Please enable JavaScript.
Coggle requires JavaScript to display documents.
Concrete technology part 2 - Coggle Diagram
Concrete technology part 2
Destructive and Non destructive test
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 are
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.
Factor affecting strength of hardened 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.
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.
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
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.
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.
Curing 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
steel
What is Steel?.
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.
Relevant Standard
Specification for high tensile steel wire and strand for the prestressing of concrete – BS 5896.
Quality Control
Mixing
To coat the surfaces of all aggregates with cement paste.
To blend all the ingredients of concrete to give a uniform consistency.
2 categories of ready-mixed concrete
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 various processes of mixing, handling, transporting, placing and compaction to required shape or form.
The important element of fresh concrete are workability, setting time and stability to prevent segregation.
Workability Test
Workability means 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;
3 types of slump observed
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.
Bleeding is caused by:
Too much water
Coarse sand grading
Not enough of fines
Bleeding 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.
This increases the porosity of the concrete and loss in strength (will be further discussed in the next section)
Has to be avoided and repair works need to kick in when segregation happens to the concrete structure.
Hardened Concrete
Main properties of hardened concrete
Strength
Durability
Strength is 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 eg. 40N/mm2.
Strength is considered as the most important property and the quality of concrete is often judged by its strength.
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.
Sustainable material
Sustainable Construction Masterplan
was developed by BCA somewhere in 2009. The objective is to steer the construction industry to achieve the following objectives;
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.
Use of Structural Steel Construction
Report has indicated that although the overall construction cost may increase around 2 to 3%, project using steel construction 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
Use of Dry Partition Walls
Advanced dry partition walls has emerged as new focus to modern design in home. 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 of waste :
Incinerator Ashes from domestic refuse.
Copper Slag.
Milled Waste
Construction and demolition (CRD) waste
Recycled Concrete Aggregates (RCA)
Wood Waste
Transport, Placing, Compacting, Finishing and Curing of Concrete
Transporting/Placing Concrete
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
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.
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.
.
Vibrators
These are mobile items of mechanical plant used to vibrate (shake) air out of fresh concrete.
There are 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.
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.
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.
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.
Curing 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 (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