Septic Tank Certification input needed

[RHTPE]

I have been asked to submit a proposal for certifying precast concrete septic tanks for compliance with ASTM C1227-09. The client is using non-metallic fibers to replace welded wire fabric in the walls of the tank. Some #3 bars are used in the top slab each side of the required openings, but that is the only rebar in the tank. I have advised my client that the most definitive method of ensuring certification is to vacuum test the tank - first to the 5" of Hg for leakage (CT requirements, not ASTM), then to an amount that would be equivalent to 1.5 times the design loading. This could likely be as much 18" of Hg.

I am seeking comments from those more knowledgeable/experienced than I on these 2 topics:

1) The actual design loading on the tanks varies - perhaps 660 psf on the top, varying to as much as 1125 psf against the walls at the bottom, with hydrostatic upift on the bottom varying with ground water level. Testing by vacuum will exert a uniform load over all surfaces of the tank, unlike what it would experience in the ground. Would you expect to apply a vacuum to reach the highest magnitude of expected design loading?

2) Does anyone know of a good resource for an analytical approach to the structural design of fiber-reinforced concrete? While it may not comply with ASTM C1227-09 (requires design per ACI 318), it could provide a good starting point, particularly for fiber dosage.

Ralph
Structures Consulting
Northeast USA

 

[Ron]

Ralph...I know that it's commonly called "fiber reinforced" concrete (even ACI Committee 544 calls it that); however, fiber only enhances the properties of the concrete itself...it does not "reinforce" in the structural reinforcement sense.

The only resistance it provides to bending for instance, is an increase in the modulus of rupture of the concrete. That's fine for plain concrete such as a pavement slab or slab on grade, but it doesn't take the place of rebar to resist bending moments. If your stresses are low, you could probably rationalize it as a higher quality plain concrete; however, your stresses are not that low.

Further, when failure does occur, it will be a fast, brittle failure, not a slow ductile failure.

Fibers can help to reduce cracking or delay the onset of cracking...in general they will lengthen the distance between cracks.

In short, I would not use fiber (steel, polypropylene, or glass) to take the place of rebar in a section subjected to structural bending.

 

[hokie66]

I concur with Ron's advice. Plastic fibres are not reinforcing. Steel fibres are, but mostly for improving toughness and abrasion resistance, with only a minor degree of improvement in tensile strength. Glass or carbon could be reinforcing, but I don't believe you mean that.

Vacuum testing tells you something, but what about when there is positive pressure inside? And handling stresses would be another issue.

 

[RHTPE]

Ron & Hokie66,

I know that fibers do little to increase the flexural strength of concrete on a macro scale. Which is why I would be pushing for the vacuum test to demonstrate this for my client, as they see fibers as the end-all simple solution for precast products. While fibers may be a good solution for small precast components, I do not believe they will provide a demonstrable strength benefit for tanks.

I would really like your thoughts on the level of vacuum necessary to satisfy a proof test, given that a load induced by vacuum will be uniform on all surfaces, while the loads applied to a buried septic tank vary on all surfaces.

Hokie66 - how would a buried septic tank experience a positive (outward) pressure? I just don't see it.

Ralph
Structures Consulting
Northeast USA

 

[Ron]

Ralph...I agree with the level of vacuum testing. Having varying pressures on the walls is just a condition, but pick the higher load and test to that plus the 1.5 factor. "Overstressing" the areas of lower working pressure is just something you'll have to endure, but the info will be good. I would also measure deflections and rebound to see how the tank is responding. If the rebound is not fully elastic, that tells you a lot as well.

Could be an equipment challenge depending on the volume of the tank. It will probably take a blower hooked to the suction side, rather than a vacuum pump.

Keep in mind that the vacuum testing will stress the panel centers, while pressure testing would be better for the corner conditions.

The outward pressure would only come from the liquid head in the tank.

 

[RHTPE]

I realize there will be outward pressure resulting from the effluent inside the tank, but the magnitude of the internal, outward load is less than the external soil load. For a full tank, the resultant load is still external, i.e. pushing inward on the tank.

The tank design must consider the loading specified in ASTM C890, Section 7, which specifies 2 conditions - one with an empty tank and the other with a full tank.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

I agree that the design pressure from the internal contents would normally be less than the design soil pressure. But I would want the tank to be strong enough to resist the condition where the tank is full and the soil around the tank is either excavated or cracked due to dry weather shrinkage.