to coat the surfaces of all aggregates with cement paste

blend all ingredients of concrete to give uniform consistency

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

ease of concrete mix handled from mixer to final compact shape

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

leaves long chains of pores and weaken concrete surface

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

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

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

as water to cement ratio increase, strength of concrete decreases

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

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

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

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

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

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

Concrete Grade : G20, G25, G30, G35, G40, G45, G50

most impt property to judge hardened concrete quaity

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

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

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

concrete with a 25 mm slump contains 20% of 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

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

mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete

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

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 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

use low pressure supply to avoid any surface damage

impermeable covering such as plastic sheet will trap moisture on concrete surface

apply after finishing when bleedwater disappears

apply twice at right angles to each other to form continuous film

alloy of an iron and carbon with some other traces of elements acting as impurities

fine granular structure with carbon contained as cementite

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)

more tough and elastic than cast iron and wrought iron

equally strong in compression, tension and shear

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

manufacturing of various tools and equipment, machine parts, towers, etc

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

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

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

reduce Singapore’s dependency on imported construction materials and enhance supply resilience

has a higher strength to weight ratio

factory fabricated and higher quality, less skilled worker on site

labour saving of around 20 to 30%

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

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

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

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

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

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

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 (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

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