Precast Ferro cement water tanks up to 10000 litres capacity
1992 Edition

IS 13356 recommends precast ferrocement water tanks with net usable capacities from 270 up to 10,000 litres. For domestic use, typical sizes include 270, 540, 750, and 1,250 litres, while industrial and community tanks range from 1,250 to 10,000 litres. Common tank configurations are cylindrical, rectangular, and square, with cylindrical tanks favored for efficient material use and eliminating sharp corners. Dimension ratios prescribe height equal to diameter for cylindrical tanks and length-to-breadth ratio of approximately 1.5 for rectangular tanks. Flat bottoms are standard, but circular tanks exceeding 2 meters in diameter should have a shallow dome or fillet at the base to reduce stress concentrations, ensuring structural integrity and ease of construction.

The tanks are constructed using a skeletal framework of steel bars shaped to the tank design, wrapped with one or more layers of wire mesh secured by binding wire. The mortar comprises a rich mix of cement and sand, initially dry-mixed then combined with water for application. Reinforcement cages are positioned on leveled platforms with debonding layers before casting. The bottom slab is cast first, followed by plastering vertical walls by forcing mortar into the cage from both sides. Cover slabs and lids, including manholes of 500 or 600 mm diameter, are cast separately with smooth finishes and optional locking mechanisms. Wall thickness varies from a minimum of 12 mm for mechanized casting of tanks up to 2,000 litres, 15 mm for hand-cast tanks up to 1,000 litres, and 20 to 40 mm for larger capacities, ensuring durability and water tightness.

Casting involves initially dry mixing cement and sand in prescribed ratios, followed by adding water to form the mortar. The reinforcement cage is placed on a flat platform with a debonding medium. The bottom slab is cast and leveled first, after which mortar is applied to the vertical walls by pressing it through the reinforcement mesh from one side, supported by plywood or GI sheets, and then repeated on the opposite side. Surfaces are smoothed inside and out after initial setting, with special care around pipe fittings. Cover slabs, complete with manholes and lids, are cast separately. Mechanized or semi-mechanized casting methods such as the tempformer system or masonry molds may be employed for full or segmented tanks to improve efficiency and uniformity. Standard curing practices require maintaining moisture—via wet burlap, spraying, or curing compounds—for at least seven days to ensure proper hydration and strength development.

Water tightness testing mandates filling each tank with water and allowing a seven-day period for absorption. Subsequently, the tank's exterior is observed for another seven days for any signs of leakage or moisture. Tanks must remain completely dry externally to pass the test, which is conducted prior to interior painting. Should leakage be detected, repairs involve exposing the wire mesh at the affected areas and patching with the same mortar used during manufacture. After repairs, tanks are retested upon purchaser's approval. This rigorous procedure guarantees the tanks are impermeable and reliable before finishing.

Quality control follows a statistical sampling approach where units are selected randomly from each production lot. The lot is arranged systematically, and every rth unit is chosen for inspection, with r calculated as the lot size divided by the sample size. Sample sizes and permissible defective counts vary by lot size: up to 50 units require 8 samples with zero defectives allowed; 51-100 units require 13 samples allowing one defective; 101-300 units require 20 samples permitting two defectives; and 301-500 units require 32 samples with a maximum of three defectives. Each sampled unit undergoes dimensional and finish inspections. The lot is accepted only if the number of defective units does not exceed the permissible limit, ensuring consistent product quality.