Cracks in Tank Walls

Hotline Question

I recently placed a cast-in-place concrete tank wall that was 20 feet high, 18 inches thick, and 150 feet long. The wall is tied into the footing with #10 dowels on 6-inch centers. The vertical bars are #10, also on 6-inch centers, and the horizontal bars and #4 on 12-inch centers. There are no control joints in this placement.

The formwork was stripped three days after placement. During stripping, we noticed vertical hairline cracks in the center of the wall. What caused these cracks? What is the best way to repair them?

Hotline Answer

Troubleshooting cracks is often difficult. Portland cement concrete shrinks; if this shrinking is resisted by internal or external means, the concrete will crack. Some contractors learn this the hard way when some of their concrete work ends up with unsightly cracks.

Troubleshooting what caused the crack often leads to the question of what caused the concrete to shrink. One important clue is time. Did the concrete crack at a relatively young age? Within three days, plastic settlement, plastic shrinkage, or early thermal contraction can occur.

Plastic settlement cracks occur only when there is a relatively high amount of bleeding and settlement and some form of restraint to the settlement. Plastic-shrinkage cracks occur when the rate of evaporation exceeds the rate of bleeding. Early thermal contraction is due to the temperature difference between the concrete and the air.

Early cracking is often thought to be due to long-term drying shrinkage. However, this usually is not the case. Most shrinkage cracks are initially formed by early thermal contraction. Later, drying shrinkage may enlarge these existing cracks. In water-retaining structures, drying shrinkage usually stops after they are put into service due to the presence of the water.

If the tank will be lined, usually no repair is needed. If no lining will be used, cracks that are 0.004 inches or less are normally considered waterproof. Often cracks will heal autogenously- that is, the further hydration of the cement or the formation of calcium carbonate will seal the crack. However, the water seepage due to autogenous healing can leave unsightly stains.

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