CHALLENGES IN DESIGNING CONCRETE DURABILITY: A SUSTAINABLE APPROACH R K Dhir M J McCarthy M D Newlands Concrete Technology Unit University of Dundee, UK
Abstract The paper considers developments that have taken place in standards for specifying concrete durability over the last 50 years. This indicates that a broader range of material options is available and wider coverage of exposure conditions is now given. While this offers scope for achieving performance through sustainable approaches, the paper suggests the best route for this lies in techniques that influence the physical and chemical properties of concrete, which may be met by exploiting available cementitious materials and mix proportioning techniques and by recycling / re-use of suitable materials. This is then demonstrated by considering the results of a series of research projects examining different aspects of concrete durability performance, including carbonation rates, chloride ingress, sulfate attack, freeze/thaw attack, and abrasion. It is suggested that the various methods considered can be used effectively to at least match the performance of conventional concrete for almost all aspects of concrete durability performance. Coverage of experience gained to date and the use of these methods in practice is also provided. It is suggested that integration of these within existing construction, is best made through developments in specifications towards performance control. Overall the paper suggests that the minor changes to concrete materials/proportioning described can satisfy both performance and sustainability issues, bringing substantial benefits to concrete construction.
1. INTRODUCTION The last 20 to 30 years have seen a growing awareness amongst engineers of the need to ensure that provisions are made for durability in concrete structures. This has arisen, largely as a result of the increasing incidence of premature deterioration over the period and the substantial repair and maintenance requirements that have resulted [1]. While the specific problems encountered and their severity have varied from region to region around the world, this has very much become an issue of global concern [2]. More recently, there has also been a growing awareness of the importance of sustainability in concrete construction and in particular the more effective and efficient use of materials [3]. In the UK, for example, clients have been encouraged, where practicable, to recycle, or re-use materials.
Furthermore, measures including landfill taxes and aggregate levies have been introduced, at government levels, in order to promote and encourage these practices and their wider use in future is likely [4]. Given this situation, where there is both need for durable concrete construction and responsible use of materials, the aims of this paper are to explore how both of these can be met through minor changes to existing concrete technology and construction practices.
2. DEVELOPMENTS IN STANDARDS FOR SPECIFYING CONCRETE DURABILITY In order to examine how concrete durability can be achieved following sustainable practices, it is appropriate to briefly review the developments that have taken place in specification documents in the UK over the last 50 years. A summary of these and their main specification parameters is given in Table 1. Table 1 Developments in standards for specifying concrete durability YEAR
1950/57 1959
STANDARDS
CP 114 CP 115 CP 116 CP 110 BS 8110
Exposure Conditions
2
1965
1972
1985
1997 BS 5328
2000
2002
BS EN206 BS 8500
2
9
5
5
6
18
18
Mix parameters considered Concrete Grade
Ψ
Ψ
Υ
Υ
Υ
Υ
Υ
Υ
Min Cement Content
Ψ
Ψ
Ψ
Υ
Υ
Υ
Υ
Υ
Max w/c ratio
Ψ
Ψ
Ψ
Υ
Υ
Υ Cement Types Permitted 1
Υ 1
Υ 1
Υ 1
Υ 3(2)
Υ Ψ (1) 3
Υ Ψ (1) 27(3)
Υ Ψ (1) 17(3)
Cover Depth
(1)
Cover is specified in appropriate structural design standard First permitted use of pfa and ggbs in standards (3) Includes use of pfa, ggbs, silica fume, metakaolin (2)
The first British Standards covering proportioning of concrete mixes were issued from the late 1940s [5]. These covered specific concrete applications, including normal reinforced (CP 114, first issued 1948), prestressed (CP 115, 1959) and precast (CP 116, 1965) and gave guidance, in terms of volume ratios, for two environments (‘internal’ and ‘exposed’) with different covers for each. In 1965, with the issue of CP 116, there was a move towards modern concrete specification practice. This defined 3 internal and 6 external environments, and linked these to both minimum strength grade and cover. In 1972, CPs 114 to 116 were replaced with a single code of practice for structural concrete, CP 110. This built on the approach of CP 116, and provided the basis for codes in current use. Minimum cement contents were specified and linked, together with minimum strength grades, to 4 of the 5 exposure conditions defined. In addition, maximum w/c ratios (linked to exposure) were introduced as an alternative approach. During the 1980s and 90s, concrete design and specification in the UK has mainly
followed BS 8110 [6] and BS 5328 [7], in which minimum cement contents, maximum w/c ratios, minimum strength grades and covers were linked. A European standard, BS EN 206-1 [8], together with a complementary British Standard, BS 8500 [9], have recently been introduced, replacing BS 5328 at the end of 2003. BS 8500 specifies minimum cement contents of between 260 and 400 kg/m³, while slightly lower values are specified in BS EN 206-1 [8]. Other requirements are similar between the European and UK Complementary standards, e.g. the use of a wide range of cement types permitted and definition of 18 exposure conditions, highlighting the significance given to and importance of designing concrete for durability. The developments in standards indicate that exposure conditions are now more clearly defined, a wider range of available materials is considered and service life is also included. While there is potential for greater efficiency in the recent documents, the approach is still prescriptive and is not based on the actual processes taking place while the structure is in service. The paper now progresses to examine the critical factors influencing concrete durability and explores how sustainable practices may be used in relation to these to more effectively achieve performance.
3. FACTORS CONTROLLING CONCRETE DURABILITY PERFORMANCE The most significant parameters defining the resistance of concrete to deterioration are the permeation characteristics of the surface and near surface of concrete. This region of concrete not only interfaces with the environment, but also provides protection to embedded reinforcement. However, a number of effects, eg bleeding, rapid drying, etc, mean that it can also be the weakest region of concrete. Deterioration of concrete normally involves the ingress of aggressive agents or water into concrete and the occurrence of physical/chemical processes thereafter. Transportation of fluid into concrete can be defined in terms of the permeation properties, which can be divided into three distinct but related transportation phenomena, covering moisture vapour, dissolved ions, gases and aqueous solutions as follows [10], Absorption The process by which concrete takes in a liquid, eg water or aqueous solution, by capillary attraction. The rate at which water enters is termed absorptivity (or sorptivity) and depends on the size and interconnectivity of the capillary pores in concrete, and the moisture gradient existing from the surface. Permeability The flow property of concrete which quantitatively characterises the ease by which a fluid passes through it, under the action of a pressure differential. This property depends on the pressure gradient and on the size and interconnectivety of the capillary pores in concrete. Diffusion The process by which a vapour, gas, or ion can pass into concrete under the action of a concentration gradient. Diffusivity defines the rate of movement of the agent and is influenced by the concentration gradient from the concrete surface, the type of ingressing agent and the amount of reaction with the hydrating cement/structure and size and interconnectivity of the capillary pores in concrete.
These transportation processes are influenced by a large number of material and environmental factors. As indicated in Figure 1, there is an extremely complex TRANSPORT PROCESS
CONCRETE / ENVIRONMENTAL CHARACTERISTICS
MIX INGREDIENTS / CONDITIONS (SPECIFIED)
Hydrate chemistry Cement type Hydrate structure
Absorption
Capillary size and interconnectivity
Water/cement ratio
Diffusion Pore fluid content Pore fluid chemistry
Permeability
Aggregate type Curing conditions
Environmental Conditions Primary Parameter
Secondary Parameter
interaction between permeation and material properties and environmental conditions. Figure 1 Interaction between material properties, environmental factors and transportation mechanisms Moreover, it highlights where the main parameters currently specified by engineers (cement type, water/cement ratio, aggregate type and curing conditions) fit in relation to the critical factors influencing durability in structures. In addition, it demonstrates where the focus should be in relation to enhancing concrete durability performance, i.e. through the use of materials influencing the physical and chemical properties, if a sustainable practices route is to be followed.
3. SUSTAINABLE METHODS OF ENHANCING CONCRETE DURABILITY Given the factors influencing durability, methods of more effectively achieving performance should concentrate on refinement of the hydrate structure of concrete and control of its chemistry and that of the pore fluids. In attempting to achieve this, within the framework of sustainable construction, three routes are considered in this paper. These can be classified in general terms under (i) cement selection, including binary and ternary combinations, (ii) material proportioning or combination and (iii) recycled / waste materials not covered in specification documents or standards. 3.1 Cement Selection There are an increasing number of different cementing materials available beyond that of Portland cement (PC). These are physically and chemically different to PC (as
shown in Table 2) and there is therefore potential for manipulating their combinations to achieve optimum binding capacity of aggressive, ingressing agents and densification of the microstructure. Among the main materials finding use in Western Europe and elsewhere include pulverized-fuel ash (PFA) and ground granulated blastfurnace slag (GGBS). More recently, other materials including condensed silica fume (CSF) and metakaolin (MK) have also been introduced. Table 2 Typical chemical and physical characteristics of common cements PARAMETERS
PC
PFA
CSF
MK
Chemical Characteristics (Oxide analysis) SiO2 20.0 37.0 Al2O3 5.0 11.0 Fe2O3 3.0 0.3 CaO 65.0 40.0 MgO 1.1 7.0 SO3 2.4 0.3 S21.0 Na2O 0.2 0.4 K2O 0.9 0.7
48.0 26.0 10.0 3.0 2.0 0.7 1.0 3.0
92.0 0.7 1.2 0.2 0.2 1.2 1.9
55.0 40.0 0.5 0.1 0.4 2.0 -
Physical Characteristics Fineness, m²/kg Loss on Ignition, % Bulk Density, kg/m³ Specific Gravity, g/cm³
380 5.0 900 2.3
15,000 240 2.2
10,000 360 2.4
340 1.0 1400 3.1
GGBS
350 1200 2.9
The characteristics of these materials mean that they can contribute one or all of the following, towards the provision of enhanced durability. High alumina content, (except silica fume; which in PFA is 6 times that of PC, in GGBS 2 times and in MK 8 times). This is also likely to be in amorphous form, with a high chloride binding capacity. Large number of well dispersed fine particles available to absorb aggressive agents. Potential for blocking and increasing the length of pathways into concrete. There has also been a growing awareness that it may be possible to get further enhancement of performance by combining two or three materials with PC. This relates both to the need for higher strength and extended durability. Developments in European cement standards (BS EN 197) where combinations beyond binary cements are permitted, means that the use of cements comprising a range of materials will start to become more commonplace. These materials should allow engineers, through careful selection and combination, to produce cement blends to achieve a required set of concrete properties. 3.2 Material Proportioning
The importance of the limits (minimum cement content, maximum w/c ratio, minimum strength) for concrete durability provisions in Standards is evident from the wide use of this approach. The basis for this has, in the main, been local experience and little work to evaluate the role of these on performance has been carried out. The influence of these parameters is clearly important for effective material use by engineers. From an environmental point of view, advantages could be gained if it was possible to reduce cement contents in concrete. In following this type of approach, it may be necessary to consider the use of fillers to achieve a closed structure and admixtures to control workability. Recent developments have also seen the introduction of mix proportioning techniques, aimed at physically minimising the void space of concrete prior to concrete production, see Figure 2. Such an approach to mix proportioning has been developed by Dewar [11], and is based on the principle that when materials of different sizes are mixed together, smaller particles attempt to fill voids between larger particles to form a mixture with minimum voids. The method seeks to achieve this by taking account of the physical characteristics of all the materials in the mix (including the mean particle size of all constituents, the voids ratio and the relative density) and provides optimum proportions of each constituent.
+
+
PC
+
PFA
+
=
CSF
Optimum Packing Concrete
Figure 2 Concept of particle packing using different sized materials to minimize voids 3.3 Recycled / Waste Materials not Covered in Specification Documents The potential for the use of domestic, industrial and construction waste products in concrete as cement components and aggregates is substantial, as shown in Table 3. Whilst this is the case, much of these materials are wasted through disposal at landfill sites. Although some materials have found use as a general fill, there is clearly scope for their use in higher value applications, such as concrete [12]. Table 3 Potential quantities of material available for use in concrete in the UK
MATERIAL Recycled aggregates Conditioned PFA (moist) Glass Incinerator ash Granulated rubber
ARISINGS (p.a.) USES IN CONCRETE 109 Mt > 4 Mt > 2 Mt 1 Mt > 40 Mt
As coarse aggregate As cement component/fine aggregate As fine aggregate As aggregate. Finer fractions have potential pozzolanic properties Specialist concretes
When used appropriately, it has been shown that these materials may, Create high performance aggregates to conserve natural mineral resources Generate sustainable construction Reduce waste disposal costs Minimise dependency on landfill Clearly, given their novelty, the route to the use of these materials in construction needs to be research and development work and thereafter through site trials.
4. CONCRETE DURABILITY PERFORMANCE 4.1 Carbonation Rates Carbonation is the process by which carbon dioxide present in air diffuses into concrete and chemically reacts with calcium hydroxide, produced through cement hydration. This leads to the formation of calcium carbonate causing an associated reduction in alkalinity. Carbonation continues from the concrete surface and in time, the neutralisation effect breaks down the passive oxide layer protecting embedded steel. In the presence of moisture and oxygen, carbon steel reinforcement will corrode. Results from a series of tests on concretes exposed for 2 years in the laboratory and under field conditions, and projected to 35 years service using modelling techniques are shown in Table 4 [13]. These indicate that at the same design strength, there was little or no difference in measured carbonation depth for different cement type concretes. Not surprisingly, the environmental conditions were the most important factor. Table 4 Comparison of long-term carbonation performance of concretes based on estimations from laboratory and field data
EXPOSURE CONDITION Outdoor Sheltered Outdoor Unsheltered (1) (2)
35 YEARS NORMALISED DEPTH OF CARBONATION OF 1 DAY STANDARD CURED 37 N/mm2 CONCRETE, mm PC PFA 30% (1) MK 10% (2) CSF 5% (1) 24 11
25 12
24 12
25 12
Increased replacement levels showed slight increase in carbonation depth. Increased replacement levels showed slight increase in carbonation depth. Notable reduction in carbonation depth with increasing exposure to moisture. Outdoor unsheltered conditions showed little difference between PC, MK 10% and 15%.
Other test results from a study examining the role of cement content on concrete performance are shown in Figure 3 [14]. These indicate that there was a gradual increase in carbonation depth with time during the 20 weeks accelerated test conditions. However, variation in cement contents at fixed w/c ratio had little influence on carbonation and, in fact, slightly improved performance was noted as cement content was reduced. These suggest less cement could be used in concrete for equivalent performance. In these cases, limestone filler and plasticizing admixture were used to
maintain the fines content and workability. Studies using particle packing mix proportioning techniques have shown that such methods can be used to bring minor improvements to carbonation resistance [15]. Work examining some of the recycled materials referred to above has shown that providing these concretes are proportioned for equivalent strength to conventional concretes, similar performance can be achieved [12]. 30 4.0% CO2 exposure, 20°C, 55% RH 1 week = approx. 1 year natural exposure CARBONATION DEPTH, mm
25 0.65 w/c ratio 20
Cement content, kg/m³ 270 270 300 205 240
15
10 Cement and water reduction 5
0 0
4
8
12
16
20
24
EXPOSURE TIME, weeks
Figure 3 Role of cement content on carbonation resistance of concrete 4.2 Chloride Ingress Soluble chlorides present in de-icing salt or occurring in seawater can enter concrete and may lead to reinforcement corrosion. In concrete subject to periodic wetting and drying, chlorides ingress under both absorption (capillary suction) when the concrete is dry and by diffusion once the pores become filled. A threshold level of chlorides must be present at the site of the reinforcement to initiate corrosion. The propagation of corrosion thereafter depends on the continued availability and passage of chlorides and oxygen to the corroding site. An example of results obtained from accelerated chloride diffusion tests, covering a range of cement combinations, with PC/PFA and PC/GGBS as the principle materials, at equivalent design strengths, are given in Figure 4 [16]. As indicated, the combinations of three materials gave substantial improvements at all design strengths compared to the control (PC) and binary cement concretes. It is not really possible to single out a mix as having overall superior performance, although no chlorides were detected during the test for high design strength concrete containing ternary cements. This particle packing method has been used effectively in a study considering the development of chloride resistant concrete where a physically dense and chemically
resistant (through use of PFA and GGBS) concrete cover was developed to slow down the advance of chlorides [17]. Similarly, work examining the effect of varying cement contents at a given w/c ratio has indicated that this has little influence on chloride resistance [18]. Results from chloride diffusion tests on conditioned (moistened) PFA concrete are shown in Figure 5 and indicate that, for a range of fine and coarse material, moistened and stored gave similar performance to dry PFA concrete and improvements compared to PC concrete [19]. 8 7
Cement type
7.2
PC control
6
PC/PFA/SF
5.1
PC/GGBS/SF 4.0
4 3
20 N/mm²
40 N/mm²
< 0.1
0.7
< 0.1
1.3 < 0.1
0
0.3-0.4 0.3-0.5
1
< 0.1
2
2.5
0.4-1.0
5
PC/PFA/GGBS
0.6-0.9 0.4-1.0
PD INDEX, cm²/s x 10
-6
PC/PFA control
60 N/mm²
CONCRETE MIX
Figure 4 Effect of multi blend cement types on chloride diffusivity
COEFFICIENT OF CHLORIDE DIFFUSION, x10 -13 m²/s
8 5M NaCl exposure 7 6
PFA Fineness
5
high
4
low
3
medium
35 N/mm²
2 1
high
0
low Dry
1 PFA STORAGE, months
50 N/mm²
6
Figure 5 Effect of PFA conditioning period and fineness on coefficient of chloride diffusion
Tests examining recycled concrete aggregate indicate that, as with carbonation, providing the concretes were proportioned to achieve a required strength, they have little impact on chloride resistance [12, 20]. 4.3 Sulfate Attack Most sulfate solutions (e.g. sodium, magnesium) in soils, groundwater and seawater react with calcium hydroxide, Ca(OH)2, and calcium aluminate in concrete to form calcium sulfate and calcium sulfoaluminate compounds. The volume of compounds formed is greater than that of cement paste, leading to its breakdown, while, in some cases, decomposition can occur. The intensity and rate of sulfate attack depends on a number of factors such as type of sulfate, its concentration and the continuity of supply to concrete. Tests with various cement components (PFA, GGBS) indicate that through the effects of dilution of the reactive components of cement (aluminate phases) and the densifying effects associated with these materials, reductions in expansion and damage due to sulfate attack may be achieved. This has also been found in work examining the role of cement content at fixed w/c ratio, where reducing expansions were noted with reducing cement content, see Figure 6 [15]. 10000
SULFATE EXPANSION, x10-6
Cement content, kg/m³ 355
5.0% Na2SO4
9000
0.55 w/c ratio
8000
320
7000
Cement and water reduction
6000 5000 280
4000
245
3000 2000 1000 0 0
20
40
60
80
100
120
EXPOSURE TIME, weeks
Figure 6 Role of cement content on expansion due to sulfates The benefits associated with PFA, have also been found with conditioned PFA, which indicates that, despite moisture and storage, performance benefits are maintained, compared to PC concrete (Figure 7) [19].
500 1 YEAR SULFATE EXPANSION, x10-6
PFA Fineness
6 g/l MgSO4 Exposure 35 N/mm² Cube Strength
High 400
Medium Low
300
200
100
0 Dry
1
6
PFA STORAGE, months
Figure 7 Effect of conditioning period and fineness on sulfate resistance 4.4 Freeze/thaw Attack Damage to concrete, may occur if it is exposed to conditions of frost during service. If concrete has a moisture content near saturation and is subjected to cycles of freezing and thawing, then the effects of ice growth in the capillaries and associated development of hydraulic and osmotic pressures, may result in the build up of expansive forces within the concrete pore system and lead to its disruption and breakdown. In the main, freeze/thaw resistance of concrete is controlled by the use of air-entraining admixtures, which introduce discreet air bubbles (air contents of typically 5%), which relieve the pressures occurring when concrete undergoes freezing. This includes tests covering a range of cement types. Data from tests using concretes without airentrainment, suggests that concretes of similar strength, but denser microstructures, may give slightly poorer resistance to freeze/thaw damage [15], for example by reducing the cement content at a fixed w/c ratio (but including filler) [14] and concretes which have been physically optimised by particle packing [15]. With the introduction of air-entrainment, the effect of using different materials in concrete tends to become secondary, in relation to freeze/thaw resistance, although, specific materials may influence the admixture dosage that is needed to achieve a particular air content in concrete. The results from a recent study using crumb rubber recovered from used car tyres are shown in Figure 8 and indicate that the introduction of small quantities of this material lead to scaling resistance of concrete approaching that achieved with air-entrainment. The beneficial effects of this material are most notable at around 6% by volume crumb rubber
and with the use of smaller size fractions [21]. The use of crumb rubber can also be useful in situations where it is difficult to use air entraining admixtures such as concrete transported over long distances, erratic air entrainment using materials with high loss-onignition or low workability mixes which are often difficult to air entrain. 1.0
CUMULATIVE SCALING, kg/m²
Crumb rubber (CR) added at 6% by volume 0.8
Concrete type Non-AE AE
0.6
1mm CR 8mm CR 20mm CR
0.4
0.2
0.0 0
10
20
30
40
50
60
FREEZE / THAW CYCLES
Figure 8 Effect of crumb rubber addition on freeze/thaw resistance of concrete 4.5 Abrasion Abrasive damage to concrete is normally caused by the action of objects or materials in contact with concrete surfaces, eg grinding action of traffic on industrial floors. Factors including finishing of concrete and curing, both of which have a significant influence on the quality of the concrete near surface, are important factors influencing abrasion resistance. Once signs have occurred, the type of aggregate and the aggregate/paste bond are factors influencing subsequent damage [22]. Work examining the effect of different cement types indicates that for a particular aggregate, there is little difference in performance providing the concretes have equal strength [23]. Results from tests examining abrasion resistance of concrete with varying cement contents at fixed w/c ratio are shown in Figure 9 [18]. These indicate that with reducing cement content minor enhancements in abrasion resistance were noted. This appears to reflect the increased aggregate contents associated with cement reductions. In studies examining the replacement of aggregate with recycled concrete aggregate, data suggests that levels up to 30% RCA gave very similar performance to that of conventional concrete, see Figure 10. Only minor increases in abrasion were noted when 50% RCA was used as aggregate [24].
0.80
0.70
390 kg/m³ cement, 195 l/m³ water
0.55
350 kg/m³ cement, 175 l/m³ water
0.60
310 kg/m³ cement, 155 l/m³ water
0.65 270 kg/m³ cement, 135 l/m³ water
DEPTH OF ABRASION, mm
0.75
M3f
M2f
M1
M6
0.50 MIX REFERENCE
Figure 9 Effect of cement content on abrasion resistance
2.0 air cured
water cured
1.5
1.5
1.0
1.0
0.5
0.5
0.0
DEPTH OF ABRASION, mm
DEPTH OF ABRASION, mm
2.0
0.0 0
10
20
30
40
% COARSE RCA IN CONCRETE
50
0
10
20
30
40
50
% FINE RCA IN CONCRETE
Figure 10 Role of coarse and fine aggregate content on abrasion resistance of concrete
5. CURRENT STATUS OF SUSTAINABLE PRACTICES IN INDUSTRY The various approaches described are clearly at different stages in their development in relation to take up by industry. This is reflected in Table 6, which summarises selected practical applications of cement combinations and recycled materials used in concrete construction or full-scale trials over the past 15 years.
Table 6 Selected practical applications of materials and mix proportioning. MATERIALS USED
LOCATION
ELEMENTS CONSIDERED
DETAILS
PFA
Sizewell B Power Station, UK, 1993
Various
100,000t PFA and 1,300t sintered PFA lightweight aggregate were used
Canary Wharf, London, UK, 1990
Various
10-30% PFA concrete used for durability purposes
Burnaby, British Columbia, Canada, 1998
Residential buildings
50% PFA concrete used in 24,500 ft2, 22-unit sustainable housing
Channel Tunnel Rail Link, Kings Cross, London, UK, 2003
Bridge piers
50% GGBS concrete used to minimize thermal cracking and improve durability
Heathrow Airport, London, UK, 2003
Airside road tunnel walls
50% GGBS concrete used to minimize thermal cracking and improve durability
Honley, UK, 1991
Reinforced concrete sewage treatment tank
70% GGBS (CEM III) mix used to combat sulfate attack.
Oresund Link, Denmark, 2001
Oresund Bridge
5% used for extreme marine conditions
311 Wacker Drive, Chicago, USA, 1990
Various
5-10% CSF used to provide high strength concrete
Stoerbelt Crossing, Denmark, 1998
West Bridge, East Tunnel and East Bridge
PC/PFA30%/CSF5% blends used to offset extreme marine conditions.
Tsing Ma Bridge, Hong Kong, 1996
Bridge piers, towers and deck
PC/GGBS/CSF mixes used in 400,000m³ of concrete
Tappan Zee Bridge Hudson River, USA, 1992
Bridge deck
PC/PFA20%/CSF6% was used to inhibit ingress of deicing salts
RCA
Dundee, UK, 2001
Concrete road pavement
100% replacement of coarse aggregate
Conditioned Fly ash
Dundee, UK, 2003
Precast seating for sports stadium
Conditioned ash used as 15% replacement for sand
Dundee, UK, 2003
Concrete beams
30% replacement for PC
GGBS
Silica Fume
Multi Blends
Cement combinations using PFA, GGBS and silica fume have been increasingly popular as advances in concrete technology increase engineers understanding of their benefits in enhancing strength and durability. Minimum cement contents remain in standards and will remain in use unless changes in specification documents are introduced. Particle packing techniques are now being used successfully in ready mix concrete production.
The use of recycled materials such as recycled concrete aggregate and conditioned (moistened) PFA have been considered in full-scale trials and in some construction applications. It is likely that there increased use in construction will be gradual, as research findings are disseminated. This process may take longer for some of the other materials referred to and is also likely to depend on factors including availability.
6. DEVELOPING SPECIFICATIONS FOR SUSTAINABLE PRACTICES An alternative approach to prescription based mix proportions for durable concrete is performance-based specification, where concrete is tested for durability to ensure achievement of a required performance. This considers concrete with respect to relevant exposure conditions, a required design life and criteria that defines the end of that life and employs a test method which directly assesses concrete resistance to a standardised deterioration process. It differs from the conventional approach in that no reference to material contents (eg cement content, w/c ratio and strength) are made, only a specified performance to an agreed test method. Prior to implementation of this approach, a number of factors must be established including the relationship between the test and real performance, sampling and test procedures and test precision. A number of other questions need to be answered [25], however, this is likely to more readily enable the application of some of the materials described earlier in this paper. A recently completed project at the University of Dundee has investigated a performancebased approach to specifying carbonation resistance [13]. The work developed a standard natural carbonation CEN test to benchmark the performance of a representative sample of cement types and combinations permitted in BS EN 197-1. Concretes containing these cement types were examined and their performance in terms of carbonation resistance compared to that of a concrete of known performance. The study highlighted the need for precision testing when considering performance based approaches. A round-robin test programme throughout Europe using a simple carbonation test had shown variability of over 30% for similar concretes and exposure conditions and this was mainly attributed to a combination of variability in concrete production and test method execution. The study reduced this variability to 6% by introducing a 3-mix normalisation procedure and modifying the test method to have active control on CO2 concentration, temperature and relative humidity [13]. Figure 11 from the study shows an example of the advantages in using a performance based approach. A series of concretes were designed on an equal strength basis for Exposure Class XC3/4 in BS 8500 [26]. Based on the prescriptive approach, two mixes (MK and CSF) would fail the mix limitation criteria in terms of maximum water/cement ratio and minimum cement content. However, when these mixes were tested for performance, all mixes performed satisfactorily with regards to meeting the maximum permitted depth of carbonation at 35 years. Further work is now underway to investigate the carbonation exposure classes given in BS EN 206-1 and to develop a rapid assessment method to determine potential carbonation performance of hardened concrete using electrical resistivity techniques. This approach will remove the constituent material element and focus on performance, provide greater flexibility in material selection and promote sustainability.
Exposure Class XC3/4 in BS 8500 Minimum intended working life = 50 years Minimum cover requirements = 30mm All mixes designed for equal strength and meet minimum strength requirements
Prescriptive Approach
Concretes performance tested for 3 years
0.60
Assuming:
ti = 35 years tc = 15 years
0.55 0.50
Thus, performance limitation of maximum 30 mm carbonation at 35 years
0.45 0.40
Cement Content, kg/m³
Maximum w/c ratio = 0.60
0.65
375 350
40 Minimum cement content = 280kg/m³
PC
PF
325
0 A3
%
G
G
B
0 S5
% M
5 K1
% CS
0% F1
300 275 250 225
Carbonation Depth, mm
Water/cement ratio
0.70
Performance Approach
35 30
35 year carbonation based on probabilistic analysis of performance testing
25 20 15 10 5 0
200 PC
A PF
% 30
G
G
50 BS
% M
K1
5%
F CS
10
%
MK 15% and CSF 10% mixes fail criteria for maximum w/c ratio and minimum cement content
PC
A PF
30
%
G
G
BS
50
% M
K1
5%
F1 CS
0%
All mixes pass minimum performance criteria of 30mm carbonation at 35 years
Figure 11 Comparison of prescriptive and performance-based approaches to concrete exposed to Exposure Class XC3/4 in BS 8500-1.
7. CONCLUDING REMARKS The paper has demonstrated that concrete for future construction needs to be durable, achieved following sustainable practices. It is suggested that the control of durability should be through the concrete microstructure and associated chemistry. Different options that may be followed, which fit within this are considered, including, cement selection, material proportioning and the use of recycled / waste materials. For specific aspects of concrete durability performance, including carbonation rate, chloride ingress, sulfate attack, freeze/thaw attack and abrasion resistance, it is demonstrated, through the work of a series of investigations, that these various sustainable options can be used effectively to match and in some cases give improvements in concrete performance compared to conventional concrete. It is illustrated that, although time is required before they are fully exploited, these various options are beginning to find use at full-scale trial and industrial level. It is suggested that the best means of using these various materials is through the
performance based specification route, for which developments are currently in progress. This requires that concrete achieves a defined level of performance and provides greater flexibility in material selection. Overall, the paper suggests that two of the most important issues in concrete construction can be considered collectively such that the aims of both can be satisfied, bringing a number of benefits to concrete construction.
8. ACKNOWLEDGMENTS The data reported in this study is based on information provided from several research projects. The Authors would like to acknowledge the contribution made to these by CTU staff in collaboration with industrial partners, the UK Government, the British Cement Association, Quarry Products Association, Scottish Environment Protection Agency and British Standards Institution.
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