Concrete armour units for breakwaters: A study on the structural integrity

Concrete armour units for breakwaters: A study on the structural integrity

Fozein Kwanke, N.J.C.

Walraven, J.C. (mentor)
Fraaij, A.L.A. (mentor)
Bouwmeester, W.J. (mentor)
Koenders, E.A.B. (mentor)

Civil Engineering and Geosciences

2007-11-01

One of the most basic types of breakwaters is the rubble mound breakwater which in essence is a heap of stones consisting of a core of fine material covered by an armour layer of big rock or concrete elements. Mostly unreinforced, these concrete elements exist in different sizes and geometries varying from massive concrete cubes to more complicated shapes like Dolos, Tetrapode, Accropode or Xbloc. The reaction of cement with water is exothermic and an unequal heating up and cooling down of the massive units can lead to too high tensile stresses which lead to cracking in case the actual tensile strength of the concrete is exceeded. Therefore, in this thesis, the main focus is on the production and hardening process of the units. What internal mechanisms during the hardening stage might lead to cracking and/or breakage of these colossal concrete armour units? For six different concrete mixes, the thermal and strength properties were experimentally determined and analyzed. The found results were used as input for a numerical hardening model. The model permits to give an estimation of the expected temperature rise, the eigenstress development as well as an indication of the possible cracking trajectories. Using the probabilistic approach of Van Breugel, an assessment on the risk of failure could be done at material level. A further analysis using the Mohr-Coulomb criterion gave also the possibility to evaluate possible breakage patterns at element level. For a reference mix, a parametrical study was done regarding the influence of specific hardening circumstances like type of formwork, moment of removal or the type of mix used. In a last step, the implemented hardening model has been used to analyze a specific case-study in the Netherlands where concrete armour units are severely damaged. The conducted research put in evidence the importance of maintaining the eigenstresses at low values in order to reduce considerably the risk on cracking. The different parametrical studies showed that different factors play a prominent role in getting a higher or lower risk of cracking, namely: the choice of a specific type of formwork, the demoulding stage, the sizes of the elements and the casting and environmental temperatures. Moreover, the implementation of the model to other concrete mixes, revealed the importance of the mix choice on the temperature and stress development. In case for instance fillers or binders are used to replace partly the needed amount of cement, it is important to make a thorough study on their quality and reactivity. It can be concluded the elaborated hardening model gives a good indication of expected temperature rise, stresses and cracking patterns and that the adopted calculation methods give a good indication of weak and critical points in the elements and possible cracking and breakage patterns and mechanisms. The results show high probabilities on crack occurrence and crack patterns that agree very well with the block damages observed at the practical case in the Netherlands. Further investigations regarding the influence of the shape, an analysis on micro scale of weak points in the structure and the effect of the quality of binders are recommended to get a broader scope and to extend the hardening model which was set-up. Moreover, it is of crucial importance designers integrate all together the aspects from concrete technology and hydraulic/ coastal engineering point of views to come up with a final design for concrete armour units of breakwaters.

breakwater
armour unit
concrete

http://resolver.tudelft.nl/uuid:1422935a-3408-4e11-bebc-667d289487b3

TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering

Student theses

master thesis

(c) 2007 N.J.C. Fozein Kwanke