The Importance of Water – In concrete, the single most significant influence on most or all of the properties is the amount of water used in the mix. In concrete mix design, the ratio of the amount of water to the amount of cement used (both by weight) is called the water to cement ratio (w/c).
- 1 What is the role of water-cement ratio in modifying the compaction factor of concrete?
- 2 What is the function of water in the concrete mix?
- 3 What is the most important component of the concrete mix design for durability?
- 4 Which is more effective ratio for durability of concrete?
What is the most important factor in concrete mix?
1. Water/cement ratio – Water to cement ratio (W/C ratio) is the single most important factor governing the strength and durability of concrete. Strength of concrete depends upon W/C ratio rather than the cement content. Abram’s law states that “higher the water/cement ratio, lower is the strength of concrete.” As a thumb rule every 1% increase in quantity of water added, reduces the strength of concrete by 5%.
- A water/cement ratio of only 0.38 is required for complete hydration of cement.
- Although this is the theoretical limit, water cement ratio lower than 0.38 will also increase the strength, since all the cement that is added, does not hydrate) Water added for workability over and above this water/cement ratio of 0.38, evaporates leaving cavities in the concrete.
These cavities are in the form of thin capillaries. They reduce the strength and durability of concrete. Hence, it is very important to control the water/cement ratio on site. Every extra liter of water will approx. reduce the strength of concrete by 2 to 3 N/mm 2 and increase the workability by 25 mm.
What is the role of water-cement ratio in modifying the compaction factor of concrete?
FAQ – What is the standard water cement ratio? Water cement ratios between 0.4 to 0.6 are used. Water cement ratio above 0.6 makes concrete too flowy and strength is reduced drastically. Water cement ratio below 0.4 gives stiff concrete which has very low workability to handle the concreting operations.
In such cases, admixtures enhancing flowability can be added to produce high strength concrete. What is the minimum water cement ratio? For concrete, the minimum water cement ratio is 0.4 and not 0.38. For cement mortar, the minimum water cement ratio is less than 0.4. Why is water cement ratio important? Water cement ratio decides the quantity of water to be added per unit weight of cement.
Higher water content reduces the strength and durability of concrete while lower water cement ratio forms stiff concrete with less workability. Many other qualities like bond strength, resistance to weathering, volume changes due to wetting and drying are also affected by water cement ratio.
What is the most important property for determining the strength of a concrete mix?
3.1 Properties of Concrete – Concrete is an artificial conglomerate stone made essentially of Portland cement, water, and aggregates. When first mixed the water and cement constitute a paste which surrounds all the individual pieces of aggregate to make a plastic mixture.
A chemical reaction called hydration takes place between the water and cement, and concrete normally changes from a plastic to a solid state in about 2 hours. Thereafter the concrete continues to gain strength as it cures. A typical strength-gain curve is shown in Figure 1. The industry has adopted the 28-day strength as a reference point, and specifications often refer to compression tests of cylinders of concrete which are crushed 28 days after they are made.
The resulting strength is given the designation f’c During the first week to 10 days of curing it is important that the concrete not be permitted to freeze or dry out because either of these, occurrences would be very detrimental to the strength development of the concrete. Theoretically, if kept in a moist environment, concrete will gain strength forever, however, in practical terms, about 90% of its strength is gained in the first 28 days.
Concrete has almost no tensile strength (usually measured to be about 10 to 15% of its compressive strength), and for this reason it is almost never used without some form of reinforcing. Its compressive strength depends upon many factors, including the quality and proportions of the ingredients and the curing environment.
The single most important indicator of strength is the ratio of the water used compared to the amount of cement. Basically, the lower this ratio is, the higher the final concrete strength will be. (This concept was developed by Duff Abrams of The Portland Cement Association in the early 1920s and is in worldwide use today.) A minimum w/c ratio (water-to-cement ratio) of about 0.3 by weight is necessary to ensure that the water comes into contact with all cement particles (thus assuring complete hydration).
In practical terms, typical values are in the 0.4 to 0.6 range in order to achieve a workable consistency so that fresh concrete can be placed in the forms and around closely spaced reinforcing bars. Typical stress-strain curves for various concrete strengths are shown in Figure 2. Most structural concretes have f’c values in the 3000 to 5000 psi range.
However, lower-story columns of high-rise buildings will sometimes utilize concretes of 12,000 or 15,000 psi to reduce the column dimensions which would otherwise be inordinately large. Even though Figure 2 indicates that the maximum strain that concrete can sustain before it crushes varies inversely with strength, a value of 0.003 is usually taken (as a simplifying measure) for use in the development of design equations. Because concrete has no linear portion to its stress-strain curve, it is difficult to measure a proper modulus of elasticity value. For concretes up to about 6000 psi it can be approximated as (1) where w is the unit weight (pcf), f’c is the cylinder strength (psi). (It is important that the units of f’c be expressed in psi and not ksi whenever the square root is taken). The weight density of reinforced concrete using normal sand and stone aggregates is about 150 pcf. If 5 pcf of this is allowed for the steel and w is taken as 145 in Equation (1), then (2) E values thus computed have proven to be acceptable for use in deflection calculations. As concrete cures it shrinks because the water not used for hydration gradually evaporates from the hardened mix. For large continuous elements such shrinkage can result in the development of excess tensile stress, particularly if a high water content brings about a large shrinkage.
- Concrete, like all materials, also undergoes volume changes due to thermal effects, and in hot weather the heat from the exothermic hydration process adds to this problem.
- Since concrete is weak in tension, it will often develop cracks due to such shrinkage and temperature changes.
- For example, when a freshly placed concrete slab-on-grade expands due to temperature change, it develops internal compressive stresses as it overcomes the friction between it and the ground surface.
Later when the concrete cools land shrinks as it hardens) and tries to contract, it is not strong enough in tension to resist the same frictional forces. For this reason contraction joints are often used to control the location of cracks that inevitably occur and so-called temperature and shrinkage reinforcement is placed in directions where reinforcing has not already been specified for other reasons.
The purpose of this reinforcing is to accommodate the resulting tensile stresses and to minimize the width of cracks that do develop. In addition to strains caused by shrinkage and thermal effects, concrete also deforms due to creep. Creep is Increasing deformation that takes place when a material sustains a high stress level over a long time period.
Whenever constantly applied loads (such as dead loads) cause significant compressive stresses to occur, creep will result. In a beam, for example, the additional longterm deflection due to creep can be as much as two times the initial elastic deflection The way to avoid this increased deformation is to keep the stresses due to sustained loads at a low level.
What is meant by water-cement ratio?
Noun. : the ratio of mixing water to cement in a concrete expressed by volume or by weight or as the number of gallons of water per bag of cement.
What is the relation between water-cement ratio and compressive strength of concrete?
If the water-cement proportion is higher, it brings about wider spacing between the cement aggregates and thus, influences the compaction. Correspondingly, concrete’s durability and compressive strength are decreased due to increased dampness levels.
What is the function of water in the concrete mix?
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.
What happens if water cement ratio is increased in a fully compacted concrete?
Compressive strength – The mean compressive strength values for the final experiment are shown in Figure 1. The w/c ratio was a statistically significant variable impacting strength ( p < 0.0001). There was not a significant difference in strength between w/c = 0.51 and 0.59 and w/c = 0.67 and 0.76. However, the strengths at w/c = 0.51 and 0.59 were significantly different from the strengths at w/c = 0.67 and 0.76. There was a 28% drop in strength from w/c = 0.59 to w/c = 0.67. According to Abram's Law, the compressive strength of concrete exhibits an inverse relationship with the w/c ratio, in the form: (1) where A and B are constants and w corresponds to the w/c ratio (Domone & Illston 2010). As the w/c ratio increases, the strength decreases. Complete hydration of cement generally only requires a w/c ratio of 0.42. Beyond this value, extra water increases the workability of concrete for compaction but is not necessary for hydration.
- Free water that does not react occupies pore space in the concrete microstructure.
- When the water evaporates, air voids remain in the microstructure.
- These air voids reduce the strength of concrete.
- Even 1% of air by volume can reduce concrete strength by 6% (Domone & Illston 2010).
- The results observed in this study, of decreasing strength with increasing water content, align with the current body of knowledge regarding concrete strength.
Aggregate segregation, where denser aggregate settles to the bottom of the cylinder, was observed for the mixes at w/c = 0.76, as shown in Figure 2. Segregation occurred to a lesser degree in the cylinders with a w/c ratio of 0.67, which can be observed in Figure 3.
- There was no obvious segregation at the lower w/c ratios.
- Aggregate segregation is an indicator of bleeding.
- Bleeding occurs when mix water is forced to the surface of the concrete due to the settling of aggregate and cement.
- Excess bleeding results in a variable w/c ratio throughout the structure, causing as much as a 20–30% difference between the top and bottom zones and a total loss of up to 30% in the recorded strength of the overall structure (Giaccio & Giovambattista 1986).
Movement of bleed water can also create a capillary network of pores that reduce the integrity of concrete within those zones. A sharp decrease in strength was also observed in Apebo et al. (2013) studies investigating the effects of using crushed gravel over burnt bricks for coarse aggregate.
What is the best principle for concrete mixing?
The most important factor governing the workability of concrete is the water content. Increasing the amount of water will increase the ease with which concrete flows and can be compacted. However, apart from reducing the strength, increased water may lead to segregation and to bleeding.
What is the most important component of the concrete mix design for durability?
Step 4: Concrete Strength and Water/Cement Ratio – The water/cement ratio is the most important parameter of the concrete mix design; it governs the strength, durability, and workability of the concrete mix. Here, you will need to enter the required compressive strength and associated water/cement ratio.
For example, reducing the water/cement ratio will increase the strength of the concrete and provide better durability. However, decreasing the water/cement ratio can also significantly reduce the workability of the concrete. In these cases, one possible solution is adding water reducer to the mix (see Step 7). Using the Help option, you can select the desired compressive strength and receive the corresponding water/cement ratio, calculated based on Table 6.3.4(a) A184.108.40.206(a)). In addition, you will receive guidelines for the maximum permissible water/cement ratio based on the structure exposition (Table 6.3.4(b)/ A220.127.116.11(b)). Using your input data, the app will calculate the amount of cement required. Note that the amount of cement can be reduced by introducing pozzolanic materials to the mix.
Source: Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) Source: Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91)
Does too much water affect the strength of concrete?
Effects of too much mixing water – Whilst adding water will in some cases facilitate easier placing, the disadvantages of this include the following:
Lower compressive strengths. Segregation of the concrete mix under certain conditions resulting in variable quality throughout the concrete mass. Cracking – with too much water, there will be lower tensile strength, and a tendency towards high shrinkage and subsequent cracking. Dusting and scaling – bleeding of excess water brings too many fines to the surface of floors. Sand streaks – excess water bleeding up the sides of forms washes out cement paste and leaves an unsightly streaked surface. Contamination – too much water in concrete placed on grades causes contamination from the subgrade with the concrete leading to an array of quality problems. Permeability – voids left as excess water evaporates invite water to seep through walls and floors. Dead losses – costly repairs, or in extreme cases, demolition and re-building at contractor’s expense.
Approximate compressive strengths for given water:cementitious ratios are shown below. Cementitious binder needs less than half its own weight of water to turn concrete into durable construction material. The “wetter” this cementitious paste is, the weaker it is. The chart below shows how strength decreases as water content of a mix increases.
Which is more effective ratio for durability of concrete?
Factors Related to Concrete Durability – High Humidity and Rain: With little to no organic content, concrete is resistant to deterioration due to rot or rusting by in hot,humid climates. Moisture can only enter a building through joints between concrete elements.
Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Walls need to breathe or concrete will dry out if not covered by impermeable membranes. Portland cement plaster (stucco) should not be confused with exterior insulation and finish systems (EIFS) or synthetic stucco systems that may have performance problems, including moisture damage and low impact-resistance.
Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing.
- It also corrodes metal framing and metal attachments.
- There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
- Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete.
Using colored pigments in concrete retains the color in aeshetic elements (walls or floors, for example) long after paints have faded due to the sun’s effects. Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both.
When it has a proper system of microscopic air bubbles, obtained through the addition of an air entraining admixture and thorough mixing, concrete is highly resistant to freezing and thawing. These microscopic air bubbles within the concrete accommodate the expansion of water into ice and thus relieve the internal pressure generated.
Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8 percent of properly distributed air voids will withstand a great number of cycles of freezing and thawing without distress. Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is regularly used for the construction of waste water transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
- However, concrete is sometimes exposed to substances that can attack and cause deterioration.
- Concrete in chemical manufacturing and storage facilities is especially prone to chemical attack.
- The effect of sulfates and chlorides is discussed below.
- Acids attack concrete by dissolving the cement paste and calcium-based aggregates.
In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments, IS001, discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack.
Read more on acid resistance, Resistance to Sulfate Attack: High amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste.
These reactions can induce sufficient pressure to slowly cause disintegration of the concrete. Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization.
What would happen if the ratio of water to cement is not balanced?
As the W/C ratio increases, strength of concrete would reduce and so the durability properties. low water to cement ratio leads to serious problems in hardened concrete. When water to cement ratio is low in a fresh mix, then less water is available for the hydration of cement.