Is there a difference between cement and concrete?
These two terms may appear the same, but they have a significant difference. However, they are two different things. In fact, cement is only a component of concrete. Concrete is a mixture of water and aggregates and other chemical components. The aggregates are gravel and sand, while water is considered the paste. The strength of concrete improves as it gets older.
Portland cement is the general term for the type of cement used in concrete. By volume, cement makes up 15 percent of the total concrete mix. The mixture undergoes the process called hydration, where the aggregates are bound together and harden. The process of hardening goes on for years, which makes the concrete even stronger as the years pass.
The proper terms are concrete mixer and concrete sidewalk and not cement sidewalk and mixer.
How is Portland cement manufactured?
Portland cement is made from ingredients that have calcium compounds, alumina and iron oxide, and silica. The components are ground and screened, then placed in a cement kiln. The kiln looks like a horizontal pipe with a diameter of 10 to 15 feet with one end slightly elevated. The mixture is put in the high end while the kiln spins and slowly moving the mix to the lower end. At the lower end of the kiln, flame jets heat the materials to high temperatures up to 3000 Fahrenheit. The extreme heat takes away calcines, carbon dioxide, and water from the materials and makes new compounds. Two-thirds of the total ton of the material that goes into the kiln comes out of the discharge end, which is called the clinker. The clinker comes in the form of pellets. The clinker is ground to very fine particles and become Portland cement. Gypsum, a small amount of it, is added in the grinding process to determine the cement’s rate of hardening.
What is “curing” in concrete?
Curing is an essential step in making concrete. Proper curing affects the strength and durability of concrete. Due to hydration, concrete hardens, but it will only take place with the help of water and the right temperature. Curing happens from five to seven days after they have been placed. The topside of concrete should be kept damp to allow hydration to happen. For new concrete, you can use sprinklers or cover them with damp burlap. You can also coat the concrete with curing compounds, which helps keep moisture in.
What is the ideal temperature to place new concrete?
Extreme temperatures can affect the curing process. Too much water evaporates when the temperature is too hot. On the other hand, hydration slows down when the temperature drops. Due to extreme temperature conditions, concrete fails to strengthen and form ideal properties. The temperature of new concrete should not fall less than 10 Celsius while it is curing.
What does air-entrained in concrete mean?
Air-entrained concrete has millions and millions of air cells per cubic foot. The air pockets release pressure on the concrete to allow small cells for water to enhance when it freezes. Air-entrained concrete is made using air-entraining Portland cement, and it has to be done under engineering supervision. The entrained air allowed should be between 4 and 4 percent of the entire amount of concrete. It may vary at times, depending on special conditions.
What is the ideal proportion for mixing concrete?
You can achieve good concrete by using a wide range of mix proportions, and the right design methods are utilized. The rule of thumb is to use the rule of 6’s:
- Minimum cement content of 6 bags per cubic yard of concrete
- Maximum water of 6 gallons per bag of cement
- Curing period of at least six days
- Air content of 6 percent, if the concrete will undergo freezing and thawing
What causes concrete to crack?
After concrete completely dries, it will have a slight change in volume. In general, the change can amount to roughly 500 millionths, which is 1/16 of one inch in 10 feet. Contractors place joints in concrete floors to let the concrete crack in a good manner due to shrinkage.
Why is testing concrete essential?
Testing concrete is crucial to ensure the components disclosed and purchased are the same ones transported to the job location. There are several ways to test freshly mixed concrete and hardened concrete. Some organizations, such as the Federal Highway Administration and departments of transportation, even have their own way to check the quality of concrete.
What are the most common tests for fresh concrete?
There are three common ways to test freshly mixed concrete: slump, air content, unit weight, and compressive strength.
- Slump- This concrete test aims to measure the concrete’s consistency or its ability to flow. There can be problems with correct consolidation if the concrete can’t flow due to too low slump. On the other hand, if the slump is too high, and the concrete doesn’t stop flowing, there may be likely issues with mortar loss through excessive formwork pressures and delays in finishing and segregation.
- Air content- the measure of the total air component of freshly mixed concrete. This measure cannot indicate the final in-place air content will be due to loss of air during transit, consolidating, placement, and finishing. The pressure meter and volumetric method and the Chace Indicator are some of the field tests used for air content measure.
- Unit weight- measures the density of fresh concrete.
- Compressive strength- This is tested by pouring fresh concrete and measuring the force required to break the concrete cylinders at prescribed intervals. The concrete is generally acceptable for as long as no test goes beyond 500 psi below the specified strength. If the average of three consecutive tests matches or goes beyond the design strength, the concrete is considered good. If in case, the strength tests don’t meet these standards, the average should be raised using acceptable procedures.
How can you make sure you are getting the right amount that you purchased?
You can base it on the volume produced on the actual batch of cement, water, and aggregates. You can use the unit weight test to determine the yield of the freshly mixed concrete when it is transported to the site. This calculation will require the unit weight of the components batched. You can either ask the concrete producer or check the delivery ticket for the total weight details. Most reputable concrete producers over yield by 50 percent to ensure they don’t fall short on their customer’s orders. However, it can also be possible that a concrete producer only produced roughly 95 to 98 of their original designed.
What causes flaking and chipping in concrete?
These happen for a variety of reasons:
Concrete should be air entrained in regions that undergo freezing and thawing to prevent flaking and chipping of concrete surface. The surface may acquire damages over time if air-entrained concrete is not used.
There should be an accurate proportion of water and cement. As much as possible, the water/cement ratio should be low to ensure the strength of the surface. If there is too much water in the mix, the concrete will end up weak and less durable. This can result in flaking and spalling of the concrete’s surface.
The process of finishing should only be started when the water on the surface has completely evaporated. If the excess water was not allowed to be gone before the finishing operations begin, the surface of the concrete would have excessive water concrete. Therefore it will be as strong and less durable.
Will the hardening of concrete take place underwater?
Yes. Portland cement is a hydraulic cement, which means it will set and harden because of a chemical reaction with water so that it will harden even underwater.
What does 28-day strength mean?
Like what we’ve mentioned earlier, concrete becomes stronger and hardens as it hydrates. The process of hydration doesn’t end in one day but continues for a period of time. At first, the process happens quickly until it slows down over time. You will be able to measure the strength of concrete after years from its placement. However, this method is rather impractical, which is why a period of 28 days was determined by authorities as the general age where the concrete should be tested for strength. During this time, the majority of the hydration process has already happened.
What is 3,000-pound concrete?
This is concrete that can carry a compressive stress of 3000 psi after 28 days, such as for a concrete driveway. There are other ways to specify the strength of concrete aside from this. In general, standard concrete has 7000 psi or fewer strengths. If the strength of concrete is somewhere between 7000 and 15,000 psi, then it is identified as high-strength.
What are the ways to control concrete strength?
If you want to add strength to the concrete, adding cement is the way to do it. The major indicator that affects concrete strength is the proportion of cement to water because this binds the aggregates together. If the ratio is high, the strength of concrete will be weak. On the other hand, if the ratio is low, the concrete will be stronger. If you add more water to the mix, it will negatively affect the physical state of the concrete. This is the reason why it is not recommended to add water to what is ideal.
Is it possible to get rid of stains from concrete, and how?
Yes, you can remove stains from concrete using mechanical methods or wet methods with water or chemical.
Some of the common mechanical or dry methods are flame cleaning, sandblasting, shot blasting, grinding, planing, and scouring. If you are using steel-wool brushes, make sure to use it with care because they can leave metal particles that later on cause rusting and staining on the concrete’s surface.
You can also use the wet method by applying chemicals depending on the type of stain. The chemical will dissolve the stain or bleach it so it will no longer be visible. For instance, you can remove blood stains with water and cover it with a layer of sodium peroxide powder. Let it sit for a few minutes, then rinse with water and scrub thoroughly.
What can decorative finishes be used to surfaces of concrete?
There are several ways to perform decorative finishes to concrete. You can add colors before or after the concrete is placed, and instead of using the conventional gray cement, you can use white cement. You can also use chemical stains or make colorful aggregates exposed on the surface. You can have a smooth polish depending on the smoothness of gravel.
You can also choose to stamp, score, or inlay patterns so the concrete will resemble materials like stone, brick, or tiles. There are available techniques to make concrete slip-resistant.
How can I protect my concrete surface from harsh materials?
Concrete is very durable, which is why many products don’t have an effect on it. But there are some materials such as acids that can have an adverse effect on it. One of the best ways to protect concrete from harsh chemicals is by using top quality concrete. Another is to apply for protection on the surface to prevent harsh materials from touching the concrete. Choose the right cement type and practice the right water/cement ratio.
Is there a standard international specification for Portland cement?
There is no standard international specification for Portland cement because each country has its own standard. For the United States, we use the standards set by the American Society for Testing and Materials Standard Specification for Portland Cement. While some countries use this as their specification, there are other standards available.
What is ASR or alkali-silica reactivity?
This is the reaction between silica in aggregates, potassium, and sodium alkalis from cement as well as from aggregates, admixtures, and mixing water. The reaction can also be affected by outside sources such as alkali from the soil and industrial processes. An alkali-silica gel forms due to the reaction. This induces pressure, cracking of the paste and the aggregate. This causes alligator pattern cracking or map pattern cracks. There are ways to avoid ASR, including using the proper aggregate, using blended types of cement, right pozzolanic materials, and water free from contaminants.
What are the different types of Portland cement?
While most Portland cement is essentially the same, there are eight types of cement produced to meet specific standards and chemical requirements:
Type I cement is basically used for general purposes and is ideal for many uses.
Type II is used when heat build-up is an issue and for structures in water or soil without excessive amounts of sulfate
Type III cement is used when you want high strength for less time
Type IV cement is mostly used for massive concrete structures such as dams because it moderates heat generation by hydration
Type V cement is chemical attack-resistant especially by soil and water high in sulfates
Types IA, IIA, and IIIA are cement types that are used to manufacture air-entrained concrete. Technically, they share the same property as type I, II, and III, but they don’t have as many air-entrained materials in their components.
White Portland cement has little or no iron or manganese that gives the cement its gray color.
If there are questions we were not able to answer here, please don’t hesitate to give us a call and we will be more than glad to enlighten you with answers.