Frequently Asked Questions (FAQs)

Technical Answers for Your Questions

Frequently Asked Questions (FAQs)

Concrete

Durability of concrete refers to its ability to resist the effects of the environment and retain its structural integrity over time. The following factors can affect the durability of concrete:

  • Water-cement ratio: The water-cement ratio (W/C) is a major factor that affects the durability of concrete. A lower water-cement ratio results in a more dense concrete that is less permeable and more resistant to water and chemical attack.

  • Aggregate: The quality and properties of the aggregate used in the concrete mix can affect the durability of concrete. Aggregates that are hard, dense, and non-reactive are preferred to provide a strong and durable concrete.

  • Cement: The type and quality of cement used in the concrete mix can affect the durability of concrete. High-quality cements provide more durable concrete than low-quality cements.

  • Curing: Proper curing is essential for the durability of concrete. Curing helps to prevent cracking and shrinkage and promotes the strength and durability of concrete.

  • Exposure environment: The environment in which the concrete is exposed to can affect its durability. Concrete exposed to harsh weather conditions, freeze-thaw cycles, salt, chemicals, and high levels of moisture can be more susceptible to damage.

  • Reinforcement: The type and quality of reinforcement used in the concrete can affect the durability of concrete. Properly designed and placed reinforcement can improve the durability of concrete.

  • Use of supplementary cementing materials: The use of supplementary cementing materials like fly ash or slag can improve the durability of concrete by providing a more dense and less permeable concrete.

  • Quality control during production: Proper control of the production process can ensure that the ingredients of concrete are correctly proportioned and that the concrete is mixed, placed, and cured correctly, which can help to improve the durability of concrete.

Overall, to ensure the durability of concrete, a combination of good mix design, quality materials, proper construction practices, and effective quality control is important.

Permeability of concrete refers to the ability of liquids and gases to penetrate through the concrete. A concrete with low permeability is considered to be more durable and resistant to the effects of water and chemicals.

The following factors can affect the permeability of concrete:

  • Water-cement ratio: The water-cement ratio (W/C) is the most important factor affecting the permeability of concrete. The lower the water-cement ratio, the lower the permeability will be.

  • Cement content: The cement content of concrete affects the permeability as well, the higher the cement content, the lower the permeability of the concrete.

  • Aggregate: The size, shape, and surface texture of aggregate affect the permeability. Coarser, rougher and angular aggregates tend to make the concrete more porous and less permeable.

  • Pore structure: The pore structure of concrete can affect permeability. The presence of interconnected pores and voids can allow liquids and gases to penetrate more easily, while a denser pore structure can impede the movement of liquids and gases through the concrete.

  • Air content: The entrained air in the concrete can increase the permeability of concrete by creating small interconnected pores.

  • Curing conditions: The curing conditions can affect the permeability. If the concrete is not cured properly, the water evaporates quickly leaving behind large pores which can increase the permeability of concrete.

  • Chloride ingress: Chloride ions can penetrate concrete, especially if the surface is not protected by coating or if the surface is exposed to marine environments, which can lead to corrosion of reinforcement steel and decreased service life.

To make concrete less permeable, mix designs with lower water-cement ratios, higher cement content, appropriate aggregate selection, proper curing and adequate protection of the surface against chloride ingress are usually employed.

The strength of concrete is affected by the air content, which refers to the amount of entrained air that is present in the concrete mix. Entrained air is air that is deliberately introduced into the concrete mix by means of an air-entraining agent.

The addition of air-entraining agent can cause the following effects on concrete strength:

  • Increase in workability: Air-entraining agents can make the concrete more workable, making it easier to place and finish.

  • Increase in freeze-thaw resistance: The entrained air bubbles act as cushioning agents and help to protect the concrete against damage caused by freezing and thawing cycles.

  • Decrease in compressive strength: The entrained air bubbles can reduce the density of concrete and decrease the compressive strength by around 5-10 %.

  • Decrease in bond strength: Entrained air can reduce the bond between the concrete and reinforcement steel, which can decrease the bond strength.

  • Increase in permeability: Entrained air can increase the permeability of concrete, which can increase the rate of water and chloride ion penetration.

The amount of entrained air that is used in concrete is typically between 2-8% by volume of the concrete. As the amount of entrained air increases, the concrete becomes less dense and weaker. However, the trade-off is that the concrete becomes more workable and more resistant to freeze-thaw cycles.

It is important to choose the right amount of air content to balance between the strength and durability of concrete and the workability requirements.

The water-cement ratio (W/C) is a measure of the amount of water used in relation to the amount of cement in a concrete mix. It is an important factor that affects the properties of concrete.

A lower water-cement ratio typically results in:

  • Higher compressive strength: Lower W/C ratios tend to increase the compressive strength of concrete. The extra water in the mix acts as a lubricant for the concrete, making it easier to place and finish. However, the extra water also causes the concrete to be weaker because it dilutes the cement paste and causes the hydration products to be less dense.

  • Lower permeability: Lower W/C ratios lead to more dense concrete, which reduces the permeability and makes the concrete more resistant to water and chemical attack.

  • Low shrinkage and creep: Lower W/C ratios lead to lower shrinkage and creep, which can reduce the potential for cracking caused by drying shrinkage and long-term loading.

  • Low thermal cracking: With lower water-cement ratio, the concrete is less prone to thermal cracking due to the reduced heat of hydration

A higher water-cement ratio typically results in:

  • Lower compressive strength: Higher W/C ratios tend to decrease the compressive strength of concrete.

  • Higher permeability: Higher W/C ratios lead to more porous concrete, which increases the permeability and makes the concrete more susceptible to water and chemical attack.

  • Higher shrinkage and creep: Higher W/C ratios lead to higher shrinkage and creep, which can increase the potential for cracking caused by drying shrinkage and long-term loading.

  • High thermal cracking: With higher water-cement ratio, the concrete is more prone to thermal cracking due to the increased heat of hydration

The optimal water-cement ratio depends on the intended use of the concrete, the environment in which the concrete will be used, and the desired properties of the concrete. Typically, a water-cement ratio of 0.4 to 0.6 is considered to be a good balance between workability and strength.

Temperature can have a significant effect on the properties of concrete. The following are some ways in which temperature can affect concrete:

  • Setting and hardening time: Temperature can affect the rate of setting and hardening of concrete. High temperatures can cause the concrete to set and harden more quickly, while low temperatures can cause the concrete to set and harden more slowly.

  • Strength development: Temperature can affect the rate of strength development of concrete. High temperatures can accelerate the rate of strength development, while low temperatures can slow it down.

  • Shrinkage: Temperature can affect the amount of shrinkage that occurs in concrete as it dries and hardens. High temperatures can cause more shrinkage, while low temperatures can cause less shrinkage.

  • Thermal cracking: Concrete is known to expand and contract with the change in temperature and these expansions and contractions can cause cracks in concrete known as thermal cracking. High temperatures during curing can accelerate the rate of thermal cracking.

  • Freezing and thawing resistance: The ability of concrete to resist the damage caused by freezing and thawing cycles is known as its freeze-thaw resistance. Low temperatures during curing can make the concrete more susceptible to freeze-thaw damage.

  • Creep: Temperature also affects the long-term deformation of concrete under sustained load, known as creep. High temperatures can increase the creep deformation, while low temperatures can decrease the creep deformation.

To mitigate the effects of temperature on concrete, it is recommended to maintain a consistent temperature during the curing process, which is usually done by providing insulation, or by protecting the concrete from direct sunlight or cold temperatures. Additionally, mix design can also be modified to optimize the properties of concrete under different temperature conditions.

Fly ash is a byproduct of coal-fired power plants and is commonly used as a supplementary cementing material in the production of concrete. The use of fly ash can have the following effects on the properties of concrete:

  • Improved workability: Fly ash can improve the workability of concrete by making it more cohesive and easier to handle.

  • Increased compressive strength: Fly ash can contribute to the strength of concrete by reacting with the calcium hydroxide produced during the hydration of cement to form additional cementitious compounds. This can increase the compressive strength of the concrete, typically at 28 days.

  • Increased durability: Fly ash can improve the durability of concrete by reducing the permeability, which can make the concrete more resistant to water and chemical attack.

  • Reduced heat of hydration: Fly ash can reduce the heat of hydration, which can help to prevent cracking caused by thermal expansion during the curing process.

  • Reduced shrinkage and creep: Fly ash can reduce the shrinkage and creep of concrete, which can help to prevent cracking caused by drying shrinkage and long-term loading.

  • Reduced Alkali-silica reactivity (ASR): Fly ash can reduce the potential for Alkali-silica reactivity (ASR) by reacting with the alkalis present in cement to form less-reactive compounds.

  • Environmental benefits: Using fly ash as a supplementary cementing material can help to reduce the amount of cement needed, which can lower greenhouse gas emissions from the cement production process and also reduce the pressure on natural resources.

However, it’s important to ensure that the type and quality of fly ash is appropriate for the intended application. Furthermore, the optimal replacement rate of fly ash to cement should be determined based on local codes and regulations and the specific characteristics of the fly ash and the intended use of the concrete.

Delayed ettringite formation (DEF) in concrete is caused by a chemical reaction between the sulfates present in the concrete and the tri-calcium aluminate (C3A) in the cement. The reaction forms ettringite, which can expand and cause cracking and damage to the concrete.

The factors that contribute to DEF include:

  • Sulfates in the concrete: The presence of sulfates, either in the form of sulfate ions in the pore water or as part of the aggregate, is necessary for DEF to occur.

  • Tri-calcium aluminate (C3A) in the cement: C3A is necessary for the formation of ettringite. Cements with higher content of C3A are more prone to DEF.

  • Moisture: Sufficient moisture and water is required for the reaction to occur.

  • Time: DEF is a delayed reaction, and typically does not occur until several months or even years after the concrete has been placed.

  • High concrete temperature and humidity during curing.

To prevent DEF, it’s necessary to avoid using aggregates that are high in sulfates, use low-C3A cement and mix design that minimize the water-cement ratio, and control the curing temperature and humidity.

Also, using a pozzolanic material like fly ash or slag cement can help to reduce the risk of DEF because they can help to consume the sulfates and C3A before they can react to form ettringite.

Rhyolite is a type of volcanic rock that is composed mainly of quartz and feldspar. It is not typically used as an ingredient in concrete, because it is not readily available and is not considered to have any significant effect on the properties of concrete.

Concrete is typically made by mixing cement, water, and an aggregate (such as sand and gravel or crushed stone) together. The cement is the binding agent that holds the concrete together, while the aggregate provides strength and bulk to the concrete. The water activates the cement and starts the chemical reactions that harden the concrete.

In general, the aggregate used in concrete can have some effect on the properties of the final product. For example, using a fine aggregate (such as sand) can make the concrete stronger, while using a coarse aggregate (such as gravel) can make the concrete more durable. The shape, size, and texture of the aggregate can also affect the workability, density, and compressive strength of the concrete.

Rhyolite, however, is not considered to be an appropriate aggregate for concrete as it’s hard to find and also not suitable for commercial use. Instead, concrete uses natural gravel, crushed rock and sand. The use of rhyolite in concrete, therefore, would not affect the properties of the final product.

Steel

Cracks in steel welds can be caused by a variety of factors, including:

  • Improper welding technique: Cracks can form when the welder does not maintain the correct welding speed, or if the welding current or voltage is too high or too low.

  • Improper fit-up: Cracks can form if the edges of the steel that are being welded are not properly aligned, if there are gaps between the edges, or if the edges are not cleaned properly before welding.

  • Improper preheat: Cracks can occur if the steel is not preheated to the appropriate temperature before welding, especially when welding thick sections.

  • Improper post-weld heat treatment: Cracks can occur if the welded steel is not cooled at the appropriate rate after welding, especially when welding with certain types of filler materials.

  • Residual stress: Cracks can occur due to the residual stress present in the welded zone due to the thermal contraction and expansion during cooling or welding.

  • Improper welding procedure: Cracks can occur when welding procedure is not followed as per the standards.

  • Improper selection of filler metal: Cracks can occur if the filler metal has different properties or composition than the base metal.

  • Improper Inspection: Failure to identify defects such as slag inclusions or porosity can lead to cracks after weldment.

Proper training of welders, using the correct welding techniques and procedures, using the appropriate filler metal, and properly inspecting the welds can help to prevent cracks from forming in steel welds.

Porosity in steel welds is a type of internal defect that occurs when small pockets of gas become trapped in the weld metal during the welding process. These pockets of gas can weaken the strength of the weld and make it more susceptible to corrosion. Porosity in steel welds is caused by a variety of factors, including:

  • Improper cleaning: If the surfaces to be welded are not cleaned properly before welding, porosity can occur due to the presence of dirt, oil, or other contaminants.

  • Improper shielding: Porosity can occur if the weld is not properly shielded from the surrounding air. Shielding gases, such as argon, can help to prevent the formation of porosity by protecting the molten weld metal from oxidation and other forms of contamination.

  • Improper technique: Porosity can occur if the welder uses an improper welding technique, such as using too high or too low a welding speed, or if the welding current or voltage is not set correctly.

  • Improper filler material: If the wrong filler material is used, or if the filler material is not heated properly before welding, porosity can occur.

  • Improper fit-up: Porosity can occur if the edges of the steel that are being welded are not properly aligned, or if there are gaps between the edges.

  • Improper preheat: Porosity can occur if the steel is not preheated to the appropriate temperature before welding.
  • Improper post-weld heat treatment: Porosity can occur if the welded steel is not cooled at the appropriate rate after welding, or if the steel is not heated properly before post-weld heat treatment.

To prevent porosity, proper surface preparation, proper shielding, and the use of the correct filler materials and welding techniques are necessary. Also, proper preheat, post-weld heat treatment and inspection of the finished weld are also important steps to prevent porosity in steel welds.

Lack of fusion in steel welds is a type of internal defect that occurs when the edges of the steel being welded are not properly fused together. This can cause a weak spot in the weld that can lead to failure under stress. Lack of fusion is caused by a variety of factors, including:

  • Improper cleaning: If the surfaces to be welded are not cleaned properly before welding, lack of fusion can occur due to the presence of dirt, oil, or other contaminants.

  • Improper fit-up: Lack of fusion can occur if the edges of the steel that are being welded are not properly aligned, or if there are gaps between the edges.

  • Improper technique: Lack of fusion can occur if the welder uses an improper welding technique, such as using too high or too low a welding speed, or if the welding current or voltage is not set correctly.

  • Improper filler material: If the wrong filler material is used, or if the filler material is not heated properly before welding, lack of fusion can occur.

  • Improper preheat: Lack of fusion can occur if the steel is not preheated to the appropriate temperature before welding.

  • Improper post-weld heat treatment: Lack of fusion can occur if the welded steel is not cooled at the appropriate rate after welding, or if the steel is not heated properly before post-weld heat treatment.

  • Welding in multi-passes: Lack of fusion can occur if the welding is not done properly in multi-passes, especially if not enough heat is generated to melt the edges for proper fusion.

Inclusions in steel welds refer to the presence of solid impurities or non-metallic substances that are trapped within the weld metal. Inclusions can weaken the weld and make it more susceptible to cracking and corrosion. Inclusions in steel welds are caused by a variety of factors, including:

  • Improper cleaning: If the surfaces to be welded are not cleaned properly before welding, inclusions can occur due to the presence of dirt, oil, or other contaminants.

  • Improper filler material: If the wrong filler material is used, inclusions can occur.

  • Improper shielding: Inclusions can occur if the weld is not properly shielded from the surrounding air. Shielding gases, such as argon, can help to prevent the formation of inclusions by protecting the molten weld metal from oxidation and other forms of contamination.

  • Improper technique: Inclusions can occur if the welder uses an improper welding technique, such as using too high or too low a welding speed, or if the welding current or voltage is not set correctly.

  • Improper post-weld heat treatment: Inclusions can occur if the welded steel is not cooled at the appropriate rate after welding, or if the steel is not heated properly before post-weld heat treatment.

To prevent inclusions, proper surface preparation, proper shielding, the use of the correct filler materials and welding techniques, and proper post-weld heat treatment are necessary.

Phased array ultrasonic testing (PAUT) is a method of using ultrasonic waves to inspect materials for defects and measurements.

It uses a phased array probe, which is made up of a number of individual ultrasonic transducer elements that are grouped together in a linear or two-dimensional array.

Each element can be pulsed independently, and the time delay between the pulse of each element can be adjusted to control the direction and focus of the ultrasonic beam.

The basic principles of PAUT are as follows:

  • Pulse-echo technique: A single ultrasonic pulse is transmitted into the material and the echoes are received by the same probe, similar to conventional ultrasonic testing.

  • Time delay control: The time delay between the excitation of individual elements can be adjusted, to steer the beam in different directions and create a focal zone at a specific depth in the material.

  • Scan pattern: By adjusting the time delay for different elements of the array, the beam can be scanned in a certain pattern and cover a large area, providing a 2D image of the inspected area.

  • Focusing: By adjusting the time delay for different elements, the phase of the waves can be adjusted to focus the beam in a specific area, providing improved resolution and sensitivity.

  • Multiple angles: By changing the time delay and angle of incidence, inspection can be done from different angles.

PAUT can provide a better resolution and sensitivity over conventional ultrasonic testing, due to the ability to focus the ultrasonic beam, and scan the inspected area with multiple angles and at different depths. It is particularly useful in inspecting complex geometries, such as welds, and for detecting small or deep-seated defects.

General

Quality control and quality assurance are two closely related but distinct concepts that are often used in industries such as manufacturing, construction, and service. Both are focused on ensuring that products or services meet a certain level of quality and that they conform to customer and industry specifications. However, the difference between the two is in how they approach this goal.

Quality Control (QC) is the process of inspecting, testing, and measuring products or services to ensure that they meet established standards and specifications. QC activities are focused on identifying and correcting defects and problems in products or services before they are released to the customer. This is accomplished through a variety of techniques such as inspections, testing, and measurements.

Quality assurance (QA) is a broader concept that includes not just quality control, but also the entire process of planning, designing, and implementing a system of processes, procedures, and standards to ensure that a product or service is of the highest quality. Quality Assurance is about the overall process, not just the final product. QA activities are focused on preventing defects and ensuring that processes are in place to consistently produce products and services that meet or exceed customer requirements.

Simply put, Quality control is the inspection process and quality assurance is the management process. Quality control is about catching defects, and quality assurance is about preventing them.

In a nutshell, Quality control is about inspecting, testing and measuring products or services to ensure that they meet established standards and specifications. Quality assurance is about the entire process of planning, designing, and implementing a system of processes, procedures, and standards to ensure that a product or service is of the highest quality.

Accuracy and precision are two related but distinct concepts that are often used in fields such as science, engineering, and manufacturing. They are both used to describe the level of agreement between a measured value and the true value, but they refer to different aspects of that agreement.

Accuracy refers to how close a measured value is to the true value of a quantity. In other words, it is a measure of how much a measurement deviates from the true or accepted value. A measurement can be accurate but not precise, or precise but not accurate.

Precision, on the other hand, refers to the degree of reproducibility or consistency of a measurement. It is a measure of how close the set of measured values are to each other. A measurement can be precise, but not accurate.

An easy way to remember the difference between accuracy and precision is by thinking about a target:

  • If all the arrows or bullets hit the bull’s-eye, the shooter is both accurate and precise.
  • If all the arrows or bullets hit the target but not the bull’s-eye the shooter is precise but not accurate.
  • If the arrows or bullets are scattered all over the target, the shooter is neither precise nor accurate.

In short, accuracy is how close a measurement is to the true value, and precision is how reproducible the measurement is.

Procedure and process are related terms that are used to describe the way in which work is organized and performed, but they have slightly different meanings.

A procedure is a set of detailed instructions that describes how to perform a specific task or activity. It is a step-by-step guide that outlines the specific actions that need to be taken, in a specific order, in order to complete a task or achieve a specific result. Procedures are often written down and may include flow charts, diagrams, or other visual aids to make them easy to follow. They are often used to ensure consistency and compliance in workplaces, and also serve as a reference guide for employees.

A process, on the other hand, is a broader concept that refers to the entire sequence of activities or tasks that are involved in achieving a specific goal or outcome. A process is a series of steps or actions that are taken to transform inputs into desired outputs. It is the overall method by which work is organized and executed, and it often involves multiple procedures and sub-processes. Processes can also involve different people and departments and can be more general and abstract than procedures.

In summary, procedures are detailed instructions on how to complete a specific task, while processes are the overarching method of how work is organized, executed and achieved. Procedures are usually specific to certain activities and tasks, while processes are more general and holistic. Procedures are an important part of processes and procedures are used to ensure consistency and compliance in executing the process.

We perform a full spectrum of physical, chemical, and nondestructive tests. We also offer in-shop, third-party inspections for your listed products. Please contact us for technical details and pricing.

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