Foundation, Concrete and Earthquake Engineering

What is Pounding? Structural and Foundation Issues of Pounding

What is pounding?

Collision of adjacent buildings during earthquake is generally called pounding. This is occurred when they have different dynamic properties and there have not sufficient or no separation distance from adjacent one. The damage due to pounding may be of local and global; local damage is associated with collision force but global damage of buildings depends on their dynamic properties at the time of taking place of collision. Global damages caused by transfer of energy and momentum between two system generated by collision.

Seismic pounding between buildings is one of the most common causes of severe structural damages due to earthquake. Pounding involves movement along or transverse to separation joints provided between neighbor structures and may cause non-structural damages. From the above discussion, seismic pounding between adjacent buildings occur during earthquake when

• Buildings have different dynamic characteristics

• They vibrates out of phase relative to each other

• The separation between buildings at rest is not sufficient

• Insufficient energy dissipation system, if any, to restrain movement within allowable separation distance. 

Why is pounding of buildings concerned?

Past building codes didn’t provide definite recommendation or guidelines to account pounding effect and to counteract this phenomenon. As a result in many densely populated areas to achieve maximum land usage and for economic consideration, many buildings over the world already built extremely close to neighbor and in some region even no space is left. These building are vulnerable to pounding damage during future seismic activity. A large separation distance is not expected from both technical point of view within same facility having large expansion joints and economical point of view considering loss of land. In many cities the highly congested structures become a major concern for pounding damage. This is why, it is now widely accepted that this unexpected phenomenon has to be mitigated or prevented (Abdel Raheem, 2006).

What is seismic gap?

The distance provided between two adjacent building structures is called separation joint; often same facility is divided into two wings and sometime more depending on dimension of buildings. This permits an independent movement of structures relative to each other. A seismic gap is nothing but a separation joint kept to provide room for relative lateral movement due to seismic agitation.

What is gel/space ratio? Its significance in concrete strength prediction

Gel/space ratio

The ratio of volume occupied by hydrated cement paste to the aggregated volume of capillary pores and hydrated cement paste is known as gel/space ratio. It is denoted by r. Powers (1958) found that compressive strength of concrete is 34000 r3 psi (234 r3 MPa) and interestingly he found no influence of mix proportion of concrete and age of it on strength prediction. To realize the definition and significance of gel/space ratio it is required to discuss about volume of hydration product.

Volume of hydration produts

The total space available to occupy by productsof hydration is the summation of absolute volume of fresh cement and the volume of mixing water. Of these, if small loss of water under the contraction of the cement paste and that due to bleeding is ignored, the water consumed by chemical reaction with C2S and C3S was found to be 21 and 24 percent (very roughly) of the mass of two respective silicates. If the final reaction of hydrate C4AF is

The respective figures of C3AF and C3A are 37 and 40 percent. Equation (1) is also vary approximately due to our inadequate knowledge of stoichiometry of the hydration products and cannot be ascertained the amount of chemically combined water. Non-evaporable water determined under specific conditions is considered as 23% if anhydrous cement (measured by mass); in case of type II, moderate sulfate resistant cement, this value may be 18%. The specific gravity of hydration products of cement becomes such that the resulting volume is more than absolute volume of anhydrous cement.
Volume of hydration product of cement
The average value of specific gravity of hydration product in saturated structure, inclusive of pores available in the possible densest structure, is 2.16. Here we are providing a demonstration of calculation of volume change during hydration.

Example 1.0

Mass of cement=100 g
Specific gravity of cement=3.15
That is absolute volume of hydrated cement=100/3.15=31.8 ml
Volume of non-evaporable water= 23ml (23% of mass of cement)
Volume of solid product of hydration of cement=31.8+0.23 X 100 (1-0.254)
=48.9 ml

The cement paste at this condition has characteristic porosity around=28%

That is 

Where wg=volume of gel water 
i.e, wg=48.9X0.28+0.28Xwg
or, wg=19ml

Thus the volume of hydrated cement paste=48.9+19=67.9 ml 

The summary of the example 1 can be drawn as in table-1
Volume of hydration product
From table -1 it can be concluded that
Total volume of water in the mix=23+19=42 ml
Water/cement ration= 42/100=0.42 (by mass)

Water/cement ration= 42/31.8=1.32 (by volume)
Actual volume cement+water=31.8+42=73.8 ml

Volume reduction during hydration=73.8-67.9= 5.9 ml
Volume of hydration products for 1 ml of dry cement=67.9/31.8=2.13 ml

Example 1.0 is ideal condition, where hydration is assumed to occur in sealed container where no water movement is allowed whether into or away from the system. It is interesting to note that reduction in volume by value 5.9 stand for empty capillary pores dispersed in the hydrated cement paste.

Effect of temperature on gel/space ratio

We know a higher temperature at the time of placing of concrete and maintaining or allowing during setting, definitely increase strength at very early stage; followed by often adverse effect on strength at and after 7 days. This is due to rapid initial reaction that occur during hydration leading to formation of hydration product of poorer quality; a poor physical structure having more pores.Thus a significant portion of pores will always leave unfilled. According to gel/space ratio rule, the gel/space ratio will be lower in this case as compared to concrete that is hydrated slowly. The latter one will have less pores, thus resulting higher gel/space ratio.

According to Verbeck and Helmuth, the explanation of low gel/space ratio due to high early temperature and subsequent adverse effects on later strength of concrete, is higher rate of hydration at initial stage under higher temperature inhibit successive hydration. . This is due to an irregular distribution of hydration products in the paste. Diffusion away of the hydration product, in this case, takes inadequate time from cement particles under initial higher temperature and cannot produce uniform precipitation into interstices of paste.

Thus high density of hydration product is produced in vicinity of particles still waiting for hydration reaction or in state of some degree of hydration and inhibit subsequent hydration. As a result gel/space ratio will be less in the interstices and adversely y affects strength of concrete in the long run. Thus the local weakness in the concrete section will permit to progress cracks and results a lower strength of it as a whole. With the help backscattered electron imaging it has been confirmed that porous C-S-H exist in between hydrated cement particles.

Influence of chemical composition of cement

As per chemical composition, only gypsum content of cement is here prime concern. For a given cement, the gypsum content defines shrinkage and setting time of cement; but the gypsum content that is satisfactory in respect of shrinkage is found not adequate for establishing desired setting time. Gypsum is added to cement clinker during manufacturing process of cement and then final grinding of clinker is done. As discussed in above section about temperature, the initial structure produced during setting determines structure of cement paste at upcoming stages of hydration. Thus gel/space ratio is dependent on optimum gypsum content to facilitate hydration of as much as cement particles to gain strength and behavior of concrete under shrinkage and creep are also affected.