Concrete is formed form a mixture of cement, water and aggregates. The cement, commonly Portland cement, and other cementitious such as fly ash and slag cement combined with water to form a paste and act as a binder for the aggregates when hardened. The cement and water is hardened by a chemical reaction known as hydration.
The graphical animation below explains the production of concrete and the mode of action of the superplasticizer Glenium:
Structures built with concrete are more durable to withstand earthquake, typhoons, hurricanes, and tornadoes. Concrete are used in one of the following forms:
- Reinforced concrete
- Prestressed concrete
A method for overcoming the concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete.
- Mass concrete
Basic desired properties of concrete:
- Good workability - When the concrete is newly mixed, it is easy to place and consolidate.
- High strength and hardness - Resistance to freezing and thawing and deicing chemicals, watertightness (low permeability) , wear resistance, and strength.
- Adequate durability - Since the quality depends mainly on the water to cement ratio, the water requirement should be minimized to reduce the cement requirement and thus reduce the cost.
6 - 16% cement
12 - 20% water
20 - 30% fine aggregate
40 - 55% coarse aggregate
The mix proportions of concrete affect the properties of concrete. It's is difficult to measure an exact volumes of the materials. Therefore, the mix proportions are usually expressed as the weight of each material required in a unit volume of the concrete production, in kg/m3.
The mix proportion by weight are:
150 - 600kg/m3 cement
110 - 250kg/m3 water
1600-2000 aggregateskg/m3 (coarse + fine)
The study of concrete concerns with:
- Properties of fresh concrete
- Early age properties of concrete
- Properties of hardened concrete
- Concrete mix design
The first 48 hours are very important for the performance of the concrete structure, between placing and setting, and during early stage of hydration. It controls the long term behavior of the hardened concrete. Let's have a look how concrete is placed and set!
The main properties of fresh concrete:
- Fluidity - The capability of being handled and of flowing into the formwork.
- Compactability - the air trapped during mixing, transporting and handling should be removed.
- Stability or cohesiveness - The concrete should remain as a homogeneous uniform mass.
Fluidity and compactability are combined into the property known as WORKABILITY.
Workability is property determining the effort required to mix, transport, place,compacted and finish a freshly mixed quantity of concrete with minimum loss of homogeneity.
Higher workability concretes are easier to manipulate and can be achieved by one or a combination of the following:
- use of a well graded aggregate.
- use of air-entraining admixtures and plasticizers and superplasticisers
- higher water/cement ratio
Use of a well graded aggregate
Total surface area of the aggregate is important. Workability decreases as surface area increases. Workability is less in a lean mix (i.e., lower cement/aggregate) than in a rich mix. Too little sand, however, produces a “harsh” mix, that is prone to segregation and difficult to finish. Aggregate porosity may influence workability. Roundness and smoothness of particles increases workability. Cement/Aggregate ratio is an important factor in determining workability.
Use of air-entraining admixtures and plasticizers and superplasticisers
Air entraining admixtures, water reducing admixtures, superplasticisers improve flowability and cohesiveness.
SCMs can also improve cohesiveness and flow. The small particle size of some SCMs, particularly silica fume, may result in decreased flowabilty. Use a water reducing or superplasticizing admixture to improve
workabililty.
Higher water/cement ratio
Flowability increases as water/cement ratio increases.Segregation and bleeding also increase as water/cement ratio increases. Strength decreases as water/cement ratio increases. Finer cement produces a less workable mix because of higher specific surface area and increase rate of hydration. The cement composition is much less important than the aggregate characteristics and mix proportioning in determining workability.
However, there is some undesirable side-effects from the use of the above methods:
- use of smooth and well rounded aggregate lead to lower strength but this can be offset by lower water/cement ratio.
- higher porosity arising from the use of air-entraining admixtures results in lower strength. However, this loss is partially offset by the gain in strength by lower water/cement ratio.
- use of higher water/cement ratio is the simplest way to achieve higher workability but there is the risk of segregation of aggregate which results in lower strength.
Workability Tests
- Slump test
The apparatus used in slump test:
Slump test is an empirical test that measure the workability of a fresh concrete. It is commonly used in the construction site as it the apparatus for the test is easy to set up and the procedure is simple. The objective of slump test is to measure the slump value and thus determine the workability of fresh concrete. Let's watch how slump test is carried out!
Description on the procedure of slump test
- From picture 1,2,3 and 4, the cone is filled with concrete in three equal layers, and each layer is compacted with twenty five tamps of the tamping rod.
- From picture 5, the cone is slowly raised horizontally and the concrete is allowed to slump under its own weight.
- From picture 6, The slump is measured to the nearest 5(or sometimes 10)mm using the upturned cone and slump rod as a guide.
According to the profile of slumped concrete, the slump termed as true slump, shear slump and collapse slump.
- Collapse slump - The concrete collapse completely. This shows that the concrete is too wet and high workability. Usually having slump value of 50-90 mm which is suitable for heavily reinforced section and flowable concrete.
- Shear slump- The top portion of the concrete sheared off or slip away. Having medium workability, usually have slump 10-40 mm, used in foundation with light reinforcement, columns, beams and retaining walls.
- True slump - The concrete simply subsides, keeping more or less to the shape. Having low workability, usually have slump 0-25 mm, used in road making, massive section, vibration, and little reinforcement.
Compacting factor test
Compacting factor apparatus:
Featured of the apparatus:
- portable
- frame is rigid and stable
- removal and cleaning of hopper and receiver is easy
- Instantaneous opening of trap doors at the back when released and during the fall of material.
- Especially useful for workability determination of concrete mixes of very low workability such as compacted by vibration.
Compacting factor test is the measure of degree of compaction for the standard amount of work and thus offer a direct and reasonably reliable assessment of the concrete workability. The compaction factor is defined as the ratio of the weight of partially compact concrete with the weight of fully compact concrete. For the normal range of concrete the compacting factor lies between 0.8 - 0.92 .The sensitivity of the compaction factor is reduced outside the normal range of workability and is generally unsatisfactory for compacting factor greater than 0.92 .
Interpretation of Compaction Factor Test:
Video on Compacting factor test~
Procedure of Compacting factor test:
- Concrete is loaded into the upper hopper.
- The trap door is opened, and the concrete falls into the lower hopper.
- The trap door is opened, and the concrete falls into the cylinder.
- The concrete is struck off level with the top of the cylinder.
- The cylinder + concrete is weighed, to give the partially compacted weight of concrete.
- The concrete is fully compacted, and extra concrete added to fill the cylinder.
- The cylinder + concrete is weighed, to give the fully compacted weight of concrete.
- The compacting factor is calculated using the formula: weight of partially compact concrete/ weight of fully compact concrete.
Vebe Test
Vebe test apparatus:
The vebe test is a more scientific workability test than slump test. It measures the work needed to compact the concrete. Vebe times range from 1 second for runny concrete to more than 12 seconds for stiff concrete. Unlike the slump test, the Vebe time test gives useful results for stiff concretes.
Procedure of Vebe test:
- A slump test is performed in the container.
- A clear perspex disc which is free to move vertically is lowered onto the concrete surface.
- Vibration at a standard rate is applied.
- After the cone has been lifted off the concrete. The time taken for the concrete to be compacted is measured.
Flow table test
Equipment of flow test:
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Procedure:
 The cone filled with concrete, prior to lifting.  The diameter of the resulting flow is measured. Flow test is a method to measure the consistency of fresh concrete. The consistence of concrete is measured before the concrete is poured into a formwork to guarantee the quality of structure built. Flow of concrete is reported as the percentage increase in average diameter of the spread over the base diameter of the cone. |
EARLY AGE PROPERTIES OF CONCRETE
Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structure.
Operations: batching, mixing, transporting, placing, compacting, surface finishing.
A) Behavior of fresh concrete after placing and compacting
1. Segregation and Bleeding
From placing to final set, concrete is in a plastic, semi-fluid state.
Heavier particles (aggregates) have tendency to move down (SEGREGATION).
Mix water has a tendency to move up (BLEEDING).
BLEEDING
A layer of water (~ 2 % or more of total depth of concrete) accumulates on
surface, later this water evaporates or re-absorbed into concrete.
Other effects of bleeding;
Surface laitance ~ water rich concrete layer hydrating to
a weak structure (not good for floor slabs that need to have hard wearing
surface)
Water-rich pockets ~ upward migrating water can be trapped under coarser
aggregate particles causing loss of strength and local weakening in transition zon
2. Plastic settlemen
Horizontal reinforcing bars may put restraint to overall settlement of concrete.
Then plastic settlement cracking can occur.
Vertical cracks form along line of the bars, penetrating from surface to bars.
3. Plastic shrinkage
- On an unprotected surface, bleed water evaporates.
- If rate of evaporation > rate of bleeding, then surface dries (water content reduces on surface) and plastic shrinkage (drying shrinkage in fresh concrete) will occur.
- Restraint of walls of concrete causes tensile strains in near surface region.
- Fresh concrete has almost zero tensile strength, thus, plastic shrinkage cracking results cracking is in fairly regular “crazing” form.
Plastic shrinkage cracking will be increased by greater evaporation rates of the surface
water which occurs, i.e. with higher concrete or ambient temperatures, or if the
concrete is exposed to wind.
4. Methods of reducing segregation and bleed and their effects
B) Curing
What is curing? Curing is the protection of concrete from moisture loss from as soon after
placing as possible, and for the first few days of hardening.
Curing methods
- Spraying or ponding surface of concrete with water.
- Protecting exposed surfaces from wind and sun by windbreaks and sunshades.
- Covering surfaces with wet hessian and/or polythene sheets.
- Applying a curing membrane, a spray-applied resin seal, to the exposed surface to prevent moisture loss.
Hydration reactions between cement and water are temperature-dependent and rate of reaction
increases with curing temperature.
- At early ages rate of strength gain increases with curing temperature (higher temperatures increases rate of reaction, thus more C-S-H and gel is produced at earlier times, achieving a higher gel/space ratio and thus higher strength).
- At later ages, higher strength are obtained from concrete cured at lower temperatures. (C-S- H gel is more rapidly produced at higher temperature and is less uniform and hence weaker than produced at lower temperatures).
- Standard curing temperature is 22 ± 1 o C.
- Hydration proceeds below 0 o C, stop completely at -10 o C
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick additives that were added to the burnt lime to obtain a hydraulic binder were later referred to as cementum, cimentum, cäment and cement.
Cement used in construction is characterized as hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g.gypsum plaster) must be kept dry in order to retain their strength.
The most important use of cement is the production of mortar and concrete—the bonding of natural or artificial aggregates to form a strong building material that is durable in the face of normal environmental effects.
Concrete should not be confused with cement, because the term cement refers to the material used to bind the aggregate materials of concrete. Concrete is a combination of a cement and aggregate.
The Cement Production
Raw Materials
The main raw materials used in the cement manufacturing process are limestone, sand, shale, clay, and iron ore. The main material, limestone, is usually mined on site while the other minor materials may be mined either on site or in nearby quarries. Another source of raw materials is industrial by-products. The use of by-product materials to replace natural raw materials is a key element in achieving sustainable development
Raw Material Preparation
Mining of limestone requires the use of drilling and blasting techniques. The blasting techniques use the latest technology to insure vibration, dust, and noise emissions are kept at a minimum. Blasting produces materials in a wide range of sizes from approximately 1.5 meters in diameter to small particles less than a few millimeters in diameter.
Material is loaded at the blasting face into trucks for transportation to the crushing plant. Through a series of crushers and screens, the limestone is reduced to a size less than 100 mm and stored until required.
Depending on size, the minor materials (sand, shale, clay, and iron ore) may or may not be crushed before being stored in separate areas until required.
Raw Grinding
In the wet process, each raw material is proportioned to meet a desired chemical composition and fed to a rotating ball mill with water. The raw materials are ground to a size where the majority of the materials are less than 75 microns. Materials exiting the mill are called "slurry" and have flowability characteristics. This slurry is pumped to blending tanks and homogenized to insure the chemical composition of the slurry is correct. Following the homogenization process, the slurry is stored in tanks until required.
In the dry process, each raw material is proportioned to meet a desired chemical composition and fed to either a rotating ball mill or vertical roller mill. The raw materials are dried with waste process gases and ground to a size where the majority of the materials are less than 75 microns. The dry materials exiting either type of mill are called "kiln feed". The kiln feed is pneumatically blended to insure the chemical composition of the kiln feed is well homogenized and then stored in silos until required.
Pyroprocessing
Whether the process is wet or dry, the same chemical reactions take place. Basic chemical reactions are: evaporating all moisture, calcining the limestone to produce free calcium oxide, and reacting the calcium oxide with the minor materials (sand, shale, clay, and iron). This results in a final black, nodular product known as "clinker" which has the desired hydraulic properties.
In the wet process, the slurry is fed to a rotary kiln, which can be from 3.0 m to 5.0 m in diameter and from 120.0 m to 165.0 m in length. The rotary kiln is made of steel and lined with special refractory materials to protect it from the high process temperatures. Process temperatures can reach as high as 1450oC during the clinker making process.
In the dry process, kiln feed is fed to a preheater tower, which can be as high as 150.0 meters. Material from the preheater tower is discharged to a rotary kiln with can have the same diameter as a wet process kiln but the length is much shorter at approximately 45.0 m. The preheater tower and rotary kiln are made of steel and lined with special refractory materials to protect it from the high process temperatures.
Regardless of the process, the rotary kiln is fired with an intense flame, produced by burning coal, coke, oil, gas or waste fuels. Preheater towers can be equipped with firing as well.
The rotary kiln discharges the red-hot clinker under the intense flame into a clinker cooler. The clinker cooler recovers heat from the clinker and returns the heat to the pyroprocessing system thus reducing fuel consumption and improving energy efficiency. Clinker leaving the clinker cooler is at a temperature conducive to being handled on standard conveying equipment.
Finish Grinding and Distribution
The black, nodular clinker is stored on site in silos or clinker domes until needed for cement production. Clinker, gypsum, and other process additions are ground together in ball mills to form the final cement products. Fineness of the final products, amount of gypsum added, and the amount of process additions added are all varied to develop a desired performance in each of the final cement products.
Each cement product is stored in an individual bulk silo until needed by the customer. Bulk cement can be distributed in bulk by truck, rail, or water depending on the customer's needs. Cement can also be packaged with or without color addition and distributed by truck or rail.
Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove. By this crude method he laid the foundation for an industry which annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. Cement is so fine that one pound of cement contains 150 billion grains.
Portland cement, the basic ingredient of concrete, is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other ingredients to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Lime and silica make up about 85% of the mass. Common among the materials used in its manufacture are limestone, shells, and chalk or marl combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore.
Each step in manufacture of portland cement is checked by frequent chemical and physical tests in plant laboratories. The finished product is also analyzed and tested to ensure that it complies with all specifications.
Two different processes, "dry" and "wet," are used in the manufacture of portland cement.
When rock is the principal raw material, the first step after quarrying in both processes is the primary crushing. Mountains of rock are fed through crushers capable of handling pieces as large as an oil drum. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer millsfor reduction to about 3 inches or smaller. In the wet process, the raw materials, properly proportioned, are then ground with water, thoroughly mixed and fed into the kiln in the form of a "slurry" (containing enough water to make it fluid). In the dry process, raw materials are ground, mixed, and fed to the kiln in a dry state. In other respects, the two processes are essentially alike.
The raw material is heated to about 2,700 degrees F in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameterlarge enough to accommodate an automobile and longer in many instances than the height of a 40-story building. Kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil or gas under forced draft.
As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces about the size of marbles.
Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process thatsaves fuel and increases burning efficiency
Chemical Composition of Portland Cement
There are four chief minerals present in a Portland cement grain: tricalcium silicate (Ca3SiO5), dicalcium silicate (Ca2SiO4), tricalcium aluminate (Ca3Al2O5) and calcium aluminoferrite (Ca4AlnFe2-nO7). The formula of each of these minerals can be broken down into the basic calcium, silicon, aluminum and iron oxides (Table 1). Cement chemists use abbreviated nomenclature based on oxides of various elements to indicate chemical formulae of relevant species, i.e., C = CaO, S = SiO2, A = Al2O3, F = Fe2O3. Hence, traditional cement nomenclature abbreviates each oxide as shown in Table 1.
The composition of cement is varied depending on the application. A typical example of cement contains 50–70% C3S, 15–30% C2S, 5–10% C3A, 5–15% C4AF, and 3–8% other additives or minerals (such as oxides of calcium and magnesium). It is the hydration of the calcium silicate, aluminate, and aluminoferrite minerals that causes the hardening, or setting, of cement. The ratio of C3S to C2S helps to determine how fast the cement will set, with faster setting occurring with higher C3S contents.
Lower C3A content promotes resistance to sulfates. Higher amounts of ferrite lead to slower hydration. The ferrite phase causes the brownish gray color in cements, so that “white cements” (i.e., those that are low in C4AF) are often used for aesthetic purposes.
The calcium aluminoferrite (C4AF) forms a continuous phase around the other mineral crystallites, as the iron containing species act as a fluxing agent in the rotary kiln during cement production and are the last to solidify around the others. Figure 1 shows a typical cement grain.
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| Figure 1: A pictorial representation of a cross-section of a cement grain. Adapted from Cement Microscopy, Halliburton Services, Duncan, OK. It is worth noting that a given cement grain will not have the same size or even necessarily contain all the same minerals as the next grain. The heterogeneity exists not only within a given particle, but extends from grain to grain, batch-to-batch, plant to plant. Types of Cement Gray Ordinary Portland Cement Our Gray Ordinary Portland Cement is a high-quality, cost-effective building material—mainly composed of clinker—that meets all applicable chemical and physical requirements and is widely used in all construction segments: residential, commercial, industrial, and public infrastructure. White Portland Cement CEMEX is one of the world's largest producers of White Portland Cement. We manufacture this type of cement with limestone, low iron content kaolin clay, and gypsum. Customers use our White Portland Cement in architectural works requiring great brightness and artistic finishes, to create mosaics and artificial granite, and for sculptural casts and other applications where white prevails. Masonry or Mortar Masonry or mortar is a Portland cement that we mix with finely ground inert matter (limestone). Our customers use this type of cement for multiple purposes, including concrete blocks, templates, road surfaces, finishes, and brick work. Oil-well Cement Our oil-well cement is a specially designed variety of hydraulic cement produced with gray Portland clinker. It usually forges slowly and is manageable at high temperatures and pressures. Produced in classes from A to H and J, our oil-well cement is applicable for different depth, chemical aggression, or pressure levels. Blended Cement Blended hydraulic cements are produced by intergrinding or blending Portland cement and supplementary cementitious materials or SCM such as ground granulated blast furnace slag, fly ash, silica fume, calcined clay, hydrated lime, and other pozzolans. The use of blended cements in ready-mix concrete reduces mixing water and bleeding, improves workability and finishing, inhibits sulfate attack and the alkali-aggregate reaction, and reduces the heat of hydration. CEMEX offers an array of blended cements which have a lower CO2 footprint resulting from their lower clinker content due to the addition of supplementary cementitious materials. The use of blended cements reinforces our strong dedication to sustainable practices and furthers our objective of offering an increasing range of more sustainable products. Water to Cement Ratio A low water to cement ratio is the number one issue effecting concrete quality. The ratio is calculated by dividing the water in one cubic yard of the mix ( in pounds) by the cement in the in the mix (in pounds). So if one cubic yard of the mix has 235 pounds of water and 470 pounds of cement- the mix is a .50 water to cement ratio. If the mix lists the water in gallons, multiply the gallons by 8.33 to find how many pounds there are in the mix. Low water cement ratio impacts all the desired properties of hardened concrete listed in desired properties of concrete. Use a maximum .50 water to cement ratio when concrete is exposed to freezing and thawing in a moist condition or to deicing chemicals per the 1997 Uniform Building Code. (Table 19-A-2) Use a maximum .45 water to cement ratio for concrete with severe or very severe sulfate conditions per the 1997 Uniform Building Code (Table 19-A-4) Water permeability increases exponentially when concrete has a water cement ratio greater than .50. Durability increases the less permeable the concrete mix is. Strength improves with lower water cement ratios. A .45 water cement ratio most likely will hit 4500 psi (pounds per square inch) or greater. A .50 water cement ratio will likely reach 4000 psi or greater. |
Aggregates
Aggregate is a broad category of coarse particulate material used in construction, including:
Sand
Gravel
Crushed Stones
Slag
Aggregates are one of the component of concrete which is then added witth water and cement. They usually occupy about 65%-80% of the tatal concrete volume.
For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.
Aggregates are divided into two categories which are coarse and fine. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch (9.5-mm) sieve. Coarse aggregates are any particles greater than 0.19 inch (4.75 mm), but generally range between 3/8 and 1.5 inches (9.5 mm to 37.5 mm) in diameter.
The reason of using aggregates in concrete is due to:
- to reduce cost
- reduce heat output and therefore reduce thermal stress
- reduce shrinkage of concrete
- help to produce a concrete(when fresh) with satisfactory plastic properties
Desirable properties of aggregates:
- Absorption, Porosity, AND Permeability
- Surface texture
- Strength and elasticity
- Aggregate voids
- Coatings
- Well graded
Aggregates contain pores which can absorb and hold water. The size, the number, and the continuity of the pores through an aggregate particle may affect the strength of the aggregate, abrasion resistance, surface texture, specific gravity, bonding capabilities, and resistance to freezing and thawing action.
Porosity is a ratio of the volume of the pores to the total volume of the particle. Permeability refers to the particle's ability to allow liquids to pass through. If the rock pores are not connected, a rock may have high porosity and low permeability. Before concrete mixing, aggregates can be in one of the 4 moisture conditions as shown below:
- completely dry
- air dry, pores partially filled with water
- saturated with water and surface dry
- wet with excess water on surface
Surface texture and particle shape
Surface texture is the pattern and the relative roughness or smoothness of the aggregate particle. Right shape and texture so that not to adversely affect the properties of fresh and hardened concrete. A rough surface texture gives the cementing material something to grip, producing a stronger bond, and thus creating a stronger hot mix asphalt or portland cement concrete. Surface texture also affects the workability of hot mix asphalt, the asphalt requirements of hot mix asphalt, and the water requirements of portland cement concrete.
Some aggregates may initially have good surface texture, but may polish smooth later under traffic. These aggregates are unacceptable for final wearing surfaces. Limestone usually falls into this category. Dolomite does not, in general, when the magnesium content exceeds a minimum quantity of the
material.
Strength and elasticity
Strength is a measure of the ability of an aggregate particle to stand up to pulling or crushing forces. Elasticity measures the "stretch" in a particle. Most aggregates are stronger than the concrete strength designed or specified. In fact, aggregates of moderate and low strength reduce the stress in the cement paste and increase durability of concrete.
These qualities minimize the rate of disintegration and maximize the stability of the compacted material. The best results for portland cement concrete may be obtained by compromising between high and low strength, and elasticity. This permits volumetric changes to take place more uniformly throughout the
concrete.
Aggregate voids
There are aggregate particle voids, and there are voids between aggregate particles. As solid as aggregate may be to the naked eye, most aggregate particles have voids, which are natural pores that are filled with air or water. These voids or pores influence the specific gravity and absorption of the aggregate materials.
The voids within an aggregate particle should not be confused with the void system which makes up the space between particles in an aggregate mass. The voids between the particles influence the design of hot mix asphalt or portland cement concrete.
Coatings
Coating is a layer of substance covering a part or all of the surface of an aggregate particle. The coating may be of natural origin, such as mineral deposits formed in sand and gravel by ground water, or may be artificial, such as dust formed by crushing and handling. Generally when the aggregates are used in hot mix asphalt or portland cement concrete mixes, the aggregates are required to be washed to remove the coating (contaminant) left on the particles. The coating may prevent a good bond from
forming between the aggregate surfaces and the cementing agent. The coating could even increase the quantity of bonding agent required in the mixture. If the quantity of the coating varies from batch to batch, undesirable fluctuations in the consistency of the mix may result. Deposits containing aggregates which display a history of coating problems require decantation.
Well graded
Grading refers to the determination of the particle-size distribution for aggregate. Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. When gap-graded aggregate are specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate are used to obtain uniform textures in exposed aggregate concrete. Close control of mix proportions is necessary to avoid segregation.
The coarse aggregate can be either;
- single size where nearly all of the particles are within 2 succesive sizes, eg 5-10mm, 10-20mm or 20-40mm.
- graded where the smallest size is 5mm, eg. 5-14mm, 5-20mm or 5-40mm.
The range for fine aggregate is wide. The BS standard subdivides this into 3 divisions of fine, medium and coarse.
For important work, tests need to be carried out to determine the grading which gives maximum workability, economy, density, strength and durability in the concrete.
General characteristics
Aggregates have three primary uses in highway construction:
- As compacted aggregates in bases, subbases and shoulders
- As ingredients in hot mix asphalt
- As ingredients in portland cement concrete
and other less significant uses.
Compacted aggregates without the addition of a cementing material may be used as a base or subbase for hot mix asphalt and portland cement concrete pavements. Portland cement concrete pavements are rigid pavements. For these types of pavements, the purpose of the base may be to improve drainage,
to prevent pumping, or to cover a material that is highly susceptible to frost. Consequently, gradation and soundness are the primary considerations in selecting or evaluating aggregates for bases under rigid pavements. The load-carrying capacity is a primary factor in the selection of aggregates for
hot mix asphalt pavements. A hot mix asphalt pavement does not carry the load; help from the underlying base courses is required. In addition to gradation requirements, the aggregates are required to also possess the strength to carry and transmit the applied loads. Aggregates are sometimes used to make up the entire pavement structure. In this type of pavement, aggregates are placed on the natural soil to serve as a base course and surface course. Again, the primary requirement is the
gradation. In many instances, compacted aggregates are also used to construct roadway
shoulders and berms. In these applications, gradation and stability are very important.
Aggregate for hot mix asphalt
INDOT uses hot mix asphalt in a number of different ways. In all cases the
aggregates used should meet five requirements:
- Strong, tough and durable
- The ability to be crushed into bulky particles, without many flaky particles, slivers or pieces that are thin and elongated
- Low porosity
- Low permeability
- Correct particle size and gradation for the type of pavement
Aggregate for portland cement concrete
Some of the major uses of aggregates in highway construction are in rigid-pavement slabs, bridges,
concrete barriers, sidewalks, curbs, slopewalls, and other structures.
Aggregates in portland cement concrete are required to always be physically
and chemically stable. Other factors to be considered include:
- The size, distribution, and interconnection of voids within individual particles
- The surface character and texture of the particles
- The gradation of the coarse and fine aggregates
- The mineral composition of the particles
- The particle shape
- Soundness abrasion resistance
- Water absorption
