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Cement Technology and Sustainable Materials Part 2 - Coggle Diagram
Cement Technology and Sustainable Materials Part 2
Quality Control
Mixing
to coat the surfaces of all aggregates with cement paste
blend all ingredients of concrete to give uniform consistency
2 categories of ready mixed concrete
Central mixed concrete
Truck mixed concrete
Properties of Fresh Concrete
important elements are workability, setting time and stability to prevent segregation
Fresh concrete is subjected to mixing,handling, transporting, placing and compaction to various shapes and form
Workability
ease of concrete mix handled from mixer to final compact shape
measurement method: slump test
Testing fresh concrete BS EN 12350
3 types of slump
True Slump
Shear Slump
Collapse slump
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
Causes
Too much water
coarse sand grading
not enough of fines
leaves long chains of pores and weaken concrete surface
Segregation
owing to the heterogeneity and complex nature of the concrete mix, 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
increases porosity of concrete and loses strength
has to be avoided and repair works need to kick in when segregation happens to the concrete structure
Hardened Concrete
Main Properties
Strength
defined as the maximum load or stress it can carry at day 28
usually use the 28-days Concrete Cube Test results as the concrete strength
considered as the most important property and the quality of concrete is often judged by its strength
Durability
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
Factors affecting strength of hardened concrete
Factors affecting compressive strength of concrete
Wet/Cement Ratio
one of the most important properties in concrete
as water to cement ratio increase, strength of concrete decreases
Gel/Space Ratio
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
increase in fineness of its cement particles will form better quality gel structure, in between the space, in the course of hydration and will increase the 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
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
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
Curing Temperature
temperature at which concrete is cured has an important bearing on the development of its strength with time
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
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
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
Destructive and Non-Destructive Tests
Destructive Tests
To measure strength of concrete
Mortar Cube Test
Concrete Cube Test
cubes cast in 100mm steel cubes moulds
demoulded after curing for 24h in humid and room temp
stored in water until tested in compression machine on 1st,3rd,7th and 28th day
Cylinder Test
used for compressive or tensile strength test
Concrete Strength
Concrete Grade : G20, G25, G30, G35, G40, G45, G50
testing ages : 1, 3, 7 & 28 days
Compressive Strength
most impt property to judge hardened concrete quaity
Tensile Strength
for design of concrete roads and pavements
Non-Destructive Tests
techniques which are used on existing structures to obtain in situ concrete material parameters with minimal damage to the structure
e.g. Rebound Hammer, Ultra-sonic tester CAPO etc
Transporting, Placing, Compacting, Finishing and Curing of Concrete
Transporting
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
can be transported by a variety of different methods ranging from wheelbarrows, dumpers and ready-mix trucks to skips and pumps
Compacting Concrete
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
amount of entrapped air related to workability
concrete with 75mm slump contains 5% air
concrete with a 25 mm slump contains 20% of air
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
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
Vibrators
mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete
2 major types of vibrators
External vibrators (Form vibrators)
Internal vibrators (Poker/Immersion vibrators)
Length of time required for full 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
when the pitch (whine) becomes constant, the concrete is free of entrapped ai
thin film of glistening mortar on the surface is a sign that the concrete is compacted
Finishing/Float
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
use a wooden float for rough or textured surfaces
for a smoother finish use an aluminium or magnesium float
Curing
curing the concrete is the process of maintaining as much moisture as possible in the mix to fully hydrate the cement
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
Ponding
Build low wall and fill with water to cover slab
Spraying
keep continuously wet to avoid crazing
use low pressure supply to avoid any surface damage
Covering
use wet straw, hessian, sand or sawdust
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
Steel
What is Steel?
alloy of an iron and carbon with some other traces of elements acting as impurities
fine granular structure with carbon contained as cementite
Grade of Steel vs Carbon Content
higher carbon content increases strength and hardness, but reduces ductility, toughness and weldability
mild steel is commonly used in construction (up to 0.25% carbon)
% of carbon affects properties of steel
Properties of Mild Steel
more tough and elastic than cast iron and wrought iron
can be magnetised permanently
malleable and ductile
easily welded, riveted and forged
equally strong in compression, tension and shear
difficult to tamper and harden
specific gravity is 7.8
uses of mild steel
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
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
secondary reinforcement is used to prevent spalling of concrete
Steel Mash Reinforcement
commonly used as reinforcement steel for slab and wall constructions
manufactured from plain round bar electrically welded to form rectangular mesh sizes ranging from 100mm x 100mm to 100mm x 400mm
High Tensile Steel
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
Sustainable Materials
Purpose
ease the impact on our limited landfill capacity
reduce Singapore’s dependency on imported construction materials and enhance supply resilience
Use of structural steel construction
has a higher strength to weight ratio
factory fabricated and higher quality, less skilled worker on site
labour saving of around 20 to 30%
Use of dry partition walls
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
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
astes can be turned into resources by recycling them to reduce disposal problems in Singapore
types of waste
copper slag
milled waste
Incineration Ashes
comprises of about 15% fly ash and 85% incineration bottom ash
largely inert and 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
treated and processed copper slag is imported from various countries by shipyards for grit blasting to remove rust and marine deposits accumulated on the ship
blasting copper slag breaks into smaller particles on impact with metal surfaces and after several rounds of reuse, the copper slag gets contaminated with rust and paints
Steel Slag
by-product formed during the steel making process and it can be used for coarse aggregates for asphalt pavement / road surfacing aggregate
formation of road mixes using steel slag as aggregates has shown better rut resistance and mechanical stability, hence, making the wearing course more durable
1994, 100% steel slag has been used in Singapore for wearing course of roads
Milled Waste
asphalt that has been machine milled from existing roads
bitumen based product is commonly recycled and reused as sub-base materials
100% of such material must pass through 63 mm sieve
Construction and Demolition Waste
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
materials has been sorted, it can be channelled to appropriate recycling facilities for further processing into reusable products
Recycled Concrete Aggregates
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
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
Wood Waste
constitutes about 3% to 30% of the C&D waste
can be converted to wood based products with enhanced properties
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