To minimize rebound during gunning, API TR 980 clearly instructs to:

“Begin at the lowest level of the installation and build up gradually to full thickness in a small area of approximately 10 ft² (1 m²), avoiding sweeping motions.”

— API TR 980, Section 4.3.4​

This method prevents rebound contamination and cold joints that could occur if a larger area is covered too quickly or in an uncontrolled pattern.

Therefore, Option C is correct, and Options A, B, and D are incorrect according to best practices outlined in API TR 980.

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According to API 936, during applicator qualification testing , the equipment used must be identical to the equipment that will be used in the actual installation . This ensures that all conditions (air pressure, nozzles, mixers, vibration tools, etc.) accurately reflect field performance and allow for a true demonstration of the applicator’s ability to meet the project’s quality requirements.

This rule eliminates discrepancies between mock-up performance and production work, which might otherwise compromise the quality of the installed refractory. Testing with alternative or idealized equipment does not represent real installation challenges.

API TR 977 defines refractory abrasion in the context of erosion resistance testing:

“Abrasion is the wearing away of the surface by the mechanical scouring action of solids moving across the surface.”

— API TR 977, Section 2​

This distinguishes abrasion from gas or chemical attack and aligns with ASTM C704 test context.

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The term "rebound" is officially defined in API Std 936 as follows:

“Rebound: That portion of a refractory that does not adhere to the surface and falls away when a material is applied by gunning.”

— API Std 936, Section 3, Definitions​

This makes "rebound" the precise and correct term describing the material that fails to adhere during gunning. The other options, such as “slurry,” refer to different material states or applications, and “trimmings” or “overspray” are not used with this specific meaning in the standard.

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Per API Std 936 Section 6.3.2.1, qualification trials for castable refractories require strict control over water content. The variation must be within ±1.0% of the manufacturer’s recommended water addition to ensure consistency and reliability in performance.

"Acceptance of the test results from the cast sample trial shall be contingent on the mix water being within ±1.0 % of that recommended by the manufacturer."

— API Std 936, Section 6.3.2.1​

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API 936 clearly states that applicators/installers must be qualified to demonstrate the ability to install refractory linings in accordance with the specified project quality standards. This includes using the correct methods, achieving proper material density, and avoiding defects like voids and laminations.

The ASTM C704 standard requires the use of two specimens per sample for testing abrasion resistance:

“The abrasion resistance shall be determined using two 4½ in. × 4½ in. × 1 in. (114 mm × 114 mm × 25 mm) specimens per test sample.”

— API Std 936, Section 6.4.4, Abrasion Resistance​

This ensures accurate and reproducible results for quality control in refractory installations.

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TR 980 outlines curing and dryout best practices for monolithic refractories. For high-purity low-cement castables, gradual heat-up is necessary to avoid thermal shock and cracking:

“A minimum 24-hour air cure at ambient temperature should be followed by a controlled dryout ramp not exceeding 50°F (28°C) per hour up to 600°F (316°C).”

— TR 980, Section 6.4.2​

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Apparent porosity is a measure of the volume of open pores (voids that can absorb water) within a refractory sample, relative to the total volume. It is always expressed as a percentage . Testing is done using standards such as ASTM C20, which API 936 references. This metric is crucial for understanding a material’s permeability, thermal shock resistance, and mechanical integrity.

API Std 936 defines the acceptance criteria for CCS under erosive service in Section 6.4.2:

“Test values for cold crushing strength shall meet or exceed 75% of the manufacturer's minimum published value for each individual specimen.”

— API Std 936, Section 6.4.2​

This criterion ensures that all tested samples meet a critical durability threshold before field acceptance.