Mortar Testing

Mortar test plays a pivotal role in securing the longevity, integrity, and warranty compliance of brick and mortar builds in New South Wales. But not all testing methods deliver reliable, actionable results. Many stakeholders — including builders, warranty managers, developers, and homeowners — default to the scratch test (also known as the “scratch index test”) because it’s fast and on-site. Yet, this test is often misunderstood, misused, or misinterpreted.

This article deconstructs how the AS 3700 scratch test works, its strengths and limitations, and — crucially — why chemical analysis is your gold-standard for defining mortar composition and proving fault (or ruling it out) in disputes. We’ll dive deep into the science, cite Australian research, industry standards, and real-world examples — while ending with an essential open invitation: contact Probrik Remedial Specialists to decode your site's mortar and secure warranty-grade documentation.

1. Understanding the AS 3700 Scratch Test

1.1 What Is the Scratch Index?

The scratch test, embedded in the Australian Masonry Structures Standard (AS 3700), measures the surface hardness and potential durability of in-situ mortar joints. The test is executed using a controlled abrasive tool that scratches the mortar surface under specified force; the depth of penetration — known as the scratch index — is recorded. A lower index represents harder, presumably more durable mortar.

Australian researchers developed performance thresholds for different mortar exposure classes (e.g., M2, M3, M4) based on this index.

1.2 What Influences Scratch Index Results?

Multiple variables affect the scratch index:

• Mortar composition (cement / lime / sand ratios, presence of fines)

• Joint finish (ironed, flush, raked)

• Sand type and grading, including clay fines

• Cement type, and

• Exposure conditions (intended durability requirement of the structure)

One Australian study tested 360 mortar joints across different sands, cements, finishes, and units; all yielded scratch indices within acceptable ranges (0.02 – 0.08 mm), regardless of harsh marine conditions — even after three years — demonstrating variability and sensitivity of the test.

1.3 Limitations of the Scratch Test

• Operator dependence: uneven joints, tooling variations, and operator technique can skew results.

• Lacks material insight: the scratch index reveals only hardness — nothing about mix proportions, contaminants, cleaning issues, or additives.

• False positives: extremely weak mortar, improperly executed, might pass the scratch test yet fail durability or structural performance.

• Limited to performance — no origin clarity: It doesn't tell you what went wrong — only that something might be off.

While quick, the scratch test is best used as a durability screening tool — but not as definitive proof in warranty claims or technical disputes.

2. The Case for Chemical Analysis

2.1 What It Reveals

Chemical analysis breaks down actual mortar composition: cement, hydrated lime, sand ratios, plus the presence of plasticizers, acid residues from brick cleaning, waterproofing agents, or other contaminants.

2.2 How It’s Conducted

Often performed on fresh, in-place mortar, these analyses can identify whether poor workmanship, raw materials, or cleaning practices caused defects — and are admissible in arbitration or legal proceedings.

Internationally, ASTM C1324 is the established petrographic and chemical analysis standard. It requires skilled petrographers and chemists, extensive instrumentation, and evaluates both composition and microstructure — and therefore provides a deeper technical foundation for claims or compliance demonstrations.

2.3 Benefits in Practice

• Quality control during bricklaying: Check mix compliance in real-time.

• Dispute resolution: Identify the responsible party (mason vs cleaner vs materials).

• Warranty defense or claim validation: Hard evidence to support or contest defect claims.

• NCC / BCA and AS 3700 compliance: Demonstrable alignment with codes — even if composition deviates from “deemed-to-comply” mixes.

3. Side-by-Side Comparison

Test Type Benefits Limitations & Risks z'

Scratch Test

BENEFITS: Fast, on-site, screens durability.

LIMITATIONS & RISKS: Operator-dependent, no mix info, false positives

Chemical Analysis

BENEFITS: Detailed composition, legal-grade evidence

LIMITATIONS & RISKS: Requires sampling / lab, slower, costs higher

Key Insight: The scratch index may alert you to a potential issue — but chemical analysis tells you why there is an issue. In technical disputes or warranty claims, you need clarity, not just a bell (pass / fail).

4. Integrating Both for a Robust Strategy

A high-performing workflow uses both tests — but in the right order:

1. Scratch Test on-site — Quick screen to catch suspect zones.

2. Targeted Chemical Sampling — Collect samples from failed or borderline areas.

3. Lab Analysis (AS 3700 or ASTM C1324 protocols) — Decode composition, identify contaminants or improper mixes.

4. Report → Repair Path — Combine the durability data with mix evidence to craft a builder-backed remedy plan.

This staged approach balances speed and accuracy — protecting your build and your warranty, while building trust with trades and clients.

5. Supporting Research & Standards

• A comprehensive Australian study (360 joints over 3 years) found all scratch indices passed within limits regardless of exposure — highlighting test variation and the need for deeper analysis.

• Additional lab and field research mapped the effect of mix proportions, sand type, joint finishing, cement type, and exposure on durability outcomes via scratch index.

• ASTM C1324 remains the global benchmark for hardened mortar composition testing — necessary when mortar needs a forensic breakdown.

• Industry feedback from testing providers confirms the scratch test’s limitations and the supreme value of chemical analysis in disputes.

7. Conclusion

Choosing between a quick scratch test and a comprehensive chemical breakdown isn't an either / or choice — it's a spectrum of clarity, risk, and defensibility. If you want to know not just if your mortar is durable but why it performs (or fails), and to get builder and warranty-ready reporting, you don’t need to decide alone.

👉 Contact Probrik Remedial Specialists today to arrange a combined scratch and chemical analysis — tailored to your site.

👉 If you’re a builder, developer, or homeowner dealing with mortar disputes, Probrik provides clear answers, fast testing, and rectification solutions.

We handle the testing, interpret the results under AS 3700 and NCC context, and deliver a defense-grade report plus reliable remedial recommendations. Let's resolve your mortar mystery together.