Benefits of gypsum capping
The most important property of concrete for an engineer is its compressive strength. But are you testing for it in the right way? Several factors influence the measurement process. The flatness of the surface, its horizontal level does have a huge impact on the measurement outcome. To ensure that such anomalies are ruled out, conventionally the concrete specimen block is ground to get a flat surface. This method however, has several shortcomings and which can be overcome with the new gypsum capping method.
Capping is the process of leveling the ends of cylindrical concrete specimens to ensure that the test cylinder or core has smooth, parallel, uniform bearing surfaces that are perpendicular to the applied axial load during compressive strength testing. This is done to ensure that the specific criteria for flatness and perpendicularity of the ends are in accordance with the applicable standards specified.
Importance of capping
ASTM C39 requires that the ends of compressive test specimens be plane to within 0.002 inch (0.05 mm) and that the deviation of end faces from being perpendicular to the specimen axis is less than 0.5° (0.12 inch in 12 inches or 1 mm in 100 mm). Similar requirements are stated in ASTM C42 for testing cores. Irregular end surfaces or when the specimen axis is not perpendicular to the end-faces will cause stress concentrations within the test specimen and reduce the measured strength. Since the end-faces of most test specimens will not meet these requirements, procedures in ASTM C617 or C1231 for capping hardened concrete cylinders are used. This ensures that the compressive load is applied evenly and consistently for every strength test.
ASTM C617 is the standard practice for capping cylindrical concrete specimen. Cores should be cut or ground to meet the end condition requirements or they may be capped in accordance with ASTM C617. Unbonded caps are currently not permitted when testing cores but may be permitted in the future. ASTM C617 permits a bonded cap to be applied to freshly molded cylinders using high strength gypsum plaster or sulfur mortar are permitted as bonded capping materials for hardened concrete cylinders.
There are two ways to get the ends of the concrete specimen flat and horizontal. One is grinding the concrete surface and other is using capping material. Let's review each one.
Grinding is a very common and standard surface treatment method but the user must spend huge capital for the machine and also has to do periodic maintenance to ensure that ground surface remains evenly flat.
There are three capping materials, which are qualified for this as per the IS 516 standard.
- Neat cement.
- Hard plaster.
Those are also qualified in the ASTM C617 standard.
Neat cement capping
Mix the cement with water in the desired water-cement ratio. A two to four hour additional waiting time is required before it can be pasted on the concrete surface. The waiting period is important as the mixed pastes tend to bleed, shrink and make unacceptable caps.
Sulphur capping requires a heater for melting the capping material first. It also generates toxic gas and bad odour in the process. Sulphur capping needs two hours of hardening time before testing. For concrete strengths of 350 kg/cm2 or greater, sulphur caps must be allowed to harden for at least 16 hrs before use. Besides, sulphur caps cannot be reused.
Hard plaster capping
The strength of the plaster material is a critical property for its use as capping material. Ordinary plaster of Paris will not serve the purpose of the capping material due to its low compressive strength. Taiwan Capstone manufactures extraordinary high strength gypsum for use as capping material in concrete compression strength testing. Generally, the common gypsum hardens to with stand 1000 psi more or less. However, our products in the Capstone series can reach 5,000-9,000 psi (350-630 kgf/cm2), 30 minutes after mixing with water. It is fast, more efficient, highly reproducible, health friendly to the user, easy to stock, no costly apparatus needed and follows IS 516 Indian Standard and ASTM C617 regulation. There is another material which is also used in the concrete compressive strength test. It is Neoprene pad capping. The major advantage is the convenience. But often the test results are not completely reliable. The users have to prepare different types of caps for variety of concrete specimen.
Gypsum capping vs grinding
The two methods can be compared on the basis of following characteristics:
- Surface flatness.
- Pressure uniformity.
- Sample applicability.
Surface flatness Gypsum
Gypsum shows better flatness after capping. The complete smooth surface is an outcome of the flowing gypsum slurry and the thick glass plate placed on it while the gypsum sets. After covered by the glass, the gypsum surface will become flat just like the glass surface on top of it.
Grinding merely reveals the surface beneath the top with all its imperfections. In this process the concrete surface is treated by the grinding knife. It is difficult to get a completely smooth surface. Without the periodic maintenance, the knife frequently gets damaged and eventually leads to uneven concrete surface.
Here, a pressure-sensitive paper is used to check the surface smoothness. As you can see from the diagram, there is a significant difference between the flatness of gypsum capped and ground surface.
Comparison of capping materials
If we compare gypsum capping, sulphur capping and the neoprene pad, it becomes obvious that gypsum has several advantages over the other two. It provides the best pressure uniformity and data accuracy. That is clear from the comparison chart below.
As the data shows, gypsum capping renders the surface extremely smooth and also makes it perfectly horizontal. Therefore, it can reach the highest compressive strength value and retain stable data at any time. From the operational point of view, gypsum only needs water for the reaction. It provides a safe working process and also increases the efficiency of the sample preparation. Sulfur has characteristics similar to those of gypsum for capping applications, but it takes a very long time to harden, with very bad odours and toxic gases let out during the process.
Neoprene pads have lower compressive strength and a higher SD value since they are often not perfectly horizontal. Furthermore, neoprene pads show higher and higher variation as the pads get reused. It is a convenient but inaccurate capping material.
Capstone compressive test data
Established in 1981, Taiwan Capstone is based in Taiwan and is the sole supplier of high strength gypsum for capping in the country. Sulfur capping is prohibited for safety reasons and rubber is banned by the market for the unstable performance.
They started their international business in April 2013 and since then, have expanded the market with competent local agent partners in India, Thailand, Singapore, Indonesia, USA, Mexico, Columbia, Canada and Israel. Now they are looking for more partners worldwide, to grow further. High-strength gypsum is absolutely the revolutionary capping material, especially considering its environment friendliness.
For more information, contact: Shailesh Chauhan Tel: 00-91-9377458606, E-mail: email@example.com Website: www.twcapstone.com