Structural Integrity

What is meant by structural integrity?

Structural integrity is the robustness or strength of a single unit: the resistance of an armour unit against forces due to handling, placement and movement under extreme wave attack [rocking].

How has the structural integrity of the Xbloc® been determined?

The structural integrity of the Xbloc® has been determined by Finite Element [FE] calculations. With the aid of FE calculations using standard load cases the tensile stresses in the Xbloc® unit and other single layer armour units were determined. Furthermore prototype drop tests have been performed to check the validity of the calculations.

How does the strength of the Xbloc® unit compare to other units?

From both Finite Element calculations as well as prototype drop tests it can be concluded that the maximum tensile stresses in the Xbloc® unit are certainly no greater than the tensile stresses in other single layer units. Due to its shape the Xbloc® is a robust unit. This requires the use of standard non- reinforced concrete with a minimum concrete tensile strength (splitting) of only 2.5 N/mm2. Other modern single layer armour units require concrete with a higher minimum tensile strength.

Why is this strength so important for a unit?

During handling and placement the unit must be strong enough to cope with the stresses imposed. Once placed on the breakwater an armour unit is subject to wave forces which can be extremely high during storms. Extreme wave action can cause small movements [i.e. rocking] of units. In the past this caused failure of other proprietary armour units and subsequent failure of the whole breakwater. An armour unit should therefore be strong enough to resist all loads that may be expected during the lifetime of the structure.

What are the weak spots of the unit considering breakage of the unit?

Following from the finite element calculations for certain standard loads it was found that the highest tensile stresses in the concrete occur at the transition between the X-shaped base and the nose. In order to decrease the tensile forces a chamfer or fillet has been introduced between the base and the nose. The prototype drop tests were however performed with units where no chamfer or fillet had been incorporated and even without the improvement the strength of the unit was excellent.

What kind of damage does occur during droptests?

In the tests it was found that the sharp edges of the concrete will crush, leading to a negligible decrease of the unit mass.

At large fall heights of approximately 4m on to a rigid concrete base a nose or a leg broke off, depending on the particular orientation of the drop. Stresses inflicting this kind of damage are not expected to occur during the life-time of an Xbloc®.

Is salty water a threat to the Xbloc® concrete?

Saltwater penetration is a threat to reinforced concrete, as the steel will corrode easier in a saline environment. The Xbloc® as is made of non-reinforced concrete, which performs very well in a marine environment provided that other properties (minimum cement content; water/cement ratio) comply with normal code requirements. There are various examples where non-reinforced concrete has been made using seawater that are known to be very durable.

Does the Xbloc® consist of reinforced concrete?

The Xbloc® consists of non-reinforced concrete, similar to other single layer armour units.

What grade of concrete should be used?

As the Xbloc® has a robust shape, a standard C25/30 (Eurocode) concrete quality with a minimum tensile strength of 2.5 MPa (tensile splitting strength) at 28 days can be used.

Can heavy aggregates be used in the concrete?

Use of heavy aggregates will increase the specific weight of the concrete which means that smaller units can be used. However this is only economical if the aggregates are readily available as hydraulic stability of the element is not related to unit weight only.

What is the risk of cracking due to heat development during hydratation?

Calculations have been made with a Finite Element Model which simulates concrete curing. These calculations have been calibrated using measurements taken during curing of a 3 m3 unit. The calculations show that hydratation cracking does not occur.  Calculations have been carried out for units up to 20 m3. For very large units in adverse environmental conditions Delta Marine Consultants can perform a numerical analysis to check whether special measures are required to prevent hydratation cracking.

For several projects with different sizes of Xblocs, temperature measurements have been carried out with thermo couples during the hardening of the fresh concrete. The temperature data show limited differential temperatures.

Is it possible to use concrete with a lower quality, since the Xbloc® is a structurally strong unit?

The use of a lower concrete quality than specified is not recommended as the tensile strength is reduced. Neither is a higher quality concrete recommended, since this material may also display more brittle behaviour.