Insulating Tanks

Foam Core Stress-Skin Shell Method for Insulated-Tanks

This  method  creates  two  very  thin  and  strong  shells  that  are  comprised  of  a  unique,  ultra high-performance cementitious  composite.  This  special  high-strength  coating is hand-applied  with  a  trowel  to  both  sides  of a  2-inch  thick  extruded  polystyrene  insulation panels and  embedded  with  layers  of  fiberglass  grid  reinforcing  and finished with a brush-on glaze of similar material. The  rigid  blue  foam  panels  are  often  called  ‘Dow  Board’  and  they come in several  thickness  and  varieties  such  as  High-Load  Dow  Board  that  has  a  higher compressive  strength  than  the  regular  15  psi Dow  Board.

The  Insul-Tanks  use 2 –inch  thick regular  Dow Board  that  comes  in 4  feet  by  8  feet  by  2 inch sheets. The  two,  very  thin, surface  shells  transform  the  foam  into  very  rigid  and damage  resistant  material  because the  high-strength  composite  and  reinforcing create a stress-skin, or structural sandwich, effect.   The   composite   wet   mix   plaster and   the   finish glaze   are   both   made   by combining special  additives  that  are  used  for  Ultra  High Performance  Concrete with pre-bagged polymer-modified cement.   

This polymer-modified cement is readily available in building materials supply outlets that carry plaster supplies.
It is used as a base-coat for Exterior Insulation and Finish System (EIFS) as is the special fiberglass mesh that is used with it. Typically the base mix and glass make a sturdy shell for EIFS but adding the water-reducer/superplasticizer called Eucon SPJ and a refined pozzolan called Micron 3, greatly reduces the porosity and increases the density of the cement based shell resulting in a harder and more chemically resistant shell.

Adhesive  Base  Coat  Mix
This  usually  comes  in  50 lb  bags  and  is  used  as  plaster  for embedding  fiberglass  mesh onto foam  board  for  the  exterior  of  buildings.    It  is  called adhesive  base-coat  because  it  adheres the  fiberglass reinforcing  to  the  foam  while  making a  hard  protective  shell.    The  finish system  that  goes  over  the  adhesive  base coat  is  usually  a sprayed  stucco  texture  of rubberized  sand  to  make  a  nice  smooth  sandy  finish.    The adhesive base  coat  mix  is  a  true polymer  modified  cement  that  consist  of  about  50  percent Portland  Cement,  40  percent fine quartzite  sand,  and  10  percent  dry  (high-solids)  acrylic usually  an  acrylic  vinyl  acetate  co-polymer. Some common  brand  names  are  Dryvit, Primus,  and  UltraKote.    The 50 lb sacks  usually cost  between  $24  and $35 each.  For this high strength version of this mix, it is necessary to only mix up a small 64 oz batch.

Crack-Free  Fiber  Glass  Mesh
This  mesh  is  specially  engineered  for  a  cementitious matrix because  it  is  has  a light rubberized  coating  to  prevent  alkali  chemical  attack  on  the fiberglass. This  fiberglass  mesh comes  in  150 feet rolls that are 38 inches to 48 inches wide and that cost between $85 and $125 each.  The  fiberglass  mesh  is  cut  to  match  the layouts,  the  planned  plaster  area  for  a  batch, so  that  two  or  three  layers will  be  used.    The layout  is  drawn  onto  the  foam  with  a marker.   The  layout  is  between  6  and  10  square  feet and will  use  the  mix  that  is  about  ½ gallon.    An  additional  line  that  is  inset  one  inch from the  edges  of  the  layout  is drawn  to denote  the  overlap  area  to  the  adjoining  pieces.    On the overlap  area,  the  fiberglass  is  not covered  with extra  composite  material  or  a  glaze,  so  the overlap  can  be  seen  and  so  the overlap  area  is  not  thicker  than  the  rest of the shell.

Other Uses of this Foam Core Stress-Skin Shell Method

Besides  the  Insul-Tanks, this  basic  shell  method  can  be  used  for  many  things  where  a durable,  water-proof,  and easy  to  shape  and  smooth  surface  is  desired.    The  insulation really  helps  the  material  to  serve  other  important purposes.  There are some other uses for the Foam Core Stress-Skin Method, one being cantilevered counter-tops and shelves. For the counter-tops and shelves, the base material is easy to shape and sand, and  the  enhanced  shell material  is  very  durable  and  easy  to  repair.    A  2  inch  shelf  can  be  fully  supported  from one side,  provided  it  is  bonding  with  the  same  material  as  a  backing  that  is  mounted  to the  wall  or  a  self  supporting unit that sits on the floor. This  allows  more  open  designs  for counters  such  as  counter  tops  that  do  not  need  cabinets  below  for  supports.   Open shelves can be placed below and above. Self-supporting  prefabbed  sections  can  be  brought  inside,  or used  as  durable  outdoor  kitchen  elements  and assembled  to  make  beautiful  counter  top  and shelf  assemblies  transforming  a  bare  room  or  patio  into  a  functional and  beautiful  kitchen.  The  cantilevered  shelves  attach  to  the  backing  with  a  small  radius  instead  of  a sharp  corner so  it  does  not have dark,  recessed,  and  dusty  corners  where  spiders  can  build webs. That  web-free  corner  combined with  the  more  open  design  enabled  by  the  stress-skin  effect,  makes  the  kitchen  and  bathroom  designs  easy  to clean and brings in a new element of simplicity to design that was not available before.



 The material   is also  well suited to create bathtubs,  showers, and bathroom sink  and shelf assemblies similar to the kitchen counter-tops and shelves. These surfaces should also be finished with a smooth  glaze  on  the  high-strength  composite  with  chlorinated rubber paint (which makes for an easy to clean and durable surface).

Additionally,this  is  a  great  medium  for  making  decorative  doors  and  windows  that  have complex  shapes.    For  making  hatch  designs  that  are  more  curvy  than  square  doors,  the medium  allows  you  to sand  precise  shapes  that  will  make  a  good  weather  seal.    You  can make  conventional  doors  and  window  frame designs as well, but the insulation here makes this medium better. For the decorative doors and windows, the surface  shell  is  very  dense  and hard  but  bonds  to  the  chlorinated  rubber  paint  better  than other  cement  shells  such  as swimming  pool  concrete.    This  is  because  the  sand  aggregate  in  the  enhanced polymer-modified  mortar  mix  is  much  smaller  and  so  will  not  flake  away  or  weather  away  as quickly.  

The harder  mix  allows  the  swimming  pool  paint  to  endure  warmer  temperatures  because  of the  better  bond.    If  the paint  does  show  signs  of  weathering  over  the  years,  a  fresh  coat can  be  applied.    If  an  epoxy  paint  is  preferred  for longevity,  the  composite  can  be  sanded and  prepped  for  an  epoxy  paint,  that  will  endure  heat  better  and  is  what  is typically  used for  hot  tubs,  but  the  epoxy  paint  could  fade  in  the  sunlight  whereas  the  chlorinated rubber endures ultra-violate  better.    The  chlorinated  rubber  swimming  pool  paint  may  last  longer on  this  particular  composite design,  even  though  epoxy  based  swimming  pool  paints  will last  longer  on  conventional  concrete.  Both  are  good options  but  on  this  shell, but it  is easier to prepare  the  rubber  painted  surface  for  a  new  coating  of  epoxy  swimming pool paint.

Installing  Hardware (such  as  hinges  and  latches)  

Hinges  and  latches  bond  differently  to this  medium  than  to wood.  The  bond  is  more durable because  the  anchor  into  the  shell material  on  the  surfaces  rather  than  in  the middle.    The pins  or  bolts  that  support  the  hinges  or  other  hardware  go  from  one  side  of the  stress-skin  shell  to the  other  and  anchor  to  the  plane  of  the  shell,  by  using  washers that  are  layered  into  the  composite  and  fiberglass.   In  wood,  a  screw  will  rely  on  a  friction bond  on  the  inside.  This  bond  can  loosen  over  time  as  repeat  strains  are placed  on  the screw  or  as  the  wood  shrinks  and  degrades  over  time.  Plastering  the  hardware  into  both of the supporting  shell  makes  it  so  that  bending  strains  placed  on  the  hardware  will  not weaken  the  connection.  This  is because  the  strains  are  more  in-line  with  the  direction  of tensile  and  compression  reinforcing  on  the  hardened shell. The  expected  deflection  caused by  strong  bending  strains,  such  as  hanging  on  a  door  or  cabinet  door,  strains strong enough  to  damage  the  stress-skin  shell  surround  the  hardware,  are  not  likely  to  dislodge the  hardware because  the  pull-out  resistance,  the  twisting  resistance  are  all  fully  countered better  than  the  breaking  resistance around the hardware, meaning the stress-skin hull would break before the hardware pulled loose.