CONCRETE

Concrete can be placed and fully com-pacted in areas of heavily congested rein-forcement, but this requires both gooddetailing on the part of the designer andgood planning and control on the part ofthe contractor to ensure that there are noproblems. Advice for designers and con-tractors is given in the following Sections.

Designers and detailers should be awareof and follow the rules given in BS 8110:Part 1 for the maximum amounts andminimum spacing of reinforcement in asection. The reinforcement should be

detailed so that concrete can be placedand compacted using normal methods.Consideration should be given to thecover in congested areas as too little mayprevent the concrete flowing around thebars into the cover zone.

Remember that a line on a drawing repre-senting, say, a 25 mm bar is not to scaleand that a nominal 25 mm diameter bar isin practice nearly 30 mm in diameter onsite due to the deformations and the use oftying wires.

Where it is believed that the reinforce-ment is congested, i.e. at a beam/beamjunction, draw out the junction to scale,putting in the bars to the overalldimensions given by the steel suppliers.Check this sketch to see if a 75 mm or 50 mm diameter poker vibrator can passthrough the congested reinforcement.

CONCRETE ADVICE No. 04

Congested reinforcement:effects on placing and compacting concrete

Dick Roberts BSc CEng MICE FCS

Congested reinforcement can lead to many problems of placing and compactingconcrete. The result is poorly compacted areas into which no concrete has pene-trated, and, in certain cases, areas where the coarse aggregate has been strainedout of the concrete by the reinforcement, resulting in the cover being a purecement/sand grout. Due to the lack of coarse aggregate this material has a higherdrying shrinkage, resulting in surface cracking and crazing and the consequentloss of durability of the structure.

Typically, heavily congested reinforcement occurs at beam/beam andbeam/column junctions and in bridge deck slabs. In these cases the congestedreinforcement is often seen to be a problem prior to placing the concrete. In othercases, such as heavily reinforced walls and columns, access for placing and com-pacting the concrete down the centre of the wall or column is easy but getting theconcrete through the congested reinforcement into the cover area can be difficult.This document is intended to provide appropriate advice to designers and con-tractors.

1 General

2 Advice to designers

Advice

Allow for the fact in this exercisethat bars vertically above each otherwill be slightly displaced in plan,and therefore three layers of 25 mm(nominal) diameter bars at 150 mmpitch vertically above each otherwill not result in a 120(150 - 30) mmgap but may result in a gap of 75 mmor less in some instances.

Do not forget that where bars arelapped the amount of steel isdoubled. In such cases consider stag-gering the laps so that they do not alloccur at the same position.Remember also that a beam stirrupshape code 61 is lapped on one topcorner, resulting in double theamount of reinforcement trans-versely across part or the whole ofthe top of the beam, depending uponthe beam width and the bar diameter.

When examining reinforcementdrawings, remember that lines ondrawings do not represent thediameter of a bar when actually inposition.

Prior to concreting, discuss with theconcrete supplier the type of concreteto be used, including size ofaggregate and consistence. The useof 10 mm maximum sized aggregateis not a cure all and can often leadto other problems such as a stickiermix that is more difficult to place andone that may not give a good finish.In situations where aggregatestraining occurs it may be the onlyoption. However, the cement content

will increase, therefore increasing therisk of early thermal contraction andall its associated problems. Re-member that even so called flowingconcrete needs some compaction.Match the pokers diameter to thecongested reinforcement, butremember that the smaller thediameter of the vibrator the smallerits radius of action, and therefore theless concrete it will be able tocompact.In conjunction with the above, moveand adjust the reinforcement to allowaccess through the reinforcement forpokers. (NOTE: This should only bedone after agreeing the moves withthe design engineers.) If necessary,mark these access points so that thepoker operators do not have to searchfor them during the placing of theconcrete. Carry out a trial run with thepokers to ensure that all sections of

the pour can be suitably compacted.

Remember that the rate of placing theconcrete must be controlled by therate of compaction, which will bereduced in heavily congested areas.

Even if the pour is to be carried outduring daylight, ensure that once thereinforcement is fixed all nooks andcrannies in the pour can be seen. Arclights suspended above the pour maynot penetrate through a heavily con-gested top mat of a deep slab, and itmay be necessary to obtain protectedlights that can be placed through thetop mat to enable the concreting gangto see below it. In large heavily con-gested slabs during placing concretecan build up on the top reinforcementso causing problems with placingand the ability to see below the topmat. This problem can be overcomeby having an operative with a stiffbrush removing this build-up as itforms.

3 Advice to contractors

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 Societys 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.

Impartial advice can be sought from The Concrete Society. Members are entitled to sub-stantial discounts on services and products including site visits and investigations,dependant on status. For publications and information, Concrete Information Ltd holds awide range of books and pamphlets which are available through The Concrete Book Shopand their library stock. They provide many services such as literature searches and notifi-cation of new references to their extensive catalogue via their Concquest scheme.

ContactsThe Concrete Society 01276 607140www.concrete.org.uk technical@concrete.org.uk

Concrete Information Ltd 01276 608770www.concreteinfo.org enquiries@concreteinfo.org

The Concrete Bookshop 01276 608778www.concretebookshop.com enquiries@concretebookshop.com

References1. BRITISH STANDARDS INSTITUTION, BS 8110, Structural use of concrete, Part 1:

Code of practice for design and construction, BSI, London, 1997.

FURTHER READING AND ADVICE

Issued: July 2003