When the Tender Says ‘or Equal’: Judging a Substitute Gel on Paper

Three bids, one specification, and a 14 percent gap between the middle and the lowest. The lowest bid offered a product with a higher absorption figure, a lower price and a certificate from an accredited laboratory. It was approved as an equal. Eight months later, 30 percent of the polymer had washed out of the hydroseeded slope, because the particle size distribution was 40 percent below 0.5 mm and the slurry carried it straight off the face. The certificate was genuine. It just did not describe the property that mattered.

The phrase “or equal” is where most gel specifications lose their teeth. It transfers the technical judgement from the person who wrote the clause to the person who reads the bids, usually under time pressure, usually without a sample.

Why absorption is the wrong headline

Absorption is the easiest property to test and the easiest to inflate. A sodium polyacrylate can post 400 g/g in deionised water and still be a poor choice on farm ground, because sodium disperses in soil, the polymer breaks down faster under UV and mechanical cycling, and the salt index is higher. A potassium salt at 250 g/g can outperform it across a five-year planting cycle.

On a slope or a hydroseeded face, the controlling property is not absorption at all. It is whether the particle stays where it was placed. That depends on particle size distribution, bulk density, and how the granule behaves in a slurry with mulch fibre.

Six lines that make “or equal” checkable

• Chemistry: potassium polyacrylate, with the cation stated, not “polyacrylate” alone.
• Absorption: at least 200 g/g in a defined test water of 1.0 dS/m, tested to a named method, at 2 hours and 24 hours.
• Particle size: at least 90 percent retained on a 1.0 mm sieve for field planting, or a stated range for the application method, with fines below 0.5 mm capped at 5 percent.
• Moisture: 8 percent maximum at packing, by oven method.
• Bulk density: a stated range, so that volumetric dosing is calculable.
• Traceability: lot number on every bag, and a 500 g retained sample available on request.

Those six lines fit on half a page and they remove most of the room for a paper substitution. The point is not to exclude competitors; it is to make the comparison factual.

Test the substitution, do not read it

For any alternate offered after award, ask for a 2 kg sample and run three tests in a day. Swell 5 g in 1 litre of site water for 20 minutes, sieve at 1 mm, and weigh. Sieve 100 g dry through 1 mm and 0.5 mm and record the fractions. Weigh 1 litre of product in a graduated container for bulk density.

Then compare with the approved product. If the alternate loses more than 15 percent on any of the three, it is not equal for the purpose of the contract, whatever the certificate says. Document the test with photographs and a dated sheet, and send it to the supplier with the query, not to the contractor.

On one municipal contract, the approved product and the alternate had almost identical absorption and bulk density. The alternate failed anyway, on a property nobody had specified: dust. Its fines content was 11 percent, and the mixing station on a windy site became unusable, with the crew stopping work twice a day. The contract had no dust clause, so the variation was arguable and the contractor wore the cost.

The lesson is to specify the properties that the site will actually punish. A windy site needs a dust clause. A hydroseeded slope needs a fines cap. A drip-fed greenhouse needs a particle range that will pass an emitter or a filter. Generic chemical specifications do not capture any of that.

What to do before the next bid round

Rewrite the clause so that the alternate has to be submitted with a sample and a test report in the specified water, and make acceptance conditional on your own test, not the supplier’s. Add one sentence: the purchaser may reject any alternate that differs by more than 15 percent from the specified product on absorption, fines or bulk density. Then keep the samples and the results in the tender file.

That sentence costs nothing to write and it converts “or equal” from an escape hatch into a test. The property values for our potassium polyacrylate SAP powders are published in the form the clause above requires, including the sieve fractions and bulk density range.