Section 1: Concrete 1-4
2022 - DE4201 Materials (Civil) & MG5107 Civil Materials
Slide 2
1 Constituents of Concrete (overview)
In its most basic form, concrete is a mixture of cement (Portland), water, and fine and coarse aggregates (sand and
crushed rock or natural gravel), which is plastic when first mixed, but which then sets and hardens into a solid mass.
When plastic, it can be moulded or extruded into a variety of shapes. When hardened, it is strong and durable, able to
support substantial loads and at the same time, resist the effects of fire, weather, wear and other deteriorating
influences. It is therefore a construction material of great versatility and wide application.
Concrete is made up of these primary constituents
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Water (drinkable)
Cement
Air
Fine Aggregate (sand)
Coarse Aggregate (crushed rock, river pebbles, lightweight)
Plus if required - Chemical admixtures - are added to adjust properties of concrete mix and are usually added to the
water Admixtures can adjust the rate the concrete sets (speed it up or slow it down – accelerators, retarders), adjust
the workability of the mix (make the mix flow more easily plasticisers and superplasticisers), make the water in the mix
go further (water reducers), add air to the mix (air entraining agents), pigments (or white cement) to change the colour
for visual effects for patios/walkways, and water proofing silicon spray to water proof heavy trafficked areas.
Concrete is infinitely variable; its properties can be adjusted for specific tasks/jobs. By changing the properties of
concrete you can alter the strength of concrete to either temporarily weaken it to make it more workable for longer or
strengthen it to extend its usable life eg bridges and buildings. Or have no fines concrete so the mix is porous.
The properties of concrete are determined by:
• the properties of the individual constituents/ingredients
• the proportions in which they are mixed
• concreting practice (mixing, transporting, placing, compaction, curing procedures).
The properties of concrete in both the plastic and hardened states are dependent on the physical characteristics,
the chemical composition, and the proportions of the components used in the mixture.
Hardened-state properties - appropriate to the purpose for which the concrete is to be used; ie strong enough to
carry the loads imposed on it, and durable enough to resist the deteriorating influences of wear and weather.
Plastic-state properties must also be appropriate to the methods of handling, placing, compacting and finishing.
If not properly mixed, placed and compacted, concrete will not achieve its potential strength and durability. It is
important that when delivered to site concrete m u s t be sufficiently workable for it to be placed and compacted.
Workability is achieved by having sufficient cement paste (cement and water) in the mixture to lubricate the particles
of aggregate and allow them to move freely as the concrete is placed and compacted.
Concrete initially hardens within 1.5-2 hrs after adding water, 7 days to harden, 28 days = full compression strength.
New Zealand Concrete Standards
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NZS 3109 :1997 Concrete Construction
NZS 3114:1987 Specification for concrete surface finishes
NZS 3104 :2003 Specification for concrete production
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 3
2 Cement
History of Portland Cement
Cement in one form or another has been around for centuries, the Egyptians (300BC) used a lime-based cementing
mortar for bedding the stones of their buildings and pyramids and the ancient Romans used c e m e n t mostly
composed of calcium carbonate (lime) to make mortars and concrete to make the Colosseum, and the Pantheon.
The type we use now was invented in 1824 in Britain, it was named Portland Cement.
In 1824 a bricklayer from Britain called Joseph Aspdin, patented the process of heating a mixture of clay and limestone
to chalk in a furnace to drive off all the carbonic-acid gas, and ground it to a fine powder. He called the product Portland
Cement. The name has continued, although modern Portland Cement is slightly different.
In 1845 Isaac Johnson improved the cement-making process by raising the temperature of the kiln until the chalk and
clay fused into a clinker. The clinkering temperature enables chemical reactions to take place and form essential
compounds. There have been many refinements, the Johnson process is the basis of modern cement manufacture.
Clinker are lumps usually 3–25mm in diameter, produced by sintering limestone and clay (alumino-silicate) during the
cement kiln stage, sintering is a method used to create lumps from powders. The clinker when cooled has 5% of calcium
sulfate (usually gypsum) added to the clinker and the mixture is ground to form the finished cement powder.
Portland Cement - Raw materials
Portland cement consists essentially of limestone (calcium oxide, CaCO3) and clay (silica - silicon dioxide, SiO2), shale and
bauxite, these are mixed with alumina (aluminum oxide, Al2O3). Iron oxide (ferrite Fe2O3) can come from the clay, or
the addition of small quantities of iron ore, it acts as a flux. During clinkering, the clinker contains a solid and a liquid
phase, the bulk of the clinker remains as a solid. The liquid is vital in that it acts a flux, promoting reactions by ion
transfer; without the liquid phase, combine-ability would be poor and it would be very difficult to make cement.
Portland Cement - Manufacturing process
You tube (How to make cement) – 5 minutes http://www.youtube.com/watch?v=n-Pr1KTVSXo
The cement manufacturing process involves four distinct stages:
Step 1 – Quarrying
The raw material for cement manufacture is a rock mixture which is about 80% limestone (which is rich in Calcium
Carbonate CaCO3) and 20% clay or shale (a source of silica, alumina and Iron(III) oxide or ferric oxide Fe2O3).
These are quarried and stored separately. The lime and silica provide the main strength of the cement, while the
iron reduces the reaction temperature and gives the cement its characteristic grey colour.
Step 2 - Raw material preparation
There are two main cement manufacturing processes currently used in New Zealand: the dry process (used by Golden
Bay) and the wet process (originally used by Milburn, now owned by Holcim). The dry process uses more energy in
grinding but less in the kiln, and the wet process has lower overheads than the dry process.
The dry process
The quarried clay and limestone are crushed separately to the size of a tennis ball. Samples of both rocks are then sent
off to the laboratory for mineral analysis. If necessary, minerals are then added to either the clay or the limestone to
ensure that the correct amounts of aluminium, iron etc. are present. The clay and limestone are then fed together into
a mill where the rock is ground until more than 85% of the material is less than 90μm (micron) in diameter.
The wet process
The clay is mixed to a paste in a washmill - a tank in which the clay is pulverised in the presence of water. Crushed lime
is then added and the whole mixture further ground. Any material which is too coarse is extracted and reground. The
slurry is then tested to ensure that it contains the correct balance of minerals.
Step 3 - Clinkering
This step is characteristic of Portland cement. The finely ground material is dried, heated (to enable the sintering
2022 Materials (Civil) – Section 1: Concrete 1-4
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reactions to take place) and then cooled down again. While it is being heated various chemical reactions take place to
form the major mineral constituents of Portland Cement. The powder from the dry process doesn't contain much
moisture, so can be dried in a pre-heater tower. As it falls through the tower (which takes 30 seconds) it is heated from
70 to 800oC. The moisture evaporates, up to 20% of the decarbonation (loss of Carbon dioxide CO2) occurs, the mixture
is then fed into the kiln. The slurry from the wet process contains too much moisture, so the slurry is fed directly into
the kiln where it is formed into dry balls by the heat and rotation of the kiln. Because of this extra role of the kiln, wet
process kilns are longer than dry: eg Milburn's kiln is more than 100m in length, whereas Golden Bay’s is 60m long.
The Kiln
The kiln shell is steel, 60m long is inclined at a slight angle and lined with heat-resistant bricks. The shell is supported on
3 roller trunions and weighs in at over 1100 T. The kiln is heated by injecting pulverised coal dust into the discharge end
where it spontaneously ignites due to the very high temperatures. Coal is injected with air into the kiln.
Reaction processes within the kiln has four simple zones:
Zone 1: 0 - 35 min, 800 - 1100oC - Decarbonation. Melting of fluxing compounds
Zone 2: 35 - 40 min, 1100 - 1300oC - Exothermic reactions and the formation of secondary silicate phases
Zone 3: 40 - 50 min, 1300 - 1450 - 1300oC - Sintering and reaction within the melt to form silicates and tetra calcium
alumino-ferrates:
Zone 4: 50 - 60 min, 1300 - 1000oC - Cooling and crystallisation of the various mineral phases formed in the kiln.
The Cooler
Immediately following the kiln is a large cooler to drop the temperature from 1000oC to 150oC. The plates within the
cooler slide back and forth, shuffling the clinker down the cooler to the discharge point.
At this point in the process the raw materials have been formed into clicker/lumpy cement. Like cement, the clinker will
react with water and harden, but because it is composed of 1-3 cm diameter fragments it is too coarse to be used.
Step 4 - Cement milling
To produce the final product the clinker is mixed with gypsum, which is added as a set retarder, and ground for
approximately 30 minutes in large rotating chamber, being first ground with 60mm then 30mm diameter steel balls. The
first grinding breaks up the material, the second grinds it to a fine powder. The cement grinding process is highly energy
intensive. The largest mill at Golden Bay Cement is some 11m in length, weighs over 230 T.
Summary
Figure 1 – Manufacturing process diagram
• The raw materials: Limestone as a source of Calcium oxide (CaO) and Clay as sources of Silica (SiO2), Alumina
(Al2O3), Ferrite (Fe2O3) are quarried from the ground.
• This rock is run through the crusher where the rocks are mixed, crushed and ground (by a Dry or Wet process).
• This is then fired to around 1500oC in rotary (constantly turning) kiln which produces the ‘Clinker’.
• Then cooled and ground into a very fine powder and mixed together with Gypsum (which delays the setting
time of cement up to two hours) and any Mineral additions (5% max) then bagged for sale.
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 4
Hydration of cement
You tube (Hydration of Cement) – 12 minutes http://www.youtube.com/watch?v=o9WAEOWv7IY&list=PLWC3iyZnooEoBZL5NzexAWgWMCukwrjl
When water is added to cement, the various components of the cement undergo a chemical reaction which causes the
cement to set and harden. This reaction with water is called hydration.
Heat is generated during hydration of cement. The amount of heat generated and the rate at which it is generated, is
dependent upon the type of cement.
The rate of hydration is also influenced by ambient temperatures. Low temperatures (eg 10°C) decrease the rate, and
high temperatures (eg 30°C) accelerate the rate of hydration.
In most concrete structures which are not very large, this heat is readily dispersed from the surface of the structure and
is not an issue.
Fineness of Cement
Fineness is measure by the specific area, which is the total surface area of all the particles in a given mass of cement. It
is expressed in square metres per kilogram (m²/kg). The finer a cement is ground, the greater the surface area
produced, and the faster the cement will set and harden. While finer cements react more quickly with water and
consequently gain strength quicker, the ultimate strength is not affected. Greater fineness also gives the cement more
cohesiveness (fattiness) and reduces the potential for bleeding. Bleeding occurs when a free water film forms on top of
fresh (wet) concrete.
Shrinkage upon drying and the consequent potential for cracking of concrete, increases with the increase in the rate of
strength development and are therefore higher with finer cements.
Slide 5
Types of Cement
The cement chemist by controlling the relative proportions of the principal compounds of cement, influence the
properties of the cement produced.
The range and types of cement manufactured will be governed by the demand.
The following types are covered by NZS 3122: Portland Cement. (the three in blue are the most common)
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Type GP – General purpose Portland cement
Type HE – High Early strength cement
Type SR – Sulphate Resisting cement
Typed GB – General purpose Blended cement
Type LH – Low Heat cement
Type SL – Shrinkage Limited cement
General Purpose Portland Cements (Type GP)
This is the cement type most widely used for normal concrete production where there is no requirement for any special
conditions for example where low heat of hydration or high early strength gain is required.
Cement classified as GP can at the discretion of the cement manufacturer, contains up to 5% of mineral additions. The
latter include fly-ash and/or ground granulated blast furnace slag (by-product of steel manufacturing process).
Uses:
•
Slide 6
ideal for use in structural concrete, mortars, renders, grouts and cement based products and can also be used
as a general binder for applications such as soil stabilisation.
General Purpose Blended Cement (Type GB)
Blended cement is cement containing Portland cement and a quantity greater than 5% of fly ash and/or slag, and/or up
to 10% silica fume.
By varying the proportions of the above mentioned constituents, it is possible to produce cements with a fairly wide
2022 Materials (Civil) – Section 1: Concrete 1-4
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range of characteristics.
In practice, however, the difference between typed GP and GB may not be great, as both are formulated to be used in
general building construction.
In general, the effect of the greater proportion of mineral additions in Type GBlend, is to reduce the rate of strength
development (compared to Type GP) making it suitable for applications such as mass concrete construction.
The ultimate strength of Type GB, however, is similar to that of Type GP.
NOTE: Blast furnace slag is a by-product of the iron manufacturing process. It is formed in the blast furnace during
smelting of the raw materials. The slag form the furnace is granulated by rapid quenching, and ground finely.
Fly-ash is finely-divided residue from the combustion of pulverised coal in power stations.
Silica fume (silica dust or silica powder), is a by-product from the production of silicon and ferro-silicon alloys.
Sulphate Resisting Cement (Type SR)
Manufactured in New Zealand only to special order and is expensive to make.
Uses:
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Slide 7
Where high resistance to sulphates (salts) is required, for example marine construction, construction in
sulphate bearing soils or waters, or concrete sewer pipes. Industrial applications like chimneys and geothermal
areas.
High Early Strength Cement (Type HE)
You tube (Rapid Set Cement, admixture) – 4 minutes http://www.youtube.com/watch?v=0bKfX3uejEM
High Early Strength cement was formerly called Rapid Hardening cement.
Manufactured in the same way as Type GP with the exception that the cement is ground more finely, hence hydration
occurs more rapidly (approx. 1hr to set), results = greater rate of strength, gain at early stages (ie in the first few days).
Other properties (setting time and ultimate strength) are the same as for types GP and GB.
Uses:
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Where early strength is desired, eg precast and for early stripping of formwork, or repairs to concrete
structures.
Construction in cold weather conditions.
Figure 2 – Rate of Strength Development
Slide 8
Low Heat Cement (Type LH)
It is characterised by low heat of hydration characteristics, hence, slower rate of strength development.
Uses: Mass concrete construction, or thick structural elements where Low heat of hydration is desirable, in order to
• Reduce potential cracking problems, that result from
• Differential temperature between interior and the surface.
Hot weather concreting
2022 Materials (Civil) – Section 1: Concrete 1-4
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Shrinkage Limited Cement (Type SL)
Can be a Portland or a Blended Cement provided it meets a specified drying shrinkage limit (750 microstrain at 28 days).
Uses: Where emphasis is place on drying shrinkage and crack control, eg. road pavements or bridge structures.
Slide 9
Other Cements
Off-white and White Portland Cement
• As the name implies, the colour is white instead of the usual grey.
• It is manufactured using raw materials low in iron oxides.
• It is used for aesthetic purposes where the white finish is required.
• It is imported into New Zealand because of small demand.
Portland Limestone Filler Cement
• It is produced by inter-grinding cement clinker with up to 15% of mineral limestone that has a minimum CaCO3
content of 75%.
• Gives concrete enhanced workability, allowing some water addition savings.
High Alumina Cement (HAC)
• High Alumina Cement is not a Portland cement.
• It is a special cement produced by blending the Limestone with Bauxite (Aluminium bearing ore).
Particular properties of HAC:
• a high rate of strength gain in the early stages
• a high ultimate strength
• high resistance to acid attack
• can resist high temperatures
Uses:
• Where high early strength is required
• where resistance to very high temperatures is required, for example, refractory concrete (heat resistant
cement eg pizza ovens) & factory floor slabs.
• HAC is not suitable in conditions which are both warm (above 25oC) and humid. In these conditions it suffers a
substantial loss in strength.
Concrete types
Normal mixes – 17.5, 20, 25, 30, 35, 40, 45, 50MPa. High MPa’s are not used very often, only for commercial buildings
with columns/beams etc. Most non-structural jobs can be achieved with 25Mpa
Special grades – anything over 50MPa or mixes requiring special properties/additives. (highest grading = prestressed
precast concrete companies who have accreditation to achieve very high MPa’s) Grades are an industry standard,
determined by Quality Control processes by the ready mix manufacturers providing the target concrete strengths. The
companies are audited to make sure they are achieving the required standards.
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Slide 10
3 Water used for concrete Manufacture
Water has two main functions in concrete manufacture.
• to hydrate the cement
• to lubricate the concrete mix
If sufficient water just for hydration purposes was added, the concrete will be very ‘dry’ (unworkable). Additional water
is required to lubricate the components and to achieve the desired workability.
The requirements for water are covers by NZS 3121: Water and Aggregate for Concrete. It states, ‘the water for
concrete must be free from significant amounts of impurities which may deleteriously affect the setting, hardening or
strength of concrete or its long-term durability.’ So: Water that is acceptable for drinking is also suitable for making
concrete. This means that water supplied by local authorities for public supply is suitable.
Potentially harmful impurities in water include:
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Dissolved salts
Chlorides (as in seawater) – may accelerate setting time and can accelerate corrosion of reinforcing steel
Sulphates – affect setting time and strength of concrete
Organic matter (in particular sugar) – affects strength and can prevent setting
Carbonates and bicarbonates – affect setting and strength
Soaps and detergents.
The use of sea water for mixing concrete should be avoided because it:
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reduces the rate of strength development of the concrete
creates an unsightly crystalline deposit on the surface of the hardened concrete (efflorescence)
enhances the rusting of reinforcing steel in the concrete.
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 11
4 Concrete Aggregates
Concrete aggregate blends are made up of several separate stone sizes and these are mixed with sand, cement and
water to make concrete. The aggregate interlocks with compaction to give concrete its strength.
Classification & properties of aggregates
Aggregates are primarily classified by maximum (largest) particle size and then by range of particle sizes (number).
Most of the qualities of aggregate depend on the qualities of the ‘parent rock’ where the aggregate was quarried from.
Single size aggregates
Are used in concrete, various sizes are put together to make up a concrete mix. Starting at larger rock pieces that give
the concrete it’s strength and then several smaller sized stones, including sand, which their purpose is to fill in the air
voids/gaps to create a dense concrete to which cement and water are added to ‘glue’ the aggregate pieces together.
Concrete aggregates - Coarse aggregates - larger than 4.75mm
Fine aggregates - 4.75mm - 2mm
Sand - 2mm - 0.06mm
Slide 12
The term aggregate means ‘stone/rock particle’ = coarse aggregate and the ‘sand’ = fine aggregate in the concrete.
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Slide 13
Aggregate makes up about 60% to 70% of the volume of concrete.
Aggregate must be strong, stable, hard, durable and clean and it must not react with the cement or the steel
reinforcing.
Most aggregate comes from crushed stone or rock or from screened river gravel.
Greywacke is by far the most commonly used aggregate source in New Zealand. Much of it is extracted from
river beds (alluvial rock or gravel), which is rounded by the rolling action in the river. Large alluvial boulders are
crushed into smaller size ranges.
The other source of Greywacke is from quarries, in this case all the material will be crushed material.
Pumice and Scoria are not generally used for concrete manufacture, except for the manufacture of lightweight
concrete.
The requirements for concrete aggregate are specified in NZS 3121: Water and Aggregate for Concrete and NZS
3111:1986 Methods of test for water and aggregate for concrete.
Aggregate Shape
The shape and surface texture of the aggregate affect the workability of fresh concrete through their influence on
cement paste (mortar) requirements. The mortar coating on the aggregate particles should be enough to provide
lubrication for mixing and placing as well as to bond the particles together when set.
Figure 3 – Basic Aggregate Shapes
Figure 4 - Below rounded, irregular and angular shaped aggregates are most desirable, left, elongated right not desrable.
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 14
From a workability point of view, the ideal aggregate particle shape for screened gravel is spherical.
For crushed aggregate, the best shape is cubical/rough textured which will give optimum strength with
interlock.
Avoid aggregates that are flat or elongated because they will not produce concrete with good workability. Elongated
particles are furthermore weak and are easily broken.
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River aggregate is usually rounded and smooth, while quarried aggregate (which has to be crushed) is sharp and
angular, with rough surfaces.
Both river and quarried aggregates can make good concrete. Crushed aggregate with their irregular and rough
surface texture enhances particle interlock and bonding with the cement mortar.
To retain the same level of workability as smooth aggregate, crushed aggregate must be combined with more sand.
•
The availability of aggregates for concrete in New Zealand is summarised in the table below:
Area
Igneous
Sedimentary
Northland
Basalt throughout but
poorest in the west
Greywacke-argillite in the east
Auckland, Waikato, King
Country
Basalt in Auckland, andesite
Widespread greywacke-argillite
quarries. used for decorative
purposes
Taranaki
Coromandel, Bay of
Plenty, Central Volcanic
Region
Metamorphic
Andesites predominent
Predominatly volcanic with
andesites common
Greywacke-argillite in east
but often of poor quality
East Coast
Greywacke-argillite both quarried
and as gravel. Limestone used in
Gisbourne area
Wellington
Greywacke-argillite both quarried
and as river gravel
Marlborough, Canterbury
Principally greywacke-argillite gravels
Nelson, Westland
Granite
Greywacke and limestone
Quartzite
Otago, Southland
Basalt and phonolite
Greywacke and schist gravels
Schist
The common types of aggregate are:
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Natural sands and gravels
Crushed rocks (igneous, sedimentary, metamorphic)
Manufactured aggregates (iron blastfurnace slag, expanded clay and shales, sintered pulverised fuel ash,
polystyrene beads)
Other properties aggregates influence:
Density, water absorption, dimensional stability, abrasion resistance, soundness, strength and rigidity, reactivity,
impurities and other harmful materials, thermal expansion and colour.
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Slide 15
Aggregate Grading
You tube (Standard Method for Sieve Analysis of Fine and Coarse Aggregates) – 5.50 minutes
https://www.youtube.com/watch?v=3Xqq1cxhD-s
Grading refers to the particle size distribution on a mass basis.
A material’s grading is determined by passing the material through a nest of standard sieves (arranged in order of
decreasing size) and determining the mass retained on each sieve. This data is then processed to draw the grading
curve. The mass retained on each sieve is recorded then % retained calculated and % age passing each size is
determined by subtraction.
The grading curve is a plot of percentage passing (vertical) vs. size (horizontal).
Figure 5 – Typical Grading Curve
Slide 16
GAP65 Blended aggregate
Grade 2 Sealing chip
The shape of the grading curve tells us something about the distribution of the different particle sizes present in the
material. The following terms are used:
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Continuously graded: material containing a good proportion of all sizes (or well graded, first picture below)
Uniformly graded: material consisting mainly of one size (or poorly graded, middle picture below)
Gap-graded: excess of two or more sizes with a deficiency (gaps) of others (as per third picture below)
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Slide 17
Aggregate testing
Sampling of aggregates
All aggregate sampling must be a ‘representative’ sample. Done in-field, using random sampling methods.
Because not all material can be tested, random sampling is used for large amounts of aggregate. Samples can be
reduced in size by either ‘quartering’ or the use of a riffle box. In ‘quartering’ a heap of aggregate is divided into four
segments and two opposite quarters are discarded, process is repeated until required sample size is obtained.
There are two classes of testing
1. Source property tests (These tests the intrinsic (belonging naturally to rock) properties of ‘parent’ rock from quarry)
2. Production property tests (determines the properties of the aggregate and reliant on production methods and
equipment).
Slide 18
Grading of Coarse Aggregate
Coarse aggregates used for concrete manufacture are normally designated by the largest particle size, 19mm aggregate.
Terminology
Maximum size: The smallest sieve size opening through which all the particles will pass.
Nominal size: The whole number above the sieve size through which nearly all the aggregate passes.
Continuously graded (well graded) aggregates are desirable for roading base course materials and concrete. However,
concrete can be produced from a wide range of fine and coarse aggregate gradings depending on the type required.
The grading for the coarse aggregate specified by NZS 3121 shows aggregates that are virtually of a single size, that is it
is fairly uniformly graded (or poorly graded).
Sometimes mixtures of two or more coarse aggregate grades are used. This increases the density and reduces the voids.
By reducing voids, the amount of cement and sand required is reduced. This reduces the cost because the cement is the
most expensive ingredient of concrete.
The volume of voids is also minimised by using the largest possible coarse aggregate. The maximum aggregate size that
can be used is governed by practical considerations such as:
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Slide 19
minimum cover over reinforced bars (C)
spacing between reinforcing bars (S)
minimum dimension of section (M)
method of placing (eg pumping)
Grading of Fine Aggregate (Sand)
Sand used for concrete should ideally be continuously graded, that is containing particles of all sizes.
A uniformly graded sand can be used, but will yield a mix which is:
• less workable
• more liable to honeycombing
• harder to bring to a good surface finish.
A small proportion of ‘fines’ in the sand is desirable. A coarse sand, without a small proportion of fines, results in a
‘harsh’ mix which is hard to place and finish and more prone to bleeding.
An excess of fines, on the other hand, tends to reduce the strength of the concrete, and results in an excess of ‘mortar’
on the surface. This mortar may craze and the surface will wear rapidly under ‘traffic’. The greater the fines content, the
2022 Materials (Civil) – Section 1: Concrete 1-4
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greater the specific surface, consequently the greater the amount of cement required for the same strength.
Natural sources of sand include river beds, pits or beaches. The material is often acceptable, but not necessarily ideal:
• River sands are generally reasonable clean (free from clay), with rounded particles which is good for
workability, but exhibit varied grading being sorted into sizes by water flow.
• Pit sands are often ‘well’ graded, but tent to contain excessive quantities of ‘fines’ (clay). Washing of sand to
remove fines is expensive.
• Unprocessed and unwashed beach sands tend to be poorly graded and contain salt which causes
efflorescence and corrosion of reinforcing steel and reduction in strength.
The best source of sand of consistent quality is crusher sand. This is the product of a properly controlled manufacturing
process which include crushing, washing and grading.
Fineness Modulus (FM)
Slide 20
Aggregate/sand - fineness modulus
The fineness modulus (FM) of an aggregate or sand is a measure (or index) of the fineness or coarseness of the
particles.
When testing, it is common to calculate fineness modulus by adding the cumulative % retained on (all the sieves used
in the analysis, do not use the pan amount) the example below uses 9500µm, 4750µm, 2360µm, 1180µm, 600µm,
300µm & 150µm sieves and divide by 100.
Fineness Modulus (aggregates) = Add cumulative % retained on only sieves sizes above the pan/100
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•
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A low FM value (1.8 – 2.4) fine sand
A high FM value (3.0 – 3.5) coarse sand
For good average concrete FM of sand should be between 2.4 and 3.2
Coarse aggregates smaller than 38.1 mm range from 6.50 to 8.00. Combinations of fine and coarse aggregates
may have intermediate values.
When two sands, say Sand A and Sand B, are blended (mixed) in order to obtain a desirable intermediate mixture, the
FM of the blend can be calculated by combining the moduli of the individual components in the ratio of the mix
proportion. Say blend (mixture) contains a% of Sand A and b% of Sand B, then:
FMBLEND = a/100 FMA + b/100 FMB
Example
Slide 21
Sieve Size
(µm)
Mass Retained
(g)
% Retained
9500
4750
2360
1180
600
300
150
Passing 150 (Pan)
Total
0
26.8
69.7
107.2
128.6
123.3
42.9
37.5
536.0g
0
5
13
20
24
23
8
7
100%
Try the above example for homework; you have to calculate
Cumulative%
Passing
Cumulative
100
95
82
62
38
15
7
0
5
18
38
62
85
93
%retained
Total
301
4 columns.
Solution: FM = 301/100 = 3.01
(modulus – a constant multiplier or coefficient. A constant that gives a ratio between an amount of physical effect and
that of a force producing it) (coefficient – quality placed before multiplying another quality)
µ = MICRON = one millionth of a metre
Slides 22, 23
Sand Sieve analysis - Class exercise 1
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 24
Water/Moisture Content and Moisture States
Definition of water (moisture) content: water content = Mass water divided by Mass solids
Water (moisture) content: w = Mwet – Mdry x 100%
Mdry
Most aggregates are slightly porous and can absorb water into the pores.
Water can also adhere to the surface of the aggregate as a film.
For aggregate containing pores, four moisture states are recognised:
•
•
•
•
Oven-dry (OD)
Air-dry (AD)
Saturated surface dry (SSD)
pile)
Wet
- No moisture at all
- Pores partly filled with water
- all pores full of water, no surface water film (Similar to aggregate from a stock
- All pores full of water, surface coated with a film of water
Figure 6 – Moisture States of Aggregates
Slide 25
The SSD state is used as the reference base for calculation of the amount of water that the aggregate will add or
subtract to the paste in concrete mix design. Three quantities are defined.
Absorption and Surface Moisture
To determine the amount of water an aggregate will add or subtract from a cement paste, three quantities are used:
Absorption Capacity:
AC = MSSD – MOD x 100%
MOD
The absorption capacity (AC) is the maximum amount of water the aggregate can absorb.
Effective Absorption:
EA = MSSD – MAD x 100%
MSSD
EA is the amount of water required to bring an aggregate from the AD state to the SSD state
Surface Moisture (free water content):
SM = Mwet – MSSD x 100%
MSSD
SM is the amount of water in excess of SSD.
Slide 26
If the aggregate used in a concrete mix is in the SSD condition, it will neither absorb any of the missing water nor add
any additional moisture to the missing water.
If the aggregate is drier than the SSD condition, an extra quantity has to be added to the required amount of mixing
water in order to allow for the water that will be absorbed by the aggregate, and vice versa.
The mass of mixing water absorbed (Mabs) by aggregate which is on the dry side of SSD, is:
Mass of mixing water absorbed = EA x Mass of aggregate (in SSD state)
The mass of additional water added by a wet aggregate which is on the wet side of SSD, is:
Mass of additional water added = SM x Mass of aggregate (in SSD state)
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 27
Density
Density measurements are important in the blending process at the batching plant because the aggregate, sand, any
additives and cement are weighed quantities and the water is measured in litres or is a ‘volume’ so the density of
aggregate is required to get the correct moisture content.
Densities are commonly measured in two states: oven dry (= rock & air, in lab) then the dried loose aggregate is
poured into a container without compaction to get the maximum air voids. Saturated surface dry (SSD) (= rock, air &
water) the aggregate is compacted in a container to get maximum density to obtain the maximum and minimum
densities of the aggregate. Saturated surface dry is commonly used because of its approximation to the state the
aggregate is likely to be if stored in a stockpile in an open yard and therefore the density will be higher than oven dry.
Slide 28
Unit mass measurements are used to determine maximum and minimum densities that a particular aggregate blend will
achieve with 1. minimal compaction and with 2. high degree of compaction.
• The calculation of unit mass of an aggregate blend will usually include an air voids volume and hence = bulk density.
• Whereas the density of an individual stone size (not including air voids volume) is a solid density where density =
mass/solid volume.
Density (Unit Mass)
Density is a defined as mass divided by volume, expressed in kg/m3 or t/m3.
ρ
Mass
=
=
Volume
ρ=
(the lower case Greek letter ‘rho’)
M
V
Because density expresses the mass of a substance per unit volume, it is sometimes referred to as unit mass.
The density of soil is therefore the mass of a given quantity of the soil, divided by total volume occupied by the soil.
ρsoil =
Mass soil
Volume soil
Slide 29
Above:Density (Unit Mass) Bulk Density
Above: Solid Density
Solid density
Solid density is the mass of solids divided by the volume of solids.
ρsolids
= Mass of solids
Volume of solids
= M solids
V solids
Solid density is a fundamental property of a material. It is a fixed value which is determined by the density of the parent
rock from which the solids are derived.
Slide 30
Concrete aggregates, classed as normal weight aggregates, have densities in excess of 1.95 t/m3, typically of 2.65 t/m3.
Lightweight aggregates are those having a density less than 1.95 t/m3. They are used to produce lightweight concrete
for special applications, and include aggregates such as scoria, other porous rock or volcanic origin, vermiculite and
perlite. Heavyweight aggregates (density ≥ 3.0 t/m3) include limonite and magnetite. They are used principally to
produce concrete for shielding against nuclear radiation.
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 31
Specific Gravity
Specific gravity is the ratio of the density of a substance compared to the density (mass of the same unit volume) of a
reference substance eg water
Specific gravity of any substance =
ρsubstance
ρwater
Eg Specific gravity of solid particles
ρsolids
=
= 2650 = 2.65
ρwater
1000
Specific gravity is dimensionless, ie it has no units. It is just a number which indicates how many times a substance is
heavier than water - specific gravity of water = 1000.
Specific gravity and density are numerically the same if density is expressed t/m3 units. The terms density of solid
particles, particle specific gravity and solids density (in t/m3) are closely associated.
Slide 32
Void content
Void content = Volume of void spaces between aggregate particles
=
Vvoids
Vtotal
Total volume occupied by aggregate
Void content, in this context, refers to the spaces between the aggregate particles, and does not include the pores that
may be present in the aggregate. It is a percentage.
Void content
ρsolids - ρagg(SSD)
=
x 100 %
ρsolids
Slide 33
Example:
The mass of aggregate in the SSD condition required to fill a 1.045 litre calibrating can, is 1620 grams.
The solid density of the aggregate is 2.63 t/m3.
Check units
Convert g to kg (/1000)
Convert kg to t (/1000)
Determine:
a. The unit mass of the aggregate in the SSD condition, and
b. The void content
Solution:
1. Unit mass = density =
mass
=
Volume
2. Void content
=
ρsolids - ρagg(SSD)
1.620(kg)
1550 kg/m3
1.045 x 10-3 (m3)
x 100%
ρsolids
Slides 34, 35, 36, 37
=
=
2.63 – 1.55 x 100%
= 41 %
2.63
Moisture Content, D, SG, V - Class exercise 2
2022 Materials (Civil) – Section 1: Concrete 1-4
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Slide 38
Bulking of sand
The ‘loose volume’ occupied by a given quantity of sand depends on the water content.
When the sand is dry, the particles are in close contact with each other and the sand occupies a relatively small volume
When the sand is wet, containing say 5% moisture, a film of water coats each particle. Menisci are formed between
these water films which allows surface tension effects to push the particles apart, thus increasing the volume occupied
by the sand. This increase in volume is known as bulking (expansion).
When the water content approaches saturation level, the menisci are destroyed/broken, and with it the forces pushing
the particles apart. The volume occupied by the sand particles return to the dry volume.
Figure 7 – Bulking of Sand
Slide 39
In coarse aggregates, the particles size is large compared to the thickness of the water film, consequently the effect of
bulking is, if anything, is negligible.
The finer the sand, the greater the effect of surface tension, and the greater will be the bulking. In very fine sands the
volume increase can be as high as 40%.
Figure 8 – Typical Bulking Curves
The bulking effect has to be taken into account when the concrete batching is done by volume (m3). Bulking is of no
consequence when batching is done by weight. That is one of the reasons why weigh-batching is preferred for all
site/engineering works.
Example: A concrete mix requires 0.065m3 of sand, in the SSD condition.
The sand in the stockpile has a water content of 5%.
It is estimated that the bulking factor for the sand at this water content is 20%.
What volume of sand should be used?
Solution: Volume of sand to be used = 1.20 x 0.065 = 0.078m3
2022 Materials (Civil) – Section 1: Concrete 1-4
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