0.3 Percent SAP by Cement Weight: Curing a Bridge Deck From the Inside

A deck pour in July started showing map cracking at the six-hour mark, before the finishing crew had packed up. The mix carried 380 kg of cement per cubic metre at a water-to-binder ratio of 0.42, and nothing in it held water back. The cracks were not structural. They were the surface losing moisture faster than bleed water could climb to replace it, and at three spots the pattern ran deep enough to sit over the top rebar.

The usual fix is a curing compound and a wet burlap blanket at dusk. That treats the surface. Internal curing treats the whole section, and it uses the same polyacrylate powder that goes into a diaper core, dosed at a fraction of one percent.

The dose is tiny, the extra water is not

A super absorbent particle pre-soaked in mixing water behaves like a small reservoir. It holds the water at low suction while the paste is still plastic, then gives it back once the internal relative humidity drops below roughly 95 percent and self-desiccation starts pulling moisture out of the capillary pores. The particle does not create water. It moves it, from a place the cement cannot reach into a place it can.

Two numbers govern the whole design. The first is absorption in cement pore solution, not in distilled water. Pore solution is a calcium-rich electrolyte at pH above 12, and a powder that takes 300 g/g in deionised water will take 15 to 25 g/g in that liquor. Anyone who sizes a dose from the distilled-water number will under-dose by a factor of ten. The second is the extra water: for each kilogram of SAP you add roughly 20 to 25 kg of water beyond the mix design, water that is bound inside the particles and never becomes part of the water-to-binder ratio.

Skipping the second number is the classic failure. A ready-mix plant added 1.5 kg/m³ of powder to a bridge mix and changed nothing else. Slump fell from 180 mm to 90 mm inside twenty minutes. The crew added 25 litres per cubic metre at the pump to keep placing, which pushed the effective water-to-binder ratio up and cost about 4 MPa at 28 days. The SAP did nothing wrong. It simply took water that the mix did not have to spare.

Grading and mixing sequence decide uniformity

Most concrete grades sit between 100 and 200 microns. Finer than that, the particles disperse well but raise water demand and drag air into the paste; coarser than 300 microns, you get reservoirs sitting in one spot and a weak zone around them. A narrow cut costs more per tonne and saves admixture, which is usually the better trade on a deck where each cubic metre already carries a high cement load.

You can add the powder dry with the aggregate and extend mixing by ninety seconds, or pre-soak it into a slurry and dose that. Dry addition is simpler and is what most plants do. Pre-soaking gives a more uniform distribution in high-performance mixes, but the gel is thick and needs a wide-mouth pump; a progressive cavity unit handles it, a diaphragm pump does not.

What belongs in the submittal

Ask for absorption measured in synthetic pore solution, desorption at 93 percent relative humidity, and a gradation curve with the fines fraction below 75 microns spelled out. Those three tell you whether the powder will actually release water in a real section rather than just hold it. A free-swell number in tap water tells you almost nothing about a deck.

Also ask how the product affects entrained air. Some surface-treated powders carry surfactant residue that pushes air content up by a point or two, and on a deck with a 5 percent target that is enough to change the finishing window. Run a trial batch at the plant before the first truck, not on the day of the pour.

For the powder itself, the water absorbing polyacrylate granule range lists the gradations used in cementitious work. Before your next deck submittal, write three lines into it: dose by cement weight, extra water in kg/m³, and the pore-solution absorption figure with the test method named. A supplier who cannot fill those three lines is selling you a diaper grade and hoping the concrete does not notice.