Concrete is a versatile and powerful tool in many forms of construction. Its strength comes from a fine balance in the different components that make it up. Understanding these ratios and the value behind each of them allows you to create the most effective form of concrete, but what if you include too much cement? Putting too much cement in concrete can result in several disadvantages.
- If too much is added to the mix, the concrete’s workability will suffer, and some of the aggregates won’t properly bond to the cement.
- If too much is used versus the aggregate, the final product’s structural integrity will likely decrease.
- We will focus on the risks of using the wrong ratios for making concrete and what each of them could lead to.
Careful attention should always be placed on how much of each of these elements are involved. Image via Shivanshu Guar via Unsplash
- 0.1 Is too much cement good?
- 0.2 How does cement content affect concrete strength?
- 1 How do you reduce cement in concrete?
- 2 What increases concrete strength?
- 3 What makes concrete so strong?
- 4 Is concrete stronger with less water?
- 5 How do you increase the flow of concrete?
- 6 Will concrete shrink as it dries?
- 7 What does too much water in concrete look like?
- 8 What is the strongest concrete mix ratio?
- 9 How cement is harmful to the environment?
Is too much cement good?
How-To – Posted on: December 01, 1990 I know if you add too much sand or lime to a mortar it can become weak. Is there any problem with adding too much portland cement to a mortar? Portland cement gives a mortar added durability, high early strength, a consistent hardening rate, and high compressive and bond strengths.
However, mortar with excessively high cement contents can affect water permeability and durability of the masonry. Water permeability is increased for three reasons. First, mortars with high cement content have greater shrinkage and more frequent shrinkage cracks. Shrinkage cracking often takes the form of evenly spaced vertical cracks in bed joints and evenly spaced horizontal cracks in head joints.
The mortar may also separate or pull away from the body of the brick at the bond interface. Although high cement content mortar itself is less permeable, water can penetrate the shrinkage cracks. The second reason that high cement content mortars may increase water permeance is that these mortars are stiff and not easily workable.
- Because of this, good bond may not be achieved at the time of construction.
- Finally, the high strength (high cement content) mortars are hard (brittle) and don’t easily accommodate movement of the masonry (due to environmental changes or foundation settlement) and therefore may result in cracking of the masonry wall.
Cracks make the walls more susceptible to freeze-thaw deterioration even though high cement content mortar itself is more resistant to frost damage. Mortar should always be weaker than the masonry unit to accommodate small movement without damage to the masonry unit.
How does cement content affect concrete strength?
Increasing cement content typically results in an increase in compressive strength, particularly early age strength, however with all other factors kept standard the w/cm has the greater effect on strength.
What is the effect of increasing cement & water on concrete?
How Moisture Affects Concrete Strength – Increased space between cement grains : Higher water-to-cement ratios result in greater spacing between the aggregates in cement, which affects compaction. Similarly, increased moisture levels reduce the concrete’s compressive strength and durability.
- As concrete’s surface area increases, particularly with the addition of fine aggregates, so does the demand for water.
- The increased water leads to a higher water-to-cement ratio.
- When excess water creates greater spaces between aggregate materials, the voids fill with air after the moisture evaporates.
The resulting inadequate compaction reduces the concrete’s strength. Concrete with trapped air levels as little as 10 percent experiences reductions in strength of up to 40 percent. pH levels : Relative humidity levels and pH in concrete are directly related.
As humidity levels increase, so does the concrete’s pH and temperature. As pH levels in concrete increase, the more likely floor covering adhesive bonds will fail. While higher temperatures allow concrete to dry faster, it results in a less structured, more porous product. Water containing bicarbonate ions and carbon dioxide causes a reaction known as carbonation in concrete.
This often happens in the presence of salts and acid rain. As the acidic substances lower the concrete’s pH, the calcium carbonate within the aggregates dissolves and reduces the concrete’s strength. Eventually, the concrete will crumble into sand and rock.
- As the concrete becomes more acidic and damage progresses, the acids will eventually affect the protective layer of iron oxide on steel reinforcements, leading to corrosion.
- Steel expands as it corrodes.
- This expansion within already weakened concrete will cause it to further break and crack.
- Microbial growth : High relative humidity levels, increased temperatures and porous concrete create the perfect breeding ground for mold, bacteria and other organisms.
While concrete doesn’t contain sufficient organic materials for mold to feed on, it traps dust, pollen, microorganism and salts, which are food sources. When mold feeds on the particles trapped within concrete, it excretes acids that degrade the building material’s strength and integrity.
How do you reduce cement in concrete?
Ways to Reduce Cement Content Optimizing aggregate gradations (1) will help increase workability and lower the paste content. Lower paste contents helps in reducing thermal and drying shrinkage (lower cracking potential). Lower paste contents also reduce permeability, thereby enhancing long term durability.
Why is my concrete staying wet?
How to Stop Concrete Sweating > Articles Have you ever experienced the phenomena of concrete sweating? Concrete sweating is often times mistaken for a water problem since it is common to see a condensation effect on the surface of the slab. Concrete sweating usually occurs when warm air comes into contact with a cooler concrete slab and it causes the concrete floor to sweat.
Continue reading to learn how to prevent and reduce concrete sweating. After a humid night, water vapor in the air will come into contact with the cooler concrete floor and will condense into morning dew. What causes concrete to sweat? SSS or sweating slab syndrome is a phenomenon where moisture intermittently develops on the surface of an interior concrete slab.
When warm air comes into contact with a colder temperature concrete slab, the condensation will build up on the surface and can commonly be defined as concrete sweating. The combination of temperature swings and humidity build up cause the concrete to sweat.
Concrete sweating, also known as sweating slab syndrome (SSS), refers to condensation that develops on the surface of the concrete. It is directly related to the dew point. “If the surface of a floor slab is colder than the dew point temperature of the ambient air above the slab, moisture will condense on the surface of the slab.
This condition, commonly called “sweating,” typically occurs when warm, moist air flows into a building that has relatively cool floors.” Kanare, H.M., “Sources of Moisture”, Concrete Floors and Moisture, page 15.
- Why is concrete sweating problematic?
- Concrete sweating make concrete slippery and can be dangerous on larger concrete slabs with a lot of traffic.
- How to stop concrete sweating
- Dry out the room by air movement: a high volume low speed fan works well.
- Turn down the air conditioning/turn up the heat to keep the concrete floor temperature and the air temperature at similiar levels.
- Use a dehumidifier to remove moisture from the air and reduce the condensation.
- Use a penetrating sealer to seal the concrete and keep out moisture. We recommend using a concrete densifier first as a primer followed up by a penetrating concrete sealer.
It is important to use a penetrating concrete sealer as soon as possible so that you do not have to deal with mold or mildew issues which are a result of concrete sweating.
- One of the best solutions to combat concrete sweating is to use a chemically reactive concrete densifier followed by an impregnating, penetrating sealer.
- The is first used as a primer to reduce the porosity of the concrete.
- By using a penetrating, water repellent sealer, the impregnates the concrete making it less susceptible to concrete sweating.
Lithi-Tek 4500 – Primer & Siloxa-Tek 8510 – Sealer STEP 1: Lithi-Tek 4500 – Penetrating Concrete Densifier & Hardener:
- First used as a primer to fill in pores and voids making concrete stronger, harder, less porous and less susceptible to concrete sweating
- Leaves a clear, natural finish that will never yellow, peel, flake or delaminate
- Lifespan: +100 years – once concrete has been treated with a densifier you do not need to retreat
STEP 2: Siloxa-Tek 8510 – Penetrating Water Repellent Concrete Sealer:
- Used as a top coat sealer to reduce sweating and water penetration
- Resists mold & mildew reducing cleaning frequency
- Makes future oil and grease stains easier to clean
- Leaves a clear, natural finish that will never yellow, peel, flake or delaminate
- Lifespan: +10 Years
Published Monday 31st of October 2022 // Updated Thursday 1st of July 2021 : How to Stop Concrete Sweating > Articles
What increases concrete strength?
Increasing Concrete Strength Compressive strength is the most common performance metric for concrete, The compressive strength of concrete can be increased by:
Including admixturesAdjusting the cement type and quantityReducing the water/cement ratioUtilizing supplementary cementitious materials (SCMs)Altering the aggregates – type and gradations
Additionally, high-early strength concrete can be achieved through a, : Increasing Concrete Strength
What makes concrete so strong?
« Back to Help & Advice Concrete is a fantastic material that is used worldwide to create some of the most impressive, strong and long lasting structures. Suitable for a whole range of constructions, including pavements, houses, skyscrapers, dams and bridges, the strength of concrete very rarely fails to hold up. The most simple answer comes down to the production process: concrete is produced when a paste of portland cement and water is mixed with both small and large aggregates. When the paste and aggregates meet, the paste coats the surface of the aggregate and a chemical reaction called hydration takes places, causing the paste to harden and the entire mixture to gain strength.
- The science The science behind the reaction is a little more complex: Concrete is a composite material that is made up of a binder (the cement paste) and a filler (aggregate), which when mixed form chemical bonds with water molecules to become hydrated.
- This hydration causes the concrete to cure.
- The tricalcium silicate compound is responsible for most of the strength of concrete, releasing calcium ions, hydroxide ions and heat, which speeds up the reaction process.
Once the material is saturated with calcium and hydroxide ions, calcium hydroxide begins to crystallise and calcium silicate hydrate forms. The production of crystallisation leads to more and more calcium silicate hydrate forming and thickening until the water molecules’ path is blocked and there are no longer any empty, weak spaces in the mixture.
This results in a very low porous, and therefore very strong, structure. Even stronger concrete Following the science behind the creation of concrete, there are simple steps that can be taken to make concrete even stronger. High-strength concrete is created by simply adjusting the ratios of cement, water, aggregates and admixtures to form stronger bonds that make for more robust, long lasting concrete.
The lower the water to cement ratio, the stronger the mix will be, but as water is an essential element to the hydration process, it is necessary to ensure that enough is still used to saturate the mixture. In general, cement is made with a water to cement mass ratio of between 0.35-0.6. Click image for full size view As well as being immensely strong and popular for use in construction, concrete has a rather interesting history; read our 8 amazing facts about concrete here! Concrete use dates back to Egyptian times Yes, even the early Egyptians saw the benefits of using concrete in their buildings, with a primitive form made from gypsum and lime being used as infill in the construction of the pyramids over 5,000 years ago.
- Mud and straw was used to form the bricks.
- The world’s largest unreinforced concrete dome still stands today Located in Rome, the Pantheon was commissioned by Marcus Agrippa as a temple to all the ancient Roman gods, but sadly both this and the second structure caught on fire.
- The third Pantheon, which is still standing today, was finished under the rule of the emperor Hadrian in 126AD.
Standing at 43 metres tall and 43 metres wide, this dome is still the world’s largest unreinforced (without steel bearers) dome today, beating out other impressive structures such as St. Peter’s Basilica. Thanks to the durability and longevity of concrete, the Pantheon is considered to be one of the best preserved buildings of ancient Rome You can get bubblegum scented concrete Yes, you read that right! Construction companies such as Quintechs LLC have been experimenting with decorative concrete, including making concrete smell more appealing. Smells available include bubblegum, vanilla, thyme, coffee and lavender.
On large concrete structures the scented form release agents are applied directly onto cured concrete where they react with the alkali present and bond, emitting a pleasant smell. Concrete makes up one of the Seven Wonders of the World The Statue of Christ the Redeemer, located in Brazil, is built from reinforced concrete and soapstone.
The statue took nine years to complete through the 1920s and weighs in at 635 tonne with a height of 30.1 metres. Concrete was used during World War II to detect approaching aircrafts Huge concrete dishes, often referred to as ‘concrete ears’ or ‘sound mirrors’, were erected along the south coast of England during the Second World War.
- Small microphones were suspended above the dishes, allowing a listener to detect approaching aircraft up to 27 miles away.
- The advancements in technology, however, soon made these systems obsolete.
- The oldest piece of concrete on record is 12 million years old The oldest ever piece of concrete is over 12 million years old and was found in Israel in the 1960s.
The natural deposit was formed when an oil shale had exploded near limestone and when the two touched, a layer of concrete was formed. Thomas Edison saw the potential of concrete Thomas Edison had 49 patents experimenting with single pour concrete houses and furniture, including refrigerators and pianos, which he thought could be built in a single pour with the right mold. Unfortunately, production scale was limited and the production wasn’t financially viable, otherwise who’d have known what we’d be using concrete for now! Man-made cement was invented by a Leeds Bricklayer In 1824, an English bricklayer named Joseph Aspdin patented portland cement after many previous failed attempts.
Aspdin began by mixing clay and limestone together, and when this mixture burned a fine powder was created, which he named after Portland because it reminded him of a stone on the isle. This creation radicalised the construction industry as it was so fast-setting and aesthetically attractive. If you’re looking for high quality concrete suppliers in Essex, look no further than Neil Sullivan and Sons.
We are well experienced in all aspects of concrete services for both domestic and commercial projects, including supplying volumetric concrete and on-site mixed concrete, floor screed and aggregates, For more information, get in contact today.
What decreases the strength of concrete?
Concrete Strength Cement like water, aggregates and some times admixtures is one of the ingredient of concrete. The mixing of these materials in specified proportions produces concrete. Accordingly cement alone is not a building material, it is the concrete which is a building material.
For a given cement and acceptable aggregates, the strength that may be developed by a workable, properly placed mixture of cement, aggregates, and water (under same mixing, curing and testing conditions) is influenced by the : a) Ratio of cement to mixing water b) Ratio of cement to aggregates, the strength of the mortar, the bond between the mortar and the coarse aggregate.
c) Grading, surface texture, shape, strength, and stiffness of aggregate particles. d) Maximum size of aggregate. Strength of concrete is directly related to the structure of the hydrated cement paste. Air in concrete produces voids. Excess of water in concrete evaporate leave the voids in the concrete.
- Consequently, as the W/C ratio increases, the porosity of the cement paste in the concrete also increases.
- As the porosity increases, the compressive strength of the concrete decreases.
- STRENGTH OF CEMENT V/S STRENGTH OF CONCRETE It is not possible to design a concrete mix of high strength with cement of low strength.
The variation in strength of cement is due largely to the lack of uniformity in the raw materials used in its manufacture, not only between different source of supply, but also with in a quarry. Further, differences in details of the process of manufacture and above all, the variation in the ash content of coal used to fire the kilin, contribute to the variation in the properties of commercial cements.
- This is not to deny that the modern manufacturing of cement is a highly sophisticated process.
- Upto 1975, the mass production of cement in India was only OPC-33 Grade.
- It was found difficulty in obtaining high strength concrete with this cement.
- The consumer has been normally finding it difficult to get consistent and ensured supply of high strength cement for prestressed concrete and certain items of precast concrete.
For these special requirements BIS published IS:8112, Specification for OPC-43 Grade cement. Now, the varieties of cement manufactured in India are: 1. Ordinary Portland Cement (Grade OPC-33, OPC-43 and OPC-53. OPC-33 Grade almost vanished from Indian market) 2.
Portland Pozzolana Cement (PPC) 3. Sulphate Resistance Cement (SRC) Test results of different brand of cement minimum to maximum compressive strength are given in the table-1. Due to variation of cement strength, the concrete made from these cement will also have variable strength. For a correct approach in the Concrete Mix Design, if the facilities at site are available, with the given set of materials, requirements and site conditions own W/C ratio v/s compressive strength of concrete curve should be developed at site itself.
It is often observed that cement bags marked as OPC-43 Grade may really be containing cement of much higher grade. PPC cement as per IS Code is only of 33 Grade. Where as on bags it is marked as 43 MPa or 53 MPa. Site cement samples should be tested for its actual strength and other properties.
- There are instances where higher grade cement is being used even for low strength concrete, as mortar or even for plastering.
- This can lead to unnecessary cracking of concrete/surfaces.
- In low grade OPC, the gain in strength will continue beyond 28th day.
- Due to early strength gain of higher grade of OPC the concrete strength do not increase much beyond 28th day.
The heat of hydration of higher grade OPC being higher, the chances of micro-cracking of concrete is much greater. Thus during initial setting period of concrete, the higher head of hydration can lead to damaging micro-cracking with in the concrete which may not be visible at surface.
Is concrete stronger with less water?
What is in This Stuff? The importance of concrete in modern society cannot be overestimated. Look around you and you will find concrete structures everywhere such as buildings, roads, bridges, and dams. There is no escaping the impact concrete makes on your everyday life.
- So what is it? Concrete is a composite material which is made up of a filler and a binder.
- The binder (cement paste) “glues” the filler together to form a synthetic conglomerate.
- The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate.
- The role of these constituents will be discussed in this section.
Cement, as it is commonly known, is a mixture of compounds made by burning limestone and clay together at very high temperatures ranging from 1400 to 1600 ]C. Although there are other cements for special purposes, this module will focus solely on portland cement and its properties.
The production of portland cement begins with the quarrying of limestone, CaCO 3, Huge crushers break the blasted limestone into small pieces. The crushed limestone is then mixed with clay (or shale), sand, and iron ore and ground together to form a homogeneous powder. However, this powder is microscopically heterogeneous.
(See flowchart.) Figure 1: A flow diagram of Portland Cement production. The mixture is heated in kilns that are long rotating steel cylinders on an incline. The kilns may be up to 6 meters in diameter and 180 meters in length. The mixture of raw materials enters at the high end of the cylinder and slowly moves along the length of the kiln due to the constant rotation and inclination. Figure 2: Schematic diagram of rotary kiln. As the mixture moves down the cylinder, it progresses through four stages of transformation. Initially, any free water in the powder is lost by evaporation. Next, decomposition occurs from the loss of bound water and carbon dioxide.
This is called calcination, The third stage is called clinkering. During this stage, the calcium silicates are formed. The final stage is the cooling stage. The marble-sized pieces produced by the kiln are referred to as clinker, Clinker is actually a mixture of four compounds which will be discussed later.
The clinker is cooled, ground, and mixed with a small amount of gypsum (which regulates setting) to produce the general-purpose portland cement. Water is the key ingredient, which when mixed with cement, forms a paste that binds the aggregate together.
The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process.
The role of water is important because the water to cement ratio is the most critical factor in the production of “perfect” concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.
- Walls, domes, etc.).
- Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.
- Aggregates are chemically inert, solid bodies held together by the cement.
- Aggregates come in various shapes, sizes, and materials ranging from fine particles of sand to large, coarse rocks.
Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used.70 to 80% of the volume of concrete is aggregate keeping the cost of the concrete low. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete.
- For example, the density of concrete is determined by the density of the aggregate.
- Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion.
- Aggregates should be clean, hard, and strong.
- The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste.
It is then separated into various sizes by passing the material through a series of screens with different size openings. Refer to Demonstration 1 Table 1: Classes of Aggregates
|class||examples of aggregates used||uses|
|ultra-lightweight||vermiculite ceramic spheres perlite||lightweight concrete which can be sawed or nailed, also for its insulating properties|
|lightweight||expanded clay shale or slate crushed brick||used primarily for making lightweight concrete for structures, also used for its insulating properties.|
|normal weight||crushed limestone sand river gravel crushed recycled concrete||used for normal concrete projects|
|heavyweight||steel or iron shot steel or iron pellets||used for making high density concrete for shielding against nuclear radiation|
Refer to Demonstration 2 The choice of aggregate is determined by the proposed use of the concrete. Normally sand, gravel, and crushed stone are used as aggregates to make concrete. The aggregate should be well-graded to improve packing efficiency and minimize the amount of cement paste needed.
- Also, this makes the concrete more workable.
- Refer to Demonstration 3 Properties of Concrete Concrete has many properties that make it a popular construction material.
- The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal.
- Good-quality concrete has many advantages that add to its popularity.
First, it is economical when ingredients are readily available. Concrete’s long life and relatively low maintenance requirements increase its economic benefits. Concrete is not as likely to rot, corrode, or decay as other building materials. Concrete has the ability to be molded or cast into almost any desired shape.
- Building of the molds and casting can occur on the work-site which reduces costs.
- Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures.
- It is resistant to wind, water, rodents, and insects.
- Hence, concrete is often used for storm shelters.
- Concrete does have some limitations despite its numerous advantages.
Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking. Concrete remains the material of choice for many applications regardless of these limitations.
Hydration of Portland Cement Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Concrete does not need to dry out in order to harden as commonly thought. The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden).
When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years.
|Cement Compound||Weight Percentage||Chemical Formula|
|Tricalcium silicate||50 %||Ca 3 SiO 5 or 3CaO, SiO 2|
|Dicalcium silicate||25 %||Ca 2 SiO 4 or 2CaO, SiO 2|
|Tricalcium aluminate||10 %||Ca 3 Al 2 O 6 or 3CaO, Al 2 O 3|
|Tetracalcium aluminoferrite||10 %||Ca 4 Al 2 Fe 2 O 10 or 4CaO, Al 2 O 3, Fe 2 O 3|
|Gypsum||5 %||CaSO 4,2H 2 O|
Table 2: Composition of portland cement with chemical composition and weight percent. When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail. The equation for the hydration of tricalcium silicate is given by: Tricalcium silicate + Water->Calcium silicate hydrate+Calcium hydroxide + heat 2 Ca 3 SiO 5 + 7 H 2 O -> 3 CaO,2SiO 2,4H 2 O + 3 Ca(OH) 2 + 173.6kJ Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH – ) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved. The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier’s principle). The evolution of heat is then dramatically increased. The formation of the calcium hydroxide and calcium silicate hydrate crystals provide “seeds” upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower. Figure 3: Schematic illustration of the pores in calcium silicate through different stages of hydration. The above diagrams represent the formation of pores as calcium silicate hydrate is formed. Note in diagram (a) that hydration has not yet occurred and the pores (empty spaces between grains) are filled with water. Diagram (b) represents the beginning of hydration. In diagram (c), the hydration continues. Although empty spaces still exist, they are filled with water and calcium hydroxide. Diagram (d) shows nearly hardened cement paste. Note that the majority of space is filled with calcium silicate hydrate. That which is not filled with the hardened hydrate is primarily calcium hydroxide solution. The hydration will continue as long as water is present and there are still unhydrated compounds in the cement paste. Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate: Dicalcium silicate + Water->Calcium silicate hydrate + Calcium hydroxide +heat 2 Ca 2 SiO 4 + 5 H 2 O-> 3 CaO,2SiO 2,4H 2 O + Ca(OH) 2 + 58.6 kJ The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Because these reactions do not contribute significantly to strength, they will be neglected in this discussion. Although we have treated the hydration of each cement compound independently, this is not completely accurate. The rate of hydration of a compound may be affected by varying the concentration of another. In general, the rates of hydration during the first few days ranked from fastest to slowest are: tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate. Refer to Demonstration 4 Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown below as a function of time. Figure 4: Rate of heat evolution during the hydration of portland cement The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage.
- The dormancy period can last from one to three hours.
- During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty.
- This is particularly important for the construction trade who must transport concrete to the job site.
- It is at the end of this stage that initial setting begins.
In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.
- Refer to Demonstration 5 Strength of Concrete The strength of concrete is very much dependent upon the hydration reaction just discussed.
- Water plays a critical role, particularly the amount used.
- The strength of concrete increases when less water is used to make concrete.
- The hydration reaction itself consumes a specific amount of water.
Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms, The water not consumed in the hydration reaction will remain in the microstructure pore space. Figure 5: Schematic drawings to demonstrate the relationship between the water/cement ratio and porosity. The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph that follows. Figure 6: A plot of concrete strength as a function of the water to cement ratio. Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability. The physical characteristics of aggregates are shape, texture, and size.
These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio. Time is also an important factor in determining concrete strength.
Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete’s strength. It is common to use a 28-day test to determine the relative strength of concrete.
- Concrete’s strength may also be affected by the addition of admixtures.
- Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process.
- Some admixtures add fluidity to concrete while requiring less water to be used.
- An example of an admixture which affects strength is superplasticizer.
This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.
|AIR ENTRAINING||improves durability, workability, reduces bleeding, reduces freezing/thawing problems (e.g. special detergents)|
|SUPERPLASTICIZERS||increase strength by decreasing water needed for workable concrete (e.g. special polymers)|
|RETARDING||delays setting time, more long term strength, offsets adverse high temp. weather (e.g. sugar )|
|ACCELERATING||speeds setting time, more early strength, offsets adverse low temp. weather (e.g. calcium chloride)|
|MINERAL ADMIXTURES||improves workability, plasticity, strength (e.g. fly ash)|
|PIGMENT||adds color (e.g. metal oxides)|
Table 3: A table of admixtures and their functions. Durability is a very important concern in using concrete for a given application. Concrete provides good performance through the service life of the structure when concrete is mixed properly and care is taken in curing it.
- Good concrete can have an infinite life span under the right conditions.
- Water, although important for concrete hydration and hardening, can also play a role in decreased durability once the structure is built.
- This is because water can transport harmful chemicals to the interior of the concrete leading to various forms of deterioration.
Such deterioration ultimately adds costs due to maintenance and repair of the concrete structure. The contractor should be able to account for environmental factors and produce a durable concrete structure if these factors are considered when building concrete structures.
Does adding more water to concrete make it weaker?
The compressive strength of a concrete mixture is reduced when additional water is added.
How do you increase the flow of concrete?
Improving concrete consistency – In a typical job site, if the slump or slump flow is low, water is often added to the mix. Unfortunately, adding water weakens the concrete. The ready mix producer can add a water reducer or superplasticizer along with synthetic macro fibers right into the concrete mix to mitigate this concern.
What happens if you add less water to concrete?
Water in Concrete Water content is the single most important factor affecting workability or the ease of mixing and placing concrete. The amount of water in concrete controls many fresh and hardened properties in concrete including workability, compressive strengths, permeability and watertightness, durability and weathering, drying shrinkage and potential for cracking.
Water-cementitious materials ratio Code requirements Water content and drying shrinkage Workability Adding water onsite If measured slumps are less than allowed by the specifications, slumps may be adjusted by a one-time addition of water. However, there are requirements associated with adding water onsite:
The ratio of the amount of water, minus the amount of water absorbed by the aggregates, to the amount of cementitious materials by weight in concrete is called the water-cementitious ratio and commonly referred to as the w/cm ratio. The w/cm ratio is a modification of the historical water-cement ratio (w/c ratio) that was used to describe the amount of water, excluding what was absorbed by the aggregates, to the amount of the portland cement by weight in concrete.
- Because most concretes today contain supplementary cementitious materials such as fly ash, slag cement, silica fume, or natural pozzolans, the w/cm ratio is more appropriate.
- To avoid confusion between the w/cm and w/c ratios, use the w/cm ratio for concretes with and without supplementary cementitious materials.
The w/cm ratio equation is: w/cm ratio = (weight of water – weight of water absorbed in the aggregates) divided by the weight of cementitious materials. Upon hardening, the paste or glue consisting of the cementitious materials and water binds the aggregates together.
- Hardening occurs because of the chemical reaction, called hydration, between the cementitious materials and water.
- Obviously, increasing the w/cm ratio or the amount of water in the paste dilutes or weakens the hardened paste and decreases the strength of the concrete.
- As shown Figure 1, concrete compressive strength increases as w/cm ratio decreases for both non-air-entrained and air-entrained concrete.
Decreasing the w/cm ratio also improves other hardened concrete properties by increasing the density of the paste which lowers the permeability and increases watertightness, improves durability and resistance to freeze-thaw cycles, winter scaling and chemical attack.
- In general, less water produces better concrete.
- However, concrete needs enough water to lubricate and provide a workable mixture that can be mixed, placed, consolidated and finished without problems.
- Because w/cm ratio controls both strength and durability, building codes have set upper limits or maximum w/cm ratios and corresponding minimum compressive strengths as shown in Table 1.
For example, concrete exposed to freezing and thawing in a moist condition or to deicing chemicals shall have a maximum 0.45 w/cm ratio and a minimum 4,500 psi compressive strength to ensure durability. Designers select maximum w/cm ratios and minimum strengths primarily based on exposure conditions and durability concerns — not load-carrying capacity requirements.
For different exposure conditions, use the code required maximum w/cm ratios and minimum strengths to reduce the permeability of the concrete. Doing so will increase the concrete’s resistance to weathering. The most important factor affecting the amount of drying shrinkage and the subsequent potential for cracking is the water content or the amount of water per cubic yard of concrete.
Fundamentally, concrete shrinkage increases with higher water contents. About half of the water in concrete is consumed in the chemical reaction of hydration and the other half provides the concrete’s workability. Except for the water lost to bleeding and absorbed by the base material or forms, the remaining water that is not consumed by the hydration process contributes to drying shrinkage.
By keeping the water content as low as possible, drying shrinkage and the potential for cracking can be minimized. The ease of mixing, placing, consolidating and finishing concrete is called workability. The water content of the mixture is the single most important factor that affects workability. Other important factors that affect workability include: mix proportions, characteristics of the coarse and fine aggregates, quantity and characteristics of the cementitious materials, entrained air, admixtures, slump (consistency), time, air and concrete temperatures.
Adding more water to the concrete increases workability but more water also increases the potential for segregation (settling of coarse aggregate particles), increased bleeding, drying shrinkage and cracking in addition to decreasing the strength and durability.
Do not exceed the maximum water content for the batch as established by the accepted concrete mixture proportions. No concrete has been discharged from the mixer except for slump testing. All water additions shall be completed within 15 minutes from the start of the first water addition. Water shall be injected into the mixer with such pressure and direction of flow to allow for proper distribution within the mixer. The drum shall be turned an additional 30 revolutions or more at mixing speed to ensure a homogenous mixture.
Before adding water onsite, the allowable amount of water that can be added must be known. This amount should be printed on the delivery ticket or be determined during the pre-construction meeting and be agreed upon by all parties. Water is a key component in concrete.
However, too much water can be detrimental to both the fresh and hardened concrete properties, especially strength, long term durability and potential for cracking. On your next job, be sure to know the water requirements for the concrete mixtures being used, especially the allowable water that can be added for slump adjustments.
References Kosmatka, S.H., and Wilson, M.L., Design and Control of Concrete Mixtures, 15th edition, (PCA), www.concrete.org : Water in Concrete
Will concrete shrink as it dries?
Why Does Concrete Shrink? – In order for concrete to hydrate and gain strength over time, the minimum amount of water that is needed is 26 gallons per yard. All water over 26 gallons is only used for pumpability and workability of the concrete mixture.
- During the mixing stages, when more water is added than the design requires for a measured slump, it helps to place and work with the concrete.
- However, the extra water is not used for the hydration process and bleeds out of the concrete.
- As the water leaves the concrete, it creates a volume change, known as drying shrinkage.
If the concrete is not strong enough during the curing process to withstand the tensile forces of this volume change, the concrete will crack.
What does too much water in concrete look like?
Summary: What If Concrete Is Too Wet? – Dry concrete is made by mixing sand, stone and cement together in a specific ratio. When water is added, it chemically reacts with cement to form a paste that binds the ingredients together. Water slowly evaporates out as the concrete dries and strengthens over a long period of time.
- This process is called curing and typically takes around 28 days to complete.
- One of the biggest mistakes you can make when mixing concrete is adding too much water.
- When concrete is too wet its final strength is greatly reduced.
- This can spell disaster for structural projects like foundations and footings that rely on strong concrete.
This is why masons carefully monitor the amount of water they add and mix concrete as dry as possible. If concrete gets too wet it causes excess shrinkage during the drying process. As a result, the concrete is prone to cracks that can be large enough to damage the structure.
- While cracks are a problem, the real issue with watered down concrete is weakness.
- A watery mix can greatly reduce the compressive strength of concrete.
- As a rule of thumb, every additional inch of slump reduces concrete’s compressive strength by about 500 psi.
- That strength reduction can spell disaster if the concrete is being used to support something heavy.
When concrete gets too wet, it becomes weak and prone to cracks, breakage, weakness and failure. The ideal consistency should feel like thick oatmeal and should never be watery. When concrete has too much water it also dries more porous. This is because of all the water that needs to evaporate out.
What is the fastest way to get moisture out of concrete?
The Basics Of Drying Your Home Out After A Leak – There are two main areas of focus when you are trying to dry up after a water leak: remove the water as quickly as possible, and ensure that the affected area is dried out thoroughly. Standing water causes damage by its very presence, and so needs to be cleared out as soon as it is safe to do so.
Many homeowners make a rookie mistake: they can no longer see any water, and so they assume that the surface is safe to start drying out. They then start the repairs, risking damp, mold and mildew becoming locked in once the new surfaces are laid – this, in turn, can cause an even more expensive headache down the line.
Getting concrete floors completely dry is the key to avoiding this issue; but just how do you achieve this?
Remove the Water
As we mentioned, the first priority is to remove as much excess water as possible, via mopping, draining or pumping. This helps to minimise the damage, and allows the drying process to occur more quickly.
Allow Air To Circulate
Once you have removed all of the water, the next stage is to air out the room completely. The easiest – and cheapest – way to achieve this is to open as many windows as possible, allowing fresh air to circulate and remove moisture from the room. Adequate ventilation seriously speeds up the natural drying process, allowing you to go in with tools to speed up the process.
Bring in The Big Guns
Once the windows are open and the fresh air is doing its thing, you can use dehumidifiers or fans to speed up the drying process. A dehumidifier is an essential tool; as the name suggests, it removes the humidity and moisture from the air, allowing the floor beneath to dry more quickly and thoroughly and reducing the risk of damp or mould having a chance to develop.
In most cases, the concrete floor will be bare and ready to work with, but there may be situations where sections are covered with carpet or other flooring. If this is the situation, make sure that this is removed as quickly as possible – a covering will prevent the concrete base from drying properly or completely, and this can slow down the process, as well as risking complications down the line.
What is the strongest concrete?
Ultra-High Performance Concrete (UHPC) is a cementitious, concrete material that has a minimum specified compressive strength of 17,000 pounds per square inch (120 MPa) with specified durability, tensile ductility and toughness requirements; fibers are generally included in the mixture to achieve specified requirements. Ultra-High Performance Concrete (UHPC), is also known as reactive powder concrete (RPC). The material is typically formulated by combining portland cement, supplementary cementitious materials, reactive powders, limestone and or quartz flour, fine sand, high-range water reducers, and water. The material can be formulated to provide compressive strengths in excess of 29,000 pounds per square inch (psi) (200 MPa). The use of fine materials for the matrix also provides a dense, smooth surface valued for its aesthetics and ability to closely transfer form details to the hardened surface. When combined with metal, synthetic or organic fibers it can achieve flexural strengths up to 7,000 psi (48 MPa) or greater. Fiber types often used in UHPC include high carbon steel, PVA, Glass, Carbon or a combination of these types or others. The ductile behavior of this material is a first for concrete, with the capacity to deform and support flexural and tensile loads, even after initial cracking. The high compressive and tensile properties of UHPC also facilitate a high bond strength allowing shorter length of rebar embedment in applications such as closure pours between precast elements. UHPC construction is simplified by eliminating the need for reinforcing steel in some applications and the materials high flow characteristics that make it self-compacting. The UHPC matrix is very dense and has a minimal disconnected pore structure resulting in low permeability (Chloride ion diffusion less than 0.02 x 10-12 m2/s. The material’s low permeability prevents the ingress of harmful materials such as chlorides which yields superior durability characteristics. Some manufacturers have created just-add-water UHPC pre-mixed products that are making UHPC products more accessible. The American Society for Testing and Materials has established ASTM C1856/1856M Standard Practice for Fabricating and Testing Specimens of Ultra High Performance Concrete that relies on current ASTM test methods with modifications to make it suitable for UHPC. The following is an example of the range of material characteristics for UHPC: Strength Compressive: 17,000 to 22,000 psi, (120 to 150 MPa) Flexural: 2200 to 3600 psi, (15 to 25 MPa) Modulus of Elasticity: 6500 to 7300 ksi, (45 to 50 GPa) Durability Freeze/thaw (after 300 cycles): 100% Salt-scaling (loss of residue): < 0.013 lb/ft3, (< 60 g/m2) Abrasion (relative volume loss index): 1.7 Oxygen permeability: < 10-19 ft2, (<10-20 m2) Figure 1. Shawnessy Light Rail Transit Station, Calgary, Canada
What is the strongest concrete mix ratio?
Strong Concrete Mix Ratio – In making concrete strong, these ingredients should usually be mixed in a ratio of 1:2:3:0.5 to achieve maximum strength. That is 1 part cement, 2 parts sand, 3 parts gravel, and 0.5 part water.
Does more cement make it stronger?
Concrete’s effectiveness depends on its ingredients and consistency. You don’t want a mixture that shrinks or becomes brittle; nor do you want it to be runny. There will be four basic materials you need in your mix: Portland cement, sand, aggregate and water.
- Adding water will form a paste that will bind the materials together until the mix hardens.
- The strength of the concrete is inversely proportional to the water/cement ratio.
- In other words, the more water you use to mix the concrete, the weaker the concrete mix.
- The less water you use to mix the concrete, the stronger the concrete mix.
A mix with little water and more concrete mix will be dryer and less workable but stronger. But of course the water makeup isn’t the only consideration. The sand and the aggregate help to reduce the cost and also limit the amount of shrinking that happens to the concrete as it cures.
- In order to produce a strong, resilient concrete mix, you need to get the ratio of aggregate to sand to cement right.
- Consider the following formulas as you mix your concrete: One standard recipe calls for one part of cement to two parts of sand to four parts of gravel.
- This results in a C20-rated concrete mix, which means the concrete will be of medium strength.
Concrete is rated on a system that indicates the strength of the mix after it’s cured for approximately a month. To make the concrete stronger, add more cement or less sand. The closer you bring the ratio to an even one-to-one of sand to cement, the stronger the rating becomes.
- This principles works in the opposite direction as well.
- If you want to get a little more technical, some concrete experts recommend going for 26 percent sand, 41 percent gravel, 11 percent cement and 16 percent water.
- The lacking 6 percent volume is air entrainment.
- Air entrainment is an admixture added to the mix during production to assist the mix in resisting the damaging effects of freeze-thaw cycles.
This admixture is required in all concrete exposed to exterior elements. Overall this makes a good general purpose mix for foundations and other structures. While Portland cement is the standard for concrete mixtures, the type of sand you use may vary.
- Unwashed beach sand creates a mixture that isn’t quite as strong as products made with sand that’s been cleaned.
- Clean sand tends to produce a more high-quality product.
- You can achieve an accurate mixing ratio by using buckets or other measuring devices to get the right quantity of each ingredient for your mixture.
Getting the right ratios throughout the process means getting consistent mix throughout your whole concrete project. For an accurate estimate of the paving materials needed for a project, please visit our calculators page.
How harmful is cement?
Health Hazards of Working With Cement in Construction Projects Workers using or supervising the use of cement should know the health hazards, understand the risks, and follow safe working procedures necessary to limit harmful exposure. Although the terms cement and concrete often are used interchangeably, cement (also known as portland cement) is actually an ingredient of concrete. Cement makes up about 10 to 15% of the overall concrete mix when added to sand and gravel or crushed stone along with water.
OSHA 1926 Subpart Q provides the specific standards for concrete including requirements for equipment and tools, cast-in-place concrete, formwork, precast concrete, lift-slab operations and masonry construction. Anyone who works with cement, or products that contain cement, is at risk of developing mild to severe skin problems, eye irritation and symptoms of respiratory distress. Wet cement can cause caustic burns on the skin, sometimes referred to as cement burns. Continuous contact between the skin and wet concrete allows alkaline compounds to penetrate and burn the skin. Skin contact with wet cement that is not washed off immediately can cause inflammation of the skin, referred to as dermatitis, with symptoms that might include itching, redness, swelling, blisters, scaling or other changes to the skin.
Cement burns may result in blisters, dead or hardened skin, or black or green skin. In severe cases, these burns may extend to the bone and cause disfiguring scars or disability. When working with cement, do not rely on pain or discomfort as the sign to get washed off! Wash areas of the skin that come into contact with wet or dry cement in clean, cool water as soon as possible.
Use a pH-neutral or slightly acidic soap. Employees working with wet cement who begin to experience skin problems, even if they seem minor, are advised to see a health care professional for evaluation. Early diagnosis and treatment can help prevent chronic skin problems.
By the time a person becomes aware of a cement burn, much of the damage has already happened, and cement burns can get worse even after there is no longer skin contact with the cement. Any person experiencing a cement burn should see a health care professional immediately. Dry cement is less hazardous to the skin because it is not as caustic as wet cement.
However, be aware of cement dust released during bag dumping or concrete cutting. When moisture from sweat or wet clothing reacts with cement dust it can form a caustic solution that will burn the skin. Only mix dry cement in well-ventilated areas. Some workers may develop allergic skin reactions to the hexavalent chromium in cement which can produce symptoms ranging from a mild rash to severe skin ulcers. When emptying bags of cement, workers may be inhaling high levels of dust which can irritate the nose and throat causing breathing difficulties. Exposure to airborne dust may cause immediate or delayed irritation of the eyes. Depending on the level of exposure, effects may range from redness to chemical burns and blindness.
OSHA has established a permissible exposure limit (PEL) to address the inhalation hazards of working with dry portland cement. (See OSHA 1926.55) Employers must limit airborne exposure to portland cement to 15 milligrams per cubic meter (mg/m3) of air for total dust and 5 mg/m3 for respirable dust. When cement dust cannot be avoided or may exceed the PEL, suitable (such as a P, N or R 95 respirator) should be worn.
Wearing proper PPE, including gloves, boots and eye protection, is critical when working with materials that contain cement, like wet concrete. Butyl or nitrile gloves (rather than cotton or leather gloves) are frequently recommended for caustic materials such as cement.
- Gloves should be alkali-resistant.
- Use only well-fitting gloves that are not loose enough to let the cement inside.
- Glove liners can be used for added comfort.
- Before putting on gloves, wash your hands and dry them with a clean cloth or paper towel.
- Before removing reusable gloves, clean them thoroughly by rinsing or wiping them off.
Wash your hands every time you remove your gloves. Always dispose of contaminated or worn-out gloves. Don’t wash your hands with water from buckets used for cleaning tools. For additional protection, wear a long sleeve shirt and tape the sleeves to the gloves to prevent wet materials from getting inside the gloves. Wear sturdy, waterproof boots that are high enough to prevent wet cement from getting inside. Where mixing, pouring, or other cement work activities may endanger the eyes, suitable must be worn.
- At minimum, safety glasses with side shields or goggles should be worn.
- Avoid wearing contacts when handling cement products.
- When kneeling on fresh concrete, use a dry board or waterproof kneepads to protect knees from the wetness that can soak through fabric.
- Change out of any work clothes that become soaked with wet cement and keep dirty work clothes separate from your street clothes.
OSHA Standard 1926.51(f)(1) The employer shall provide adequate washing facilities for employees engaged in the application of paints, coating, herbicides, or insecticides, or in other operations where contaminants may be harmful to the employees. Such facilities shall be in near proximity to the worksite and shall be so equipped as to enable employees to remove such substances. Weeklysafety.com is giving away 10 free safety topics, no credit card required! Take advantage and grab your free set of safety meeting topics today by clicking the button below. A membership to Weeklysafety.com comes at a very low price that never goes up no matter how many employees you have and no matter how many awesome safety topics you use.
How cement is harmful to the environment?
Health and Environmental Effects of Cement Plant Emissions – Cement plants are a significant source of sulfur dioxide, nitrogen oxide and carbon monoxide, which are associated with the following health and environmental impacts:
Nitrogen oxide (NO x ) can cause or contribute to a variety of health problems and adverse environmental impacts, such as ground-level ozone, acid rain, global warming, water quality deterioration, and visual impairment. Affected populations include children, people with lung diseases such as asthma, and exposure to these conditions can cause damage to lung tissue for people who work or exercise outside.
Sulfur dioxide (SO 2 ) in high concentrations can affect breathing and may aggravate existing respiratory and cardiovascular disease. Sensitive populations include asthmatics, individuals with bronchitis or emphysema, children, and the elderly. SO 2 is also a primary contributor to acid deposition, or acid rain.
Carbon monoxide (CO) can cause harmful health effects by reducing oxygen delivery to the body’s organs and tissues, as well as adverse effects on the cardiovascular and central nervous systems. CO also contributes to the formation of smog (ground-level ozone), which can cause respiratory problems.
What are the harmful effects of cement?
What you should know – Skin problems are not just a nuisance, they can be very painful and sometimes debilitating. Cement and cement-based products can harm the skin in a number of ways. Wet cement is highly alkaline in nature. A serious burn or ulcer can rapidly develop if it is trapped against the skin.
- In extreme cases, these burns may need a skin graft or cause a limb to be amputated.
- Cement can also cause chemical burns to the eyes.
- Cement also causes dermatitis,
- It can abrade the skin and cause irritant contact dermatitis.
- Cement also contains hexavalent chromium (chromate).
- This can cause allergic contact dermatitis due to sensitisation.
Manufacturers add an ingredient to lower the hexavalent chromium content and reduce this risk. This ingredient is only effective for a limited period as indicated by the shelf date. After this period, the level of hexavalent chromium may increase again.
Worker’s story Dermatitis pictures Skin at work