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Er. KAUSHAL KISHORE
Materials Engineer, Roorkee
Ferrocement as a construction material has now gained acceptance in different applications, namely : housing, agriculture, marine, water supply, sanitation, water proofing treatment etc. Numerous studies published have built up confidence in the material resulting its wider application,
Traditional methods of roof water proofing by lime concrete and mud phuska with thin burnt clay tiles are very cumbersome, time consuming involved high labor cost and also due to non availability of traditional skills and good materials these methods of water proofing are now not very popular. Though bitumen felts are also provided for water proofing their life is less than five years and need frequent replacement.
Ferrocement has very valuable characteristics in that, it is highly crack resistant and have excellent bond with the surface over which it is laid, its highly impervious nature does not require any treatment over it. These characteristics make ferrocement an ideal lining material over cement concrete, bricks masonry, stone masonry or even wooden structure making them leak proof against water. Ferrocement for waterproofing was successfully used upon old buildings, particularly constructed with lime concrete upon which no other water proofing treatment was found successful.
Ferrocement provides higher tensile and flexural strength, better resistance to impact, fracture and failure. It also provide a crack free, tough dependable surface free, from danger of leakage and corrosion. These improvements in the characteristics are due to the fact that concrete can undergo large deformations in the neighborhood of reinforcement. Since reinforcement in ferrocement structure are provided in the form of well distributed wire mesh layers, it can carry large strains without cracking during its services. As the material provides a surface free from danger of cracking and offers a highly impervious layer, it can be safely adopted for water proofing treatment of structures.
1 Cement: The cement for general construction is to be ordinary Portland cement of 43-grade conforming to IS: 9112-1989. It should be compatible with the admixture used.
2 Sand: Sand shall be obtained from reliable supplier. It should be clean, hard, strong, free of organic impurities and deleterious substances. It should be inert with respect to other materials used and of suitable type with regards to strength, density, shrinkage and durability of the mortar made with it. Grading of the sand is to be such that a mortar of specified proportions is produced with a uniform distribution of the aggregate, which will have a high density and good workability of the aggregate, which will have a high density and good workability and which will work into position without segregation and without use of a high water content. The grading of the sand shall be as given in Table 1.
Table 1: DESIRABLE SAND GRADING
|I.S. Sieve||Percentage passing by weight|
50 – 70
25 – 45
10 – 20
2 – 5
3 Water: Water used for making and curing shall be clean and free from injurious amounts of oil, acids, alkalis, salts, sugar, organic materials or other substances that may be deleterious to mortar or steel. Potable water is generally considered satisfactory for making cement : sand mortar and its curing.
4 Reinforcing Mesh: One of the essential components of ferrocement is wire mesh. Different types of wire meshes are available almost everywhere. These generally consists of thin wires, either woven or welded into a mesh, but the main requirement is that it must be easily handled and flexible enough to be bent around sharp corners. For the general construction 20 gauge 12 mm x 12 mm galvanized wire mesh may be used which comes into size of 1 x 30 m roll.
a. Non-shrink grout : For injection grouting of porous, honeycombed etc., areas of roff slab.
b. Polymer modified mortar : Polymer modified mortar for sealing cracks of roof slab and patch work repair of vertical surfaces.
c. Superplasticizer : High ranger water reducer for making cement slurry.
d. Liquid integral water proofer: Liquid integral water proofing compound to be admixed with mortar/concrete for base course.
e. Plaster Plasticizer and Water proofer : Liquid water proofer and plasticizer for ferrocement mortar.
6 MIX PROPORTIONS
a. Mix for base course: For 6 mm thin layer of base course, the sand should be as given in Table 1. The mix ratio of Cement : Sand should be 1 : 3. Free water / cement ratio maximum 0.45. The mixing water admixed with liquid water proofing compound at a dosage of 140 ml/50 kg of cement. However at some locations due to bad shape of main roof slab thicker base course may be required to give proper slope towards the drain. In such cases the maximum size of aggregates may be as per the requirement of base course thickness. Suppose a thickness of 25 mm is required for the base course, then the mix ratio will be Cement : Sand 1 : 3, sand 100% passing on 8 mm. Free water / cement ratio maximum 0.4. The dosage of liquid waterproofing compound will be the same.
b. Mix for ferrocement mortar: The sand gradings shall be as given in Table 1. The cement : sand ratio will be 1 : 2.5. Free water / cement ratio maximum 0.4. The mixing water shall be admixed with liquid water proofer and plasticizer for mortar at a dosage of 10 ml / 50 kg of cement.
The mix proportions are given by weight. However, sand may be taken by volume. Mix ratio of sand by weight may be converted by volume by dividing its ratio by the room dry bulk density of actual site sand. The bulk density of cement may be taken as 1.44 kg/lt.
PROCEDURE OF WATER PROOFING TREATMENT
Ferrocement can be used for water proofing of new as well as old flat/sloping roofs. In case of the roofs, which are not giving satisfactory performance, the roof treatment is removed to expose the concrete/brick surface.
The entire roof surface including parapet walls are to be cleaned with wire brushes and washed with water. Any cracks found on the surface should be investigated, if they are not static suitable remedial measures be provided before undertaking any water proofing treatment.
The porous and honeycombed surfaces of the roof should be grouted by Non-Shrink grout. Any cracks of more than 1 mm should be chased out to “V” shaped groove and filled with Polymer modified mortar. For entire work the roof surface and parapet walls be kept in saturated and surface dry condition.
On the above saturated and surface dry surface thin cement slurry be applied by brush. Mixing water of which admixed with superplasticizer at a dosage of 200 ml / 50 kg of cement. Upon the wet slurry a mortar layer as per mix ratio mentioned at 6 (a) be laid and provide a suitable slope towards the drains. The layer surface is to be made rough with small hard coconut brush. For fixing wire mesh, wire nails were fixed in this mortar layer at appropriate spacing with projection from the plaster surface of about 4 mm. Proper curing should be done for 3 days.
Two layer of 20 gauge 12 mm x 12 mm galvanized wire mesh are stretched over the above prepared surfaces. At the joints of wire meshes, 150 mm overlap is provided. The joints in the two layers of wire mesh are staggered. The mesh then fixed to nails. 5 mm thick cover block or 5 mm dia G.I. wire pieces are inserted between roof and the mesh and in between the two meshes for keeping the meshes at desired positions and for maintaining proper cover. Thin cement slurry, the mixing water of which admixed with 200 ml superplasticizer 50 kg of cement is sprayed over the surface and then immediately apply the mortar. The mortar shall be as mentioned at (b). The mortar is thoroughly mixed and applied with trowels to finished thickness of about 20 mm. While applying the mortar, the wire mesh layers are lifted up using a hook so that a cover of about 4 mm could be provided below the bottom layer of the wire mesh and between the two meshes. Hard trowelling is carried out over the surface for leveling and finishing is done using a wooden float. At the junction of roof slab with parapet walls or at roof projections, water proofing treatment is done as shown in Fig. 1.
At very low temperature and high humidity, twelve hours after finishing the mortar, the roof should be covered with wet gunny bags. However, at higher temperature and low humidity the surface should covered with wet gunny bags after forty minutes of finishing the mortar. Twenty four hours after laying the treatment the roof surface may be ponded with water for two weeks.
Curing is a very important operation in ferrocement construction. Ferrocement structures are thin and may dry at faster rate hence need more attention. Therefore surface of all ferrocement structures should be kept moist for at least two weeks. At no time the mixing water is allowed to be dry.
It should be remembered that the process of hydration (the process responsible for gaining strength for cement/mortar or concrete) stops once the structure is dry. It does not restart again if surface is wetted again.
For large surfaces of roof expansion joints at proper spacing are to provided. The gap of the joints are to be filled by Polysulphide Sealants.
For thermal comfort, the roof top may be lined with thin brick tiles with joints grouted with cement : sand mortar. Alternatively for pleasant appearance and also thermal comfort, the roof top may be lined with scrapped/broken glazed ceramic tiles.
Note: Also refer dosage of admixtures recommended by manufacturer.
The total cost of ferrocement treatment amounted comparatively less than the other methods of water proofing, i.e. tar-felt treatment. A 30 mm thick ferrocement treatment will have a load of only 60 kg/m2 against brick bat coba load of 350 kg/m2.
Ferrocement water proofing treatment has long effective service life, where, as tar-felt life is less than five years and needs regular replacement. Further where even the tar-felts are laid the surfaces are so spoiled that no other treatment can be done without its removing and cleaning the surface. The cleaning of bitumen coated surfaces are very cumbersome and involved higher labour cost. On the other hand, if need arises chemical water proofing treatment can be easily done upon ferrocement surface.
Large number of leaking roofs are existing in important structures for want of economical methods to stop water leakage. Ferrocement method of water proofing can be applied on old as well as new buildings. Numerous old leaking buildings constructed with lime concrete needs proper water proofing treatment. On these buildings ferrocement water proofing treatment can be most successful.
1. Pama, R.P.- “Ferrocement Constituent Materials and Mechanical Properties” Asia Pacific Training Course on Ferrocement Construction, April, 1984, pp. 17-34.
2. Sharma, P.C. (1980) – “Use of Ferrocement for Water Proofing” Journal of Ferrocement, Vol. 10, No. 2, pp. 127-141.
3. Sharma, S.P., Sharma, P.C., Singh, K.P. and Batra, S.S. “Ferrocement Treatment for Repairing a 50,000 Gallons Over head Water Tank”, Journal of Ferrocement, Vol. 14, No. 3, pp. 241-247.
4. Rai, Mohan, Jaisingh, M.P. – “Advances in Building Materials and Construction”, C.B.R.I., Roorkee, 1986, pp. 312-314.
We at engineeringcivil.com are thankful to Er. Kaushal Kishore for submitting the paper on Water Proofing By Ferrocement. This will not only be a great help to fellow civil engineers who are looking for more information regarding ferrocement and how water proofing can be done by it.