Road Works

Concrete crash barriers – its application

Concrete crash barriers are not considered as the best barrier design because of the following reasons when compared with flexible barriers:

(i) Concrete barriers possess rough surface which, when impacted by moving vehicles, tend to cause considerable damage to the vehicles.

(ii) Since concrete is a rigid material and the deceleration of collided vehicles is comparatively large when compared with flexible barriers.

However, concrete crash barriers have particular application in locations where the deflection of barriers is not allowed. For instance, in the central divider of a carriageway, if flexible barriers are adopted and vehicles crush into the barriers, the deformation resulting from the hitting of vehicles would result in an intrusion to the adjacent carriageway. This is undesirable because this may trigger further collisions in the adjacent carriageway and hence rigid barriers like concrete crash barriers should be adopted in this scenario.

Direction of placing the main weight of reinforcement in concrete pavement

The reinforcement of concrete pavement is usually in the form of long mesh type. A road usually has length is generally much longer than its width and therefore cracking in the transverse direction has to be catered for in design. Reinforcement is required in the longitudinal direction to limit transverse cracking while transverse steel acts to provide rigidity to support the mesh fabrics. For long mesh in concrete slab, the main weight of reinforcement should be placed in the critical direction (i.e. longitudinal direction) to control cracking. However, if the concrete road is quite wide, certain reinforcement has to be placed in the transverse direction in this case to control longitudinal cracking.

Function of waterproof (or separation) membrane for concrete carriageway

A layer of waterproof (or separation) membrane is normally placed between sub-base and concrete slab for the following reasons:

(i) It prevents the loss of water from cement paste which affects the strength of concrete slab.

(ii) It enhances the movement of concrete slab relative to sub-base layer and reduces the frictional forces developed at their interface.

(iii)It avoids the possibility of active aggressive agents from soil water being attached to the concrete slab.

(iv) It prevents the intermixing of freshly placed concrete with loose materials on the surface of sub-base.