A.B. Broughton (7223981)

Stephen R. Nash (7218515)

English

Loughborough University Institutional Repository

G   WATER SUPPLY: BROUGHTON and NASH27023rd WEDC Conference Durban, South Africa, 1997Ferrocement reservoirs: how appropriate?A.B. Broughton and S.R. Nash, South AfricaWATER AND SANITATION FOR ALL: PARTNERSHIPS AND INNOVATIONSFERRO-CEMENT RESERVOIRS WERE virtually unknown inthe South African water engineering field until thenineties. Prior to that their use had been restricted mainlyto spring protection and rainwater harvesting work, andthe sizes built were typically 5kl and 15kl. With thedramatic upsurge in the amount of rural water supplywork being done in South Africa in the last six years,ferrocement techniques are now better known. Reser-voirs have been built up to 400kl in size, and the 100klferrocement reservoir is becoming common.Ferro-cement is labour intensive, and the skills re-quired are easily assimilated in rural areas. They can bebuilt either with or without shutters, but the formermethod is recommended for controlling quality andappearance. Ferrocement also lends itself to the construc-tion of attractive domed roofs, which are more durablethan the corrugated iron roofs often used over masonryreservoirs.Built correctly ferrocement reservoirs are strong, dura-ble and watertight. But are they cost-effective? Costs areaffected by context. In water projects where large num-bers of these reservoirs are built, with simple designspecifications and appropriate levels of supervision, theyare most cost-effective.  However in water projects whereonly one or two reservoirs are built, with complex designspecifications imported from standard practice and sev-eral layers of supervisory personnel, the percentage costsaving compared with conventional reinforced concreteis unimpressive.The history of ferrocementThe development of ferrocement technology began in the1840s with J.L. Lambot who constructed a rowing boatusing a composite of wires and cement. At the same timeothers were developing conventional reinforced con-crete. Further development of ferrocement did not occuruntil the early 1940s when Pier Luigi Nervi resurrectedthe original ferrocement concept. The development offerrocement technology has primarily been done in theboat building industry although ferrocement has success-fully been used for many other applications such as roofsystems and silos (ACI Manual of Concrete Practice).What is ferrocement?True ferrocement differs from conventional reinforced con-crete in that it consists of closely spaced, multiple layers ofmesh or fine reinforcing bars completely impregnated withcement mortar. Reinforcing requirements are specifiedas a minimum total volume fraction (3.6 per cent volumeof steel per unit volume of composite) and a minimumtotal specific surface area of steel (0.16mm2/mm3). Theresult is a thin walled composite material with a muchhigher volume fraction of steel than conventional rein-forced concrete. The mechanical characteristics dis-played approximate that of a homogeneous material andare different to conventional concrete in terms of strengthand deformation. The effect is not unlike that achievedwith fibre glass reinforced resins. Walls are usually muchthinner than conventional reinforced concrete and themaximum cover on the reinforcing is as little as 5mmwith 2mm being the average recommended cover (ACIManual of Concrete Practice).Ferrocement reservoirsAn adaption of ferrocement technology is now becomingcommonly used to construct water reservoirs. Most of theso called ferrocement reservoirs constructed differ from trueferrocement in that they include a lower volume fractionand specific area of reinforcing and a higher maximumcover. The quantities of steel wire reinforcing provideddo however provide ample strength for the purpose ofwater retaining structures. The relatively densely distrib-uted wires spread loads through the mortar, away fromplains of weakness, preventing failure. The resultingmaterial, which could be termed mesh reinforced mor-tar, is closer to ferrocement than traditional reinforcedconcrete (Watt. 1993).Ferrocement reservoirs have been well proven overmany years in extremes of climatic conditions in manycountries including the United States of America, theUnited Kingdom, New Zealand, Thailand, Mali, Zim-babwe and Namibia. Reservoirs with capacities up to150kl can be constructed with confidence althoughferrocement reservoirs with capacities of over 450klhave been successful (Watt. 1993).Ferrocement reservoirs in South AfricaFerrocement reservoirs were virtually unknown in theSouth African water engineering field until the nineties.Prior to that their use had been restricted mainly to springprotection and rainwater harvesting work, and the sizesbuilt were typically between 5kl and 15kl. With thedramatic upsurge in the amount of rural water supplywork being done in South Africa in the last six years,ferrocement techniques are now better known. Reser-voirs up to 400kl have been constructed with 100klG   WATER SUPPLY: BROUGHTON and NASH271reservoirs becoming common. Although the vast major-ity constructed use mesh reinforced mortar, some haveused a purer ferrocement technology.Over 300 ferrocement reservoirs providing total stor-age of over 7ml have been constructed in KwaZulu/Natal over the last six years. Most of these have beenconstructed on projects funded by the former JointServices Boards (now Regional Councils) and the MvulaTrust, but they are now also being used by authoritiessuch as Durban Metro Water and Umgeni Water. Anestimate of the total number of ferrocement reservoirsconstructed in South Africa to date might exceed onethousand, but no national data is available. Most of thesereservoirs have been constructed by rural communities inrural water schemes.Advantages of ferrocement reservoirsFerrocement is labour intensive, and the skills required areeasily assimilated in rural areas. In fact construction ofa ferrocement reservoir is not unlike the building of atraditional wattle and daub hut, a procedure still widelypractised. This allows a high percentage of local labourto be used and at the same time reduces the amount ofsupervision required.Materials such as sand, stone, reinforcing wire and ce-ment are commonly available. During the conflict inBosnia aid workers constructed ferrocement reservoirs asthe materials required were all that were available(Deverill. 1997). In many places local communities areable to contribute by collecting local sand for thereservoirs. The sand used should be well graded and passthrough an 8mm sieve. If the sand is too coarse it shouldbe blended with plaster sand. Sands that are too fine arehowever prone to cause cracking.Only simple inexpensive tools are required such as spades,buckets, hand hocks, floats and pliers. It is stronglyrecommended that all mixing of the mortar be done byhand. It has been found that when using a mechanicalmixer labour tend to add too much water to the mortarin an attempt to prevent it from sticking to the sides of themixer. Ferro-cement reservoirs can be built with or withoutshutters. Experience has shown that good shutteringmakes construction more foolproof and is thereforerecommended for controlling quality and appearance.Prefabricated shutters made from rolled corrugated ironsheets have been very successful. They are relativelyinexpensive and their cost can be shared over a numberof reservoirs. If it is not possible to use prefabricatedshuttering cheap local material can be used such astimber, iron sheets or even adobe. The main requirementof the shuttering is that it remains rigid enough to supportthe mortar while it is being applied preventing cracking(Watt.1993). Ferrocement also lends itself to the con-struction of attractive domed roofs, which are moredurable than the corrugated iron roofs often used overmasonry reservoirs.Key specificationsThe life of a ferrocement reservoir is expected to exceed50 years, assuming sound construction procedures arefollowed (Watt. 1993).A rich sand cement ratio of 1:3 by volume is recom-mended. The mortar should be neither too dry nor toowet. A dry mortar is difficult to work into the reinforcingand is likely to leave air voids while wet mortar will bemore permeable and is prone to developing slumpcracks. It is recommended that the water:cement ratioshould be less than 0.5:1 (Watt. 1993). The water:cementratio should be carefully supervised and the labour willdevelop a feel for the correct mix. Cube tests taken fromreservoirs constructed in Fredville outside Durban had a28 day strength of 30Mpa. The mortar mix employedused a sand cement ratio of 3:1 while the sand was ablend of 50 per cent river sand and 50 per cent plastersand.The wall thickness is usually built up by applying anumber of layers of mortar. By applying subsequent layersof mortar onto “green” plaster a better bond betweenlayers can be achieved. Mortar layers which are toothick are susceptible to slump cracking.Proper curing is often difficult to achieve. Each layerof mortar should be covered as soon as possible withplastic to prevent drying and cracking. At the very leastthe mortar should be wetted at least three times per day.Care should also be taken at the joint between the baseand the wall as leaks can develop at this point. Asuccessful method of achieving a water joint is to providea key for the wall in the base. The wall is then thickenedat the joint by providing a mortar collar on both sides ofthe wall. Shear reinforcing is also fixed between the baseand the wall on the larger reservoirs. A rigid joint is thenachieved.Provided that these simple precautions are adhered to astructure of sound quality is the result.Cost effectiveness of reservoirsBuilt correctly ferrocement reservoirs are strong, durableand watertight. But are they cost-effective? Costs areaffected by context. In water projects where large num-bers of these reservoirs are built, with simple designspecifications and appropriate levels of supervision, theyare most cost-effective as shown in Figure 1.However, a number of factors can largely influence thecost effectiveness of ferrocement reservoirs. These fac-tors are the design specification for the base and thepipework, the location of the reservoir and number ofreservoirs to be built in the project.A typical 100kl ferrocement reservoir of height 2m willplace up to 50kPa bearing pressure on the soil. Suchbearing pressures are in the same range as those producedby an average man walking. Complicated foundationspecifications are unnecessary as most soils will handlebearing pressures of at least 100kPa. Ferrocement reser-voirs usually have bases with a diameters of less thanG   WATER SUPPLY: BROUGHTON and NASH27210m. A raft foundation is usually constructed by provid-ing weldmesh in the base. Provided one takes the precau-tion of encasing all pipe work under the base in concrete,thus preventing leaking pipes wetting the founding ma-terial, this type of foundation will work well. If thefounding material is of a reasonable quality and there isadequate drainage, failure of the base is unlikely. Theover-designing of the foundation and base decreases thecost effectiveness of the reservoirs, in some cases by asmuch as 20 per cent.Complex pipework specifications can also negativelyeffect the overall cost effectiveness of the reservoir. Insome cases the cost of the pipework has doubled theoverall cost of the reservoir. Outlets constructed of uPVCpipe can be extremely cost effective and have the poten-tial to last longer than coated steel outlets provided theyare not exposed to sun-light. Expensive control valvesare often required for inlets, however there are manycases where the use of a manifold of brass float valves ismore appropriate and less costly. Overflows and scourvalves must be included but are used infrequently and aresubjected to low pressures. uPVC can be used for theoverflow provided that it is not exposed to sunlight.Scour can be economically provided by providing theoverflow with a removable riser pipe.Ferrocement reservoirs are easily constructed by rela-tively unskilled labour. Training is simple and can bedone on the job. Once a team has been trained in theconstruction procedures, they can with limited supervi-sion continue to build other reservoirs. Ferrocementreservoirs can therefore have a high local labour contentwhich is not only desirable but is usually more costeffective. The use of local labour or even local contrac-tors does not need increased supervision if specificationsare kept simple. Without affecting the quality of the finalproduct it is possible to specify simple mix designs suchas a 2:2:1 mix for the base which would be easilyunderstood by a local contractor and would in most casesproduce 30MPa concrete. Ferrocement reservoirs alsolend themselves to self help water schemes where contri-butions from communities can be in the form of work andnot capital. This can provide significant cost savingswhich are often required in such schemes.Exorbitant supervision costs can dramatically reducethe cost effectiveness of ferrocement reservoirs. Factorswhich can negatively effect supervision cost are: thenumber of reservoirs being built; complex supervisionstructures with multiple contractors and consultants; andthe skill of the labour.Ferrocement reservoirs lend themselves to the con-struction of more distributed storage. Constructing moredistributed reservoirs in a project not only improves thecost effectiveness of the ferrocement reservoirs but alsoreduces peak factors in the pipelines, reducing the overallcost of the project.ReferencesACI MANUAL OF CONCRETE PRACTICE, 1987,American Concrete Institute. Detroit, Michigan.DEVERILL P., 1997, Personal communication. Partnersin Development. Pietermaritzburg.WATT S.B., 1993, Ferrocement Water Tanks. Interme-diate Technology Publications. London.ANDREW BROUGHTON, Project Manager, Partnersin Development cc, South Africa.STEVEN NASH, Project Engineer, Partners inDevelopment cc, South Africa.Figure 1. Breakdown of costs for typical ferrocement reservoirs in rural water projects, KwaZulu/Natal, South Africa.Note that these costs exclude supervision and training costs, which can vary from 20 per cent to 50 per cent of thetotal cost depending on circumstances.

Ferrocement reservoirs: how appropriate?

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Ferrocement reservoirs: how appropriate?

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