Reflexiones finales, conclusiones y recomendaciones
4.3. Relación entre variables catastrales e indicadores socioeconómicos
complying with BS4027 andBS 4248.
6.1.5.2 Admixture spec~flcations
6.1.5.3 Approval andperformance ofadmixtures 6.1.5.4 Concrete durability
Any chloride ion in the admixture should be included in the calculation of the total chloride contents in Table 6.4. The Na:O equivalent in any admixture should be included in the calculation of the total alkali content (6.2.5.4).
6.1.5.5 Air-entraining agents 6.2 Durability of structural concrete
Thissection treatsall aspectsofdesign toachieve durabilityandtherefore gives abroader perspective than, for example. Sections 3.3 and 4.12 which are concerned specifically with the requirements forcover andconcrete quality.particularlyastheyinfluencesizing ofsections at the design stage. Design also means identifying the structural form and constituent materials appropriate to the life-time and environment. ~freezing (6.2.3.3) are identifiedas other broad typesof exposure condition. In addition tothegeneral environment (6.2.3.1and asdefined in moredetail in3.3.4). andthawingandde-icingsalts’ (6.2.3.2land~-exposure toaggressive chemicals”
6.2.1 General
6.2.2 Design for durability
As indicated in the Code in Clause 2.1.1. durability is an aspect of the structure that has to be considered carefully and consciously taken account of in design. This implies a consideration of facets of design. materials and construction. and a convenient check list for these in the different stages Is:
Desi~n — assessment of environmental conditions during expected life — geometryofstructure andsectionstoimproveweatheringproperties. — cover to reinforcement and its adequacy — dependingonseverityofenvironment. surfaceprotectiontoconcrete i.e. control offlow of ~vater Materials — constituents and theirquality — mix proportions
Construction — mixing. placing and compaction of concrete — curing — accuracy of form’.~’ork
131Ha,Iabook w B58110:1985
— achieving the specified cover
— appropriate quality assurance procedures.
6.2.2.1 Shape andbulk of concrete
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The emphasis should be on ensuring good drainage of water and the avoidance of standing pools and rundown water. Equally the cracks referred to are not those controlled by the clauses in Section 3 but those which may occur when the chosen geometry and bulk of the section make them virtually unavoidable— in other words, badly designed!
The particular aspects requiring attention as regards the cover have been taken into account in deriving Table 3.4 and no further adjustments are necessary.
6.2.2.2 Depth of concrete cover and concrete quality
The alkaline environment provided by fresh concrete protects reinforcement. From experience. appropriate combinations of cover and concrete quality ensure that in wellr defined environments the effects of carbonation of the concrete and of penetration of I:
chlorides do not lead to unacceptable corrosion of the reinforcement during the expected life of the structure or component (2.1.1). Within limits, a trade-off is possible between
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free water/cement ratio. cement content and thickness of concrete cover to achieve the same nominal protection, except that for more severe exposure conditions the available combinations become more restricted.
The cover for a given strength, using the reduced concrete grades given in 3.3.51.
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and exposure condition. e.g. Table 3.4, is broadly in line with that in Table 19 of CP11O except for increases of 5mm for mild and moderate exposure of lower concrete grades.
These increases reflect concern that the durability of some buildings is proving less than had been anticipated. Variability of strength tends to be greater at lower grades and typical variations in cover have proportionately greater effect at lower covers. These influences are significant together only for mild and moderate exposure. Inaddition.
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however, restrictions are placed on minimum cement content and maximum free water!
cement ratio to provide adequate impermeability for the particular thickness of cover.
Although compliance with compressive strength can be demonstrated. compliance with limits for water/cement ratio and cement content is difficult to demonstrate, especially~
in hardened concrete. Based on analysis of a substantial number of records from readvL mixed concrete plants (6.5) it is possible to specify a lowest grade of concrete which. if achieved, will ensure the limits on cement and water/cement ratio for 95% of materials in current use. The inclusion of these ‘lowest grades’ in Tables 3.4 and 4.8 represents a practical approach to achieving compliance with the necessary quality of concrete.
although it should not be taken to imply that durability is a function only of compressive strength. It follows that the reduced values of grade in Clause 3.3.5.2 do not represent relaxations as such but are values that it will seldom be possible to use because of the
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difficulty of demonstrating compliance with the other limitations.
Clause 3.3.5.2 does not permit these values of grade to be used for mixes containing
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pfa or ggbfs even though 3.3.5.5 indicates that the protection to reinforcement should be equal to that of Portland cement concrete if the 28 day strengths are equal. The restriction arises because data for maximum water/cement ratio and minimum cement content in relation to durability are not available for concretes containing pfa or ggbfs
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in the same way that they are for Portland cement concrete. Although some pfa or ggb mixes may conform to the limiting values in Table 3.4 and 3.3.5.2 the wide range in percentage additions permissible means that this is not generally true. For all but the
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lowest percentage additions a proportionately greater mass of pfa or ggbfs will almost certainly be required. Put another way. the strength equivalence data are based on assumptions about minimum cement content in Table 3.4 for Portland cement concrete
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which are not necessarily true for- pfa or ggbfs concrete.
Because the equivalence concept is based on broad comparisons with limited data it is reasonable to exercise some caution, given the concern to avoid premature deterioration,~
in seryice. This concern extends to sulphate resisting Portland cement in3.3.5.6for yeryf severe or extreme exposure conditions even though increased cover is recommended forL~
achieving equivalent protection to reinforcement.
6.2.3 Exposure conditions
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Part!: Section 6 6.2.3.1 General environment
6.2.3.2 Freezing and thawing and tie-icing salts Air-entraining agents
The resistance of concrete to freezing and thawing depends to a large extent on its permeability, the provision of adequate curing and the degree of saturation of the concrete when exposed to freezing, concrete with a higher degree of saturation being the more liable to damage. The use of salt for de-icing roads greatly increases the risk of damage from freezing and thawing.
The use of de-icing chemicals can cause concrete deterioration through two different mechanisms. Firstly, the melting of ice and snow produces pools of water available to be absorbed by the concrete. This can raise the level of saturation in the concrete and the salt solution remains liquid at lower temperatures than pure water. Thus concrete may be subjected to many cycles of freezing and thawing at a much higher degree of saturation than if the de-icing salt had not been used. Secondly, de-icing salts increase the presence of chlorides which, in reinforced concrete, can pose a corrosion risk.
Air-entraining agents entrain controlled amounts of air in concrete, and greatly improve its durability and in particular its resistance to damage on freezing. Air-entrainment causes some loss in strength but, as a designed mix is required. this will be offset automatically. The engineer should specify air-entrainment where the concrete will be in contact with de-icing salt and should specify the average air content of the concrete in accordance with 6.2.3.2. Site control of air content is covered by BS 1881: Part 106.
Care is required in the selection of air-entraining adinixtures. It is recommended that products be obtained from reliable firms having a technical department capable of advising on the use of the product. The admixture must not only cause the entrainment of the air in the required amount, despite varying mixing and agitating times, but must also lead to the correct size and spacing of the air bubbles in the freshly mixed concrete.
When those requirements are met, the air is reasonably stable in the fresh concrete.
which can then be handled and compacted by vibration without serious loss of air. It should be noted that difficulties may be met in entraining air into mixes containing pfa.
6.2.3.3 Exposure to aggressive chemicals
This Clause is concerned with aggressive chemicals external to the concrete. The same chemicals may be introduced in the mix constituents (61.5) and have an aggressive internal effect. Concrete used in agricultural situations may be subject to acidic solutions, e.g. food processing, silage effluentt66’6.7), Engineers should be particularly wary of old industrial tips and the chemicals they may containt68~.
The omission of values for cement content and free water/cement ratio against class 1 in Table 6.1 is covered by the footnote to the Table and arises because different values may be appropriate and are stated elsewhere in the Code. For concrete in contact with non-aggressive soil (i.e. class 1 of Table 6.1). Table 3.2 defines the environment as
‘moderate’; for a moderate environment Table 3.4 giving cover to reinforcement requires a minimum cement content of 300kg/in3 and a maximum water/cement ratio of 0.60:
these values then apply to class 1 of Table 6.1. However, unreinforced concrete is treated in 6.2.4.2. and for a moderate environment. Table 6.2 requires a minimum cement content of 275kg/in3 and a maximum free water/cement ratio of 0.65.
Based on longstanding practice and absence of durability problems in class 1 non-aggressive soil conditions. concrete made with normal-weight aggregate and used for foundations (strip and trench-fill) to low-rise structures (6.2.4) may have a lower cement content not less than 220kg/in3 if the grade is not less than C20. Under these conditions the recommendations for increased cover to any reinforcement in 3.3.1.4. for concrete cast against uneven surfaces. will usually’ apply.
The presence of water is necessary for sulphate attack to occur: attempts to dry one surface of concrete can exacerbate flow of moisture and the rate of attack.
6.2.4 Mix proportions 6.2.4.1 General
this Clause picks up the general principles for achieving durable reinforced concrete.
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Handbook to BSSJIf.J:I98~
given in 6.2.1. and focuses on mix proportions by reference to Tables 3.4. 4.8. 6.1 and
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6.2. It emphasises the importance of achieving the lowest free water/cement ratio ~ compatible with producing placed concrete of uniform consistency and of ensuring the specified minimum cement contents.
If it is necessary to use admixtures it should be ensured that the limiting values are still met because the values in Tables 3.4, 48. 6.1 and 6.2 are based on data on concretes made without admixtures.
It is equally important to be aware of the behaviour during curing of concretes containing high cement contents. particularly in excess of 550kg/rn2, when high drying shrinkat~e or thermal stresses may be induced.
6.2.4.2 Unreinforced concrete
Table 6.2 is analogous to Table 3.4 except ofcourse there are no requirements for cover.
6.2.4.3 Mix adjustments in Tables 6.1 and 6.2
The changes or adjustments which may be made to values in Table 62 are again analogous to those relating to Table 3.2. However, recognizing that in some cases it may be
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appropriate to specify prescribed mixes, recommendations are given for mixes described in BS 5328 which will provide the necessary cement contents and meet the free water/
cement ratio limits.
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6.2.5 Mix constituents
6.2.5.1 General
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The importance of proper selection and control of materials is emphasised.
6.2.5.2 Chlorides in concrete
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It with by the Control of the risk of corrosion of embedded metal by’ chlorides is dea
limits in Table 6.4. which represent a small modification to the stricter limit of 0.06%
introduced in 1977 which excluded some inland aggregates previously regarded as completely satisfactory. Although a very low limit for chloride is required in this category it is considered that the risk of corrosion would not be increased by raising the limit from 0.06% to 0.1%. To achieve the revised limits, washing of sea-dredged aggregates is essential.
It is considered that there is sufficient information and experience of the use of cements complying with BS 4027 or BS 4248. for the chloride limit to be set at 0.2%, subject to continuing review. The 0.2% limit applies to both plain and reinforced concrete. Itis [ needed in plain concrete for sulphate resistance purposes and in reinforced concrete for ‘-‘
both sulphate and corrosion resistance. Where the type or use of concrete lies in more
than one category. e.g. steam cured concrete using a sulphate resisting cement. the more
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onerous limit should be applied. The value of 0.4% for most reinforced concrete represents a simplification for the previous method of expression.
6.2.5.3 Sulphates in concrete
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6.2.5.4 Alkali-silica reaction
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A revised edition of the Guidance notes on tninimising the risk of alkali-silica reaction.
together with a set of Model Specification Clauses was published for public comment in October I985£~.4£. K
It. must be emphasised that the recommendations relate to conditions found in mainland
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Britain. and before using them Engineers working outside that area should satisfy L themselves that local conditions are comparable.
The recommendations in the Code are in line with those given in the September 1983
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Guidance Notes. The revised edition includes some important changes with the current
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advice being as follows:
As the three elements of moisture, high alkali levels and reactive silica aggregates all have to be present for damage to occur. it is only necessary to eliminate one of them to minimise the risk of ASR. The Guidance Notes recommend various ways in which this ma~ be achieved, but stress the importance of giving as wide a choice of methods
34
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Part!: Sectionoas possible to the contractor to minimise costs.
Taking the four sub-paragraphs in Clause 6.2.5.4 in turn:
(1) Controlling moisture will only be successful ifthe equilibrium relative humidity in the concrete is less than 75%.This can be the case in dry, well-ventilated parts of
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buildings. It will not apply to foundations evenifwaterproofed. to external members.or to those subjected to condensation.
(2) Guaranteed low alkali cement to BS 4027 has less than 0.6% alkali content. This
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requirement has to be specified at the time of ordering. Provided that their water-soluble alkali content is taken into account, either ggbfs or pfa can be used as a partial replacement for BS 12 Portland cement to reduce the alkali content of the cementitious materials below 0.6%.ui
(3) When avoidance of ASR is based on limiting the alkali content of the concrete to a maximum of 3k~ in~. all sources of alkali have to be taken into account. In particular the contribution of sodium chloride whether from aggregates or from mixing water
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must be included.(4) If ggbfs or pfa are included in the concrete mix as a partial replacement for Portland cement, the revised Guidance Notes require the inclusion of the water-soluble alkali content of whichever diluent is used.
In the case of ggbfs.the control of alkalis can be achieved in one of two ways:
(a) replacement of cement by ggbfs at a minimum level of 50% so that the combination has an acid-soluble alkali content of less than 1.1%,
(b) replacement of cement by ggbfs ata level greater than 30% such that the acid-solublealkali inthe ggbfs when combined give a total alkali content of not more than 0.6%.
Suitable pfa can be used as a replacement of 30% or more of the Portland cement. provided that the total alkali level in the concrete does not exceed 3kg/in3 ~vhenthe acid-soluble and water-soluble alkalis of the Portland cement and pfa respectively are taken into account.
There areothermatters covered in the Guidance .Notes ~vhichthe Code refers to but does not cover in detail. In the absence of a recognised test a list is given of those aggregates which are considered to be non-reactive.In addition, the reactive rock types chert and flintare considered to be safe provided that they are present at a level greater than 60% of the combined coarse and fine aggregates.
Structures which are considered to be particularly vulnerable to attack by ASR include those subjected to high humidity and those buried in waterlogged ground. Highway structures come into this category and are in addition subjected to frequent saturation with de-icing salts. In such cases, more rigorous precautions may be necessary.
For further information. see also references 6.2.6.3. 6.4.
6.2.6 Placing, compacting, finishing and curing
6.3 Concrete mix specification
6.3.1 General
Following the publication of CPI1O in 1972. the British Standard Methods for specifying concrete (BS 5328) was published in 1976. and revised in 1981. It was intended that BS 5328 should provide a single standard for concrete to be referred to in all codes and specifications for concrete. Unfortunately’ the publication and revisions of these documents have not kept in step and different terminology has been used for the types of concrete mixes as shown in Table H6.S.
Irrespective of the detailed terminology, the fundamental difference between a designed’ mix and a ‘standard’ or (special) ‘prescribed’ mix lies in the responsibility for selecting the mix proportions. the form of specification. the materials which can be used and the parameters for judging compliance. These differences are shown in Table H64.
It is the Engineers responsibility to select the concrete grade together with any limits required on the mix proportions. the requirements for fresh concrete and the types of materials which ma’- or may not be used to meet his strength. durability and any other 135
Hanabook to BS8IIO:1985