Extraction Of Minerals For Concrete Manufacture Biology Essay

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There are two main construction materials that are used by construction industry; steel and concrete. The construction industry has significant effects on environment and one of the reasons of that is the consumption of concrete. The resources that are used for concrete production are twice greater as compared to steel manufacture. The concrete components require large amount of energy to present them in that kind in which we have got used to see and use them. Carbonfoot print reduction for many countries (governments) in our days is a high priority for all products and industries. The construction industry for many years has been trying to reduce carbon dioxide emissions in different ways and one of them is reuse of construction wastes.

However, extraction of minerals for concrete manufacture is another concern that shows an effect not only in the form of carbon dioxide emissions but also in destruction of historically valuable earth landscapes.

This part of report presents general view of concrete constituents, their production and environmental.

1.1 Concrete and constituents

'Concrete is one of the most basic building blocks of modern life that most people take for granted' [Prof. Adam Neville, 1963].

Concrete is the most commonly used structural material in the world that is primarily consists from three general constituents; water, Portland cements and aggregates. When Portland cement mixed with water and granular aggregates, the chemical reaction between cement and water occurs known as hydration. The result of hydration is interlock of separate particles of sand and gravel in the form of the firm composite material known as concrete. Concrete can be mixed on site or delivered by truck mixer from one of the suppliers, cast on site or precast in a factory, and designed with specific strength. One of the reasons why concrete is commonly used is shape factor. The shape factor plays significant role of concrete popularity because concrete takes any form which is set by the mould (formwork).

The concrete hardens rapidly at first, after a day or so it is usually strong enough to remove the mould. After 28 days it reaches its design strength but continues to gain strength at a rapidly reducing rate for years.

Ready-mixed concrete 0.95kg/CO2 per tonne

1.3 Cement

Cement conforming to BS EN 197-1, termed CEM cement, is a hydraulic binder, i.e. a finely ground inorganic material which, when mixed with aggregate and water, forms a concrete or mortal which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability for a long time under different environment conditions. Cement comprises from 10 to 15 percent of the concrete mix, by volume.

ASTM Standards define cement as

1.3.1 Portland cement

First Portland cement was made at the beginning of 19th century by Joseph Aspdin. This bricklayer from Leeds (England) applied crude method; he burned mixed together clay and limestone powders in his kitchen stove and produced gray substance, known as clinker. For many years this crude method was a fundamental method for cement manufacturing and it still remains the common method in a cement industry for cement production.

The Portland cement consists from precise combination of raw materials; limestone and clay. At high temperatures (1430-1650 °C) these raw materials are blended in rotary kiln to produce hard granular substance, known as clinker. Then cooled clinker is combined with gypsum and milled together to produce a fine gray powder, this powder is Portland cement.

Combination of Portland cement in different proportions with mineral admixtures (supplementary cementing materials) is used to improve properties of final product, concrete.

The American Society for Testing and Materials (ASTM C 150/C 150M) classifies Portland cement into five general groups.

Cement types

Sub classes

CEM I- Portland cement

Strength characteristics

Content of cement clinker

CEM II- Portland -composite cement

N-indicates normal early strength

A- high content

CEM III- Blastfurnace cement

B-medium content

CEM IV- Pozzolanic cement

R-indicates rapid (higher) early strength

CEM V- Composite cement

C-Low content

The chemical composition of cement is varied depending on the application. A typical example of element contains presented in Table 2.

Mineral

Percentage Range (%)

Chemical Formula

Abbreviation

Tricalcium silicate (alite)

50 - 70

Ca3SiO5

C3S

Dicalcium silicate (belite)

15 - 30

Ca2SiO4

C2S

Tricalcium aluminate

5 - 10

Ca3Al2O4

C3A

Tetracalcium aluminoferrite

5 - 15

Ca4AlnFe2-nO7

C4AF

Other additives or minerals - 3.8 % (such as oxides of calcium and magnesium)

Table Chemical formula for major constituents of Portland cement [Andrew R. Barron].

1.3.2 Composite cements

Some types of cement are mixture of Portland cement with other materials such as blastfurnace slag from iron production and pulverised fuel ash from coal-fire electricity power stations. The content of added materials varies; some examples are presented in Table 2. These widely-used mixtures are called 'composite' cements.

Main types

Content [percentage by mass]

blastfurnace slag

pulverised fuel ash

CEM II

6 - 35

6 - 35

CEM III

36 - 95

-

CEM IV

-

11 - 55

CEM V

18 - 50

18 - 50

Table Modified Table 1 from BS 197-1:2000,

Non-Portland cements

There are other types of cement apart from Portland cement. Important examples include:

High-Alumina cement/Calcium Aluminate cements - concrete made with High-Alumina cements usually used when require development of rapid strength and resistance to chemical attack.

Modified Portland cement/Expansive cement - these are special cements designed to exert an expansive force on their surroundings after the cement has set.

Lime concrete/mortar - lime mortar and concrete are used mainly in the rebuilding or repair of historic or ancient buildings.

The detail description of Composite and Non-Portland cements is not presented in this paper as they are not used within this project.

Aggregates

Aggregates are inert granular material such as sand, gravel, crushed recycled materials or crushed stone that, along with water and Portland cement, are an essential ingredient of concrete. Aggregates, which account for 60-75 percent of total volume of concrete have strong impact on concrete's freshly mixed and hardened properties, mixture proportions and economy. Fine aggregates generally consist of natural sand or crushed stone with particles size less than or equal to 4mm (0.16 inch) in diameter [BS EN 12620:2002+A1]. The purpose of the fine aggregate is to fill the voids between coarse aggregate and to act as a workability agent.

Coarse aggregates are any particles greater than 4mm (0.16 inch) [BS EN 12620:2002+A1], but generally in range between 4mm to 37.5mm (0.16 to 1.48 inches) in diameter.

Aggregates may be classified into other categories such as natural or artificial, both with respect to source and to method of preparation.

The acceptance of an aggregate for use in concrete on a particular job or in meeting a particular specification should be based upon specific information obtained from tests used to measure the aggregates quality such as:

Grading

Durability

Particle shape and surface texture

Abrasion and skin resistance

Unit weight and voids

Absorption and surface moisture

The aggregates that are used in construction should comply with all the requirements of the European Standards, regarding to that numbers of tests were performed concerning to The British Standards Institute released document BS EN 12620:2002+A1 to obtain aggregates properties.

(http://www.cement.org/basics/concretebasics_aggregate.asp) 23-1-11

Natural aggregate

The British Standards Institution defines natural aggregate as 'aggregate from mineral sources which has been subjected to nothing more than mechanical processing' [BS EN 12620:2002+A1, p.6].Leaton-Quarry.jpg

Figure Leaton Quarry, Shopshire

http://www.heatonplanning.co.uk/services/minerals-planning-aggregates/ (25-1-2011)

Aggregates are essential construction material to built development, construction projects, and maintenance of infrastructure. The UK aggregates industry extracted 207 million tonnes (Mt) of primary aggregates in 2008. Whilst sand and gravel production declined slightly between 1965 (100 Mt produced) and 2008 (79 Mt produced), crushed rock production has shown a considerable increase over this period, from 60 Mt in 1965 to 128 Mt in 2008 [Mineral Production Association report 2009]. Demand has increased mainly because roads and new developments construction requires a high proportion of aggregates.

Natural aggregates such as gravel and sand are usually dug or dredged form a quarry, river, lake, or sea bed. Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Aggregate processing consists of crushing, screening, and washing the aggregate to obtain required cleanliness and gradation. If necessary an additional processes such as jigging or heavy media separation can be used to obtain better quality.

Increased environmental awareness has led to pressure to re-use construction materials rather than classifying them as waste. Using redundant materials as an aggregate for new concrete is technically viable and may, in some circumstances, be environmentally beneficial.

Manufactured Aggregates

Recycled concrete

Recycled aggregate

The British Standards Institution defines recycled aggregate as 'aggregate resulting from the processing of inorganic material previously used in construction' [BS EN 12620:2002a, p.7].

The recycled aggregates (RA) for this experimental work were sourced from the Ellel Recycling Ltd plat, which located in Lancaster (Lancashire). At Ellel Recycling Ltd construction and demolition waste is processed into quality recycled aggregates. The process of converting construction waste into recycled aggregates on this plant involves some operations such as: crushing, ferrous metals separation from general waste and washing. Recycled aggregates with nominal size of 10 mm were collected on recycling plant for further experimental works.

Determination of recycled aggregates composition was processed in order to evaluate quantity of different components. The composition of recycled aggregates has fundamental effect on recycled concrete properties and behaviour. For example: recycled concrete using bricks as recycled aggregates will have higher permeability comparing to recycled concrete with usage of glass as aggregates, but will have opposite effect on compressive strength. DSC01368.JPG

Random sample of 1 kg was taken from sack in order to evaluate presence of different components. Different components were visually detected, recognised and weighted; recorded data is presented graphically in Figure1.

Natural aggregates were recognised after careful exploration. Several washes were applied before visual survey and usage of hand tools (hammer). Typical rounded texture and gray colour which refer to natural (river) aggregates were established as well as high hardness which is typical for this sort of aggregates was determined.

Slate was found in random sample with properties of hard stone, dark gray colour and typical elongated texture that used for roofing. It is not surprising as slate is a material that widely used in construction industry as roofing material.

Brick was generally recognised through texture and colour. The usual crushed brick irregular texture and a light orange-red brick colour was visually identified (commonly used in UK construction industry).

Concrete crashed was classified in accordance to adhered cement paste and crumbling surface. The cement paste was noticed that adhered to the surface from original concrete and slight crumbling of old cement paste (common for exposed concrete).

Ceramic was determined through texture and decorative surface. Repeated texture explained by manufacturing process, because all particles had the same thickness. Smooth decorative and glazed (pottery) surface which is typical for decorative tiles.

Glass in small quantity with maximum size of 7 mm.

Asphalt (bitumen macadam) was identified as bitumen coated aggregates. The black surface aggregates were placed in preheated oven and after exposure to high temperatures the black substance, presumable bitumen, started melt distributing specific smell.

Aggregates properties

Natural aggregates such as siliceous river sand, coarse aggregates natural and recycled with nominal maximum size of 10 mm were used for cylindrical specimens preparation. Some general laboratory tests such as sieve analysis and water absorption were carried out in order to evaluate the aggregates properties.

Sieve analysis tests

Sieve analysis in accordance with ASTM C136-06 Standards (Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates).

This test method is used primarily to determine the particle size distribution of fine and coarse aggregates by sieving. The results were used to determine compliance of the particle size distribution with applicable specification requirements and to provide necessary data for mix design of the concrete specimens with purpose to achieve expected compressive strength.

Procedure

The following equipments were used throughout the test: electronic scales with accuracy of two decimal places, mechanical sieve shaker, sieves, and oven that uniformly distributed temperature of 110°C (± 5°C).

The sieve frame was constructed conforming to the requirements of Specification E11 ASTM standard with some adjustments. In addition to conservative sieves that are specified in Specification E11 ASTM extra 3 and 4 sieves were added for fine and coarse aggregates analysis, respectively, with purpose to get better understanding of particles grading.

The sizes of the samples were chosen as 300g for fine and 2500g for coarse aggregates in accordance with ASTM C136-06 (clause 7.3 and 7.4). The coarse aggregates samples were split into two portions and sieved individually in order to prevent an overload of material on an individual sieve.

The subsequent procedures were followed to evaluate required data from tests. The sample was dried to constant mass at a temperature of 110°C (± 5°C) for 24 hours. After 24 hours the sample was extracted from oven and left to cool down for 2 hours at room temperature. The sieves were nested in order of decreasing size of opening from top to bottom and the sample was loaded on the top sieve. The sieve frame was agitated by mechanical sieve shaker for a sufficient period to minimise error. The mass of the retained aggregates of each sieve was weighted on electronic scales and recorded. Then for calculation of percentage passing in relation to total sample mass.

Sieve analysis data for siliceous sand is graphically presented in Figure 1. The remained graphical interpretations of results with calculation sheets for natural and recycled coarse aggregates are attached in the Appendix.

Further analysis of the presented graph (Fig. 1) was made in order to determine gradation of particles. In Table 1 are represented the Coefficient of Uniformity (Cu) and the Coefficient of Curvature (Cc) for three types of aggregates.

Aggregate type

Coefficient of Uniformity (Cu)

Coefficient of Curvature (Cc)

Fine (siliceous sand)

1.23

1.06

Natural coarse

1.52

0.94

Recycled coarse

1.35

0.89

In accordance with the Unified Soil Classification System well-graded gravels must have a Cu value > 4, and well-graded sand must have a Cu value > 6. For well-graded sand and gravel, a Cc value from 1 to 3 is required. Siliceous sand and recycled coarse aggregates did not meet required values and in addition to that regarding to BS 5930:1981 can be classified as gap-graded aggregates. Natural coarse aggregates similar to others aggregate did not meet necessary values, but in accordance with BS 5930:1981 (Figure 32) were classified as uniformly-graded.

Water Absorption

Water absorption conducted in accordance with ASTM C127-07 Standards [Water Absorption of Coarse Aggregate] and ASTM C128-07a Standards [Water Absorption of Fine Aggregate].

It was proved that water absorption of aggregates is one of the most important factors which has significant effect on the concrete properties. The recycled aggregates tend to absorb a higher percentage of water comparing with natural aggregates.

During literature review was found that the traditional testing approaches for water absorption cannot give accurate results for recycled aggregates and errors in concrete mix design may result. The researcher Dr. Vivian W.Y Tam (2006) proposed a new approach in measuring water absorption of recycled aggregates. However this method is not approved by BS and ASTM committee yet and decision was made to continue to use the ASTM Standards.

The following equipments were used throughout the test: electronic scales with accuracy of two decimal places, oven that uniformly distributed temperature of 110°C (± 5°C), absorbent cloth, sample container and water tank, drying fan.

The subsequent procedures were followed to evaluate required data from tests. The sample was dried to constant mass at a temperature of 110°C (± 5 °C). After 24 hours sample was extracted from oven and cooled in air at room temperature for 2 hours. The sizes of the samples were chosen as 1000g for fine and 2000g for coarse aggregates in accordance with ASTM C128-07a (clause 7.1) and C127-07 (clause 7.3) standards. The samples weights were recorded at this point and referred to as mass 'A' (ASTM C127-07, clause 9.4). After that the oven dried samples were immersed in water at room temperature for a period of 24 hours (± 4h).

The coarse aggregates were then transferred onto a large absorbent cloth to soak away the visible films of water on the surface of aggregates. A moving steam of air was gently assisted to achieve surface-dry condition, but preventing evaporation of water from aggregate (surface remained damp). This condition of aggregates is known as saturated-surface-dry condition.

Similar procedures were followed for the fine aggregates. The only difference was in aggregates drying process. To minimise lost of fine particles the aggregates were placed in drying pan with paper towel at the bottom instead of using soaking cloth.

The mass of saturated-surface-dry aggregates was taken at this stage, and referred to as mass 'B' (ASTM C127-07, clause 9.4) and the following equation was implement to calculate the percentage of absorption:

The results are presented in Table 6 (below).

Aggregate

Natural

Recycled

Coarse (10mm)

Fine

Coarse (10mm)

Water Absorption (%)

2.54

1.23

2.28

The test results indicated that recycled aggregate exhibited lower water absorption, when compared with corresponding value of natural aggregate. In many publications was found that recycled aggregate tend to have higher value of water absorption comparing to natural aggregate. In our research constituents of recycled aggregates include small presence of ceramic tile and glass. Thus may explain why test results diverge with expected data.

4 Making and curing concrete specimens

Concrete test specimens were prepared and cured in the laboratory under accurate control of materials and environment conditions in accordance with ASTM C192/C192M-7 Standard [Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory].

The strength of concrete is one of its most important and useful property which gives an indication of concrete properties like compressive strength, permeability, resistance to aggressive environments, resistance to high temperatures etc. These properties generally vary considerably depending upon the curing and making conditions.

Joseph F. Lamond and James H. Pielert in their book (Significance of tests and properties of concrete and concrete-making materials, 2006) stated that making and curing 'include size of the aggregate, size and shape of the specimen, consolidation of the concrete, type of mould, capping procedure, curing and temperature'.

The above mentioned variables that may affect experimental work results were carefully controlled under laboratory conditions.

Before mixing concretes the mix proportions were evaluated in accordance with publications of the Department of Environment UK. The workability test was performed on freshly mixed concrete before it was placed into the mould.

4.1 Evaluation of mixture proportions

Concrete mix design in accordance with Department of Environment UK (1988). (Appendix 11)

'Concrete is composed principally of aggregates, Portland cement, and water, some may contain other cementitous materials and/or chemical admixtures' (ACI 211.1-91, reapproved 2002). The mix design of concrete proportions usually is a balance between economy and requirements of workability, durability, strength, density and appearance.

The concrete mix design was based on publication of the Department of Environment UK (1988) that considers following factors:

Workability

Grading and type of aggregate

Water/cement ratio

Target compressive strength

Concrete density

The assumption of slump test for RAC-0 was made at this point as 75mm in order to continue mix design.

The selection of replacement levels was based on literature review (Ryu J.S, Zaga J.C and Yang Keun-Hyeok et al). The typical substitution proportions of natural aggregates by recycled aggregates are 30, 50, 70 and 100 %. For this experimental work the replacement levels of coarse aggregates were chosen as 50 and 100% (notated as RAC-50 and RAC-100 respectively) by volume of natural coarse aggregates by the recycled aggregates.

In accordance with ASTM Standard the minimum cement content for the specified water/cement ratio of 0.5 and a maximum aggregate size of 10mm is 320kg/m3. The mixture proportions of concretes shown in Table 2. The general calculations and tables are attached in Appendix 11.

Type of concrete

Mixture proportions per unit volume (1m3) - (kg/m3)

Fine aggregates Coarse aggregates Water Binder Natural Recycled (cement)

RAC - 0

1017

678

0

205

410

RAC - 50

1017

339

339

290

410

RAC - 100

1017

0

678

205

410

4.2 Mixing and making concrete specimens

The standard method of mixing and making concrete specimen for concrete specimens was applied as mentioned above in accordance with ASTM C192/C192M-07 Standard.

Three different concrete mixtures were prepared using different replacement levels of natural coarse aggregate with nominal maximum size of 10mm. The constituents mentioned in Table 3 were used to mould 24 cylindrical concrete specimens (8 specimens for each mixture). The constituents; Ordinary Portland cement, coarse and fine aggregates were stored in laboratory room at temperature of 20°C (± 5 °C) in order to prevent aggregates segregation and cement hydration.

The original moisture content in coarse and fine aggregates was determined 24 hours before mixing procedure. The samples of 1000g for fine and 2500g for coarse aggregates were stored in an oven at a temperature of 110°C (± 5 °C) for 24 hours. Then samples were extracted after 24 hours and moisture content was evaluated as change in weights. This moisture content was used to modify water amount for batch.

Procedure

The following equipments were used to mould concrete specimens: scales, cylinder mould (100mm-200mm in diameter and height, respectively), pan, vibrating table, concrete mixer, and shovel.

Prior to starting rotation of the mixer the batch of granular materials for RAC-0 was determined in accordance with Table 1. Initially the granular materials were mixed in machine mixer without addition of water until they were uniformly distributed throughout the batch. Then water was added taking into consideration original moisture content. The concrete ingredients were mixed all together for 5 minutes. As mixing process was completed the slump was measured.

The concrete specimens of RAC-50 and RAC-100 concretes were mixed similarly as described above for RAC-0 concrete.

4.2.1 Workability test

The concrete slump test was conducted in accordance with ASTM C143C-08 [Standard Test Method for Slump of Hydraulic-Cement Concrete].

The slump test was performed as a part of concrete mix design with interest to check indirectly the quantity of water and therefore on the water/cement ratio of concrete. On the other hand, this test is a prediction model of concrete workability. Workability of concrete is an important indicator of compatibility, this relates to amount of water, cement, aggregates in concrete mix.

Procedure and results discussion

The following equipments were used throughout the test; metal mould (frustum shape), tamping rod, ruler, scoop.

The freshly mixed RAC-0 concrete was compacted into metal mould in shape of frustum of a cone with the base 200 mm in diameter, the top 100mm in diameter and the height 300mm, in three equal layers. Each layer was tamped 25 times by the tamping rod. After the compaction of top layer the excess concrete was removed and levelled at the end of the mould. Then the mould was removed immediately from the concrete and the difference in levels between the height of the mould and highest point of the concrete was measured (Fig.3). The 'true slump' condition of the concrete which described in ASTM C143-08 was observed (Fig.3). This difference in height in millimetres was recorded as 83, 80 and 78 for mixes RAC-0, RAC-50 and RAC-100, respectively.

slump test.jpg

Figure BS EN 12350-2:2009

The test results shown that water content in concrete mixtures was at normal level and process of making concrete specimens was conducted further.

4.3 Placing and Curing

The different mixtures of concrete RAC-0, RAC-50 and RAC-100 were placed likewise in the cylindrical moulds (100mm-200mm) in laboratory room at temperature of 20°C (± 5 °C). The cylindrical moulds were slightly coated with mineral oil before placing concrete into them, it was made to simplify extraction of concrete specimens. The concrete batches from mechanical mixing pan were placed in the cylindrical moulds in two equal layers as requires ASTM C 192/C 192M-07 Standard [table 1, see appendix X]. The duration of concrete compaction on vibrating table depends from workability which was evaluated for each concrete mix. When first layer was placed the moulds were moved on vibrating table and vibrated for 5 seconds. Then last layer was applied and compaction occurred on vibrating table until large air bubbles stopped coming out on the top surface. The time of compaction was accurately controlled to prevent overvibration, because it could cause segregation. As compaction was finished the moulds from vibrating table were relocated on clean horizontal surface and immediately covered with polyethylene in order to minimise water evaporation. In that position concrete specimens were cured for 24 hours, this stage is known as initial curing.

Initial curing after 24 hours was completed and concrete specimens were removed from the moulds and transferred to the curing room for standard 28 days curing time. The curing room with a constant temperature of 22°C (± 5°C) and relative humidity (RH) of 50 % (± 5%) was used to provide standard 28 day curing regime.

The concrete test specimens were used for experimental work when was finished the standard curing regime of 28 days.

5 Compressive Strength Test of Concretes

Testing compressive strength of concrete in accordance with ASTM C39/C39 M-05 [Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens]

The compressive strength (f'c) of concrete is the most common performance measure used by the engineers in designing buildings and other structures. In construction industry compressive test results are primarily used to determine achievement of required strength, but in this experimental work in addition to that this parameter was used to evaluate changes in concrete properties after exposure of concrete specimens to high temperature.

The experimental programme involved testing the concrete specimens with different mixture proportions after standard 28 days of curing. Once the curing period was complete, the cylindrical specimens 100mm - 200mm in size (4-8 inch) were separated into two main groups:

Specimens used as control: tested with no subjection to high temperature- control group.

Specimens exposed to 270°C for 3 h (T) - exposed group

In both groups each concrete mix (RAC-0, RAC-50 and RAC-100) had three concrete specimens in order to average test results.

The compressive strength tests for both groups were performed in accordance with ASTM C39 standard by employing a hydraulic machine with automatic deformation control. The load was applied at a rate of 140 kN per minute that corresponding to 0.3 MPa per second as requires ASTM C39 standard.

Heon-Soo Chung et al (2008) investigating effect of recycled aggregates replacement level on the properties of concrete gained reduction of compression strength by 20% with replacement level of natural aggregates by 75%. Regarding to literature review the compressive strength of concrete samples RAC-50 and RAC-100 comparing with concrete sample RAC-0 has to be lower.

Results discussion

Control sample

The design compressive strength was expected to be 35 N/mm2 for control concrete sample with no content of recycled aggregates (RAC-0). From the test result of control group the RAC-0 sample achieved average compressive strength of 33.7 N/mm2. The decrease in compressive strength is not significant 3.7% (1.3 N/mm2) comparing with expected 35 N/mm2 and the decision was made to continue experiment.

As expected the recycled concretes RAC-50 and RAC-100 showed a decrease in compressive strength with respect to RAC-0 by 13% and 14.4%, respectively. The averaged test results are presented in Table 4 and graphically in Fig. 5. Typical fracture patterns of failure that are described in ASTM C39/39M and defined as Types 1, 2, 3 and 4 can be seen on pictures in the Appendix 1.

Concrete sample

Control

Exposed to 270°C

RAC-0

RAC-50

RAC-100

RAC-0

RAC-50

RAC-100

Average compressive strength,

(N/mm2)

33.7

29.3

28.9

32.9

26.8

24.9

Exposed sampleDSC01311.JPG

Researcher from Hong Kong Polytechnic University Dr. Y.XU doing experiments with concrete found, that compressive strength of concrete improves after exposure to 250°C temperature. Based on Dr. Y.Xu experiment the decision was made to subject exposed group concretes to temperature of 270°C and then evaluate changes in concrete properties.

An electrical oven with a maximum temperature of 330 °C was used to heat the concrete specimens as shown on Fig.6. The oven from inside is covered with insulation materials, heating element built in back wall, ventilated in the top in order to allow the evaporation of gases, and is equipped with temperature control equipment (mercury thermometer).

The concrete specimens were heated at 270 °C temperature for 180 minutes and the compressive strength test was repeated after cooling. The compressive strength was repeated in the interest of evaluation of changes in relation to control specimens.

After subjection concrete specimens from exposed group to high temperature the compressive strength decreased with respect to control group by 2.4% for RAC-0, whereas for recycled concretes RAC-50 and RAC-100 the decreases were 8.4% and 13.8% respectively. As stated Cho-Liang Tsai 'when the heating temperature is roughly lower than 300 °C, the reduction of strength due to heating is not big '. For the control specimens, the recycled concretes RAC-50 and RAC-100 show a decrease (lower than 15%) in the compressive strength with respect to RAC-0 (conventional concrete). The averaged results are presented in Table 4 and graphically in Fig. 5. The full results sheet is attached in the Appendix 1

The decrease in compressive strength in our experiment generally can be explained by the quality of recycled aggregates and quality of new Interfacial Transition Zone (ITZ). Interfacial Transition Zone is bound between bulk cement paste and aggregate (rock and sand) particles in concrete as shown in Fig. 5. The quality of ITZ plays an important role in determination of RAC performance. The new ITZ between bulk cement paste and recycled aggregate with old ITZ (the cement paste from original concrete that is adhered to the recycled aggregate) creates weak bound and as consequence reduction in compressive strength of recycled aggregate concretes (RAC) as can be seen from compressive test results in Table 4.ITZ.jpg

Absorption Test (Sorptivity Test)

Testing water absorption rate of concrete in accordance with ASTM C 1585 - 04 [Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic Cement Concretes]

Durability of material like concrete and especially of reinforced concrete is typically associated with a microstructure compaction. The dense concrete matrix expected to have lower permeability, which has direct relation to durability. The permeability is usually strongly affected by the microcracks that are present in the transition zone between the aggregates and the cement paste. Due to high permeability the transport of water, chloride ions, oxygen etc. will cause deterioration of concrete and reduction of concrete service life.

The principal mechanisms of concrete deterioration are sorption and diffusion. These two methods describe the movement of chloride ions and other aggressive elements through concrete, and as consequence reduction of service life.

In this study, sorptivity test (ASTM C1585) was conducted to determine the vulnerability of an unsaturated concrete to the penetration of water. The test was repeated before and after subjection of concrete specimens to high temperature and in result to determine effect of high temperature on concrete durability.

Procedure

The following equipments were used throughout the test: electronic scales with accuracy of two decimal places, oven that uniformly distributed temperature of 50°C (± 2°C), absorbent cloth, water container, stopwatch, water prove tape and plastic net.

Two cylindrical specimens 100mm (diameter) - 50mm (height) were extracted, using water-cooled saw, from middle section of larger concrete samples 100mm (diameter) - 200mm (height) cylinders. Single sample from each concrete mix was used for sorptivity test, because another extracted cylinder was used in following experiment.

DSC01361 - Copy.JPG

Prior to testing, the specimens were stored in an oven at a temperature of 50°C for 3 days. After that specimens were sealed in plastic containers and stored in the environmental chamber at 23°C for 3 days. The sides of the specimens are then sealed with tape and the ends of the specimens opposite the absorbing surface are covered to impede evaporation from this surface during the test (Fig.). The specimens before test started were weighed and referred as initial mass (M1), and then absorbing surfaces were exposed to water by immersion into a reservoir. At increasing time intervals, the specimens were removed from exposure to water, the surfaces were swept with absorbent cloth to remove excess surface water, and after that the specimens were reweighed and referred as secondary mass (M2). Frequent measurements were made during the first 6 hours of testing, followed by daily measurements for next 8 days. The change in mass over time was used to calculate the sorptivity. Typically, the rate over the first 6 hours is higher than the rate over the succeeding days. These are expressed as initial and secondary rates, respectively.

'The absorption, I, is the change in mass (Mt = M2 - M1) divided by the product of the cross-sectional area of the test specimen and the density of water' ASTM C1585 (clause 10.1). The equation is presented below

Where:

I = the absorption,

Mt = the change in specimen mass in grams, at the time t,

a = the exposed area of the specimen in mm2, and

d = the density of the water (0.001) in g/mm3.

Results and discussion

The initial and secondary rates of water absorption (mm/s1/2) are defined as the slope of the line is the best fit against the square root of time (s1/2). The graphical result for control specimen RAC - 50 is presented in Fig. 5 (below). The test results for both groups (control and exposed) are presented in Table 3, the graphical interpretation attached to Appendix W.

For initial absorption points measured up to 6 hare used.

The R- squared value on chat was fitted in order to determine accuracy of rate of water absorption prediction model. 'The R2 (coefficient of determination) of a regression model is a measure of the fit with the sample data. It is the proportion of Y variability that can be predicted from X in the sample' (Schulman, 1992). As the R2 increases, the fit of the model improves.

Sample

Absorption, mm/s1/2

Initial

Secondary

I

R2

I

R2

Control

0.0109

0.96

0.0031

0.98

RAC-0

RAC-50

0.0095

0.94

0.0024

0.98

RAC-100

0.0089

0.94

0.0026

0.99

Exposed

0.0259

0.99

0.0009

0.95

RAC-0

RAC-50

0.0189

0.99

0.0008

0.88

RAC-100

0.0179

0.99

0.0008

0.86

Water absorption test results of exposed group specimens in relation to control group were compared in order to evaluate high temperature effect on water penetration to the unsaturated concrete specimens.

The increases of initial water absorption rate of exposed group in relation to control group were evaluated from results that are presented in Table 4 and obtainable below as:

Initial absorption increased by 138% for RCA-0, 99% for RCA-50 and 101% for RAC-100.

Absolutely opposite effect on water absorption rate been observed for secondary absorption the following decreases were evaluated as:

Secondary absorption decreased by 71% for RAC-0, 67% for RAC-50 and 69% for RAC-100.

In accordance to literature review, the rate of initial absorption as been expected increased after concrete specimens were subjected to high temperatures. The main reason why conventional concrete mix RAC-0 presented larger rate of absorption comparing to recycled concretes RAC-50 and RAC-100 can be explained by the quality of coarse aggregates. The orthoradial stresses generated on cement paste from thermal expansion of the aggregates and shrinkage of the cement paste could be the reason of increase number of microcracks and permeability as a result. Smith J.T. et al (2009) testing recycled aggregates in concrete found that the coefficient of thermal expansion improvers (decreases) in relation to conventional concrete as the amount of recycled aggregates increase.

Another reason of water absorption rate increase between control group and exposed group can be explained by water evaporation from porous networks, this was investigated by Sercombe J. et al (2001). Sercombe J. et al experimentally proved that permeability increase of subjected to high temperatures concretes 'is related to water removal from the porous network, to adsorbed water release and to cement hydrates dehydration. These phenomena contribute to the increase in capillary pore size and propagation of fine cracks' and as result enlarge penetration of water.

The secondary water absorption rate of exposed group decreased relatively to control group. This can be explained by the differences between initial and secondary absorptions. The greatest amount of water through microcracks from totally aborted water of exposed group was assimilated at first 6 hours (initial absorption) 53 - 56 percentages and 44-47 percentages during secondary absorption. The control group had opposite effect of water absorption during initial and secondary absorptions were 41-44 percentages and 56-59 percentages from totally aborted water, respectively.

Modified Rapid Migration Test (MRMT)

The Rapid Chloride Migration Test (MRMT) proposed by Tang and Nilsson (1991) was applied to determine chloride ingress into concrete specimens.

Concrete is a stiff material and as many others stiff materials contains microcracks. When these microcracks are combined in a network with macrocracks, the prevailing transport mechanism is not diffusion but permeation of water and aggressive agents through the network.

Resistance of concrete specimens to water penetration through fine cracks was conducted and described in relevant part. Another important mechanism of concrete deterioration is diffusion that affects durability of concrete and specially reinforced concretes. The decision was made to perform chloride penetration test in order to evaluate effect of high temperature on RAC in consequence of diffusion. Diffusion occurs when the concentration of chloride on the outside of the concrete member is greater than on the inside this results in chloride ions moving through the concrete. The chloride ions can result from sea water, de-icing salt, contaminated aggregates etc.

This method (MRMT test) was chosen to determine concretes resistivity to chloride ions diffusion after consideration of criteria such as:

Test terms. In this experimental work test was processed in 3 days comparing to 35 days of ASTM C1556-04 chloride diffusion test.

External factors. The external factors such as temperature oscillation may have effect on results. An applied voltage of 30 V did not raise temperature to level which could have effect on result. Constant room temperature couldn't have effect on results as experiment was conducted in university laboratory.

• Equipment. Tang and Nilsson MRMT test didn't require involvement of special equipment or machinery.

Procedure

The following equipments and chemical reagents were used throughout the test: water containers, stopwatch, sealant (water prove), voltage converter, silver nitrate solution (AgNO3 0.01M), calcium hydroxide (slaked lime), sodium chloride (NaCl, salt),tap water, plastic sheets, metal wire and cooper sheet.

Cylindrical specimens 100mm (diameter) - 50mm (height) were extracted, using water-cooled saw, from middle section of larger concrete samples 100mm (diameter) - 200mm (height) cylinders. These cylindrical specimens were left from previous extractions that are mentioned in relevant part of Absorption test description. Single sample from each concrete mix was used for MRMT test.

composition.jpg

Prior to testing, the sides of the specimens were sealed with water prove sealant (Fig.1, a). After 24 hours all 6 concrete specimens from both groups, control and exposed, were immersed in the limewater (calcium hydroxide water) water container at room temperature with a tightly closed top (Fig.2, b). The container was filled to the top to prevent carbonation. After 3 days of immersion the specimens were removed, because full saturation was achieved. Then the sides of fully saturated samples were sealed with sealant and plastic sheets to provide adequate waterproofness. Then specimens were placed in the half full limewater bath, to prevent evaporation, for another 24 hours for solidification (Fig.1, c). When solidification was over concrete specimens from both groups were placed in plastic buckets with exposure liquids (Fig.1, d). The exposure liquid was 3% NaCl (salt) in limewater, this aggressive agent was made in proportions of 30g of NaCl to 1000g of water. This exposed liquid was employed in the cathode side and limewater in the anode side. Applied voltage of 30 V dc at 8 hours was employed, as shown in Figure 2.

CTH test.jpg

After 8 hours of testing the specimens were removed and split into half. Then a colorimetric analysis was used to determine chloride ions penetration depth by applying a silver nitrate solution (AgNO3-0.01M) as colorimetric indicator. Colorimetric technique defined as quantitative chemical analysis by colour using a colorimeter.

When a silver nitrate solution was sprayed on a concrete a chemical reaction occurred indicating chloride ions penetration with a whitish substance and a brownish colour in the absence of chlorides. Border of chloride ions was not clear when a silver nitrate solution was sprayed on concrete specimens immediately after testing as proposed Tang and Nilsson (Fig.1a). The decision was made to indicate border and sprayed with silver nitrate solution concrete specimens were placed in an oven at 110°C for 24 hours. After 24 hours the specimens were extracted from oven and border of chloride ions diffusion was measured using a ruler (Fig.1b).

chloride pen.jpg

Regarding to literature review (Stanish K. D et al, 2004) this depth of penetration can be used to derive a chloride ion diffusion coefficient (D). Tang and Nilsson (1991) derived a following chloride ion diffusion coefficient equation from the Nernst-Einstein works.

Where:

D = the non-steady-state diffusion coefficient (cm2/s)

R = the universal gas constant (R=8.314 J/K mol)

T = the room temperature (T=295 °K)

L = the thickness of specimen (L=5 cm)

F = the Faraday's constant (F=9.65-104 J/V mol)

E = the electrical potential (E=30 V)

z = the ion valence (z=1)

t = the time (t=28800 s)

xf = the penetration depth (cm)

Results discussion

Chloride ion diffusion depths with coefficients were measured and calculated for both group (control and exposure). The chloride ion diffusion coefficients were calculated regarding to equation above. The data from MRMT test is presented in Table 5 (below), pictures are attached to Appendix X.

Concrete sample

Control

Exposed to 270°C

RAC-0

RAC-50

RAC-100

RAC-0

RAC-50

RAC-100

Diffusion depth, cm

2

2

2.2

5

5

5

Diffusion coefficient, cm2/s

2.94

-10-7

2.94

-10-7

3.24

-10-7

7.35

-10-7

7.35

-10-7

7.35

-10-7

The MRMT test results show that high temperature has high effect on chloride ions diffusion. The chloride ions penetration depth after subjection exposed group to 270°C for 3 hours was overall thickness of concrete specimens (50 mm).

The significantly higher coefficients of chloride ion diffusion of exposed group as compared to control group are explained due to the propagation in the microcracks size at cement paste and at the interfacial transitional zone. This can be proved by similar effects between RA concretes and conventional concrete specimen of exposed group, because high temperature had the same effect on RAC-50 and RAC-100 as on RAC-0 concrete. Despite of replacement of NA by 50% and 100% levels to RA the resistance to chloride ions diffusion reduced to approximately same level after concrete were exposed to temperature of 270°C for 3 hours.

Sieve analysis Appendix

Sieve analysis of siliceous sand (300g mass)

Dim (mm)

Retained mass (g)

Total retained (g)

Retained (%)

Passing (%)

9.5

0.00

0.00

0.00

100.0

4.75

0.00

0.00

0.00

100.0

2.36

0.00

0.00

0.00

100.0

1.18

0.20

0.20

0.067

99.933

1.0

4.850

5.050

1.683

98.317

0.71

102.88

107.93

35.977

64.023

0.6

143.03

250.96

83.653

16.347

0.425

46.70

297.66

99.22

0.780

0.3

1.00

298.66

99.553

0.447

0.15

0.00

298.66

99.553

0.447

tray

0.00

298.66

99.553

0.447

Sieve analysis of natural coarse aggregates (mass 2500g)

Dim (mm)

Set 1

Set 2

Retained mass (g)

Total retained (g)

Retained (%)

Passing (%)

14

0.00

0.00

0.00

0.00

0.00

100

12.5

0.00

0.00

0.00

0.00

0.00

100

10

1.32

1.33

2.65

2.65

0.106

99.894

9.5

27.34

39.70

67.04

69.69

2.788

97.212

8

219.34

285.85

505.19

574.88

22.995

77.005

6.3

350.65

293.76

644.41

1219.29

48.772

51.228

5.6

213.70

137.44

351.14

1570.43

62.817

37.183

4.75

420.90

206.01

626.91

2197.34

87.894

12.106

2.36

163.94

102.09

266.03

2463.37

98.535

1.465

1.18

7.07

11.16

18.23

2481.60

99.264

0.736

0.3

4.22

9.55

13.77

2495.37

99.815

0.185

tray

0.00

0.00

0.00

2495.37

99.815

0.185

Sieve analysis of recycled coarse aggregates (mass 2500g)

Dim (mm)

Set 1

Set 2

Retained mass (g)

Total retained (g)

Retained (%)

Passing (%)

14

0.00

0.00

0.00

0.00

0.00

100

12.5

0.00

0.00

0.00

0.00

0.00

100

10

0.00

3

3

3

0.12

99.88

9.5

40.03

46.33

86.36

89.36

3.574

96.426

8

286.39

194.05

480.44

569.80

22.792

77.208

6.3

478.16

258.52

736.68

1306.48

52.259

47.741

5.6

215.95

186.70

402.65

1709.13

68.365

31.635

4.75

470.05

241.60

711.65

2420.78

96.831

3.169

2.36

31.92

18.06

49.98

2470.76

98.83

1.170

1.18

11.05

9.08

20.13

2490.89

99.636

0.364

0.3

2.14

2.41

4.55

2495.44

99.818

0.182

tray

0.00

0.00

0.00

2495.44

99.818

0.182

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