Kolkata, India-based lubricant and grease maker Balmer Lawrie has been focusing part of its research and development efforts on greases used for concrete reinforcement, specifically for use with steel cables used for post-tensioning systems. In recent years, use of a pre-stress technique has increased because it can actively prevent cracks in the finished concrete, said N. Parameswaran, chief manager of R&D at the company.

With the conventional method of reinforcing concrete with steel rebar, the concrete slab or beam starts to bend when heavily loaded, and the concrete structure gradually starts to crack, Parameswaran explained at the National Lubricating Grease Institute India Chapters annual meeting in February. While the steel holds the cracks together, it cannot prevent them.

Pre-stressing the reinforced concrete uses steel strands, typically referred to as tendons, to hold the concrete together and prevent cracks from forming even under heavy loads. Pre-stressing can be done by applying tension to the steel tendons either before the concrete is poured or after it has hardened.

In pre-tensioning, tension is applied to the tendons using a jack against end abutments to which the tendon is secured. The concrete is cast, then the ends of the tendon are cut free from the abutments. In post-tensioning, the concrete is cast and the jack is placed against the concrete beam itself. Tension is applied to the tendon, which is then held in place by anchors at the ends of the beam.

The popularity of the post-tensioning method has risen recently due to its inherent advantages in the construction process, Parameswaran said at the meeting in Guwahati, India. In the U.S., Europe and also in China, the post-tensioning process is widely used in the construction industry, and in the last 10 years, the volume of construction by post-tensioning process has tripled, he said, adding that the method is also gaining ground in India.

Advantages of post-tensioning include the ability to use less concrete, significant reduction in building weight, higher load-bearing capacity with thinner beams and slabs, longer structure life, better corrosion resistance and flexible construction options, he noted.

Post-tensioning can be used in all facets of construction, such as office and apartment buildings, parking structures, bridges, and rock and soil anchors.

Basic Requirements

Post-tensioning tendons are manufactured from seven strands of steel wire. These strands are coated with corrosion-inhibiting grease during tendon formation and encased in a high-density polyethylene protective sheath.

In addition to preventing corrosion of the steel tendons once they are inside the concrete, the post-tensioning grease must also prevent corrosion at the anchorage; reduce the friction between the strands as well as between the tendon and the polymer sheath; resist hardening or softening the plastic sheath; and otherwise be compatible with the plastic material.

Other important properties include minimal oil separation, resistance to water emulsification, and a passing score on long-duration salt-water corrosion testing and salt-water soak testing, Parameswaran said.

Performance requirements for post-tensioning grease are governed by the United States-based Post-Tensioning Institute and Switzerland-based International Federation for Structural Concrete (FIB).

Parameswaran said the selection of thickener, base oil and additives are critical considerations for meeting PTI and FIB specifications. Lithium or lithium-calcium thickeners can be used to meet the required dropping point of 150 degrees Celsius or higher (ASTM D566).

The right combination of base oils is required for compatibility with the polymer sheath material, causing less than a 15 percent change in hardness, 10 percent change in volume and 30 percent change in tensile strength (ASTM D4289, PTI only). Base oil also affects oxidation stability, which the FIB requires to be less than 0.06 megapascal pressure change at 100 C in ASTM D942 after 100 and 1,000 hours.

A superior type of antioxidant is required to meet the severe oxidation resistance requirements, Parameswaran noted. Sturdy corrosion inhibitors allow the grease to meet PTI and FIB limits in several different corrosion tests, including ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus).

Polymer Compatibility Study

Balmer Lawrie, which is one of Indias largest grease suppliers, formulated lithium grease with API Group I, Group II and naphthenic base oils to evaluate their compatibility with the HDPE polymer sheath used in post-tensioning systems.

The government-run company developed greases with four different combinations of base oils: Grease A (Group I only), Grease B (Group II only), Grease C (naphthenic only), and one with 25 percent each Group I and Group II and 50 percent naphthenic base oil-Grease D.

Results from ASTM D4289 (Standard Test Method for Elastomer Compatibility of Lubricating Greases and Fluids) showed that the greases with Group I and II oils increased shore hardness and reduced the volume of the polymer, while the naphthenic grease softened the polymer and marginally increased the volume. The grease with a mixture of all three base oils produced the least change in hardness and volume.

Parameswaran noted the polymer compatibility values of all four greases were within the acceptable limits, but the grease based on a mixture of all three base oils showed the best results. The company selected Grease D for corrosion resistance and emulsion test using selected additives.

Corrosion & Emulsion Test

The researchers developed post-tensioning grease based on Grease D with two different corrosion protection additives. Three greases with different amounts of corrosion inhibitors were tested: 3.5 percent CP Additive I (Grease E), 3 percent CP Additive II (Grease F) and 4 percent CP Additive II (Grease G). All three formulations contained 1 percent of the same antioxidant additive.

The company evaluated all three greases for corrosion protection with the ASTM B117 method, using salt-water spray and distilled-water spray, and with the Emcor rust test, an FIB requirement. The greases were also assessed using salt fog and a soak (emulsion) test required by the PTI specification.

The experiment showed that Grease E, based on CP Additive I, met FIB requirements but failed to meet corrosion and emulsion test specifications required by PTI. Grease F and Grease G, which included different amounts of CP Additive II, passed the requirements for both sets of specifications.

Grease G far exceeded the number of hours to failure as required for salt fog test and emulsion test as per PTI requirement, Parameswaran concluded.

The fully formulated Grease F and Grease G were further tested for all PTI and FIB parameters, such as oil separation, dropping point, oxidation stability and polymer compatibility. Parameswaran said the results showed these two greases had the best properties for the post-tensioning process.

Parameswaran believes that use of the post-tensioning method in Indias construction and infrastructure segment will double or triple over the next few years. Suitable greases will ensure long-term protection of steel tendons used in such applications, he noted.