foundation system of burj al arab

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

Trail Embankment for Preloading

Detailed investigation is required for a preloading project which may involve penetration testing (may include piezocone), boring, sampling and extensive laboratory experiments. The especial emphasis should be given to

• Presence of microlayer that are permeable i.e. stratification of ground underneath project

• Drainage conditions below and above the consolidation susceptible clay layer

• Stress-strain and strength characteristics i.e. stress history

• Permeability and consolidation characteristics both in horizontal and vertical directions. 

Trail embankmentHorizontal co-efficient of consolidation, Ch is required to design vertical drains. Ch determination usually is not considered part of regular foundation investigations.

This can be determined in laboratory or in field. The in-situ methods are:

• Piezocone test introduced by Torstensson (1975)

• Permeability test: constant/falling head tests can be used to determine both Cv and Ch (Wilkinson,1968)

• Consolidation (Clarke et al.,1979)

While preloading with or without drains some uncertainties are found during installation due to wrong assumption about soil based on disturbed sample and different non-ideal conditions during installation of drains. Trail embankment is constructed to find out feasibility of the preloading project considering disturbance and malfunctioning or misusing of installing equipment of vertical drain.

It provides us an option to directly evaluate, in addition, various types or spacing patterns of drains. Guidance about bearing capacity of soil can also be found from trail embankment.

While construction of trail embankment, one portion of it may be intentionally overbuilt to produce shear failure; But good knowledge is very important about site conditions in order to determine representative test location for whole project.

A minimum monitoring for six month of trail embankment is required, as per Forrester (1982). If it is not possible, the construction of trail embankment is considered worthless. According to report of Forrester, following recommendations can be concluded for trail embankment:

1. Location of trail embankment has to be selected in such way that it becomes a part of earth structure that will be used as final construction. Thus extra costs are saved and project becomes economical.

2. The shape of trail embankment has to be selected so that it follow the assumption of drainage and strain conditions of final structure

3. When trail embankment is to check feasibility of land reclamation project, the fill have to be placed in such way that increase in linear stress is imposed over wide area. This is achieved by providing width across top of embankment equal or more than depth of underneath soft soil. But where it is not possible or feasible, analysis of results of preloading test to predict future performance of structures of different sizes may become more complex. 

4. Height of fill for trail embankment has to be ideally equal to height of actual embankment to be constructed. This is done to render similar stress condition on soft soil as co-efficient of consolidation and permeability of it reduced with increase in applied load.

5. Slip-circle analysis and bearing capacity have to check to ensure safety of trail embankment to avoid any type of failure.