Nanotechnology in concrete has become more popular over the last 25 years as a means to enhance the density of the matrix, thus reducing the permeability of concrete, increasing its strength, and increasing its durability to physical and chemical attack. The enhancements imparted to concrete’s hardened properties from nanotechnology have led to a growing acceptance of nanomaterials in the concrete and construction industry.
However, despite the rise in the popularity of the nanotechnology’s use in concrete, the lack of adequate case studies hinders the ulitization of this innovative technology into day-to-day concrete is, despite the decades of research that has been invested in proving this technology. Engineers, superintendents, architects, and concrete producers are often reluctant in employing a novel concrete product unless there have been repeatable field applications quantifying and qualifying the features and benefits of such technology. This tech note aims answer whether or not colloidal silica can increase the resistacnce to deicing salt and brine attack by presenting a case study on to validate and facilitate its wide implementation for preventing the deletiorious mechanisms initiated by deicing salt attack. This document is part of a set of tech notes that provide concrete producers and engineers tools to enhance concrete structures and infrastructure throughout the world.
Case Study -
Colloidal silica was used to increase the resistance of ready-mixed concrete to deicing salts and brines during the winter season. The nano-enhanced concrete (NEC) needed to be easy to batch and place as conventional concrete. Particularly, it had to develop fresh and hardened properties suitable for all construction sites and applications. Further, the ready-mixed producer required a significantly higher compressive strength and decreased permeability. The end result translated directly to greater resistance to the damage caused by deicer applications.
Materials, Proportions, and Mixing
1.1 Nanotechnology Type
Colloidal silica was in liquid form and deployed in a similar manner to concrete chemical admixtures. The colloidal silica appearance was clear to milky with specific surface areas of 300 m2/gram. The colloidal silica (nano silica) solid content 30% by mass of the dispersion. These nano silica particles are stabilized in solution by an electrical double layer that prevents the nano silica particles in dispersion from agglomerating
1.2 Concrete Mixture Proportion
The mixture proportion of the reference (REF) concrete is shown in Table 1 and was used as a baseline. To create the NEC mixture, 6.0 fluid ounces per cementitious hundred weight (fl oz per cwt) of colloidal silica was added to the proportion of the REF mixture, and the water content was adjusted to take into consideration the water content of the colloidal silica, this nano-enhanced concrete mix is referred to as NEC.
1.3. Using Colloidal Silica in the Ready Mixed Plant
The colloidal silica additive was sequenced into the batched concrete like a standard concrete admixture.
An automated concrete admixture distribution was used where the colloidal silica additive dosage rate was batched at the tail-end of mixing after all other admixtures had been introduced into the concrete. The colloidal silica was released with the remaining tail-water (approximately 25-50% of the batch water). The water content of the colloidal silica was take into account when weighing out batch water. The same number of revolutions used to mix the REF was used. The concrete was mixed for 70 revolutions at approximately 18 RPM before being tested at the batch plant for fresh properties. After the fresh properties of the concrete were verified and samples collected, the concrete enhanced with colloidal silica was sent to the jobsite.
2. Concrete Fresh Properties
The colloidal silica admixture had little to no impact on the fresh properties of the concrete. The
measured fresh properties for both the REF and the NEC mixtures are presented in Table 2.
3. Hardened Properties after Deicing Salt Environment
Deicing salt testing consists of concrete cylinders being cured according to ASTM C31/C31M in a temperature-controlled lime-water bath for 28-days. After 28-days the cylinders are placed in a deicing CaCl2 solution and the freeze and thaw cycling started. The zero-day (0-day) represents the beginning of freeze-thaw cycling. The cycles consisted of freezing at −18 ± 3°C [0 ± 5 °F] for 16 to 18 hours, followed by thawing a laboratory environment at 23 ± 2°C [73.5 ± 3.5°F] with a relative humidity of 45 to 55% for 6 to 8 hours. After the 28-days, the 0-day measurement was taken before placing the samples in a CaCl2 bath for 28-days followed by a MgCl2 bath. This method is consistent with a scaling test and needs to be updated throughout the document. The procedure consisted of one-cycle of 16 to 18 hours in a freezing environment and 6 to 8 hours of thawing in a brine consisting of one-day.
3.1 Compressive Strength
At different intervals, the specimens were tested according to ASTM C39/C39M at the end of the thawing period. The concrete mixture without the colloidal silica (REF) started experiencing a reduction in compressive strength after 7-days in the freeze-thaw and deicing brine environment illustrated in Figure 1. At 90-days, the REF concrete lost more than 15% of its original compressive strength measured after 28-days of curing. The concrete with the colloidal silica admixture (NEC) exhibited an increase in compressive strength up until the 56-day measurement when the strength seemed to plateau.
3.2 Mass Loss
A modified version of ASTM C672/C672M, Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals, was employed for this analysis. The concrete slabs commonly used were replaced with concrete cylinders (4-inch diameter by 8 –inch tall) and were frozen and then cured in a deicing brine environment. The concrete cylinders were weighed at the end of every thaw cycle (see above) over the 100-days.
Like the compressive strength, the mass loss data in Figure 2 illustrates an enhancement of the concrete mixture with the addition of colloidal silica. The mass loss over 100-days was 10-25% lower in the concrete mixture with the colloidal silica (NEC) when compared to the concrete mixture without the colloidal silica (REF). While mass loss in the NEC still occurred, this is an inevitable reality with the concrete composite and its susceptibility to the damage induced by the deicing brine.
3.3 Abrasion Resistance
The abrasion resistance of the specimens was tested after 56 (Figure 3a) and 90-days (Figure 3b) in the freeze-thaw and deicing brine environment, in accordance with ASTM C779/C779M, Procedure C. At both 56 and 90-days the concrete mixture with the colloidal silica (NEC) exhibited a greater resistance to the induced abrasive wear. At 90-days, the NEC mixture maintained most of the durability to abrasive wear that was exhibited at 56-day. The concrete mixture without the colloidal silica showed a significant reduction in abrasion resistance at the 90-day test when compared to the 56-day test.
Summary
Despite the rise in the popularity of using nanotechnology in concrete, the main obstacle that prohibits the entry of these novel technologies into day-to-day concrete is the lack of adequate case studies. As field applications of colloidal silica and other nanomaterials continue grow in popularity, the industry must take advantage of the opportunity to document their measurable effects.
Colloidal silica had a positive effect on the ready-mixed concrete subjected to freezing-thawing in the presence of deicing brine. Adding colloidal silica to ready-mixed concrete resulted in the following changes of material properties:
Increased resistance to strength degradation due to deicing brine attack,
Increased resistance to mass-loss due to deicing brine attack, and
Decreased the abrasive wear of concrete in severe freeze-thaw and deicing brine conditions.