Why Gypsum Capping

Instruction Video
Gypsum capping instruction
Fast! Efficient! Best capping method!
Incoming quality control
Gypsum Cube test
Capping gypsum introduction

Around 30 mins, the strength of the gypsum can reach 5,000~9,000 psi (350~630 kgf/cm2). This type of gypsum is mostly used in the capping material for concrete compression test according to ASTM C617 standard, so our high strength Capping Gypsum the offer the convenient and excellent testing material for civil engineering industry.

Gypsum Pure sulfur Pad
Human Friendly No Harm Highly Toxic No Harm
Pressure Uniformity High Medium Low
Data Accuracy High Medium Low
Surface Condition After compressive test Good Cracked Cracked after periodic used
Sample amount/one time 10~15 samples 1 sample 1 sample
Average operation time 2 mins 5 mins 1 mins
Hardening Time 30 Min 2 Hours No need
Operation Range 0 ~ 9000 Psi 0~3000 Psi 0~9000 Psi

Test Data Record From Thailand

  • Theoretically compressive strength 400 kg/cm 2
  • Relative percentage for the result
  • Gypsum capping (103%)
    Sulfur capping (96%)
    Neoprene Pads (80%)

    * Gypsum capping got the highest value because complete flatness perpendicularity and horizontality.

Bar Chart for the test result:

  • Average
  • Gypsum capping (411 kg/cm 2 )
    Sulfur capping (382 kg/cm 2 )
    Neoprene Pads (320 kg/cm 2 )

  • Standard Deviation
  • Gypsum capping (18.5)
    Sulfur capping (27.0)
    Neoprene Pads (67.0)

Six Sigma analysis:

  • Gypsum capping shows the highest average value and smaller deviation.
  • The test result for Sulfur capping is slightly worse than Gypsum capping.
  • Neoprene Pads have lower compressive strength and higher SD value because of the incompleteness perpendicularity and horizontality.


As at the the data shows, Gypsum capping could make the best capping surface condition with complete flatness, perpendicularly and horizontally.Once it it will able to reach the highest compressive strength value and one will be able to obtain stable stable and consistent data at anytime . From the operational point of view, Gypsum only needs water to react. It provides a human friendly environment and also increases the efficiency of the sample preparation.

Sulfur Capping is a similar capping material to gypsum, but it takes a very long time to harden it and it is harmful for humans and the environment.

Neoprene Pads would have lower compressive strength and higher SD value because of the incomplete perpendicularity and horizontality. , neoprene pads would have higher and higher variation as the used times increase. It is a convenient but inaccurate capping material.

Strength performance

Our high strength capping gypsum are strictly qualified that all specimens can reach the target strength within a very short time.Once, its strength is never less than our concrete cylinder. It can provide the best test result of the concrete tests.

Pressure distribution analysis

As the data shows, Gypsum capping provides an ideal test surface with complete flatness perpendicularity and horizontality.Therefore, it can reach the highest compressive strength value and retain stable data at anytime.

The flatness can reach as low as 2.7 μm.(see the fig.2) The complete smooth surface is coming from the flowing gypsum slurry and thick glass plate. After covered by the glass, the gypsum surface will get flat just like the glass similar To a mirror.

Fig.1: Gypsum Capping pressure uniformity: Providing a perfect load distribution

Grinding pressure uniformity: It shows a obvious knife pattern

The grinding surface is merely similar flatness to the bottom end. The concrete surface is treated by the grinding knife. It is difficult to get a completely smooth surface. Without the period maintenance, the knife has high possibility to get damaged causing a poor surface.

Regulations of ASTM

According to ASTM CNS 11297 Standard Practice for Capping Cylindrical Concrete Specimens, there are some key factors for using capping materials :

1. ASTM c617 4.1: Capping Plates—Neat cement caps and high-strength gypsum paste caps shall be formed against a glass plate at least 6 mm [1⁄4 in.] thick.
2. ASTM c617 4.2: Suitable alignment devices, such as As: As guide bars or bull's-eye levels, shall be used in conjunction with capping plates to ensure that no single cap will depart from perpendicularity to the axis of a cylindrical specimen by more than 0.5° (approximately equivalent to 3 mm in 300 mm [1 ⁄8 in. in 12 in.]).
3. ASTM c617 5.1: The strength of the capping material and the thickness of the caps shall conform to the requirements of Table 1.

Table 1

Cylinder Compressive Strength kgf/cm2 (psi) Minimum Strength of Capping Material Maximum Thickness
Average Thickness of Cap
Maximum Thickness
Any Part of Cap
3.5 to 50 MPa [500 to 7000 psi]

35 MPa [5000 psi] or cylinder strength whichever is greater

6 mm [1⁄4 in.] 8 mm [5⁄16 in.]
greater than 50 MPa [7000 psi]

Compressive strength not less than cylinder strength, except as provided in 5.1.1

3 mm [1⁄8 in.] 5 mm [3⁄16 in.]
4. ASTM c617 5.1.3: The compressive strength of capping materials shall be determined by testing 50 mm [2 in.] cubes following the procedure described in Test Method C109/C109M.
5. According to ASTM c617 5.1.4: The strength Of the capping material shall be determined on receipt of a new lot and at intervals not exceeding three months. If a given lot of the capping material fails to conform to the strength requirements, it shall not be used, and strength tests of the replacement material Shall be made into four four consecutive determinations conform to specification requirements.

For the Hardened Concrete Specimens:

1. According to ASTM c617 6.2.1 General—If an end of a specimen has a coating or deposit of oily or waxy materials that would interfere with the bond of the cap, remove such coatings or deposits. If necessary, the ends of a As desired, capping plates may be coated with a thin layer of mineral oil or grease to prevent the capping material .
2. According to ASTM c617 6.2.2 End Condition—The distance of any point on an uncapped end from a plane that passes through the highest point of the end surface and is perpendicular to the axis of the cylinder shall not exceed 3 mm [1 ⁄8 in.] (Note 12). If the end exceeds this limit, the end of the cylinder shall be cut, lapped or ground prior to capping.

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