Compressive Strength – ASTM C-109-86
|1 Day Cure||5,250 psi|
|3 Day Cure||8,075 psi|
|28 Day Cure||12,100 psi|
Compressive Strength – ASTM C-109-86
|1 Day Cure||5,540 psi|
|3 Day Cure||6,985 psi|
|28 Day Cure||10,375 psi|
Compressive Strength is the resistance of a material to breaking under compression. Much credence is given to compressive strength. Although it is an indicator of strength/ruggedness as in steel wheeled traffic, it is also an indicator of brittleness. Materials that have an extremely high compressive strength often tend to crack when there is movement in the substrate caused by a vibrating/shifting environment and during thermal expansion and contractions. Pre-Krete is formulated to offer sufficient compressive strength while allowing for movement in operating processes.
Flexural Strength (ASTM C580)
Flexural Strength, also known as Modulus of Rupture, is a mechanical parameter for brittleness, defined as a material’s ability to resist deformation under load.
|Pre-Krete G-8||1715 psi average|
|Pre-Krete C-17||1885 psi average|
Specific to Pre-Krete, Flexural Strength may be a factor in a ‘form and pour’ application in casting a self-supportive structure. This is not a common application for Pre-Krete. Contact our office for further details..
Maximum Service Temperature – ASTM C24
* Special Curing may be required for temperature greater than 2500F – Consult Factory
Pre-Krete Curing Methods
NOTE: CONCRETE SEALER SHOULD NOT BE USED IN POTABLE WATER SERVICE.
Use of a resin-based membrane compound such “Concrete Sealer”, as supplied by Pocono Fabricators, can be spray, brush, and/or roller applied to the fresh mortar lining. It should be applied as soon as possible after the surface moisture has disappeared. Coverage is approximately 150-200 sq. ft./gallon. Mortar lining should be allowed to set untouched for 24 hours.
Pre-Krete Systems can be moisture cured by creating a moist environment within the lined equipment. This is done by securely covering/closing ALL manholes and nozzles with plastic and tape. It is imperative that ALL openings be tightly sealed. In large equipment, a fine mist or a simple garden sprinkler of the oscillating type can be placed in the upper elevations of the equipment. This will create the moist environment required. In extreme conditions of very low humidity (less than 40%) and high temperatures (skin temperature above 900F) humidifiers are required to prevent water loss during application.
After the tank is completely lined, let it set untouched for 4 to 8 hours. When the lining has firmed, SLOWLY fill the tank with water, making certain not to “wash out” any of the lining. Allow the lining to set for 12 to 24 hours completely immersed in water. Drain and flush the tank.
When a Pre-Krete lining is installed in a boiler stack, breeching, etc. there is always some residual moisture remaining after a 24-hour curing period. When operating temperatures are above 2500F and a 5 to 7 day cure at 600F is unattainable, it is important that the installed Pre-Krete lining be heated up gradually so that the residual moisture is not steamed. This could result in spalling and other damage. The following heat-up procedure is only required when a newly lined unit is first placed into service: After you apply the lining allow it to cure for 24 hours using Concrete Sealer or allowing it to set in a moisture laden atmosphere. After that time, bring the temperature up to 2500F and allow it to remain there for 1 hour, then increase the temperature by 500F and again hold for 1 hour for each inch of thickness. Continue that procedure until you reach the operating temperature.
Coefficient of Thermal Expansion
|Pre-Krete G-8||6.4 x 10-6/°F|
|Pre-Krete G-17||6.2 x 10-6/°F|
Coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure. Pre-Krete has been formulated with specific materials that congregate to achieve a Coefficient of Thermal Expansion similar to that of steel and concrete – our most common substrates. This allows Pre-Krete to move with the substrate during processes with fluctuating temperatures avoiding disbondment and cracking.
Tensile Strength – ASTM C-109 (28 Day Cure)
|Pre-Krete G-8||850 psi average|
|Pre-Krete C-17||660 psi average|
Tensile strength measures the force required to pull something such as rope or a structural beam to the point where it breaks. Pre-Krete and other mortar type materials do not elongate significantly and break. This is rarely a consideration in design as opposed to polymer type materials that have the ability to stretch.
In determining if Pre-Krete would be an effective solution to a specific corrosion problem, the make-up of the corrosive material, pH, temperature and the frequency of exposure (immersion, intermittent, or splash and spillage) is required. In determining pH, be certain the pH is representative of the operating pH. Often plant personnel are aware of the materials being purchased. However, when placed in the process, the concentration is greatly reduced. For example, 36% sulfuric acid (H2SO4) is a common industrial strength acid. This carries a pH of less than 1. In a waste treatment system, small amounts of the 36% H2SO4 are mixed with large amounts of water to reduce bacteria. The pH of this buffered solution is in the 5 – 7 range which would be suitable for Formula G-8 Pre-Krete.
Formula G-8 Pre-Krete
Resistant to solutions having a pH of 3.5 – 7. Offers excellent resistance to the sulfurous and sulfuric acid associated with the burning of various fuels in breechings, stacks, scrubbers, etc. At temperatures above 350 0F, G-8 is resistant to environments having a pH of 2 – 7. G-8 is resistant to dilute tannic and lactic acid, carbonic acid, petroleum oils, vegetable, mineral and animal oils. Resists dilute ammonium sulfate, ammonium nitrate, ammonium chloride, seawater and acid salts. Also resists hydrogen sulfide as found in sewage and waste treatment facilities. Provides long term protection to vessels and equipment containing all types of potable water regardless of pH, hardness, and/or softness.
*Note: Pre-Krete G-8 is not recommended in hydrofluoric acid (halogen group) regardless of concentration.
Formula C-17 Pre-Krete
Resistant to solutions having a pH of 7 and above. Greatly extends the service life of equipment exposed to salt brine. Resists dilute sodium hydroxide, sodium peroxide, and potassium and sodium chlorides. Also resists gasoline, toluol, xylol, cumol, benzol, and fluorides. Resistant to ammonia and sodium carbonate. Calcium and potassium hydroxide, calcium and potassium nitrate and naphtha.
*Note: Pre-Krete C-17 is not recommended in hydrofluoric acid (halogen group) regardless of concentration. also not recommended for use with any sulfates or in acid service.
*** Pre-Krete test blocks are available to place into process environments to determine the suitability of Pre-Krete for the application.
Thermal Stress Index
The Thermal Stress Index is a numerical method of rating materials in terms of the thermally induced stresses that can cause crack formation and/or spalling. If the fracture stress σ is the strength of the material, the following equation can be used to determine the temperature differential Δtf, which will produce fracture:
Δtf = σ(1-μ)/α E = f(S/E α)
Where S is the strength of the material
The value of tf is also an inverse function of thermal diffusivity h, which is equal to K/cp ρ:
Δtf = f(1/h) = f(cp ρ/K)
In this relationship, K is the thermal conductivity, cp is the specific heat capacity and ρ is the density. This is a significant relationship because materials with high thermal conductivities and/or low-volume heat capacities (cp ρ), permit rapid temperature equalization and reduced stresses.
It is sometimes convenient to use spalling resistance indices, SRI, as the following:
SRI = hS/α E
As such, the SRI is the ratio of (S/E α) and (1/h). This can be therefore re-written as:
SRI =( K S/α E cp ρ)
Since this is only an index and not a fundamental property, variants of this equation are often used, such as:
SRI = TSI =(K S/α E)
S is sometimes represented as σ.
Using this index, the TSI can be simply calculated as:
TSI = (6 Btu-in/hr-ft²-⁰F)(850 psi)/(8.9 x 10⁻⁶/F⁰)(2.75×10⁶ psi) = 208.4 Btu-in/hr-ft²
This indicates that Pre-Krete is highly resistant to thermal stress cracking.
Pre-Krete offers excellent resistance to Thermal Shock. ASTM has no Standard for Thermocycling/Thermal Shock in masonry materials. Pocono Fabricators created a test procedure where Pre-Krete was applied to a steel angle iron, heated to 350oF and plunged into 40oF water @ 100cycles with no adverse effects.
K-Factor or Thermal Conductivity
This represents the material’s thermal conductivity or ability to conduct heat. It is offered as:
BTU / Sq. Ft. /Hr. /Deg. F/Inch.
Pre-Krete has a K-Factor of 6 at 1” thickness. Most often this information is utilized to determine the outside steel temperature of a tank lined with Pre-Krete. This information is important to determine if the tank requires insulation or barrier to protect individuals from being burned. The following graph will provide hot face / cold face temperatures for various thicknesses.
Density – Graded Aggregate
Pre-Krete is formulated to produce a dense, low permeability system. It carries a wet density of 140 Lbs. /Cu. Ft. A well graded aggregate is essential to achieving this goal. The image below demonstrates the effects of a well graded aggregate minimizing permeability.
In addition to a well graded aggregate, it is imperative that the aggregate makeup be pure with no impurities that would affect the corrosion resistance of the Pre-Krete. Pre-Krete incorporates silica as one of its aggregates. Impurities such as lime (CaO) commonly found in silica would be detrimental in an acidic environment.
The physical properties of the aggregate are also important. Pre-Krete incorporates an abrasion resistant chip that enhances its abrasion resistant properties. Pre-Krete has been tested in accordance to ASTM C704-88 (Abrasion Resistance of Refractory Materials). In laboratory conditions, our systems have an average abrasion loss of 6 – 10 cc’s. As a gunned system slightly higher results were achieved. The average abrasion loss was 12 – 15 cc’s. In general, anything below 20 cc’s is considered “abrasion resistant” for refractory materials.
However, Pre-Krete is not a true “refractory”. It is classified as a “corrosion resistant masonry” product. For these types of products, abrasion resistance is measured using a Taber Abrasion Machine. This utilizes a rotating disc of Pre-Krete being subjected to a carbide tipped pressure point with a constant mass for 10,000 revolutions. The depth of the groove is measured. The maximum depth of the groove measured was .0035”. This result is 6 times greater than 6000 psi concrete.
Absorption Factor – Barrier Linings
Pre-Krete lining systems are not “barrier” type linings. A barrier lining, such as a rubber lining, acts as a barrier to keep the contained substance from reaching the steel or concrete substrate.
Pre-Krete is a cementitious type lining with an absorption rate of 6 – 8%. Therefore, the contained liquid is absorbed by the Pre-Krete and becomes saturated. When applied at a minimum ½” thickness the saturated lining does not allow flow through the lining and prohibits free or dissolved oxygen from reaching the substrate. It is the oxygen that causes oxidation or rust/corrosion.
In order to insure there is no oxygen exchange, it is important that the substrate be sound and leak free. A leak in the steel or concrete will allow oxygen to enter the structure and facilitate corrosion between the tank shell and the Pre-Krete. Rusted steel expands (at a minimum) 6 to 1. This expansion will push against the Pre-Krete causing it to crack and become disbonded from the substrate.
Cracking and Self-Healing Properties
Hairline cracks in Pre-Krete are common. Our Pre-Krete system has an exceptionally high percentage of cement in the cement to aggregate ratio. This causes an accelerated rate of hydration that can result in the cracking of the Pre-Krete.
This does not present a problem. In the curing process of all cement based systems, there is always some unreacted cement. When the cement based system, in this case Pre-Krete, is placed into service the unreacted cement reacts with moist gasses and/or liquid causing the un-hydrated cement to hydrate or form insoluble salts which will fill and seal the cracks. These processes will not only cause hairline cracks to close but will sometimes close cracks with openings of as much as 1/4 of an inch. This is combined with the fact that Pre-Krete has an initial absorption rate of approximately 7% by weight. The absorption causes the Pre-Krete to swell and expand. This will further cause any cracks to close or, in concrete terms, “self-heal”.