Table of Contents
Definition
The gel space ratio in concrete refers to a measurement that involves calculating the ratio of the volume of the hydrated cement paste to the combined volumes of the hydrated cement and the capillary pores.
Formula Of Gel Space Ratio
The formula for calculating the Gel Space Ratio is:
Gel Space Ratio = (Vw – Vg) / Wg
where:
- Vw is the volume of water absorbed by the gel or hydrogel
- Vg is the volume of the dry gel or hydrogel
- Wg is the weight of the dry gel or hydrogel
The Gel Space Ratio is typically expressed as a percentage, and indicates the degree of swelling of the gel or hydrogel in water. A higher gel space ratio indicates a greater water-holding capacity and swelling ability of the gel.
Importance Of Gel Space Ratio
The gel/space ratio is an important parameter in determining the strength of cement and measuring the porosity of concrete. This ratio takes into account the compaction and the volume of capillary pores in the material.
By considering the volume of capillary pores, the gel/space ratio is able to provide a more accurate measurement of the compressive strength of cement at any age. This is particularly useful as it allows for the strength to be determined even for the fraction of cement that is already hydrated.
The gel space ratio is a useful tool for measuring the porosity of concrete. Porosity is a critical factor in determining the durability of concrete, as it affects the permeability of water and other substances that can cause damage to the material.
Validation of Abrams Law In Context of Gel Space Ratio
There have been many debates among researchers about the validity of Abrams’ concept as a law. Some have argued that it lacks the necessary qualifications to be considered a law and should instead be classified as a rule.
The w/c ratio law has been criticized for its limitations, such as its dependence on various factors that affect strength. These factors include the degree of hydration of cement, the chemical and physical properties of concrete, the temperature at which hydration occurs, changes in the effective w/c ratio, bleeding and shrinkage in concrete, and the air content, if it is entrained concrete. Strength decreases as the w/c ratio increases, but the relationship between the w/c ratio and concrete strength is complex.
Moreover, the w/c ratio law is only useful in determining concrete strength after 28 days and for fully compacted concrete. In contrast, the gel space ratio is independent of the age of the concrete and can be calculated at any time and for any fraction of hydrated cement. In the gel/space ratio, the strength of concrete is related to the solid products of hydration of cement in relation to the available space for the formation of these products. Therefore, it has been suggested that concrete strength is related to the solid products of hydration and the space available for their formation.
Relation Between Gel Space Ratio & Compressive Strength Of Concrete
The relation between gel space ratio and compressive strength of concrete is given by the formula as given below:
S = 240x3
Where S = strength of concrete.
x = gel space ratio.
240 = intrinsic strength of gel in N/mm2 for the type of cement and specimen used.
An typical graph depicting the correlation between gel space ratio and compressive strength is shown below.
Effect Of Temperature On Gel Space Ratio
The temperature at the time of placing and setting concrete can have a significant impact on its strength. While a higher temperature can initially increase strength, it can have adverse effects on strength after 7 days due to the rapid initial reaction during hydration. This leads to the formation of a poor physical structure with more pores and hydration products of lower quality, leaving a significant portion of pores unfilled. As a result, the gel/space ratio, which is a measure of the amount of solid gel material compared to the volume of the concrete, is lower in this case than in concrete that hydrates slowly. The latter type of concrete will have fewer pores and a higher gel/space ratio.
Verbeck and Helmuth suggested that the low gel/space ratio in high-temperature concrete occurs because the higher rate of hydration at the initial stage inhibits successive hydration. This is due to an irregular distribution of hydration products in the paste, and the diffusion of the hydration product away from cement particles takes inadequate time under initial high temperature. As a result, a high density of hydration product is produced in the vicinity of particles still waiting for hydration reaction or in a state of some degree of hydration, inhibiting subsequent hydration. This results in a lower gel/space ratio in the interstices, which adversely affects the strength of the concrete in the long run. The local weakness in the concrete section can allow cracks to progress and lead to lower overall strength. Backscattered electron imaging has confirmed that porous C-S-H exists between hydrated cement particles.
Effect of Chemical Composition of Cement
The chemical composition of cement plays a significant role in determining its properties. Among the various components of cement, the gypsum content is of prime concern. The amount of gypsum in cement affects its shrinkage and setting time. However, the amount of gypsum that is satisfactory for reducing shrinkage may not be enough to achieve the desired setting time. Gypsum is added to cement clinker during the manufacturing process, and the final grinding of clinker is done after that. The initial structure produced during setting determines the structure of the cement paste in the upcoming stages of hydration. Therefore, the gel/space ratio is dependent on the optimum gypsum content, which facilitates the hydration of cement particles to gain strength. The behavior of concrete under shrinkage and creep is also affected by the gypsum content.
Benefits Of Using Gel Space Ratio
The strength of concrete is generally correlated with its water-cement ratio (W/C) for a specific type of cement, at a particular age of strength prediction, and under specific curing conditions such as wet curing. However, the strength-gel/space ratio correlation is more generalized, as the volume of gel production within the cement paste after any time of placing is a function of the cement type and age. Therefore, the gel-space expression accounts for the fact that different amounts of gel can be produced at the same length of curing for different types of cement.
Some Limitations
Gel space ratio has some limitations as it is affected by the bonding capability of the material, which has resistance to propagate cracks. Poor bonding between adjacent crystals can lead to an easy path for propagating cracks.