Foundation, Concrete and Earthquake Engineering

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.

What are the Loads Acting on Marine Concrete Structure?

Concrete has wide variation of use as construction materials, as its behavior can be modified as per design consideration. Concrete can be said ideal material for marine structures where any marine environments like submerged, complete and partial exposure, tropical, temperate and arctic may exist. Marine structures in ocean, coastal or harbor can be constructed economically with concrete offering expected strength and durability. 

Loading of concrete structure on marine environment

The primary limitations of concrete structure in marine environment are basically related to exposure of structure to high periodic bending forces like storm waves of ocean and impacts from collision and ice etc. Before determining the loads acting on marine structure a consideration on environment is required which must include waves, winds, seismic effects, ice and currents. The last four terms are more significant for fixed off-shore structures then floating vessels. This information about environment then can be used to determine loads acting on marine structure. The loads that should be considered in designing of marine structure are:

Pressure and submergence

At first let us discuss about floating concrete structure; the structure that are considered as floating, the dead weight of it is approximately compensated by the buoyancy. When a structure is submerged in sea water it is subjected to continuous hydrostatic pressure. These hydrostatic pressure definitely exerts loads on structure additionally it force water to penetrate into the concrete through Through any weak point, if any. Microcracking in concrete is very common; though durability is not concern of this post, the weight of concrete will be increased with this penetration leading an increase in density by 2% upon absorption, which may increase the net buoyant weight up to 40 to 50 percent.

Impact loads from debris and ice

Moving debris and ice may produce impact loads directly on concrete structures. These materials may accumulate on structure and increase effective area, thus exert additional load. When ice is accumulated as solid sheet, it will fail by crushing. In this case ice breakers should be installed to pile up ices until it fails in tension. 

Debris is that mass that can float in flow path with an inundation depth Which cannot withstand water flow. When debris are carried with tsunami wave, they may be trees, cars, wooden poles used for utility supply, complete or fraction of timber framed houses. Sometimes some debris that don’t float may transported by wave when they are very strong like pieces of concrete and boulders. In normal case, debris are bottles, waste of ships, cans, fishing equipment, automobile tires, nets and abandoned fishing line.

Debris and ice cakes often become wedge within adjacent piles which exerts torsion and may lead to break of piles. An example may be included here, broke down of better piles adjacent to piles where large mass of ice were accumulated; during spring the mass was thawed and at low tide it slide down to exert load to that piles. Break water may be also effected by formation of ice cakes resulting wedging units apart from each other and during high tide they are lifted up.

It seems that the loading of debris and ice is same on different structural type like concrete, steel and timber, but the consequences of it are significantly different, as it depends on physical and structural characteristics in many cases. A concrete pile may be more susceptible to failure under impact and its resultant torsion whereas reinforced concrete sea walls or caissons often have more resistance as they possess local rigidity, mass and to some extent have ductility at low temperature.

When water is allowed to enter inside concrete structure and get frozen it will exerts a bursting pressure. If a pile or caisson have hollow-core, must have means to impede freezing like providing Styrofoam or wood log to float side and/or provision to connect with unfrozen water beneath surface like vents. If such precaution is not taken serious bursting and subsequent failure may occur.

Load exerted by current

Movement of the sea water produces different types of loads on the concrete structure which are required to evaluate overall stability of marine structure.

Nature of current

Speed of current in ocean usually run from 0.5 nautical miles per hour (knot) which attain maximum value of about 2 knots. In restricted channel or narrow inlets, even higher velocity may occur.