Self-healing strain hardening concrete

Demonstrator Bridge TU Delft

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In the year 2006, Delft University of Technology started researching ‘Strain hardening Self-healing concrete’ (SSC). The reason for this research was the desire to increase the durability of concrete. This depends mostly on the reinforcement.
To ensure a certain lifespan of concrete structures, a minimum cover depth has to be applied. However, cracks occur in the concrete and cause a significant decrease in durability.
Research by M. Sierra-Beltran in 2014 resulted in SSC that has the capacity to seal these cracks due to the addition of a healing agent. Also, fibers are added to keep the cracks small and speed up the healing process.

In this thesis SSC is compared with traditional concrete (TC). A structural design is made with both materials based on assumed characteristics. Also, tests are performed to check whether those assumptions are right and a short calculation with the results is made.

When making a structural design with the assumed characteristics for TC and SSC, one finds that it is possible to design more slenderly with SSC than with TC. This is caused by the decrease in cover depth and the absence of extra reinforcement to control the crack width. The only exception is the height of the bridge deck; the height of the bridge deck is smaller in a TC design, caused by the low Young’s modulus of SSC.
The positive effect of SSC is expected to be most noticeable on the bridge deck at the locations with negative bending moments. Other locations where the positive effects can be noticed are at the beams and the tensile bars.
The material costs of the bridge designed with SSC are significantly higher than the material costs of the bridge designed with TC. However, it is expected that the maintenance costs of the SSC bridge are much lower.

The results of the tests for the structural characteristics show that SSC mortar has a lower compressive strength and a lower Young’s modulus than TC mortar. Especially, the lower Young’s modulus affects the structural design. The flexural strength of SSC mortar is higher than the flexural strength of TC.

The tests on durability give positive results. The chloride migration shows that, before healing, the SSC and TC have similar migration coefficients. However, after the healing period the migration coefficient of the SSC is lower than the migration coefficient of TC, at the location of the crack as well as the uncracked surface. Although, the difference is not as large as expected, this might change with longer healing periods. Therefore, it is recommended to do more tests with longer healing periods.

When making a short calculation with the characteristics found in the results of the test, one sees that the height of the bridge deck and the beams increases a bit more than assumed due to the lower Young’s modulus. Furthermore, no large changes are found.

Therefore, it is recommended to research the possibility of changing the mixture in a way that the Young’s modulus is increased.