test
The details of the tests can be found in a publication by SCI known as “non destructive testing of concrete”, 1995.
Transport, Placing, Compacting, Finishing and Curing of Concrete
Process
1.Raw materials
2.Batching
The process of measuring ingredients or materials to prepare concrete mix is known as batching of concrete
3.Mixing
See Quality control
4.Transporting/Placing
To deposit the concrete as close as possible to its final position as quickly and efficiently as you can, so that segregation is avoided and it can be fully compacted.
Concrete can be transported by a variety of different methods ranging from wheelbarrows, dumpers and ready-mix trucks to skips and pumps.
5.Compacting
After concrete has been mixed, transported and placed, it contains entrapped air in the form of voids. The objective of compaction is to get rid of as much as possible this unwanted entrapped air.
The amount of entrapped air is related to the workability
Concrete with a 75 mm slump contains about 5 % air.
Concrete of 25 mm slump contains about 20 % (requires a longer compacting time).
Reasons for Removing Air
Voids reduce the strength of the concrete. For every 1 % of entrapped air, the strength falls by about 5 to 6 %.
Voids increase the permeability, which in turn reduces the durability.
Moisture and air are more likely to get to reinforcement and cause it to rust.
Fully compacted concrete are denser, stronger, impermeable, hence stronger.
Poorly / badly compacted concrete are weaker, more porous (honeycombed), hence not durable
Vibration
Concrete mix is vibrated ("fluidised”) to allow entrapped air to rise to the surface, so that the concrete becomes denser.
With a properly designed cohesive mix, segregation and bleeding will be minimised.
With an over-wet mix, the larger aggregate pieces may settle during compaction, with the result that a weak layer of laitance will finish up on the surface(due to bleeding) and the laitance must be removed.
Equipment
Vibrators
These are mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete.
Types of Vibrators
External vibrators (Form vibrators).
Internal vibrators (Poker/Immersion vibrators).
Length of compaction
Initial consolidation is rapid and the level of the concrete drops, but the entrapped air has still to be removed.
As the concrete is vibrated, air bubbles come to the surface. When the bubbles stop it can be taken as a sign that not much more useful work can be done on the concrete.
Sound : When the pitch (whine) becomes constant, the concrete is free of entrapped air.
Surface appearance : A thin film of glistening mortar on the surface is a sign that the concrete is compacted.
6.Finishing
Floating
The finishing process involves creating a dense hard surface to the concrete then applying the desired surface profile.
Finish the surface as required eg broom, wooden float, exposed, polished.
For rough or textured surfaces -> Use a wooden float.
For smoother finish -> Use a magnesium or aluminum float.
7.Curing
Curing the concrete is the process of maintaining as much moisture as possible in the mix to fully hydrate the cement.
The cement-water reaction takes weeks to fully complete. The longer the time taken to maintain the water molecules in the concrete, the better the reaction.
Methods
1.Ponding
Build low wall and fill with water to cover slab.
2.Spraying
Keep continuously wet to avoid crazing.
Use low pressure supply to avoid any surface damage.
3.Covering
Use wet straw, hessian, sand or sawdust (avoid staining).
Impermeable covering such as plastic sheet will trap moisture on concrete surface.
Curing Compounds
Apply after finishing when bleedwater disappears.
Apply twice at right angles to each other to form continuous film.
8.Striking formwork
Steel
What is it
Alloy of iron, and carbon with traces of other elements present as impurities.
Fine granular structure with carbon contained as cementite, i.e. chemical combined with the iron.
Relevant Standards
Hot rolled products of structural steels – BS EN 10025.
Steel for reinforcement of concrete – Weldable reinforcing steel – SS 560.
Grade of Steel Vs Carbon Content
The percentage of carbon affects the properties of the steel.
A higher carbon content increases strength and hardness, but reduces ductility, toughness and weldability.
Hence, mild steel is commonly used in construction.
Properties of Mild Steel
It is malleable and ductile.
It is more tough and elastic than cast iron and wrought iron.
It can be magnetised permanently.
It can easily be welded, riveted and forged.
It is equally strong in tension, compression and shear.
It is difficult to harden and tamper.
Its specific gravity is 7.8.
Uses of Mild Steel
As rolled structural sections like I-section; T-section; channel section; angle iron; plates; round rods in construction works.
Mild steel round bars are also used as reinforcement steels.
Mild steel tubes are also used as structural steel.
Corrugated mild steel sheets roof coverings.
Manufacturing of various tools and equipment, machine parts, towers, etc.
Reinforcement Steel for Concrete
To take the tensile stresses in concrete beams or slab.
To withstand shear stresses in beams which are greatest near the supports
To carry a proportion of the compressive stress and to withstand tensile stresses which may arise due to eccentric loading, as in columns.
Reinforcement may be used near the surface of mass concrete structures to control cracking by drying or carbonation shrinkage.
In some cases, secondary reinforcement is used to prevent spalling of concrete.
Steel Mesh Reinforcement
Manufactured from plain round bar electrically welded to form rectangular mesh sizes ranging from 100mm x 100mm to 100mm x 400mm.
Commonly used as reinforcement steel for slab and wall constructions.
High Tensile (Yield) Steel
This is normally used in prestressed concrete, and is usually in the form of cold-drawn high-tensile wires or alloy steel bars. The wires can be used singly or twisted together to form strand.
Specification for high tensile steel wire and strand for the prestressing of concrete – BS 5896.
Sustainable materials
Purpose
Ease the impact on our limited landfill capacity
Reduce Singapore’s dependency on imported construction materials and enhance supply resilience
Hence, the industry needs to choose materials that can reduce burdens to the environment.
All developers, designers, builders and suppliers have a responsibility to develop systems, products and methods that are environmentally friendly
Usage of structural steel in construction
the overall construction cost may increase around 2 to 3%
in general can complete faster and has labour saving of around 20% to 30%.
As the steel components are factory fabricated, they are generally of better quality and less dependent on the skill of workers at site
In addition, structural steel has a higher strength to weight ratio, hence, less requirement for foundation
Usage of Dry Partition Walls
Such dry partition wall are able to meet high level specifications in the areas of acoustic insulation, thermal insulation and resistance to fire and damp condition.
High performance dry wall are tested to severe duty standards in strength and is able to withhold minimum loading of 25kg in each point.
The fire rating for the board is about 1 hour and it is only around 10 to 15% of the weight as compared to conventional brick wall.
Recycling of Waste Material
Critical shortage of land space and the heavy reliance on the import of raw materials.
Wastes can be turned into resources by recycling them to reduce disposal problems in Singapore.
Types
Incinerator Ashes from domestic refuse.
Incineration ash comprises of about 15% fly ash and 85% incineration bottom ash.
can be possibly use for backfill materials for trenches, shore protection, land reclamation, concrete block, base and sub base for road construction.
Copper Slag.
by-product formed during copper process.
The molten copper forms at the bottom of the furnace while molten slag is formed on the top
The molten copper slag is then drained off and quenched with water or left in the air to cool down.
They can then be used or treated to replace up to 10% by mass of sand in the production to structural grade concrete subjected to some quality acceptance criteria.
It can also be used as sand replacement for Eco Concrete for non-structural component such as partition walls and road kerbs etc.
Milled Waste
Milled waste is asphalt that has been machine milled from existing roads
bitumen based product is commonly recycled and reused as sub-base materials. However 100% of such material must pass through 63 mm sieve.
Construction and demolition (CRD) waste
Recycled Concrete Aggregates (RCA)
Recycled concrete aggregates (RCA) is derived mainly from the crushed concrete from demolition waste. 70% or more demolition waste is made up of crushed concrete.
A typical process to reclaim RCA includes primary crushing (using jaw crushers), removal of ferrous metals (using magnetic separators), screening and removal of foreign materials such as bricks, plastics and asphalt, secondary crushing and screening of RCA into various sizes.
RCA may be used in Eco Concrete for constructing of non-load bearing walls, drains, kerbs, footpath, non suspended slab, lean concrete and etc. It can also be used for sub base and sub grade for road construction.
Wood Waste
Wood waste constitutes about 3% to 30% of the C&D waste and with technological means, it can be converted to wood based products with enhanced properties.
The process involves shredding of wood waste into chips, mixing and binding them using high heat and pressure to become compressed wood
Compressed wood product is pest free, fire retardant, denser, stronger, more consistent in texture and colour and has lower moisture content as compared to natural wood.
Construction and demolition waste (C&D) consist of mixture of hard-core (concrete, masonry, bricks, tiles etc), reinforcement bars, dry walls, plastic, glass, wood, scrap iron and other meals.
Hard-core makes up to about 90% of the total weight of construction and demolition waste.
For such waste to be useful, it has to be sorted. Once the materials has been sorted, it can be channelled to appropriate recycling facilities for further processing into reusable products.