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INTRODUCTION
CONCRETE
Concrete is a composite material which is produced by mixing aggregates, cement, and water. It’s one of the most versatile construction materials. Concrete can be used for massive constructions such as dams and power stations or for slender highly stressed structures such as bridges and also for paved areas such as roads and airports. Yet it is equally suited for the manufacture of building blocks, roofing tiles, paving slabs and pipes.
Constituents of concrete
Generally, concrete consists of three basic ingredients:
a) Aggregate b) Cement c) Water
However, most of concrete used in the construction industry is usually reinforced with the most common type of reinforcement being steel reinforcement. However, there are other types of reinforcement e.g. sisal fibre, plastics and grass fibres.
Concrete properties can also be varied to suit the circumstances under which it will be used. This is usually achieved through the use of admixtures. Another unavoidable constituent of concrete is air; this is usually undesirable although in some cases it may be deliberately introduced, for example, in air-entrained concrete.
Functions of the constituents
a) Aggregate
This consists of at least two parts, i.e. coarse and fine with each of these two sizes consisting of various sizes. (Any aggregate passing through 5mm BS sieve is fine). The coarse aggregate consists of gravel, crushed gravel or crushed stones graded to the required specification. The fine aggregate is either obtained from crushing plant or from natural sources such as dry or wet river beds. The combination of the coarse and fine aggregate produces the filler in concrete.
b) Cement
Cement reacts with water to form cement paste and it is this paste which binds the particles of both the fine and coarse aggregate. The cement paste also influences the workability of the concrete by providing the required lubrications which is required to handle, place, compact and finish the concrete.
c) Air
Air is inevitable in concrete and a certain amount of air is usually trapped even after thorough compaction has taken place. When excess water dries out it leaves voids and pores which usually contain air. This air has influence on the density and consequently on the strength of the concrete.
NB: the purpose of the filler material is to create an economical material and to reduce the changes in volume which may occur when neat cement and water react together.
HYDRATION
The practical use of concrete as a construction material depends on the fact that it is plastic in its fresh state and becomes hard with considerable strength. This change in the physical properties of concrete is due to the chemical reaction between water and cement. This process is known as hydration
Hydration is a chemical process and is not a drying out process. It is irreversible and gradual fast causing stiffening of the concrete and then developing into considerable strength with time. At a certain ideal conditions, concrete will gain strength indefinitely.
The reaction between cement and water produces heat and this heat must be taken into account during construction. At very low temperatures the reaction will not take place or it will be so slow that the concrete will not stiffen within a reasonable time.
Properties of the constituents materials
When the constituents material of concrete are measured out the quantities are conveniently measured by weight and conveniently expressed in terms of kg/m^3 for normal concrete this should be the range of
Compaction of concrete has a very important effect on the strength it can develop. Any air which may be trapped in the concrete can affect the strength. Compaction of concrete removes most of the air but it virtual impossible to remove all the air.
WATER / CEMENT RATIO
Another important factor affecting properties of concrete is water content of the cement paste. The water used in production of concrete is not all used for hydration but some of it is needed for lubrication. The excess water will form capillaries or very small pores which will weaken the concrete produced. These capillaries may also allow potential damaging substances to enter into the concrete if it is exposed to an aggressive environment. The measure of the water cement ratio of the concrete gives an indication of the protection offered by the cement paste.
Water cement ratio of concrete is normally between 0.4 and 0.6 by weight. If the ratio exceeds 0.77 the capillary formed will never fail and the concrete will continue at risk of attack. With ratio of 0.7 and over the capillaries will only fail after several years of hydration. It is therefore unwise to produce concrete with a water cement ratio exceeding 0.65.
CURING
It is very important to cure all concrete works. Hydration of cement depends upon the presence of water and in a concrete mix there will usually more than sufficient water for hydration.
However, there are situations where water can dry out at an excessive rate and hydration may stop if this loss is not prevented. This can happen on the surface of slabs and every around columns and beams.
Hydration and strength development depend also on temperature and it may also be necessary to maintain the temperature of concrete at the desired level. Curing therefore is the term given to the practical measures which are taken to ensure that the moisture and temperature conditions of the concrete are such that they will promote proper hydration of the cement.
MATERIALS FOR PRODUCING CONCRETE
1) AGGREGATE
Although mixtures of cement and water will harden in the shape of any mould in which they are cast, they have very few practical uses because they are relatively expensive and their shrinkage is unacceptable on drying. Aggregates are therefore required in concrete so as to modify properties such as strength, shrinkage and cost of concrete.
Aggregates in concrete normally constitute between 50% and 80% of the volume of conventional concrete and may thus influence the properties of the concrete. The aggregate used in concrete making are divided into two categories, i.e. fine and coarse aggregate. The fine aggregate which is normally natural sand should consist of particles mainly passing a 5mm sieve while the coarse aggregate which is usually crushed stone, crushed or uncrushed gravel should consist of particles mainly retained on a 5mm sieve. When choosing materials to be used as aggregates in concrete, the following requirements are important:
i. Durability
All aggregates used in concrete should be inert and durable. Should not contain any chemical which may cause aggregate to decompose or react with the cement paste.
ii. Strength
Aggregates should be of good strength and their strength should equal or exceed the compressive strength which the cement can attain. For large and very important jobs the crushing strength of the aggregate should be determined but for medium and minor works it is sufficient to judge the rock or gravel on its hardness, durability and porosity.
iii. Shape and surface texture
Both shape and surface texture can influence the strength of concrete. A rough surface of the aggregate particles increases the bond between the cement paste and aggregate resulting in higher strength. Shape of aggregate particles influences the workability of concrete.
If the cement content is the same an angular aggregate would require a higher water cement ratio than a rounded one. For a given workability and the same cement content, a mixture with
Note: if the two materials are mixed the aggregate will be classified as partially crushed gravel. The term all-in-aggregate is used to describe a mixture of coarse and fine aggregates.
ii. Light weight aggregates.
A variety of porous solids both natural and man made are available for use as light weight aggregate and is a general rule the higher the porosity of aggregate the lower the thermal conductivity, density and strength of the light weight concrete made with it. Aggregates having high porosity make low weight concrete which has excellent thermal insulating value to little resistance to stress. The less porous light weight aggregate produce concrete strong enough to resist structural stresses but denser and less efficient thermal insulators than those made with high porosity aggregate.
iii. High density aggregate
Any aggregate with a density in excess 2000kg/m^3 may be classified as high density aggregate. These aggregates are used for producing high strength concrete and for special purposes such as radioactive screening. The most common types of aggregates in this range are ferrous metal granules and barium sulphate rock. The concrete density in this range is up to 4800kg/m^3. Aggregate in this class are only required in very special circumstances.
iv. Fibres
Asbestos which is a natural occurring material has been used as an aggregate in the production of asbestos cement goods such as roofing sheets and thin panels. Wood fibre is also commonly used and when bonded with cement it produces low mass thermal insulating blocks. Steel and glass fibres are also commonly used to produce special products and so are polymers. However, these types of fibres require factory conditions as opposed to a normal construction site.
v. Gaseous aggregates
In special concrete, gases may be formed in fresh cement paste with special forming agents and special mixing equipments. If a suitable stabilizing agent is used, the bubbles do not rise to the surface and do not burst and they remain evenly disbursed throughout the cement paste until when the cement paste has hardened. This process is used to produce light weight blocks and
panels which are commonly referred to as aerated cement products and are commonly used where stresses are low and thermal insulation is important.
Choice of aggregates
The properties of an aggregate may greatly influence the properties of concrete. However, these properties depend less on the type of rock than on other factors such as shape and size of the particles and hence concrete of an adequate quality can be made from almost any natural stone, rock or gravel provided that enough of the material chosen is available. However, in some applications special properties may be required e.g. the case of flooring a hard wearing aggregate such as granite is desirable. The aggregate for highway construction, may require not being self polishing so that it does not become slippery. The chemical composition of the aggregate only becomes important when the environment is likely to be corrosive.
Deleterious substances in aggregate
Impurities – impurities interfere with the process of hydration of cement by chemically reacting with it
Coatings – which prevent the development of good bond between aggregate and the cement or certain individual particles which are unsound and may lead to disruptive expansion of concrete and failure in themselves. Theses are more prevalent in natural sand which may contain organic impurities usually consisting of products of decay of vegetable matter and appears in the form of humus or organic loam.
Clay, silt and dust – may also be present in aggregate in the form of surface coatings which prevent the development of bond between aggregate and the cement paste. Sands containing excessive deleterious substances are therefore to be avoided.
Advantages of good aggregates
Increased bond strength Reduced water requirement Produces high strength concrete Encourages proper hydration of cement
If an aggregate is subjected to prolonged drying then all the water would be removed. When such a state is reached, the aggregate is said to be in a bone dry condition. In this condition the aggregate will absorb water from the concrete mix.
2. WATER
Water for producing concrete must be clear, pure and fresh. If proper cement and aggregates are used, dirty water will still render the concrete useless. Any water that is suitable for drinking is suitable for producing concrete. The significance of water in concrete mix lies firstly on its ability to enable chemical reactions which cause setting and hardening to proceed and secondly its ability to lubricate the mixture of aggregate and cement in order to facilitate placing and adequate compaction.
Cement requires approximately 25% of its weight in water to hydrate it i.e. water/cement ratio of 0.25. Some water is required as lubricant to make the concrete workable. The lubricant water dries out leaving small voids in the concrete. The more voids there are in finished concrete the weaker it is. The concrete must be well compacted to prevent voids and the drier the mix subject to workability the stronger the concrete. A drier mix which cannot be compacted by hand is often suitable for compaction by variation and the concrete strength is increased.
3. HYDRAULIC CEMENTS
Hydraulic cements may be considered as most versatile binders known to man as they enable him to make massive construction such as dams and foundations or slender, highly stressed structures such as bridges or high-rise buildings or even fittings. Hydraulic cements react exothermically with water to form hard strong masses with extremely low solubility. A number of calcium silicates, calcium aluminates, and other related compounds are able to react in water in this way and are the active components of commercial hydraulic cements. The world demand of hydraulic cement is of hundreds of millions of tonnes per year and for this reason hydraulic cements are to be made as cheap as practicable.
The compounds which are commonly found in cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate (this is sensitive to sulphate attack) and calcium aluminofferite.
Types of cement All over the world there are 4 main categories. a) Portland cement b) Blended Portland cements c) Portland cements with additives d) High alumina cements The 1st^ two categories are the most commonly used while the last two required for special purposes. High alumina cements differ completely from Portland cement, both in composition and manufacture. Portland cements The predominant active constituents in all Portland cements are calcium silicates formed by clinkering materials which are rich in calcium and silicon in specially designed kilns. i. Ordinary Portland cements This is the commonly used cement all over the world for all normal construction. Other types of Portland cements are: ii. Rapid hardening Portland cement (RHPC) iii. Sulphate resisting Portland cement (SRPC) iv. Extra RHPC v. Ultra RHPC vi. Low heat PC vii. Hydrophobic PC viii. Water repellant PC ix. Air entraining PC
ii. For coloured concrete with addition of pigments of required colours
Hydrophobic cements
This is cement whose particles are coated during manufacture with a water proof material or water repellant substance to resist hydration in case is stored for long time in moist atmosphere. The coating is rubbed off during mixing due to friction leaving the cement to act like any other.
Blended P C
i. Portland – blast furnace cement
This is made by incorporating a controlled proportion of ground granulated blast furnace slug during manufacture. The slug is added to a proportionate not exceed 65%. The cement hardens more slowly than OPC because the slug particles hydrate more slowly than cement particles. This type of cement resists sulphate attack better than OPC.
ii. Portland – pfa cement
Pfa (pulverized – fuel ash) is a byproduct of electricity generation
iii. Low heat Portland blast furnace cement (is similar to the above)
Cement with additives
i. Masonry cement
Manufactured to BS 5224 is cement with additives which aid water retention and limit strength development in the cement mortar. The cement is suitable for mortars for brick /block and rendering. It should not be used for structural concrete.
ii. Pozzolanic cement
This is manufactured by adding about 40% pozzolanic material to OPC. It makes the cement low heat producing and suitable for mass concrete. Pozzolana is a fine material containing silica which combines with lime to form cementitious calcium silicate with cement characteristics, hence reducing the quantity of the more expensive cement required in the mix.
MANUFACTURE OF OPC
Raw materials
i. Calcium carbonate from limestone or chalk ii. Silica iii. Alumina iv. Iron oxide – from clay or shale
The raw materials are mixed in proper proportions then ground. The mixture is burned at a high temperature in a kiln to produce clinker. The clinker is then ground into powder form-cement.
There are 2 methods used for mixing:-
i. Wet process ii. Dry process
Wet process
Used for softer materials – chalk or clay. Water is added to proportioned mixture of crushed chalk and clay to produce slurry. Slurry led into a cylindrical steel kiln through upper end and discharged through the lower end where firing is fully injected.
Temperature of kiln varies from 100oc at upper end entry point to 1400oc at lower end exit point at 100oc – water is driven off
850 oc – carbondioxide is driven off
1400 oc – calcium silicates and aluminates are formed in the clinker.
Clinker is allowed to cool then ground finely with 1-5% gypsum to control setting time. To obtain different types of OPc, the following are varied:
i. Proportions of raw materials ii. Burning temperatures iii. Fineness of grinding
Figure 2: dry process of cement manufacture
Errors affecting cement quality
i. Effects of kiln operation
The skill and experience of kiln operator or ‘burner’ are important in achieving good control of the kiln. Too low temperatures may lead to high insoluble residues and low C 3 S contents. Instrumentation in modern plants has positively regulated the kiln operations.
ii. Effects of storage and handling
Although clinker has low reactivity, very prolonged outdoor storage in wet condition could lead to a significant reduction in cement properties. Once the clinker has been ground to form cement then good storage conditions become very important. Bags of cement should be stored clear off the ground not more than 8 bags high in a weather proof shed to guard against air setting. The
Shale (silica/alumina)
Raw mill Silo
Gases
Gypsum
Mill Mill^ Kiln Ball mill
Dry raw mill
Cement silo
Packaging
Mill
Iron ore
Limestone (lime)
bags should be arranged such that they are used in the same order in which they are delivered i.e. those new deliveries are placed on top of existing stacks. Cement should not be kept too long; cement in bags may loose 20% of their strength after 2 months storage and 40% after 6 months storage
iii. Effects of grinding gypsum addition
The fineness of cement is a major contributing factor towards strength. It is required that the raw materials should be ground very fine in order to promote reactions occurring in the kiln; to neglect this may lead to incomplete compound formation with resulting high insoluble residues and free lime. The fineness of grinding is therefore required to be within reasonable limits or else variations may arise in concrete construction especially in precast concrete. It is also required that gypsum additions (to prevent false setting) during manufacture of cement be carefully controlled, failure to do so could lead to problems of setting or expansion.
iv. Effects of basic raw material
These may not be always ideal, at times containing magnesium or phosphates in excess quantities. If the raw materials are not blended correctly or efficiently, the balance between major compounds in cement (c 3 s) and (c 2 s) may be upset with a consequent reduction or variability in strength.
Chemical composition of Portland cement
Name of compound Abbr Oxide composition Tricalcium silicate C 3 S 3CaO.SiO 2 Dicalcium silicate C 2 S 2CaO.SiO 2 Tricalcium aluminate C 3 A 3CaO.Al 2 O 3 Tetracalcium Aluminoferrite C 4 AF 4CaO.Al 2 O 3 Fe 2 O 3
- High C 2 S content
- Low C 3 A and C 3 S content
- Low rate of strength development
- Increased resistance to sulphate attack
Properties of cement
1. Fineness
The finer the cement particles are the greater the surface area of the cement that will be in contact with water during mixing therefore the more efficient the hydration that will take place. This results into more efficient strength development. Therefore, increase fineness, increase:
- Surface area per volume
- Contact area with water
- Rate of hydration
- Early strength development
- Cohesiveness
- Resistance to bleeding
- No. of particles per volume of cement
- Shrinkage cracking
- Cost – extra energy required to grind clinker to a finer state.
Measurements of finess of cement
This is done to BS 4550 part 3. Measures permeability of cement layer i.e. the resistance of a layer of cement to the passage of air – called the specific surface – m^2 /kg .the specific surface of cement can be determined by the air permeability method which measures the pressure drop when dry air flows at a constant velocity through bed of cement of known porosity and thickness. From this, the surface area per unit mass of the bed can relate to the permeability of the bed.
Specific surface of some cement types:-
Cement type SS (m^2 kg-^1 ) OPC 225 RHPC 325
2. Hydration This is the chemical combination of cement and water. Liberate heat measured in calories per gram (jgm-1). Rate of hydration depends on:-
Relative proportions of silicate and aluminate Cement fineness Ambient conditions – temperature and moisture Heat of hydration may cause cracking and loss of durability
Autogenous shrinkage
This is self produced by the hydration of cement i.e. is caused by the chemical combination of cement with water (hydration) which results in a net decrease in volume since the hydrated cement gel has smaller volume than the sum of the cement and water constituents in an environment where the water content is constant e.g. inside a large mass of concrete. However, when the concrete is cured under water, the water taken up by cement during hydration is replaced from outside and also the gel particles absorb more water, thus producing a net increase in volume of the cement paste and an expansion of the concrete. Factors influencing the rate of autogenous shrinkage include:
Chemical composition of cement Initial water content Temperature Time – 75% of autogenous shrinkage occurs within the first 3 months
Setting and hardening
Setting and hardening occurs after hydration. The term setting time is used to describe the stiffening of the cement paste i.e. from a fluid state to rigid state. The test for setting time
