In a message dated 06/06/2008, Paulc@........ writes:

I wonder if anyone has had  any experience using standard concrete blocks to 
form the "L" framework  for a horizontal sensor.

Hi Paul,
 
    Ordinary concrete with stone aggregate in it can be  quite noisy as the 
temperature changes. The expansion coefficients of the cement  and the stones 
are different. For seismic use, a 50:50 mix of sand and cement is  recommended 
- definitely no aggregate!

>>    For the pivots I would be using crossed  rods top and bottom.  I would 
cement in place the mounts for these and make  the top adjustable for 
levelling.This amounts to taking my horizontal  sensor which constructed on an AL 
channel frame, and moving it over to the  concrete frame.  I have the space to do 
this.
 
    You will need to  cover the concrete and keep it wet for at least a 
fortnight to cure. Then allow  it to dry out slowly and coat it with a sealing 
paint - concrete can absorb  quite a lot of water as the humidity changes. Damp 
concrete can be very  corrosive to metals.
    I am doubtful  about trying to cement the mounts in place. They are 
usually secured with bolts.  Concrete takes compressive forces OK, but it is not 
strong in tension and it  does not adhere well to metals. Adhesives are 
generally used.

The aluminum frame is  currently apart and was being readied for  welding.

    Why weld the Al U channel frame at all? I use 1/8"  thick triangular Al 
plates and 4 stainless steel bolts to secure the corners.  This is simple to do 
and works well.
    Welding Al can be OK, but you have to know your  alloys - not all of them 
weld well and quite a bit of skill is required. Welded  joints also have a 
lot of stress frozen in, which can cause problems if it is  not relieved. Welded 
metal is usually quite soft. You stress relieve the  joints at 180 C for 
maybe an hour.

Any comments or  suggestions.?


    You can also stick Al frame components together  with two part acrylic 
adhesives. Don't try to use Epoxy. Al sticks better if it  is heated first to 
drive off the water absorbed on the surface.
 
    Regards,
 
    Chris Chapman
 
 



   





In a message dated 06/06/2008, Paulc@........ writes:
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I wonder if anyone ha=
s had=20
  any experience using standard concrete blocks to form the "L" framewo=
rk=20
  for a horizontal sensor.
Hi Paul,
 
    Ordinary concrete with stone aggregate in it ca=
n be=20
quite noisy as the temperature changes. The expansion coefficients of the ce=
ment=20
and the stones are different. For seismic use, a 50:50 mix of sand and cemen=
t is=20
recommended - definitely no aggregate!
 
>>    For the pivots I would be using cro=
ssed=20
rods top and bottom.  I would cement in place the mounts for these and=20=
make=20
the top adjustable for levelling.This amounts to taking my horizont=
al=20
sensor which constructed on an AL channel frame, and moving it over to the=20
concrete frame.  I have the space to do this.
 
    You will n=
eed to=20
cover the concrete and keep it wet for at least a fortnight to cure. Then al=
low=20
it to dry out slowly and coat it with a sealing paint - concrete can ab=
sorb=20
quite a lot of water as the humidity changes. Damp concrete can be very=20
corrosive to metals.
    I am doubt=
ful=20
about trying to cement the mounts in place. They are usually secured with bo=
lts.=20
Concrete takes compressive forces OK, but it is not strong in tension and it=
=20
does not adhere well to metals. Adhesives are generally used.


  
The aluminum frame is=
=20
  currently apart and was being readied for=20
welding.
    Why weld the Al U channel frame at all? I use 1=
/8"=20
thick triangular Al plates and 4 stainless steel bolts to secure the corners=
..=20
This is simple to do and works well.
    Welding Al can be OK, but you have to know your=
=20
alloys - not all of them weld well and quite a bit of skill is required. Wel=
ded=20
joints also have a lot of stress frozen in, which can cause problems if it i=
s=20
not relieved. Welded metal is usually quite soft. You stress relieve th=
e=20
joints at 180 C for maybe an hour.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
Any comments or=20
  suggestions.?

    You can also stick Al frame components together=
=20
with two part acrylic adhesives. Don't try to use Epoxy. Al sticks better if=
 it=20
is heated first to drive off the water absorbed on the surface.
 
    Regards,
 
    Chris Chapman