Concrete’s cobra effect: unintended results of embodied carbon reduction

Silicate curing

There are many different ways to cure concrete, but the vast majority of concrete is chemically cured topically with some variety of silicate. This may be a sodium, lithium, or other type, but they are usually spray applied to the surface of the concrete in an effort to keep water in the slab to facilitate curing. Despite the many different forms and various new brands of silicate curing products that seem to come out each year, silicates are actually very old. They are also a highly inefficient technology for curing. Potassium and sodium silicates themselves were first discovered between the end of the dark ages and the beginning of the renaissance by alchemists such as Giambattista della Porta.

In more modern commercial concrete applications, the first documented use was by William Michaels describing  silicate’s-based hardening characteristics in 1893. Silicate cure and seal style approaches gained more acceptance in the early middle part of last century, particularly after the publication of Dr. James Vale’s Soluble Silicates.12 However, because of the lack of curing performance the American Concrete Institute (ACI)’s endorsement of sodium silicates was withdrawn in 1949 (ASTM-C111). The newer concrete material section 5.2 of ACI 301-16 also states “silicate-based liquid surface densifiers are prohibited curing compounds.” Despite their significant lack of performance , silicates continue to populate specifications, making them a requirement on job sites all over world.

Silicates do react with available free lime and calcium to form additional C-S-H with the surface of the pores, raise abrasion resistance, and reduce some of the permeability, but not sufficient enough for proper curing. Dr. Tyler Ley, professor of structural engineering at Oklahoma State University, created a short video13 sharing his research on comparative curing methods with lithium silicate curing rating only slightly above not curing at all with regards to both mass loss (stopping the damaging physical manifestations of volume loss, such as cracking, curling, and undulation) and chloride penetration and reduced strength and serviceability due to increased corrosion in any matrix reinforced with steel.14 ACI 302.1R-15, Guide to Concrete Slab and Floor Construction states silicates do not meet the requirements for ASTM C309 or ASTM C1315 for liquid membrane forming compounds on fresh concrete.

Combining low-carbon mix designs with silicate curing

Taking a holistic look at using new generation low-carbon mix designs used in conjunction with 100-year-old approaches to curing, the industry is aware they are doomed to fail in the field. Less water is being kept in the slab and the non-cement pozzolans, now making up more and more of the mix design, are gobbling up whatever water is left or simply not reacting at all. This does not just create a more porous, weaker concrete, with horrible curling and dry-shrinkage cracks, it also consumes time. Owner groups are being told they must anticipate longer construction schedules if they want to remove meaningful amounts of cement.

Extended curing schedules demand more energy from the grid to operate cranes, and stacked lighting. The cracked, porous floors have a shorter life-cycle on top of needing additional repairs to joints, and concrete in the form of epoxies, additional densifier coats, toppings and overlays to make up for deflected or spalled concrete. These longer schedules affect not only operational costs, but also construction financing and insurance making it further cost prohibitive.

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