General Concrete Information
Concrete is a composite
building material made from the combination of aggregate and cement
binder. The most common form of concrete is Portland cement concrete,
which consists of mineral aggregate (generally gravel and sand), Portland
cement and water. It is commonly believed that concrete dries after mixing
Actually, concrete does not
solidify because water evaporates, but rather cement hydrates, gluing the
other components together and eventually creating a stone-like material.
When used in the generic sense, this is the material referred to by the
term concrete. Concrete is used to make pavements, building structures,
foundations, highways & roads, overpasses, parking structures, bases for
gates/fences/poles, and cement in brick or block walls. An old name for
concrete is liquid stone.
The Assyrians and Babylonians
used clay as cement in their concretes. The Egyptians used lime and gypsum
cement. In the Roman Empire, cements made from pozzolanic ash and an
aggregate made from pumice were used to make a concrete very similar to
modern Portland cement concrete. In 1756, British engineer John Smeaton
pioneered the use of Portland cement in concrete, using pebbles and
powdered brick as aggregate. In the modern day, the use of recycled/reused
materials as concrete ingredients is gaining popularity due to
increasingly stringent environmental legislation. The most conspicuous of
these is pulverized fuel ash, recycled from the ash by-products of coal
power plants. This has a significant impact in reducing the amount of
quarrying and the ever-attenuating landfill space.
During hydration and
hardening, concrete needs to develop certain physical and chemical
properties, among others, mechanical strength, low permeability to ingress
of moisture, and chemical and volume stability. Concrete has relatively
high compressive strength, but significantly lower tensile strength (about
10% of the compressive strength). As a result, concrete always fails from
tensile stresses - even when loaded in compression. The practical
implication of these facts is that concrete elements that are subjected to
tensile stresses must be reinforced. Concrete is most often constructed
with the addition of steel bar or fiber reinforcement. The reinforcement
can be by bars (rebar), mesh, or fibres to produce reinforced concrete.
Concrete can also be pre-stressed (reducing tensile stress) using steel
cables, allowing for beams or slabs with a longer span than is practical
with reinforced concrete.
The ultimate strength of
concrete is related to water/cement ratio and the size, shape, and
strength of the aggregate used. Concrete with lower water/cement ratio
(down to 0.35) makes a stronger concrete than a higher ratio. Concrete
made with small (1/2" or 12mm) smooth pebbles is much weaker than that
made with larger (1" or 25mm) rough-surfaced broken rock pieces for
Certain shapes are very
strong in compression, such as arches and vaults, and are therefore
preferred for concrete construction.
Concrete is placed in a wet
or plastic state, and therefore can be manipulated and molded as needed.
Hydration and hardening of concrete may lead to tensile stresses at a time
when it has not yet gained significant strength, resulting in shrinkage
cracks. However, when concrete mix is placed in accordance with the best
recommended practice, cracking may be minimal.
Additives are organic or
non-organic materials in form of solids or fluids that are added to the
concrete to give it certain characteristics. In normal use the additives
make up less than 5% of the cement weight. The most used types of
* Accelerators: Speed up
the hydration (strengthening) of the concrete.
* Retarders: Slow the
hydration of concrete.
* Air-entrainers: Add and
distribute air to the concrete.
* Plasticizers: Increase
the workability of concrete.
Workability is the ability of
a fresh (plastic) concrete mix to fill the form/mould properly with the
desired work (vibration) and without reducing the concrete's quality.
Workability depends on water content, additives, aggregate (shape and size
distribution) and age (level of hydration). Raising the water content or
adding plasticizer will increase the workability. Too much water will lead
to bleeding (loss of water) and/or segregation (concrete start to get
inhomogeneous) and the resulting concrete will have reduced quality.
Workability is normally
tested by slump measurement. High flow concrete, like self compacting
concrete, are normally tested by one of several flow measuring methods.
Concrete slump is a
simplistic measure of fresh (plastic) concrete's workability. Slump is
normally determined by the ASTM C 143 or EN 12350-2 slump test standards,
using the Abrams cone, into which concrete is placed for testing. When the
cone is carefully lifted off the enclosed material, it will slump a
certain amount due to its water content. A relatively dry sample will
slump very little, and be given a slump of one or two inches (25 or 50
mm), while a relatively wet concrete sample may slump as much as six or
seven inches (150 to 175 mm).
To increase the slump, the
rule of thumb is:
Add 1 US gallon of water
per cubic yard of concrete in the mixer truck to increase slump by 1
inch. Adding 27 US gallons to 9 cubic yards of batched concrete will
therefore increase the slump by about 3 inches.
(converted from US rule of thumb)
Add 2 litres of water per
cubic metre of concrete in the mixer truck to increase slump by 1 cm.
Adding 54 litres to 9 cubic metres of batched concrete will therefore
increase the slump by about 3 cm.
Slump can also be increased
by adding a plasticizer, without changing the water/cement factor.
During the 1990s a number of
countries including Japan, Sweden and France developed a range of
concretes that were self-compacting. These 'SCC's are characterised by
their extreme fluidity (using plasticizers), behaving more like water than
the traditional viscous concrete.
SCCs are characterized by
* extreme fluidity measured
by flow or slump, typically measured between 700-750 mm.
* no need for vibrators to
compact the concrete, which can be noisy
* no or little need for
expensive concrete pumping equipment
* no bleed water (excess
water draining out of the concrete)
SCC can offer benefits of up
to 50% in labour costs, due to it being poured up to 80% faster and having
reduced wear and tear on formwork.
As of 2005, self compacting
concretes account for 10-15% of concrete sales in some European countries.
Shotcrete uses compressed air
to shoot (cast) concrete to a frame or structure. Shotcrete is mostly used
for rock support, especially in tunneling. Today there are two application
methods for shotcrete: the dry-mix and the wet-mix procedure. In Dry-mix
the dry mixture of cement and aggregates is filled into the machine and
conveyed with compressed air through the hoses. The water needed for the
hydration is added at the nozzle. In Wet-mix the mixes are prepared with
all necessary water for hydration. The mixes are pumped through the hoses.
At the nozzle compressed air is added for spraying. For both methods
additives such as plasticizers and accelerators may be used. Shotcrete is
normally reinforced by fibers.