self healing of concrete

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CONCRETE

In the 1990s, the Cement and ConcreteAssociation's Technical Report No. 559(1)

reported on site measurements and asso-ciated laboratory work of water leakingthrough cracks. The site work consisted ofmonitoring the leakage through twocracks in the walls of a service reservoir.The results showed that the flowmeasured through the cracks had reducedconsiderably during the monitoringperiod (about 18 days). However,although the flow had reduced to a smallamount it was still measurable and thesurrounding concrete surface remainedwet.

Conclusions drawn from the site and lab-oratory research were as follows:

� Autogenous healing reduces theleakage of water through cracks atrates primarily dependent upon theinitial effective width of the crack. Thesmaller the initial effective width the

faster the crack will seal.

� The effect of other factors such as thetype of water and the cementitiousmaterial in the concrete are onlyminimal.

� The rate of healing is slightlyincreased by reducing the differentialpressure across the crack.

� Ensuring that the concrete structureunder test is filled slowly can signifi-cantly reduce the total loss of water.

Part of the work involved breaking openthe cracks to see how they were sealed. Itwas found to be by a combination ofmechanical blocking by particles, eithercarried into the crack with the water orloose material produced during thecracking process, and the deposition ofcalcium carbonate from the cementitiousmaterial.

No reference is made in the report to thepossible hydration of unhydrated cementparticles on the face of the crack, althoughthis is often mentioned in other publi-

CONCRETE ADVICE No. 09

Autogenous healing: the self sealing of fine cracks

Dick Roberts BSc CEng MICE FCS

Autogenous healing is the natural process of crack repair that can occur inconcrete in the presence of moisture and absence of tensile stress. It has practicalapplications for closing dormant cracks in a moist environment, such as may befound in mass structures and water retaining or watertight structures.

1 Background

Advice


cations as one of the reasons whycracks autogenously heal. Neither isthere any indication given as to theamount of particles larger than0.01mm that were filtered out of thewater before it was passed throughthe crack. It is logical to assume thatif these particles had not beenremoved they would have aided andspeeded up autogenous healing.

The work carried out by TheCement and Concrete Associationwas relatively short term and doesnot give any indication of how longit takes various width cracks to com-pletely seal. Work carried outelsewhere has suggested that:

� The lower the pH value of thewater the greater the amount ofleakage. In cases where the pHvalue is less than 6, the acidicnature of the water may inhibitautogenous healing. This riskmay be reduced by the use of flyash or ground granulated blast-furnace slag (ggbs) in the mix.

� Placing a pozzolanic powder,e.g. fly ash, in the waterimproves the autogenous healingof the concrete. It is likely thatthe addition of a fine materialinto the water increases mech-anical blocking of the cracks. Inaddition there may be somechemical action.

Both BS 8007(2) and the WaterServices Association's Specification(3)

imply that cracks up to 0.2 mm widewill autogenously seal within 28days; cracks up to 0.1 mm wide willseal within 14 days.

Both publications require a waterretaining structure to be tested tocheck that it is watertight. The testprocedures are identical andbasically consist of cleaning thestructure then filling it with thespecified liquid (usually water) at auniform rate. After filling, the liquidlevel is maintained by the additionof further liquid for a stabilisingperiod while absorption and auto-genous healing takes place. Thepublications suggest that a sta-bilising period for a maximumdesign crack width of 0.1mm maybe 7 days, or 21 days for a crackwidth of 0.2mm or greater. After thisstabilising period the test is con-tinued for a further 7 days with nofurther liquid added to the structure;the permissible drop in level duringthis period, after allowing for evap-oration and rainfall, should notexceed a specified amount.

BS 8007 states that even though thetest may have been completed satis-factorily, any evidence of seepageon the outside faces of the structureshould be assessed against therequirement of the Specification. Ifany necessary remedial treatment ofthe concrete is required it should,wherever possible, be carried outfrom the liquid face. The WaterServices Association's Specificationis even more onerous; even thoughthe test is completed satisfactorily,any leakage visible on the outsideface of the structure must bestopped, and wherever possible thissealing shall be carried out from theinside face.

This poses a question; why, if theleak test proves that the loss of wateris less than the allowed amount, is itnecessary to seal any visible

seepage or leakage? Until thisproblem is clarified, contractors ten-dering for structures designed tothese codes or specifications shouldassess the cost implications of suchremedial works and the possibilityof having to retest the structures.

Autogenous healing will take placeon cracks caused by any phe-nomena, not just early thermal con-traction as is implied in BS 8007.Healing will not have stopped at theend of the testing period but willprobably continue until a full seal isobtained. The only rider is that suchcracks must be 'dead', that is, nofurther movement is taking place.'Live' cracks, such as those causedby loads applied to the structure,will open and close, so destroyingthe healing effect. 'Dead' cracksmust be in a moist condition or sub-merged to enable the autogenoushealing process to take place.

In structures where the aestheticappearance of the structure isimportant, such as elevated watertanks where the white staining alongeven fine cracks may not be per-missible, it will be necessary tospecify the use of a suitable flexibleinner liner.

From the evidence available itwould appear that cracks do autoge-nously heal. It appears that cracks aslarge as 0.3mm wide are likely toseal themselves, but that the time forthis sealing may be much longerthan the 21 to 28 days inferred byBS 8007 and the Water ServicesAssociation Specification.

2 Code requirements

3 Conclusions


FURTHER READING AND ADVICE

CONCRETE Advice Sheets are produced and published by The Concrete Society. The information and advice contained inthe Advice Sheets is based on the experience and knowledge of the Concrete Society�s Technical Staff. Although TheSociety does its best to ensure that any advice, recommendation or information it gives is accurate, no liability or responsi-bility of any kind (including liability for negligence), howsoever and from whatsoever cause arising, is accepted in thisrespect by The Concrete Society, its servants or agents. Readers should also note that all Concrete Society publications aresubject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Issued: July 2003

ContactsThe Concrete Society 01276 607140www.concrete.org.uk [email protected]

Concrete Information Ltd 01276 608770www.concreteinfo.org [email protected]

The Concrete Bookshop 01276 608778www.concretebookshop.com [email protected]

Impartial advice can be sought from The Concrete Society.Members are entitled to substantial discounts on services andproducts including site visits and investigations, dependant onstatus. For publications and information, Concrete Information Ltdholds a wide range of books and pamphlets which are availablethrough The Concrete Book Shop and their library stock. Theyprovide many services such as literature searches and notifi-cation of new references to their extensive catalogue via theirConcquest scheme.

1. CEMENT AND CONCRETE ASSOCIATION, The effects of autogenous healing upon the leakage of water through cracks in concrete,Technical Report No. 559, The Association (now British Cement Association), Camberley.

2. BRITISH STANDARDS INSTITUTION, BS 8007, The design of concrete structures for retaining aqueous liquids, BSI, London.

3. WATER SERVICES ASSOCIATION, Civil engineering specification for the water industry, Fourth Edition, The Association, London.


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