foundation system of burj al arab

Defects of Slurry Displacement Pile

Slurry displacement piles are cast in drilled hole pile (CIDH) where drilling fluid is used concurrently with drilling operation to excavate drill hole. Drilling fluid is a slurry introduced in drilling to prevent caving in unstable ground and to restrict groundwater to penetrate into drilled hole.

The slurry remains in drilled hole is displaced by concrete that are delivered by rigid supply tube and placed below drilling slurry. The causes of defects in slurry displaced piles are:

a. Settlement of materials form suspension:

One of the main causes of pile defects is problem associated with settlement of materials at drill hole. The concerned materials are that held during drilling within the suspension of slurry and settled from suspension either during or before placement of concreting.

These materials may also be come into suspension as a result of inadequate or improper cleaning of base of drilled hole. Materials can be entrapped at the below tip of pile with placement of concrete as shown as Fig-1 or can be covered and lifted by fluid concrete to get caught by cage of reinforcing bar of pile or caught against walls of drilled hole but not displaced by fluid concrete as shown in Fig-2.

Excess settled materials are observed in mineral slurries which are not cleaned properly or agitated and contain excess amount of materials and carried with suspension.

Such excess amount of materials can also occur in slurries of synthetic origin if these slurry contaminated by the encapsulation of clay particles or when enough time is not provide to settle materials out prior to final cleaning of bottom of drilled hole.

Causes of Microcracking in Concrete

Microcracking in concrete is very important issue which is formed in microstructure. The causes of microcracking are manifolds; in simple word, the main causes are:

• Large variation in mechanical properties of constituents (specially hydrated cement paste and coarse aggregate)

• Thermal and shrinkage movement

The first one is the main cause; as this cracking is formed at interface between mortar and embedded aggregate.When failure is initiated the cracking pattern follows this interface. This zone is known as interface zone, sometimes also called transition zone.

Behavior of transition zone has to be realized to find cause of microcracking. You should keep in mind that microstructure of bulk cement paste is quite different form hydrated cement paste around coarse aggregate.

The main cause behind this is that dry particles of cement, during mixing, cannot be reached to comparatively large aggregate particles i.e. cannot be packed closely. Thus less cement particles are available to be hydrated and filled the actual voids present due to improper packing which yields a zone of higher porosity than that of bulk cement paste hydrated at relatively greater distance; thus high porosity arises in interface zone.

The higher the porosity, the lower will be the strength. Let take a look at microstructure of interface zone:

• Immediate around aggregate surface, there has an about 0.5 µm thick layer layer comprising of oriented crystalized calcium hydroxide.

• Behind that there has identical thickness of C-S-H layer which is known as duplex film.

• Next layer is 50 µm thick, this layer is the actual interface zone which contains hydration products of cement having lager crystals of calcium hydroxide; but no unhydrated cement particles are found there.

The significance of furthermost layer is two folds; these are

  1. Absence of unhydrated cement means complete hydration of the same which indicates that water/cement ratio at the interface zone is higher than other portion.