The Dam That Cracked from the Inside

Rihand Dam in Uttar Pradesh is a 300 MW concrete gravity dam commissioned in 1962. For the first decade, the concrete appeared to perform as designed.

Then the cracking started.

Not surface cracks from thermal stress or drying shrinkage. These were map cracks: a network of interconnected fractures with no preferred direction, covering exposed surfaces in a pattern that resembles dried mud. White and amber gel deposits seeped from the crack intersections. The concrete was expanding from within.

The cause was alkali-aggregate reaction (AAR): a chemical reaction between the alkalis in the cement and reactive silica minerals in the aggregate. The reaction produces a gel that absorbs water, swells, and generates internal pressure that eventually exceeds the tensile strength of the concrete.

At Rihand, the consequences were severe. In one penstock gallery column, 9 of 10 reinforcement bars were found snapped due to the expansion forces. Seepage through the dam body reached 38,000 litres per minute. Structural elements in the powerhouse became inoperable.

Many Indian OPC cements of that era exceeded the 0.6% Na2Oe threshold recommended by ACI for concrete with potentially reactive aggregates, with some reaching 0.8-1.0% or higher. Combined with untested river-bed aggregates containing reactive silica, the conditions for AAR were built into the concrete at the time of mixing.

The rehabilitation programme brought seepage down from 38,000 litres per minute to 60 litres per minute through grouting, HPC lining, and surface protection. But AAR cannot be reversed. The reactive aggregate and gel remain in the concrete permanently.

Rihand is not isolated. Hirakud Dam in Odisha developed AAR after approximately 30 years, with spillway cracks measuring 25 mm wide. Nagarjuna Sagar required crack sealing and HPC lining. Multiple dams in the Bundelkhand region show AAR from reactive local aggregates.

The lesson: AAR takes 10-30 years to produce visible symptoms. By the time map cracking appears, the reaction has been progressing internally for decades. If your dam is over 25 years old and has never had a petrographic examination, the absence of visible symptoms is not evidence that the concrete is sound.

Read more: AAR in Dam Concrete: Prevention and Management →