CONCRETE
DEFINITION
Concrete can be defined as a construction material composed of cement as well
as other cementitious materials such as fly ash and slag cement, aggregate
(generally a coarse aggregate such as gravel, limestone, or granite, plus a fine
aggregate such as sand), water, and chemical admixtures.
The materials in it can be differing in strength, elastic modulus, texture, colour
and resistance to different environment.
The properties of concrete are good in compression and weak in tension when
covered by steel. If properly specified and made, it can be very durable material
which can be fashioned or designed in wide variety of shape.
TYPES OF
CONCRETE
In Situ Concrete Precast Concrete
Poured on site Cast to shape and
bring to site
CONSTITUENTS OF CONCRETE
1. Cement
- Define as material which hardens from a plastic state to be used to
bind together aggregate particles.
2. Cement Replacement
- Add to the cement compound for economic/technical reasons
- Made up of pulverized fuel ash, silica fume, granulated blast furnace slag
3. Aggregates
- Most volume of concrete (3/4) consist of aggregate. Types of concrete
are:-
Coarse aggregate (<1/4’’ – 2’’ in size) gravel, crushed rock, blended
course
Fine aggregate (<1/4’’ in size) sand, crushed gravel fine, crushed rock
fine, blended fine aggregate
- Play vital role in concrete, determining :-
Density
Strength
Durability
Shrinkage
Creep
Thermal
- Aggregate must fulfill these requirements :-
Be stable within the concrete in a particular environment and for a
design time period
Not adversely effect the properties or cost either fresh or hardened
concrete
It should be well graded, shape, surface, texture, workable mix to
produce strong concrete
4. Admixtures
- Compound, liquid, agent, chemical add to concrete for certain technical
reasons :-
Air entraining agent
Small uniformly bubbles effect in concrete to improve
durability/resistance to freeze
Retarders
Admixtures to retard/delay the hardening of fresh/green concrete –
applied at hot/humid dry site
Accelerators
Admixtures which accelerates the setting of mortars or concrete/speed
up the hardening process – applied at low temperatures/freezing site.
Under close monitor because using calcium chloride and nitrite which
can effect the reinforcement.
5. Reinforcement
- Concrete is strong in compression but weak in tension. Thus,
reinforcement were added to strengthen the concrete
- Concrete resist the compression, yet the reinforcement resist the tension
CONCRETE
CURING
Traditional Method Latest Method
Cover green/fresh Spray on with resin
concrete with damp solven solution wet
gunny bag, damp emulsion, metallic
sand, spray water silicates
Concrete shall be protected from
damage by shock, falling earth
and water flowing. Also avoiding
crack, honey comb, creep and
deformed.
CONCRETE TESTING
Slump Test Compactig Test
For the green/fresh Lab test for
concrete compression/strength
COMPRESSIVE
STRENGTH TEST
28 Days Break 7 Days Break
100% days break 60%-70% of final strength
Method
1 Deliver a small sample from the transit vehicle into a wheelbarrow or
dumper truck – transfer it to a bucket if required.
2 Prepare a flat clean surface and stand the slump cone on it.
3 Stand on the two foot tabs (or ask an assistant to hold the handles to keep
the cone firmly in place).
4 Using a scoop or brick laying trowel, deposit the fresh concrete into the
open ended cone to about the 75 mm level.
5 Using the tamping rod round end downwards, rod 25 times to remove
any trapped air voids.
6 Repeat the procedure at the 150mm level and tamp 25 times.
7 Repeat the procedure at the 225mm level and tamp 25 times.
8 Finally fill the cone (300mm) and tamp 25 times.
9 Flush off the top with the trowel, and carefully remove the cone by slightly
twisting it vertically to help it release.
10 Invert the cone and stand it next to the sample; place the tamping rod
across the wide end of the cone and over the sample.
11 Measure the difference (if any) between the underside of the tamping rod
and the top of the sample: this is known as the slump.
The greater the slump, the more workable the mix. Slump testing fresh
concrete from an initial discharge has an upper and lower limit. Where, for
instance, a stiff mix is required a specified slump of up to 40mm can range from
10mm to 80mm at time of discharge. Again a specified target slump of 50mm to
90mmcould range from 10mmto 140mm. Although concrete is a complicated
material its manufacture is dependent on many factors; therefore the batcher’s
experienced eye still is considered the best gauge. Eurocode EN 12350 for
testing fresh concrete identifies slump ranges for a specified slump class.
