CONCRETE
CANCER
EXPLAINED.
Alkali-Silica Reaction is a chemical reaction that works inside concrete, out of sight, long before anything shows at the surface. Here is what it is and how it works.
Alkali silica reaction is an internal expansive reaction between reactive silica in the aggregate and alkalis in the cement paste. It is the most serious mechanism of concrete failure, and it is sometimes called concrete cancer.
ASR is a mechanism, not a symptom. Concrete fails. Map cracking, spalling and dimensional movement are what an owner sees on the surface. The reaction is what is producing them. Every term on this page is defined once, as a standalone sentence, in the PSIP definitions, and the wider failure picture is at The Problem.
WHAT IS ACTUALLY
HAPPENING
INSIDE YOUR CONCRETE
Concrete cancer, formally called Alkali-Silica Reaction (ASR), is not a manufacturing defect, a cheap contractor, or bad luck. It is chemistry. Specifically, it is a reaction between two materials that are both inherent to concrete: the alkali hydroxides (sodium and potassium) in Portland cement paste, and the reactive amorphous silica present in many common aggregates.
When water is present, which it always is in pools, driveways, and any structure exposed to weather, these materials react. The result is a reaction product: sodium silicate hydrate. As that product precipitates inside the confined space of a concrete aggregate particle, it generates solidification pressure. The reaction occurs in aqueous pore solution throughout, but the pressure does not come from the product absorbing water.
Research using the extended Surface Force Apparatus measured a solidification pressure of 6 to 13 MPa as ASR products precipitate in confined conditions. The authors identify that solidification pressure, and not water absorption by the reaction product, as the mechanism that cracks aggregate particles and expands ASR affected concrete. This is a laboratory model of an aggregate's internal surfaces, not a field measurement.
ASR NEEDS THREE
THINGS. ALL THREE
HAVE TO BE THERE.
Per ACI PRC-221.1-98 and the American Concrete Institute's documented research, ASR requires exactly three conditions.
Amorphous (non-crystalline) silica is present in most aggregate sources, quartzite, chert, opaline materials, volcanic glasses, and certain types of sand.
Portland cement produces a highly alkaline pore solution. The alkali content, primarily sodium oxide (Na₂O) and potassium oxide (K₂O), is inherent to the cement manufacturing process. ASTM C150/C150M designates cements with more than 0.6% Na₂O equivalent as high-alkali cements. The alkalis that drive ASR are not contamination, they are a fundamental product of cement chemistry.
Water is the catalyst. Per peer-reviewed research published in PMC (2024), ASR stops when internal relative humidity drops below 80%. The reaction only needs enough time and enough water to do permanent damage.
THE IRREVERSIBILITY PROBLEM: Once ASR begins, the alkali hydroxides (NaOH and KOH) are continuously regenerated by the reaction itself, they react with calcium hydroxide (portlandite) in the cement paste, which releases them back into solution to attack more silica.
FIVE FRAMES.
CHEMISTRY IN PROGRESS.
Continuing from the interfacial transition zone shown above, the active alkali-silica reaction proceeds through a documented chemical sequence.
The five frames below visualize each step, from the alkali pore solution filling the open interfacial space, to silica attack, to gel formation, to pressure expansion, to the bond failure that initiates surface-visible cracking.
WHAT ASR
LOOKS LIKE
ASR produces a distinctive cracking pattern that sets it apart from other causes of cracking. Normal shrinkage cracks follow straight lines along control joints or slab edges. Freeze-thaw damage produces surface scaling. ASR looks different.
Random, multi-directional cracking that resembles a road map or alligator skin. Unlike shrinkage cracks that follow straight lines, ASR cracks radiate in all directions from aggregate particles. This pattern is caused by expansion occurring simultaneously throughout the concrete matrix rather than at a specific stress concentration point.
A white, gel-like substance that appears wet or glistening at crack faces. This is the sodium silicate hydrate gel that has expanded beyond the aggregate boundary and pushed through cracks to the surface. In pools, this substance may be washed away by water, but its presence in a core sample confirms ASR definitively. Lab confirmation requires petrographic analysis of 30-micron thin sections under polarized light.
Visible lifting or displacement at crack edges, one side of a crack is higher than the other. This occurs because the aggregate expansion is creating net outward pressure that causes sections of concrete to shift relative to adjacent sections. In pool shells, this manifests as tile loss, coping displacement, and structural failure at wall-to-floor transitions.
Studies published in the Concrete Society's Magazine (2024) report up to 35% loss in compressive strength and up to 24% loss in tensile strength once ASR gel reaches the concrete surface. The critical problem: by the time surface symptoms appear, significant internal damage has already occurred.
ASR-cracked concrete absorbs water far more readily than intact concrete. In Tennessee and the mountain South, this means that winter freeze-thaw cycles operate inside a concrete matrix that ASR has already compromised. The two failure mechanisms accelerate each other. What might have been surface scaling becomes structural failure.
ASR is definitively confirmed by petrographic examination of thin concrete sections (30 microns) under plane-polarised and ultraviolet light, identifying ASR gel in aggregate pores. X-ray diffraction (XRD) can quantify crystalline ASR products. ASTM C1293/C1293M (2-year concrete prism test) and ASTM C1260 (16-day mortar bar test) are the standard methods for new aggregate evaluation. Core drilling from an affected structure followed by lab analysis is the only definitive confirmation method.
THE SUPPLY CHANGED.
NOBODY HAD TO
WRITE DOWN THE MIX.
ASR has been documented since Thomas Stanton's foundational research in 1940. Fly ash was one of the materials used to reduce ASR risk.
4,273 megawatts of Texas coal capacity retired in 2018: Monticello, Big Brown and Sandow, per EIA generator data. They generated fly ash. When they closed, that supply shrank. A ready-mix producer facing that smaller supply had two paths. Reformulate the mix with an alternative supplementary cementitious material, or place concrete without it. This is not about anyone's bad faith. There was no requirement anywhere in the chain to document that the chemistry had changed. Not on the batch ticket. Not in the contract. Not at the pour.
PUBLISHED IN ASTM: "It is well chronicled that coal fly ash can mitigate the risk of alkali-silica reaction (ASR) in concrete structures." The mechanism: fly ash dilutes pore solution alkalis, consumes calcium hydroxide (portlandite) through pozzolanic reaction, and reduces the availability of alkalis for ASR.
ASR incubates, and it can take years to show at the surface. By the time it does, the question is what was in the mix. If nobody wrote it down at the pour, nobody can say.
Texas in 2018 was not the last of it. 2.6 GW of coal capacity retired in 2025, per EIA. Fly ash used in concrete rose from 11.9 million tons in 2023 to 14.6 million tons in 2024 (American Coal Ash Association, 2024 Production and Use Survey, released 4 December 2025). A national total does not say what went into any one pour.
WHAT THE
BATCH TICKET SHOWS.
These are the supplementary cementitious materials and admixtures a batch ticket may list. A PSIP record holds a photograph of the batch ticket for every delivery. The photograph records what the ticket shows. Tickets vary, and many do not list everything in the load. PSIP does not pull values out of the photograph. The contractor adds what he knows in the notes, recorded as his own statement.
ASTM C618 covers Class F and Class C fly ash and Class N natural pozzolan, such as calcined clay, shale or volcanic pozzolan. It separates Class F (low calcium, high silica) from Class C, and they are not interchangeable at the same replacement percentage. Where the ticket does not name the class, the contractor adds what he knows in the notes. Where the ticket says Class C, the photograph shows Class C.
Ground granulated blast furnace slag, specified under ASTM C989/C989M. The batch ticket photograph shows it when the ticket lists it.
Colloidal nano silica is an admixture, batched at the plant. If the ticket does not list it, the contractor records what was added in the notes, as his own statement.
The ticket may list no SCM, and the contractor may have nothing to add. If the line is blank, that is what the record says. A verbal assurance is not a record. PSIP does not stop a pour.
PSIP does not recommend a material. Not a class, not a replacement level, not a brand. Those are mix design decisions that belong to the people specifying concrete for your site and your exposure.
When you buy a bottle of supplements, the label tells you what's in it. A concrete batch ticket is the closest thing your concrete has: the ready-mix producer's own record of what was delivered. Unlike a supplement label, it may not list everything in the load. If your contractor can't show you the batch ticket, you have nothing on paper about what's in the structure you just paid for. A PSIP record holds a photograph of that ticket, with the contractor's notes on what it does not show.
WHAT YOU DO WITH
THIS INFORMATION
DEFINES EVERYTHING.
You just read the science of concrete cancer. What was in your mix is the one part of it nobody can see later. That is the reason to ask for the record.
QUESTIONS TO ASK
BEFORE YOU SIGN
WHAT YOU'RE CARRYING
RIGHT NOW
ASR CANNOT BE
REVERSED.
THE MIX CAN BE RECORDED.
What was in the mix can only be recorded at the pour. That is what a PSIP record is for: the batch ticket as photographed and the contractor's own notes, made at the time. PSIP records the work and certifies that it was recorded. The contractor is responsible for local building codes, permits and licensing. PSIP certification is not an inspection, a code approval or a warranty.