FAQ

What is the Xbloc®?

The Xbloc® is a pre-cast concrete structural element used in the construction of certain types of marine structures. It is an innovative interlocking concrete block, made from non-reinforced concrete, designed to protect rubble mound breakwaters and shores over the long term and in extreme conditions. The unit is placed in a single layer as the outer layer [i.e. armour] of the structure.

What is a rubble mound breakwater?

A breakwater is a structure made of natural stone or concrete elements which gives shelter to a shoreline, harbour, anchorage, or basin from the direct impact of incoming waves. A rubble mound breakwater normally consists of a core of small size rock covered with large [heavy] rocks or concrete elements. This outer layer is called the armour layer. An under-layer of rock is provided between the core and the armour layer.

How does an armour layer of Xbloc® units protect the breakwater?

The Xbloc® armour derives its hydraulic stability from its self weight and by interlocking with surrounding units. Due to the highly porous armour layer constructed with Xbloc® units the energy of the incoming waves will be absorbed. The Xbloc® armour layer is therefore able to protect the rock in the under layer from erosion due to waves.

What determines the choice between rock and concrete as armour layer?

For design wave heights below 3 m [significant wave height], rock is often economically viable for use as armour layer. For design wave heights above 4 m extremely large rock sizes are required which are difficult to obtain, handle and place. Limited availability of such material leads to greater expense. In areas where good quality rock is difficult to obtain concrete armour may also be preferred for smaller wave heights.

How big is an Xbloc® unit?

The size of an Xbloc® unit depends on the design wave height expected at the breakwater location. The Xbloc® unit has been developed for use in sizes varying from 0.75 m3 up to 20 m3. Using the design table, provided in the Xbloc® folder or at the internet site, the appropriate unit size can be found, for any design wave height.

What does interlocking mean?

Interlocking is the mechanism by which armour units can hook into each other due to the shape of the unit. This increases the resistance against wave attack. The advantage of units shaped to allow a high degree of interlock over other shapes where less interlocking is possible, is that smaller and lighter elements can be placed. This reduces the cost of a structure as lighter equipment is required for handling and placement and less concrete is required for the armour layer.

On what type of construction are the Xbloc® units applicable?

The Xbloc® units are applicable for all marine structures in which a stable armour layer is required. Examples are breakwaters, groins, shore protection and harbour entrances. The Xbloc® units can be applied for new construction works as well as for rehabilitation of existing structures.

Why are Xbloc® units placed as a single layer?

The armour layer of a breakwater is relatively expensive as it either consists of large size rocks or concrete units. It is therefore economical to reduce the thickness of the armour layer by the application of a single armour layer. In the past concrete armour units were placed in double layers as this was the common practice with rock armour layers. More recent concrete armour units are designed to be placed in a single layer. It was found that a single layer of Xbloc® armour units will result in a highly stable structure. Besides a major saving in the total concrete volume the construction time is also significantly reduced when compared to construction with a double layer of concrete armour units.

Can the Xbloc® system be used by all contractors?

The Xbloc® system has been developed for application by all qualified marine contractors. Normal pre- qualification of contractors is all that is necessary.

What are the advantages of the Xbloc® compared with other units?

The most obvious advantage of the Xbloc® unit is the cost benefit of the unit. Compared to other single layer units, cost savings in the range of 15 % may be anticipated due to the low concrete usage of the Xbloc® unit. Compared to double layer elements the cost savings are much larger. Additional savings are achieved as less units are placed. Furthermore the unit is easier to place than other single layer interlocking units and this results in decreased construction time. Another major advantage over other units is the large hydraulic stability [high interlocking] combined with a highly robust shape.

Does the Xbloc® unit have disadvantages?

Compared to other armour units, the Xbloc® has no disadvantages. The Xbloc® is designed as a balance between good hydraulic stability and still remains a strong and robust unit. If for instance the interlocking were to be increased by making the unit more slender the risk of unit breakage would increase. With all interlocking units concrete quality needs to be well assured as tensile stresses do occur.