ACCURACY OF UK OPENROAD FORECASTS
4.1 STUDY OF CHEMICAL ADDITIVES TO DATE
4.1.1 SODIUM FORMATE
Sodium formate (HCOONa) is the sodium salt of formic acid. It is produced commercially by absorbing carbon monoxide under pressure in solid sodium hydroxide at 160oC and usually comes in the form of a white deliquescent powder.
CO + NaOH HCOONa
Screening tests conducted by Baum et al. (1992) indicated that sodium formate at 10% and 20%
concentration to water by weight had a slush rating of between 9 and 10, indicating that ice is likely to be easily removed. At 5% concentration, values between 4 and 7 were observed indicating moderate ease of ice removal. Freezing point measurements indicated that water freezes at -3oC, -5oC and -13oC for 5%, 10% and 20% concentrations respectively. These values are comparative with that of sodium chloride and indicate the potential use of sodium formate for anti-icing purposes.
Further test results demonstrated that sodium formate exhibited high moisture pick up reaching a value of 126% moisture pickup after 2 weeks, however this effect leveled off after approximately 5 weeks.
Sodium formate demonstrated low ice adhesion with the shear strength to remove a disc of ice measured at 172.4KPa (25 psi), 20.6KPa (3 psi) and 6.9KPa (1 psi) for 5%, 10% and 20%
concentration by weight, respectively. The results were more promising when asphalt concrete briquettes were modified by blending the asphalt with sodium formate powder. For these modified specimens, a value of 117.2KPa (17 psi) was observed at -5oC for a 6.4% addition to the aggregate. However a simple washing experiment to determine such effects over a long in-service period showed ice adhesion increased from 117.2KPa (17psi) to 275.8KPa (40psi) after 4 washes compared to a control specimen exhibiting shear strength values of 441.3KPa (64 psi).
Preliminary studies on the effect of the sodium formate on the in-service performance of the asphalt pavement surface course would appear to indicate that there are no apparent negative effects. Friction measurements highlighted negligible differences with a control surface. This was demonstrated with a British pendulum number of 50BPN in the dry and 35BPN in the wet compared to the control (with no additive) of 49BPN in the dry and 35BPN in the wet. Marshall testing to determine the mechanical influence on asphalt indicated a Marshal Flow of 3.8mm (0.15) Inches and Marshall Stability of 11.1kN (2500lb). These values comply with the Conneticut material specifications.
Considering the Baum et al. (1992) study as a whole, it would appear the sodium formate has the potential to promote anti-icing with indications that sodium formate does not have adverse impacts on the pavement surface life. Background research on sodium formate indicates it is suitable for such an application based on economic, toxicity and corrosivity factors. Sodium formate is not listed as part of the EU classification of hazards and has a low toxicology meaning low concentrations are compatible with sewage systems. The high melting point of 253oC makes sodium formate suitable for asphalt production. These factors, combined highlight the potential for sodium formate to be used as an anti-icing pavement additive relative to other chemical additives.
Sodium formate is significantly less corrosive than sodium chloride. Studies by Hassan et al.
(2002) focusing on the effects of runway de-icers on pavement materials when applied to the pavement surface course showed some positive results. The study presented a comparative indicator between the effects for sodium formate, sodium chloride and distilled water and highlights the effects of the pavement composition on degradation.
The first test measured the effect de-icing solutions have on the durability of aggregates during freeze-thaw cycles. The test involved saturated aggregates in de-icing solutions, and subjecting the aggregates to 30 freeze-thaw cycles. The amount of damage experienced by each sample was quantified in terms of weight loss. This showed that sodium formate was initially more damaging to limestone aggregates than sodium chloride, however similar results were experienced for the two de-icers when a quartzite was used.
The effects of de-icers when applied to asphaltic concrete specimens were also considered.
Indirect Tensile Strength was carried out on samples after 25 and 50 cycles freeze-thaw cycles.
After 25 cycles `the average ITS was almost constant for all four deicers and slightly higher than that corresponding to distilled water only` (Hassan et al 2002). This was demonstrated for distilled water, sodium chloride and sodium formate by ITS values of 0.588, 0.657, and 0.675MPa respectively. This would indicate that, initially distilled water would appear more damaging to the strength of the mix than the de-icing chemicals. However, results for distilled water at 50 cycles showed almost no additional damage with a value of 0.572 units, unlike sodium chloride and sodium formate with values of 0.620 and 0.547 respectively.
The penetration of recovered bitumen from the asphalt cores was also considered after 25 and 50 freeze-thaw cycles. This showed significant hardening for road salt and urea, whilst values for sodium formate remained relatively unchanged. Gradation of the aggregate indicated moderate damage for sodium formate and road salt.
The important conclusion to be taken from this study is that `no significant increase in deterioration to the mechanical properties was reported due to the exposure to potassium acetate, sodium formate, or road salt relative to distilled water` (Hassan et al. 2002). While this study did not focus on blending sodium formate into the asphalt it provides further indication that sodium formate is not detrimental to the service life of asphalt.
The initial cost estimates of sodium formate is between £250 and £350 per tonne. This is significant cost and at a 3% addition by weight could increase the cost of the asphalt surface course by over 20%.