A1 Problem of chemical attack A2 Scope and structure of the guidance Chemical agents that are destructive to concrete may be found in the ground. In the UK, sulfates and acids, naturally occurring in soil and groundwater, are the agents most likely to attack concrete. The effects can be serious Figure A1 resulting in expansion and softening of the concrete to a mush. A substantial number of other substances are known to be aggressive, most resulting from human activity, but collectively these are a lesser problem as they are encountered only rarely by concrete in the ground.

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A1 Problem of chemical attack A2 Scope and structure of the guidance Chemical agents that are destructive to concrete may be found in the ground. In the UK, sulfates and acids, naturally occurring in soil and groundwater, are the agents most likely to attack concrete. The effects can be serious Figure A1 resulting in expansion and softening of the concrete to a mush. A substantial number of other substances are known to be aggressive, most resulting from human activity, but collectively these are a lesser problem as they are encountered only rarely by concrete in the ground.

BRE has underpinned this approach by issuing a series of guidance notes and Digests, dating back to , on the causes of chemical attack and how to specify chemically resistant concrete. SD1 provides guidance on the specification of concrete for installation in natural ground and in brownfield locations. The definition of a brownfield location adopted here is one that has been subject to industrial development, storage of chemicals, or deposition of waste, and which may contain aggressive chemicals in residual surface materials or in ground penetrated by leachates.

The procedures given for ground assessment and concrete specification cover the fairly common occurrence of sulfates, sulfides and acids. They also cover the more rarely occurring aggressive carbon dioxide found in some ground and surface waters. Consequently, most concrete installed in the ground has performed entirely satisfactorily and is expected to do so for its required working life. Occasionally, however, cases of chemical attack have come to light and have been subject to research by BRE and others.

Some of these cases have been attributed to rarely occurring chemicals not specifically covered by BRE Digests: some to natural ground conditions for which there was insufficient guidance, such as occurrence of pyrite; and some to the emergence of previously unrecognised attack mechanisms, such as the thaumasite form of sulfate attack TSA which has been extensively reported in the last decade[1].

Guidance in BRE Digests has necessarily evolved to cater for successive adverse field findings; to take advantage of the emergence of new concrete constituents and construction methods; and to maintain harmony with newly published standards, latterly European ones.

In order to be both comprehensive and flexible, Digests have tended to become longer and more complex. One objective of this third edition of Special Digest 1 SD1 is to simplify the guidance. Other aims and changes are discussed later. Figure A1 Extreme example of sulfate attack in a year-old highway bridge sub-structure exposed to wet, pyritic clay fill 2 Part A While SD1 discusses several aggressive agents eg ammonium salts and phenols occasionally found in heavily contaminated ground, no specific procedures are included for dealing with these.

Specialist advice should be sought if they are encountered. Part B describes modes of chemical attack and discusses the mechanisms of the principal types, including sulfate and acid attack, and the action of aggressive carbon dioxide. Part C deals with assessment of the chemical aggressiveness of the ground. It gives procedures for the determination of Design Sulfate Class DS Class from soluble sulfate and magnesium, and from the potential sulfate eg from oxidation of pyrite.

Part D gives recommendations for the specification of concrete for general cast-in-situ use in the ground. It explains how to derive an appropriate quality of concrete, termed the Design Chemical Class DC Class , from a consideration of the ACEC Class together with the hydraulic gradient due to groundwater, the type and thickness of the concrete element, and its intended working life. In some cases, where conditions are highly aggressive, additional protective measures APMs are recommended.

Part D follows this with guidance on the constituents of concrete required to achieve the identified DC Class. Part E gives recommendations for specifying surfacecarbonated precast concrete for general use in the ground. An essential requirement for compliance with this part is that surface carbonation is assured by exposure of the precast concrete to air for a minimum of 10 days after curing.

Since such carbonation provides a degree of resistance to sulfate attack, the recommendations for the derivation of DC Class in respect of sulfates is relaxed by one level. Other than this, the recommendations of Part D are followed for concrete specification. Part F includes design guides for specification of specific precast concrete products, including pipeline systems, box culverts, and segmental linings for tunnels and shafts.

These products are manufactured under rigorous quality control to ensure appropriate mix composition and achieve relatively low concrete permeability. Together these provide an inherently high quality in respect of chemical resistance.

Consequently, a further relaxation beyond that allowed for surface carbonation is permitted in respect of specification of DC Class for aggressive sulfate conditions. In practice this relaxation is used to offset the general-use recommendation that a higher DC Class should be specified where concrete is of thin cross-section, or will encounter a relatively high hydraulic gradient.

Part F also covers specification of precast concrete masonry units concrete blocks for aggressive ground conditions. The guidance is based on Design Sulfate Class rather than ACEC Class as there is currently no correlation of block performance with the latter, though work on this is ongoing.

A glossary of terms is included as Appendix A1 on page 6. This is arranged in four stages according to the construction sector that has key responsibility. Within each of these stages, the principal tasks are shown in boxes with references to the relevant sections of SD1.

While most steps are equally applicable to all uses of concrete, there is a differentiation in Stage 3 for the determination of DC Class and APM between the three categories of concrete element dealt with in Parts D, E and F.

Introduction 3 Stage 1 Designer of building or structure Consider design options for building or structure and prepare specification for site investigation. Inform geotechnical specialist of design concept and site investigation requirements Stage 2 Geotechnical specialist Carry out site investigation to determine chemical conditions for concrete, including water mobility.

See Section C5 Determine the intended working life of proposed building or structure, and the form and use of specific concrete elements. Yes Accept concrete mix design for specific use. Implement any APM specified for DC Class or in contract documents Figure A2 Procedure for design of buried concrete for use in an aggressive chemical environment No Parts D, E and F 4 A3 Background to guidance on sulfate attack One of the key drivers for revision of BRE Digests dealing with concrete in aggressive ground since the s has been a growing recognition of the occurrence of the thaumasite form of sulfate attack TSA in UK buildings and structures.

It has long been known in the UK that concretes made with Portland cements are vulnerable to attack by sulfates in the ground. For many years it was considered that the affected components of the concrete matrix were the calcium aluminate phases and calcium hydroxide, and that the minerals formed by this attack were ettringite and gypsum. Later the benefits of using fly ash or pulverized fuel ash pfa and blastfurnace slag-based cements were appreciated.

Guidance on designing concretes to resist conventional sulfate attack was developed in a series of BRE Digests, the most recent of which was Digest , Sulfate and acid resistance of concrete in the ground, first published in Since the late s, however, deterioration of concrete as a result of the formation of thaumasite has become recognised as a separate form of sulfate attack. A growing number of cases of TSA have been identified worldwide, although the majority have been found in the UK.

In all three cases, the TSAaffected concrete contained carbonate-bearing limestone aggregates and was exposed to moderately aggressive Class 3 sulfate conditions in a seasonally cold, wet environment. The concrete encountered in each case satisfied the recommendations of the then-current version of Digest It therefore became apparent that the Digest needed to be revised to take account of the risk of TSA occurrence.

Part A Accordingly a new version of Digest was issued in January which drew attention to the risk of TSA in concretes containing internal calcium carbonate and promised further guidance based on ongoing research.

Subsequently, in , several cases of TSA were identified in the foundations to motorway bridges in Gloucestershire.

As in the previous cases, the concrete contained carbonatebearing aggregates[1,4—6]. The most severe occurrence had resulted in severe concrete deterioration to a depth of up to 50 mm, exposing steel reinforcement to corrosion[7].

The high profile of these cases ensured a co-ordinated national review, culminating in with a report from a Thaumasite Expert Group[1] set up by Government. This report gave interim guidance on specifications to minimise the risk of TSA in new construction and on the management of existing structures affected by TSA. It also gave recommendations for further research on the occurrence of TSA and mitigating measures. Following publication of the Thaumasite Expert Group report, BRE guidance was revised to incorporate the interim recommendations.

This was published in as Special Digest 1, Concrete in aggressive ground. Hereafter Special Digest 1, and other editions, will be shown as SD etc. There were minor revisions to the guidance in a new edition published in These were principally to bring nomenclature used for cements and combinations into line with newly published European Standards.

Most of the subsequent research recommended by the Thaumasite Expert Group has been completed. Together with other findings, such as deficiencies in guidance for ground assessment, the new knowledge has prompted this major revision of SD1. The change stems from findings of several research investigations on ground carried out by BRE and others[9—11].

The following key changes have been made to the procedure for concrete specification. A further consequence is that starred and double-starred concrete qualities that related to restricted aggregate carbonate content are no longer included. This follows from a higher level of confidence in the provisions for the concrete. Further detail in respect of these changes is included in Parts C and D.

Consequently there has been a basic harmony between these documents in respect of concrete specification for general use in the ground. BRE guidance has presented more background information on chemical attack, given detailed guidance on ground assessment, and included dedicated guidance for the specification of concrete in certain precast concrete products such as pipeline systems and masonry blocks.

In contrast, BS guidance for concrete has integrated the provisions for resistance to chemical attack into the numerous other requirements for practical concrete specification; for example, strength class and consistence, resistance to alkali—silica reaction and chloride content in respect of corrosion of reinforcing steel.

However, a revision of this is underway that will include bringing it into line with SD in respect of resistance to aggressive ground. It is expected that an amended document will be issued for public comment in the first half of , followed by publication some months later. They comprise five options for extra measures that can be taken to protect concrete where it is considered that the basic provisions of the concrete specification might not provide adequate resistance to chemical attack for some uses of concrete Section D6, and Table D4 on page Aggregate carbonate range A term formerly used in previous editions of SD1.

It is not used in SD This portion of the dissolved carbon dioxide is termed aggressive carbon dioxide. Aggressive carbon dioxide is usually only present to an appreciable extent in rather pure natural waters since in most cases, where the water contains dissolved salts, sufficient calcium carbonate is available to combine with the carbon dioxide as calcium bicarbonate. Brownfield sites A brownfield location is defined as a site or part of a site that has been subject to industrial development, storage of chemicals including for agricultural use or deposition of waste, and which may contain aggressive chemicals in residual surface materials or in ground penetrated by leachates Section C5.

Cements and combinations Cements are pre-blended from appropriate cementitious materials and are supplied by cement manufacturers. Combinations comprise similar cementitious materials that are combined in the concrete mixer. Cements and combinations prepared from the same ingredients, taken in the same proportion, are equivalent for resistance to sulfate attack. Part A Conventional form of sulfate attack This is a form of sulfate attack in which sulfate ions that have penetrated concrete react with calcium aluminate hydrate to form calcium sulfo-aluminate hydrate ettringite , or with calcium hydroxide to form gypsum.

Initially these reactions may result in non-destructive void filling. Attack is distinguished by onset of expansion and related cracking of the concrete Section B2. It is derived from the ACEC Class but takes into account a number of other factors, including the type of concrete element and its intended working life, and any exposure to hydraulic gradient due to groundwater Section D4 and Table D1 on page Design Sulfate Class DS Class This is a five-level classification for sites based principally on the sulfate content, including total potential sulfate, of ground or groundwater, or both.

It is dependent on the presence or absence of substances including magnesium ions, pyrite, and, for pH less than 5. Disturbed ground This is, initially, natural ground that is substantially disturbed; for example, by cutting and filling to terrace a site, or by excavation and backfilling, so that air can enter and oxidise any pyrite contained therein.

Simply cutting through ground without opening up the ground beyond the cut face eg piling operations or excavation without backfill does not generally result in disturbed ground. Enhanced concrete quality This is an increase in concrete quality used as an APM. The necessary enhancement may be determined from Table D2 on page In this table, bold horizontal lines separate the various concrete qualities in respect of aggressive ground.

Flowing groundwater Flowing groundwater is defined in this Special Digest to cover the following two conditions Section C3.


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Applies to cast-in-situ piles only and for other types of pile refer to BRE Special Digest 1 or follow specialist advice. It takes into account the site natural or brownfield and the mobility and pH of groundwater. Additional Protective Measures APM - These are defined as the extra measures that could be taken to protect concrete where the basic concrete specification might not give adequate resistance to chemical attack. The DC Class is derived from the ACEC Class of the ground and other factors including the type of concrete element and its required structural performance. It is also dependent on the type of site, presence or absence of magnesium ions, pyrite and for pH less than 5. Five levels of classification are given that are equivalent to those given in BRE Digest now superseded. Each increment in concrete quality is counted as an extra APM.


Concrete in Aggressive Ground:3rd edition

This version dates from February and corrects two errors in the edition. Chemical agents that can destroy concrete may be found in the ground. In the UK, sulfates and acids naturally occurring in soil and groundwater are the agents most likely to attack concrete. The effects can be serious leading to expansion and softening of concrete. Many other substances are aggressive, most resulting from human activity, but they present less of a problem since they only rarely come into contact with concrete in the ground. It replaces the edition and has been revised to reflect new thinking and changes to British Standards. The main changes are: - a new ranking of cements with respect to sulfate resistance - removal of the aggregate carbonate range - revision of sulfate class limits - simpler requirements for additional protective measures.


2.1 Concrete and its reinforcement


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