Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D1670 – Standard Test Method Technical Guide
📐 Scope and Significance
ASTM D1670/D1670M – 11 (Reapproved 2023), titled “Standard Test Method for Failure End Point in Accelerated and Outdoor Weathering of Bituminous Materials,” establishes a precise electrical procedure for determining the failure of bituminous coatings due to cracking. Under the jurisdiction of ASTM Committee D08 on Roofing and Waterproofing, this test method utilizes a spark-generating apparatus on materials undergoing accelerated or outdoor weathering on electrically conductive backings.
The extent of cracking or pitting is a critical measure of the deterioration of bituminous films. A key assertion of this standard is that failure due to cracking is more accurately determined electrically than visually. The test outcome is influenced not only by the material’s characteristics and the duration of weathering but also by the film thickness and the amount and type of mineral filler present. To ensure validity, the standard strongly recommends exposing a control material of known weathering characteristics concurrently with the test specimens.
⚡ Safety Precaution: As noted in Section 1.3, this method involves high-voltage spark-generating equipment. Users must establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Proper grounding of the apparatus and test panel is essential.
⚙️ Test Procedure Overview
The procedure begins with dry, weathered, bituminous-coated test panels prepared according to Practice D1669/D1669M. The panels are grounded, and a sheet of linagraphic or ultra high sensitivity thermal fax paper is placed on the bituminous surface with the emulsion side facing down. A feeler electrode connected to a spark-generating apparatus is passed systematically over the back of the paper. Where cracks or pinholes exist, the high-voltage spark arcs through the defect to the conductive backing, permanently marking the paper.
Following the sparking phase, photocopies are made of the front side of the paper to record the spots caused by the sparks. An acrylic grid is then placed over the photocopies. The standard provides a specific configuration for the feeler electrode (Fig. 1), though other configurations meeting the operational conditions of Section 5 are not prohibited.
💡 Methodological Tip: Ensures the test panels are completely dry before grounding and passing the feeler electrode. Moisture can cause spurious sparking, leading to false positive failure determinations. Adhering strictly to the panel preparation practices outlined in D1669/D1669M is critical for reproducibility.
📊 Failure End Point Determination
The failure end point is defined by a quantitative analysis of the spark paper. The number of grid squares containing one or more spark records is counted. The failure end point is scientifically defined as the state where sparking is observed in 26 or more grid squares.
| 🟦 Test Element | 📏 Requirement / Specification | 🎯 Reference | ⚡ Key Value |
|---|---|---|---|
| Detection Principle | Spark-Generating Apparatus | Feeler Electrode (Fig. 1) | Arcing through defects |
| Specimen Substrate | Electrically Conductive Backing | Practice D1669/D1669M | Panel preparation |
| Recording Medium | Linagraphic / Ultra High Sensitivity Thermal Fax Paper | Emulsion side down | Spots from sparks |
| Failure Criterion | Grid Square Count | Section 3.1 | 26 or more squares |
| Weathering Methods | Xenon-Arc, Fluorescent UV, Outdoor | D4798, D4799, D1435 | Exposure protocol |
This quantitative approach removes the subjectivity associated with manual crack inspection, providing a reliable and reproducible end point for weathering studies. The reporting must include the count of grid squares exhibiting sparking, clearly defining the point at which the bituminous coating is considered to have failed under the given weathering regime.
❓ Frequently Asked Questions
🔍 How does the spark test accurately identify coating failure?
The apparatus creates a potential difference between the grounded conductive substrate and the feeler electrode. An intact coating acts as an insulator. When a crack, pit, or pinhole breaches the coating, the electrical circuit is completed via a spark that arcs through the defect, marking the recording paper. This provides direct, electrical evidence of a pathway through the film.
💡 Why is electrical detection preferred over visual inspection?
According to Section 4.1, failure due to cracking is more accurately determined electrically than visually. Microscopic or tightly closed cracks may be invisible to the naked eye but can still compromise the integrity of the coating. The spark test detects these defects, offering a far more sensitive and objective failure assessment.
📌 What factors influence the failure end point beyond the material itself?
The failure end point is critically influenced by the applied film thickness of the bituminous material and the amount and type of mineral filler present (Section 4.2). Thinner films and specific filler compositions may fail more rapidly. Using a control specimen of similar material with known weathering characteristics is strongly recommended to validate the test results (Section 4.3).
⚡ What is the specific quantitative criterion for a failing result?
The failure end point is reached when the sparking observed on the recording paper occupies 26 or more grid squares on the standard acrylic counting grid. This specific numerical threshold (defined in Section 3.1) represents the precise point established by the standard to define functional failure of the bituminous coating due to weathering.