Reinforced Earth® was invented in 1963 by French architect-engineer Henri Vidal. This construction technique combines frictional backfill with either polymeric straps or metallic strips as reinforcement. Civil Engineering spoke with Louwtjie Maritz, Managing Director of Reinforced Earth South Africa (RESA), to explore the durability of steel reinforcing strips.
Galvanised steel reinforcing strips play a key role in Reinforced Earth® structures. What is the function of these strips and how do they extend the longevity and reliability of the structures you build?
Reinforced Earth® is a coherent gravity structure, comprising a frictional backfill, metallic or polymeric reinforcements and a facing. During construction, the reinforcements are placed in horizontal layers at vertical spacings on the backfill. Normal backfill has compressive strength, but no tensile strength capacity. The function of the steel reinforcements is to capture the horizontal strain of the backfill by means of friction, and to transfer the tensile forces to the reinforcements. The strips are ribbed to ensure that the full internal angle of friction of the backfill is mobilised since the shear plane is between backfill and backfill and not between steel and backfill.
The longevity of steel reinforcements can be extended by increasing the sacrificial thickness of the strips. For a required service life of 70 years, a 4 mm thick strip could use 3 mm for the design strength with 1 mm of sacrificial thickness, whereas for a service life of 120 years, a 5 mm thick strip could have a design strength based on 3.5 mm with a sacrificial thickness of 1.5 mm. Because of uneven pitting corrosion, the minimum design thickness should not be less than 3 mm.
The reliability of steel strips is due to the creation of a highly indeterminate structure. This is due to their high modulus of elasticity (>200 GPa) and their high elongation after yield (>15%). Provided their connection to the facing is stronger than the strip itself, a steel strip, if overstressed, can shed its load to neighbouring reinforcements.
Why are the reinforcing strips galvanised?
The strips are galvanised with 80 microns of zinc which provides a cathodic protection to the steel. This smooths out the corrosion and avoids pitting. Only once all the zinc is corroded does corrosion of the bare steel commence. The corrosion products then inhibit corrosion of the bare steel. The galvanising thickness is assumed not to contribute to the strength of the strip.
What precautions do you take to ensure that aggressive backfill materials do not adversely affect the durability of the steel strips?
Research and experience have produced mechanical and electro-chemical specifications for Reinforced Earth® backfill. Samples of proposed backfill are tested in laboratories to determine whether the backfill is acceptable. As more than 95% of the structure is backfill material, the rules regarding aggressive fills cannot be compromised on.
Corrosion is a natural concern in steel-reinforced structures. What other factors affect the rate of corrosion and how do you combat these?
The reinforcements are buried in backfill and are not affected by atmospheric conditions. Should there be a risk of ingress of corrosive fluids into the backfill then suit-able precautions, such as an impermeable surface membrane and/or drainage behind and beneath the structure, should be specified.
Louwtjie Maritz, Managing Director of Reinforced Earth South Africa
Stray currents in the earth generated from direct current railway lines, for instance, could corrode steel reinforcements. Corrosion of steel reinforcing strips is not affected if they are discrete and not connected to each other. Damage from the use of coarse rocky backfill will not damage galvanised strips since the cathodic protection of the galvanising bridges damaged areas.
In your experience, has the cause of corrosion of steel reinforcing strips placed in Reinforced Earth® backfill ever been inexplicable?
We had an interesting case. Protective structures were constructed around the hangars and taxiways of a military air base. Severe corrosion after a period of five years caused the failure of part of one of these structures. Other structures on the base, constructed at the same time, with the same backfill, showed negligible corrosion.
After ruling out all conventional causes, attention turned to the large presence of ants on and within this structure. An ant specimen was sent to a laboratory in France which identified the culprits as pugnacious ants (Anoplolopis custodiens). These ants are native to dry and hot sandy areas in eastern and southern Africa and can release up to 20% of their body weight in pure formic acid when highly agitated. It is believed that the ants, burrowing along reinforcing strips, were subjected to intense sound blasts from Mirage jets. This agitation likely triggered the release of their corrosive acid, causing damage to the strips.
Are there any instances in which galvanised steel reinforcing strips should not be used?
Steel reinforcing strips should not be used if the backfill falls outside the electro-chemical specifications. In such cases, Reinforced Earth® polymeric straps are used. We are not tied to any specific factory or material type; our decisions are driven primarily by engineering require-ments, with cost considerations being secondary.
RESA conducts regular monitoring of the steel strips placed in its structures through its Durability Programme. How does this ensure that early signs of corrosion are detected?
Since 1978, all Reinforced Earth® structures have included special test strips installed to monitor the corrosion of the steel reinforcing strips over time. These strips are cut from the same material as the reinforcing steel, with their initial strength and mass recorded. Four test strips are placed in each structure and can be extracted and tested at any time to address client or consulting engineer concerns, or to assess the potential for extending the structure’s service life.
RESA arranges for regular extraction and testing of samples, which is carried out by laboratories in accordance with RESA’s procedures. The results are added to an ever-growing databank used to confirm and enhance our understanding of steel corrosion in Reinforced Earth® backfill.
What steps are taken if unexpected levels of corrosion are found?
This would depend on the severity of the corrosion and the structure’s required service life. If the corrosion is critical the structure could be demolished or reinstated by replacing the reinforcing strips with soil nails, drilled in through the facing.
With the installation of test strips in all structures, how do you manage the testing and extraction processes across such a vast portfolio, and what are the most common issues that arise during this monitoring?
issues that arise during this monitoring? Testing is conducted randomly across various structures, with 90 tested to date. Each test requires client permission. Occasionally, issues arise where test strips have either not been installed as per the drawings or have been omitted entirely. In such cases, we’ve had to excavate down to the strip level, sacrificing 500 mm for testing purposes. This approach is less than ideal.
Your team has gathered nearly five decades of data on the corrosion of galvanised steel strips. What insights have you gained from this extensive data bank?
90 test results have been obtained to date. After cleaning, 85% of the samples show no sign of bare steel. Of the remaining samples, 5% show the percentage area
of bare steel to be: 8% after ten years, 1% after nine years, 2% after five years (due to blast furnace slag backfill being out of specification because of high pH), 7% after eleven years (due to steel slag backfill being out of specification because of high pH) and 2% after four years.
The results indicate that corrosion has only occurred within the 80-micron galvanised layer, with no loss of strength in any of the strips. In fact, the actual thickness of the strips is generally greater than the nominal thickness due to the difference in tolerance of the roll and the nominal design thickness of the strips. Of the 90 strips tested, the lowest recorded elongation is 15%, with an average of 22%. This confirms the indeterminate nature of the structure.
The consistent monitoring of test strips strongly supports the prediction that galvanised steel reinforcing strips placed in backfill that meets RESA specifications and is protected from aggressive fluids should comfortably meet their required service life, potentially lasting for many additional decades.
