WATER TANK/RESERVOIR CONSTRUCTION

1. INTRODUCTION
2. A BRIEF OVERVIEW OF LAYERED SYSTEM
3. COMMONLY ASKED QUESTIONS
4. ANSWERS

INTRODUCTION

Typically concrete water tanks are manufactured using moulds or mortar applied in layers over a temporary base panel – In Australia the latter is by far the most popular.

A BRIEF OVERVIEW OF LAYERED SYSTEM

• Set up base mould and introduce rebar
  
• Pour concrete base
  
• Set up frame (usually tubular steel) aligned with outside of base
  
• Set up timber or steel panels to frame
  
• Apply steel mesh and wire to panels
  
• Apply first layer of mortar
  
• When mortar has set (say 12 hours) remove panels and frame
  
• Continue to apply layers – in/outside
  
• Similar process for roof - All layer applications typically shotcrete sprayed

COMMONLY ASKED QUESTIONS

1. Why place the roof before spraying the in/outside Coats?
   
   
2. It is claimed that spraying time is 1.5 hours – could this be broken up into the various areas and times?
   
   
3. How and why do you use steel fibres?
   
   
4. What are the cost savings in materials through spraying?
   
   
5. What other benefits are to be expected through spraying?
   
   
6. What other applications can our spray equipment handle?
   
   
7. How difficult is it to train operators and maintain quality?
   
   
   Note: All following values relate to a typical 5000 GAL – 23000 LTR tank.

ANSWERS

1. It is proven that by placing the roof before spraying inside and outside coats the moisture liberated by the inside coat within the tank hydrates the cements and helps to “feed” the outside coat – through the wire coat. This overcomes the problem or shrinkage cracking, thus less leaks and just as importantly minimises stresses within the structure that could develop into leaks or failure at a later time. Following this procedure, advantages are even more apparent when A/ Only “fatty: sands are locally available B/ Very low local humidity C/ Hot dry winds are prevalent.
   
   
2. On a 5000 gallon, 23000 Ltr tank the typical application times are:
   
   Wire Coat 20 minutes
   Roof 20 minutes
   Inside & Fillett 15 minutes
   Outside 10 minutes
   
   TOTAL 65 minutes
   
   Allowing sufficient time for moving from one job to the next, 1.5 hours is in fact quite liberal.
   
   
3. Normal mortars generally have quite adequate compressive strength but minimal tensile strength. Steel fibres added to the mix impart a significant tensile and flexural strength to the mix; thus there is less need for use of mesh or netting.
   
   The most acceptable procedure is to bind the “build” with plain wire in the normal manner, spray the “wire coat” using a steel fibre mix. The roof inside and outside coats use a normal mix. Subject to STRICT ADHERENCE TO ENGINEERING DESIGN it is also possible to omit the binding wire and use steel fibre mixes only.
   
   
4. Since sprayed concrete is much denser than hand applied or poured concrete, less material may be used with equal or better results.
   
   Note: - Spraying significantly reduces the air entrained in the mix, plus it gives a better mechanical bond.
   
   Typical volumes of material –
   
   Wire Coat 13 cub ft 368 L
   Inside Coat 10 cub ft 283 L
   Roof Coat 14 cub ft 396 L
   Outside finish 10 cub ft 283 L
   Fillet 9 cub ft 255 L
   
   Total volume not including Base, 56 cub ft (1585 L) say 2 cubic yards (1.6 Cubic Metres).
   
   The above table applies to the typical water tank using a sand to cement ratio of 3 to 1. These volumes may be compared most favourably with the volumes required by the use of other methods of application. Other cost savings may be calculated by the deduction of cost of mesh and its application time, should steel fibres be employed on the “wire coat”.
   
   
5. Further cost savings may be enjoyed by the complete elimination of the “build”. Since spraying does not require a rigid structure, it is quite feasible to employ the following method.
   
   • Pour and screed off base.
     
   • Place heavy mesh, F72 etc into the concrete and around the outer edge of the base – standing upright to form the basic tank shape
     
   • When initial or final set is complete and therefore the mesh is standing solid –tie wire the ends of the mesh
     
   • Apply hessian or shadecloth etc wrapping around the F72.
     
   • Apply a spray coat to inside of build
     
   • When initial set complete, remove wrapping
     
   • Finish off in the previously described manner (2, 3)
     
   A variation of the above procedure would be to use a much lighter mesh than F72 but to “bind” in the normal manner after removal of wrapping. One particular advantage is that any size tank may be built without the need for expensive and heavy moulds.
   
   WARNING: - Since tanks above 136,000 litres generally require that the wall be sealed but free to move radially relative to the base, it is strongly advised that RELIABLE ENGINEERING ADVICE BE SOUGHT BEFOREHAND.
   
   Note: - Whilst the above procedure was developed by this company in 1983 and has been promoted since without patent cover or royalty claim, all intending users may feel free to use the system without charge by this company.
   
   
6. Most first time buyers have their equipment commissioned by someone from within our company. This generally takes about 4-6 hours when mixing pumping and spray techniques are addressed as also equipment lubrication and maintenance. After commissioning most operators build up their techniques and confidence to the point where they “throw away the trowel” after about a week.