Tests for Surface cracking in concrete structures

Problem of concrete cracking

The wing wall to a highway bridge abutment shows random surface cracks and spalling several years after construction.

Aims of testing

The principal aim will be identification of the cause of deterioration followed by an assessment of present and future serviceability. Appointment of blame may follow.

Proposals

Visual inspection of crack patterns, and their development with time, may permit preliminary classification of cause as (a) structural actions, (b) shrinkage, or (c)material deterioration. This may be followed by strength assessment as in A2 if structural actions are suspected, or chemical/petrographic testing if material deterioration is likely. Cores may conveniently be used to provide suitable samples and should be taken from the areas most seriously affected. Chemical testing to detect chlorides or sulfates will be selected according to the crack pattern whilst microscopic examination can check for frost action, alkali/aggregate reaction and entrained air content. Serviceability will be determined on the basis of the extent of deterioration and the ability to prevent worsening of the situation.

Interpretation of testing results

Shrinkage cracks are likely to occur at an early age and follow a recognizable pattern, as do cracks due to structural actions. Material deterioration is therefore indicated in this case, and may be due to chemical attack from internal or external sources or due to frost action. Chloride attack is unlikely since the cracks do not follow the pattern of reinforcement, thus initially test for sulfate and cement content. If the results ofthese tests indicate acceptable levels, petrographic examination will be necessary to attempt to identify aggregate/ alkali attack or frost action. If frost action is indicated, micro-metric examination will yield an estimate of the entrained air content for comparison with the specified value.

Expansion and alkali-immersion tests on cores may be required if alkali/aggregate reaction is found. If future deterioration can be prevented by protection of the concrete from the source of attack, this should be implemented after such cutting out and making good as may be necessary. If the source of deterioration is internal and not of a localized nature it may prove necessary to replace the member once it reaches a condition of being unfit for use.