Gel Space Ratio in Concrete – All You Need to Know | Is It More Reliable than W/C Ratio?

Gel space ratio in concrete is a measure of voids in concrete. It can be taken as an alternative
parameter to the porosity of concrete.

What is Gel Space Ratio?

Gel space ratio is the ratio of the volume of cement gel to sum of the volume of cement gel and
that of the capillary pores.

Gel/Space Ratio Formula

From the above equation, we can notice that the gel space ratio takes into account the pores in
the concrete. Hence, it can be used as an option to porosity.

Abrams Law and Gel/Space Ratio

Many researchers have questioned the validity of Abrams concept as law. They argued that it
does not have the qualifications necessary to call it a law. Rather it can be considered as a rule.
These researchers presented that the w/c ratio law has many limitations. These limitations
include the dependency of strength on various factors as given below:

Strength at any water cement ratio depends on the following criteria:

• Degree of hydration of cement
• Chemical and physical properties of concrete
• Temperature at which the hydration of cement is done
• Change in effective w/c ratio
• Bleeding and shrinkage in concrete
• Air content; if it is entrained concrete

With an increase in the w/c ratio, the strength decreases. To know the type of relationship between
the w/c ratio with the strength of concrete, Read – Relation between W/c Ratio & Strength.

Another limitation of the w/c ratio law is that it is good for determining strength for 28 days only.
And that too for a fully compacted concrete.
In contrast, the gel space ratio is independent of the age of concrete. It can be calculated at any
age of concrete and for any fraction of hydrated cement.
In gel/space ratio, the strength of concrete is related to the solid products of hydration of cement
with respect to the space available for the formation of these products. Hence, it was concluded
that the strength relates to solid products of hydration to the space available for the formation of
the products.

Relation Between Gel/space Ratio and Compressive Strength

Powerr and Brownyard worked to establish a relation between gel space ratio and compressive
strength.
Powerr carried out an experiment and proved that the relation between the strength of concrete
and gel space ratio-

This relation given by Powerr holds true for ideal cases.
A typical curve showing the relationship between gel space ratio and compressive strength is
given below:

The relationship between strength & w/c ratio of a fully compacted concrete holds true for 28
days. The relationship between strength & gel space ratio does not depend on the age of
concrete. g/s ratio can be calculated for any fraction of hydrated cement.

Importance of Gel/Space Ratio

Importance of gel/space ratio is listed below:

• Strength measured using gel/space ratio takes compaction into account. So, the volume of capillary pores is considered
• It is used to determine the compressive strength of cement at any age
• Strength can be found even for the fraction of cement that is hydrated

Is Actual Strength Same as Theoretical Strength?

No, actual strength is always less than theoretical strength. The reason is explained below:

• Theoretical strength is estimated based on the molecular cohesion and surface energy of a solid.
• This solid is assumed as perfectly homogenous and flawless.
• While in actual condition, the ideal conditions assumed above doesn’t exist.
• The flaws present in concrete give rise to increased stress concentration in the material of the concrete. This stress is very high than the average stress on the material. (Average stress is taken across the cross-section of material as a whole.)
• The stress concentration takes place in and around the flaw of concrete. This results in the fracture of material around the flaw.

The flaws in concrete vary in size. The more the size of the flaw is, the more is the stress
concentration in the material. Some of the flaws of cement paste in concrete arise from
discontinuities caused by:

1. Voids
2. Fissures
3. Bleeding channels
4. Rupture of bond because of drying shrinkage and temperature stresses

The flaw of concrete results in the failure of the concrete as concrete is a brittle material.

How is Gel Space Ratio Calculated?

Gel space ratio can be calculated for complete as well as partial hydration. To calculate the gel
space ratio, apply the following formulae:

1. For Complete Hydration

Let us assume that 1 ml of cement gives 2.06 ml of gel upon hydration.

Therefore,

Hence,

2. For Partial Hydration

and

and

and

Therefore,

The above strength represents the theoretical strength of concrete.

Gel Space Ratio Problems (Limitations)

The theory of gel space ratio has the following limitation-

The actual strength is much less than the theoretical strength. This is because of the assumption in the theory that concrete is homogenous and flawless. These flaws decrease the strength of concrete.

Griffith’ Theory

Griffith worked upon the limitations of gel space ratio theory and postulated a theory. His theory is based on the flaws in concrete.

Important Points of Griffith’s Theory

Flaws lead to high stress concentrations to the materials subjected to load, hence high stress concentrations are accumulated in and around the flaws.

The average stress on the material taking the cross section of material remains low.

But the high stress concentration around the flaw result in the fracture of material around this flaw.

Flaws differ in sizes. Larger flaws create very high stress concentrations. At this point, material fails at much lower stress intensity than estimated.

Thus, flaws are the reason why the actual strength is much lower than the theoretical strength.

There are various flaws in concrete. It is difficult to describe how these flaws cause reduction in strength of concrete.

But, Griffith’s theory provides a satisfactory explanation on failure of concrete as a brittle material.

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