Picking the right annular space grouting mix design can be tricky. Many DOTs have very low strength requirements for annular space grouts. For example, Virginia DOT is 120psi, North Carolina DOT is 125psi (flowable fill), and Florida DOT allows 80psi.
There are a few general schools of thought for annular space grouting mix design:
Very High Strength:
Premise: Some designers specify 4000psi concrete or grout for annular space grouting. A common thought here is that if the carrier pipe were to fail, that it would be effectively encased in a concrete pipe. As there’s usually a pretty got reason that a carrier pipe wasn’t direct buried, thinking goes that if it’d be hard to excavate and repair or replace, that some redundancy from a very high strength encasement is advantageous.
Weakness: High strength grouts generally require aggregate. Mixes with aggregate (particularly coarse aggregate) are much harder to pump than mixes with no aggregate. The harder it is to pump an annular space grout, the higher the pressures in the annular are, and therefore the higher the likelihood of damaging the carrier. Very high strength grouts are also very dense, which increases carrier buoyancy. You can only go so low on slump, as well, which increases the likelihood of deadheading an annulus during the grouting process.
Premise: As long as the casing is filled with something that’s roughly similar to the soil outside of it, it’s going to be fine. To this end, sand or pea gravel can be blow into the annulus to fill it. This also has an advantage in that the carrier pipe can be pulled out in the future, the granular backfill material cleaned out, and a new pipe can be inserted.
Weakness: Sand and pea gravel can only be blown so far. This method can work well for relatively large, short annuluses. Materials also have to be very dry to perform well during blowing, which can cause issues with silica dust exposure for workers and the adjacent public. Additionally, ensuring a complete fill can be difficult. Since the products are blown in and are not self-consolidating, if they were for some reason to see loads, they would likely consolidate, and if not bulkheaded, could be prone to erosion or consolidation from ground water flows.
Premise: Leaving the annulus unfilled is not an option, so whatever the contractor thinks may work needs to be placed to refusal.
Weakness: Many pumpable or flowable fill grouts require significant amounts of water, which can lead to segregation in the annulus. Aside from issues arising from lack of uniformity and accompanying variances in strength, segregating aggregate can plaque off the flow path within the annulus, leading to a short fill, or significant pockets of trapped air. Specifiers who have experienced this in the past tend to push significant amounts of plasticizer, fly ash, and other components to try to ensure a highly fluid grout that won’t segregate or stop short of a complete fill.
The Better Way:
Cellular concrete is an exceptionally mobile, non-segregating, low density, self-consolidating grout. Comprised of cement slurry (most commonly 0.50 water : cement ratio) and a preformed foam, cellular concrete is installed at low pressure, reduces carrier pipe buoyancy, and virtually eliminates trapped air.
Typically, 30lb/cuft cellular concrete is used for annular space grouting. This annular space grouting mix design provides 125psi break strength at 28 days, virtually eliminates carrier pipe buoyancy when carriers are water filled, and shows excellent flow characteristics.