Concrete Mixing – Sub grade Preparation

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When stationary mixers are used, the first cubic meter of concrete should be mixed for 1 minute and each additional cubic meter should be mixed for 20 seconds. Mixing time starts when all cementing materials and aggregates are added to the mixer provided all of the water is added before 1/4 of the mixing time has elapsed. Concrete that is kept agitated does not get very stiff in the first 1/2 hour and usually can be placed and compacted up to 1 1/2 hours after mixing.

Moisten the sub grade when concrete is placed in warm or hot weather, and keep it unfrozen in cold weather. The sub grade should have even bearing, be drained, have no plant matter or frost, and should have the correct level or sloped contour. Undisturbed soil is generally better at supporting footings and slabs than compacted soil.

Angular & Smooth Aggregate Properties

Angular & Smooth Aggregate Properties

Rough, angular and elongated aggregate particles require more water than smooth rounded aggregates to make the concrete workable. Angular aggregates require more cement than smooth aggregates to maintain the same cement to water ratio. Rough and angular aggregates form larger voids in the concrete mixture than smooth aggregates. More water and cement is used to fill the larger voids in rough and angular aggregates. Rough and angular aggregates form a stronger bond than smooth aggregates. Aggregates should be stored to reduce segregation. Aggregates stored in uniform thin layers segregate less than when they are stored on conical piles. Rough and angular, and small aggregates segregate less than smooth, and large aggregates.

Types of Portland Cement – Concrete Moisture & Cracking

Types of Portland Cement – Concrete Moisture & Cracking

Different types of Portland cement can be used to regulate the amount of hydration heat released when concrete sets. Type 10 Normal Portland cement releases half of its heat in three days. Type 20 Moderate Portland cement takes more than three days to release half of its heat. Type 30 High Early Strength Portland cement releases half of its heat in less than three days. Type 40 Low Heat of Hydration Portland cement releases very little heat of hydration. Type 50 Sulphate Resistant Portland cement is used to protect concrete from degradation when it is placed in soils with high sulphate content. Concrete must remain moist when it is curing. Hydration continues only as long as there is moisture in the cement paste. Concrete that has dried out before it is completely cured shrinks and cracks, and can disintegrate into dust, especially at the surface. Control joints and reinforcing steel can be used to control cracking. If concrete loses it’s moisture or the relative humidity goes below 80 percent, or if it freezes, concrete stops curing and gaining strength.

Ordering & Mixing Concrete

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A concrete order must include the amount of concrete, cement type and supplementary cementing materials, necessary slump at discharge point, coarse aggregate maximum size, air content, mix proportions or exposure class, admixture type, concrete temperature, concrete density, and 28 day compressive strength and/or ratio of water to cementing materials. The ingredients for a concrete batch are measured by mass or volume and added to the mixture. Most batches are measured by mass because of the volume inaccuracies of measuring aggregates, especially wet sand. Concrete should be mixed until it appears uniform and the contents are distributed evenly. Concrete can be stationary mixed at the job site, ready mixed and transported to the job site, continuously mixed as ingredients are added to a job site Mobile Batcher Mixer truck, and mixed by High Energy Mixers.

Sub base Preparation – Concrete Placement

Sub base Preparation – Concrete Placement

Vapour barriers should be placed under all concrete ground level floors to prevent the transfer of moisture and soil gases such as radon gas. Sloping the landscape away from the building, placing a 100 mm granular sub base between the soil and the slab, adding a sub base drainage system, and using drain tiles and vapour barriers will reduce moisture problems.

After new concrete is poured and hardened it is roughened to make a better bond with the next placement. Wood forms should be oiled or treated with form release agent to prevent damage to the concrete when they are removed. Reinforcing bars should be clean and without loose rust and scale. Concrete should be placed as close as possible to its final destination. The placed concrete layer, usually 300 mm in depth, should be consolidated, and made uniform and horizontal, before the next layer is added. Concrete should not be poured into piles and placed by moving it horizontally because it will segregate as the cement and water mixture moves ahead of the aggregate. 

Concrete Curing

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Curing maintains concrete moisture content and temperature, after placing and finishing have been completed, to obtain desired concrete properties. Curing methods such as ponding or immersion, spraying or fogging, and saturated wet coverings, maintain the presence of mixed water in the concrete during the early hardening period. Covering the concrete with impervious paper or plastic sheets, or by applying membrane forming curing compounds, prevent loss of mixing water from the concrete by sealing the surface. Live steam, heating coils, or electrically heated forms or pads, accelerate strength gain by supplying heat and additional moisture to the concrete.

Supplementary Cementing Materials

Supplementary Cementing Materials

Fly ash and ground slag reduce the heat build up in concrete because their heat of hydration is lower than Portland cement. Ground granulated blast furnace slag, fly ash and natural pozzolans slow the setting time in concrete. Concrete with Portland cement and supplementary cementing materials can take longer to cure and gain strength than concrete with Portland cement alone. Supplementary cementing materials improve concrete finish ability. Silica fume is good at increasing concrete pump ability. Silica fume may color concrete blue or dark grey, and fly ash may give it a tan color. Ground granulated blast furnace slag and fly ash help increase the strength of concrete and reduce permeability. Silica fume also reduces permeability. Slag, fly ash and silica fume improve concrete resistance to sulphate corrosion. Fly ash and silica fume provide corrosion protection to steel embedded in concrete.

Admixtures

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Admixtures are ingredients, other than Portland cement, that are added to the concrete mixture just before or during mixing. The types of admixtures used are air entraining, water reducing, retarding, accelerating and super plasticizer admixtures. Air entraining admixtures entrain or suspend, and spread small air bubbles throughout the concrete. Entrained air makes the concrete more resistant to damage caused by freezing and thawing, and surface scaling caused by deicers. It also increases concrete workability, and reduces bleeding and segregation. Water reducing strength increasing admixtures reduce the water needed to produce concrete at a required slump, reduce the ratio of water to cement, reduce the total amount of water and cement needed, and increase the slump. When the water to cement ratio is reduced, concrete strength increases. Water reducing admixtures can cause an increase in drying shrinkage, and, if the water to cement ratio is maintained while water is lost, a decrease in concrete compressive strength.

Concrete Finishing

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Concrete should not be placed any faster than it can be spread, struck off, consolidated, and bull floated or derbies. Spreading concrete over too large an area can allow too much bleed water to accumulate before bull floating. Screening must be completed before too much bleed water accumulates. The concrete can be finished by strike off or screening to bring the top surface of the slab to proper grade, and bull floating to level the high and low spots and submerge large aggregates. Other finishing techniques can be applied if they are required such as booming for slip resistance, edging  for compaction, jointing for control joints, floating for leveling, and adding patterns and textures for aesthetics. Newly placed concrete should be cured and protected from rapid drying, extreme temperature changes, and damage from traffic. In winter, concrete can be heated, covered, insulated and enclosed. In summer, concrete can be cured by covering it with wet burlap to protect it from drying out too quickly.

Concrete Consolidation

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Concrete must be consolidated by rod, or by internal or external vibrators to prevent stone pockets, honeycombs and entrapped air. Repeated spading between the form and the concrete allows trapped air to escape and keeps the coarse aggregate away from the forms. Internal vibrators should be lowered vertically, at regular intervals under their own weight, into the concrete layer or lift so that the vibrator penetrates 150 mm into the layer below it. A vibrator being inserted into thin slabs should enter on an angle to keep the head submerged. Vibrators should be held stationary in concrete for 5 to 10 seconds until consolidation occurs. Vibration is complete when the aggregate particles are embedded, the surface is level, the vibrator head has a thin film of glistening paste around it, and entrapped air ceases leaving the surface. Too much vibration causes the concrete to segregate. 

Concrete Aggregate Properties

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Fine and coarse concrete aggregates fill 60 to 75 percent of the concrete volume. Fine aggregates are less than 5 mm and consist of natural sand and crushed gravel. Coarse aggregates are larger than 5 mm but are usually between 20 and 40 mm in size. They consist of  gravel or crushed aggregate such as recycled concrete, slag, quarry rock, boulders, cobbles and large gravel. Aggregates must be clean, hard, strong, durable, abrasion resistant, and free from absorbed chemicals and clays that could affect concrete hydration and bonding. Aggregates should be acid and alkali resistant. Aggregates containing shale, soft and porous rocky material and chart should not be used because they weather and cause surface defects on the concrete. Porous aggregates can absorb water, freeze and cause disintegration of the concrete.

Chemicals That Damage Concrete

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Natural drinkable water without a pronounced odor or taste is suitable as mixing water for concrete. Mixing water should contain less than 2000 ppm of dissolved solids. Sodium carbonate in the water causes rapid setting. Sodium and potassium bicarbonates can speed up or slow down setting times. Large concentrations of carbonates and bicarbonates in the water can reduce concrete strength. Chloride ions from salt can corrode embedded steel and sulphates can cause expansion and deterioration of the concrete. Manganese, zinc, tin, copper and lead salts can reduce concrete strength and change setting times. Sea water will corrode reinforced steel and prestressed concrete, and react with the alkalies in some aggregates.  Water containing alkali, organic material, sugar, oil and algae can reduce concrete strength.

Concrete slump test

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The concrete slump test is an empirical test that measures the work ability of fresh concrete.

More specifically, it measures the consistency of the concrete in that specific batch. This test is performed to check the consistency of freshly made concrete. Consistency is a term very closely related to work ability  It is a term which describes the state of fresh concrete. It refers to the ease with which the concrete flows. It is used to indicate the degree of wetness. Work ability of concrete is mainly affected by consistency i.e. wetter mixes will be more workable than drier mixes, but concrete of the same consistency may vary in work ability  It is also used to determine consistency between individual batches.

The test is popular due to the simplicity of apparatus used and simple procedure. Unfortunately, the simplicity of the test often allows a wide variability in the manner that the test is performed. The slump test is used to ensure uniformity for different batches of similar concrete under field conditions,  and to ascertain the effects of plasticizers on their introduction

What are the Structural Basics for Concrete?

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Concrete is strong in compression. So what does that really mean?

To understand compressive strength, think about several packs of crackers sitting on the floor. If you carefully stand on those packs of crackers, your weight will probably be supported, but you are putting those crackers in compression. Your weight tends towards crushing those crackers. If you jump up and land on those packs of crackers, you will increase the force applied and probably crush the crackers. You will have made the crackers fail in compression.

Now try to jump on a concrete sidewalk. You’d have to jump pretty high to make that sidewalk crush under your weight. In fact, you probably couldn’t make that sidewalk fail in compression. That’s why concrete gets used so much in construction. But the story doesn’t end with compression.

Grab a piece of string and pull in either direction. You’ve just put the string into tension. If you can pull hard enough, the string will fail in tension by snapping. Concrete, while quite strong in compression, fails quickly in tension by cracking. The resistive strength of concrete for compression is around 4,000 pounds per square inch, while the resistive strength for concrete in tension is probably less than 400 pounds per square inch. Generally, the tension strength of concrete is less than 10% of its compression strength.

Builders in the past understood these properties of concrete and stone and typically used those materials only in compression. So walls could be concrete and stone, as could foundations, since both primarily resisted downward compression loads.

Arches are an interesting structural form because arches also act totally in compression. Therefore, arches above windows in old buildings could be concrete or stone because the load transferred around the arch keeping the structure in compression, so tension cracks didn’t occur in the concrete or stone. Barrel vault ceilings are really just three dimensional arches, so they also worked as compression members only.

If an arch above a window got too flat, however, it would stop working as an arch and the bottom of the member would go into tension. So, regular concrete cracks at the bottom of the beam, near the center, in this scenario. The cracking then causes the beam to fail. This example illustrates how concrete fails in tension, which had traditionally been a major design shortcoming for concrete.

The advantages of aerated concrete

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If thoughtful – in modern construction are not so many materials that can be used without any pretreatment. People found ways to improve materials: brick now has excellent insulation and hardwood handle special structures Which protect from moisture, heat and insects. Accounted for all: bearing characteristics of materials, availability of price, ease of operation. Pores (cells) of cellular concrete, such as saturated air, and air – the ideal heat insulator. That is why the cellular concrete prober such an important quality as excellent thermal insulation, which distinguishes it from conventional concrete. Modern buildings, erected from cellular concrete, much warmer than brick or wood.

Of course, homes and cottages of brick and wood are sufficiently thermally insulated but for their warm-up spent a lot of energy. Energy consumption for heating the rooms in the building of cellular concrete is considerably lower than for heating buildings of brick. Heating country house can be very costly. For what would be a brick house and a building made of cellular concrete were heated with an equally low power consumption, the walls of the house of bricks must be thick as 1.9 meters, and cellular concrete 0,5 meters. From this it follows that for heating in the brick building is necessary to use heaters and expend more energy. Accordingly, to increase the price of the entire structure. The thickness of walls in buildings made of cellular concrete standard that allows for 20-40% reduction in spending on heating. Porosity of the material can also maximize soundproofing.

Building Concrete Slabs

Building Concrete Slabs

Concrete slabs are used to support everything from patio furniture, to foot traffic, to semi-trailer trucks. With such a wide range of purposes and support requirements, concrete slabs present many construction variables that must be considered before concrete placement begins.

A slab pour requires efficient planning so that all of the elements that go into producing a high-quality slab are done in time (before the concrete sets) and done correctly. Knowing the right finishing tools to use and the right time to start bull floating and final toweling are essential to preventing dusting, scaling and craze cracking of the slab.

You also need to provide a firm and stable base for the concrete slab by compacting the sub grade properly. Neglecting this critical step can result in serious slab settlement and cracking problems, especially in slabs placed on poor subsoil or exposed to heavy traffic conditions.

Determining the right concrete mix design and reinforcement requirements for the anticipated slab exposure and traffic conditions is essential as well. You’ll need to calculate the proper water-cement ratio and air-entrainment requirements for the concrete mix to ensure that the slab will perform as intended. Proper positioning and support of wire reinforcement is also important to control and minimize cracking.

After concrete placement, you have a whole new set of issues to address, such as proper placement and spacing of control joints and adequate curing. The timing and execution of these post-pour activities are equally essential to good slab performance, since rapid drying of a slab and improper installation of control joints can lead to inadequate strength and unwanted cracking. Concrete that is moist cured for at least seven days is about 50% stronger than uncured concrete.

Here are some useful links to information that can guide you through the steps required to build high-quality concrete slabs on grade and help you avoid mistakes that can lead to poor performance, or even worse, slab failure. You’ll also find advice on concrete mix design and calculating the water-cement ratio.

Concrete Mix Design Methods

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The basic objective of concrete mix design is to find the most economical proportions (Optimization) to achieve the desired end results (strength, cohesion, workability, durability, As mentioned earlier the proportioning of concrete is based on certain material properties of cement, sand and aggregates. Concrete mix design is basically a process of taking trials with certain proportions. Methods have been developed to arrive at these proportions in a scientific manner. No mix design method directly gives the exact proportions that will most economically achieve end results. These methods only serve as a base to start and achieve the end results in the fewest possible trials.

The code of practice for mix design-IS 10262 clearly states following: -

The basic assumption made in mix design is that the compressive strength of workable concretes, by and large, governed by the water/cement ratio. Another most convenient relationship applicable to normal concrete is that for a given type, shape, size and grading of aggregates, the amount of water determines its workability. However, there are various other factors which affect the properties of concrete, for example the quality & quantity of cement, water and aggregates; batching; transportation; placing; compaction; curing; etc. Therefore, the specific relationships that are used in proportioning concrete mixes should be considered only as the basis for trial, subject to modifications in the light of experience as well as for the particular materials used at the site in each case.

Different mix design methods help us to arrive at the trial mix that will give us required strength, workability, cohesion etc. These mix design methods have same common threads in arriving at proportions but their method of calculation is different. Basic steps in mix design are as follows:

a. Find the target mean strength.

b. Determine the curve of cement based on its strength.

c. Determine water/cement ratio.

d. Determine cement content.

e. Determine fine and coarse aggregate proportions

Precast Concrete Manufacturing and Erection

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Precast Concrete is a construction product produced by casting concrete in a reusable mold or “form” which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and cured on site. Precast stone is distinguished from precast concrete by using a fine aggregate in the mixture so the final product approaches the appearance of naturally occurring rock or stone.

By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete. The production process for Precast Concrete is performed on ground level which helps with safety throughout a project. There is a greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced which allows cost of formwork per unit to be lower than for site-cast production.

There are many different types of precast concrete forming systems for architectural applications, differing in size, function and cost. Precast architectural panels are also used to clad all or part of a building facade free-standing wall used for landscaping, soundproofing and security walls and some can be Pre stressed concrete structural elements. Storm water drainage, water and sewage pipes and tunnels make use of precast concrete units.

Guidelines On Use Of Ready Mixed Concrete

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Basic considerations 

• The proportioning of concrete mixes consists of determination of quantities of different concrete-making materials necessary to produce concrete having the desired workability and 28-days compressive strength of concrete for a particular grade of concrete and durability requirements. Emphasis is laid on making the most economical use of available materials so as to produce concrete of required attributes at the minimum cost.
• Concrete has to be satisfactory both in fresh and hardened states.    The proportioning of concrete mixes is accomplished by the use of certain established relationships from experimental data which provides reasonably accurate guidance for selecting the best combination of ingredients so as to achieve the desired properties of the fresh and hardened concrete. Out of all the physical characteristics of concrete compressive strength is often taken as an index. Therefore , the mix design is generally carried out for a particular compressive strength of concrete coupled with adequate workability so that the fresh concrete can be properly placed and compacted. In addition the mix proportions are also checked against the requirement of adequate durability for the type of exposure condition anticipated in service. The following basic assumptions are made in design of concrete mixes of medium strength:

1. For given aggregate characteristics, the workability of concrete is  dependent on its water content.
2. The compressive strength of concrete is related to its water-cement ratio.

• For high strength concrete mixes, considerable interaction  occurs between these two criteria and validity  of. such  assumptions may become limited. Moreover, there are various other factors which affect the properties of concrete e.g. the quality and quantity of cement, water, aggregates and admixtures (if used); procedures of batching, mixing, placing, compaction and curing etc. Therefore, the specific relationships that are used in proportioning concrete mixes, should be considered only as a basis for trial mixes .  Concrete mix design on the basis of recommended guidelines is really a process of making an initial guess at the optimum combination of ingredients and final mix proportions are    arrived at, only on the basis of further trial mixes.

Carbon Strategy

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Climate change is considered one of the most critical global challenges of our time. We acknowledge the local and global challenges posed by climate change, and are committed to applying our skills, technologies, and determination to reduce the contribution of our operations and our industry to climate change. Climate change is caused by increasing concentrations of greenhouse gases, primarily CO2, in the Earth’s atmosphere. It is widely believed that this phenomenon is the result of human activity, including the burning of fossils fuels for energy and also emissions derived from a variety of agricultural and industrial processes. Minimizing climate change and its consequences is a critical global challenge. The global cement industry produces about five percent of all manmade CO2 emissions.1 This greenhouse gas is generated chiefly in the production of clinker (the main ingredient in cement). Clinker is produced in large rotary kilns by processing limestone, clay, and other minerals under very high temperatures (>1,400 °C or 2,500 °F). CO2 results from the fuel combustion required to achieve such high temperatures and from the chemical decomposition of limestone into lime and CO2. Compared to these emissions, other sources, mainly the transport of raw materials and final products and emissions related to the generation of electricity that Mastour ReadyMix consumes, are very small, but still offer an opportunity to reduce our total footprint.

What Materials Can Be Added to Concrete to Make It Stronger?

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Concrete is often a strong enough product on its own. However, the material does crack under too much pressure. Similarly, concrete can crack under the wrong kind of pressure. Concrete is known to withstand compression, but it bends or cracks under tension weight. To make the concrete stronger and last longer under tension, companies add materials to the concrete during various stages of the process.

Nano-concrete

Molecular materials are poured into the concrete to add strength while the concrete is being mixed. Some of the nano-concrete mixtures are comprised of polyethylene or ethylene particles such as being developed by The Massachusetts Institute of Technology. Other nano-concrete, like that developed by the University of Florida, use the silicates from finely ground clay. According to MIT’s “Technology Review Magazine,” nano-molecules work their way into the microscopic holes found in the cement. As a concrete slab is poured, the nano-molecules inside the holes make it harder for salt and other contaminants to enter the holes and cause the concrete to break.

Post Tensioning (Rebar or Cable)

Another way to strengthen concrete is to incorporate steel into the mix. This is not done using fine particles. Instead, the concrete is poured over steel bars or cables that are woven into a mesh. The mesh is completely encapsulated into the concrete. The steel adds reinforcement to the concrete, providing extra help in handling tension. The concrete slabs made in this manner are called post tensioning slabs.

Fiber Reinforcement

Fibers applied to the concrete during the mixing stage are yet another strengthener. Steel, polypropylene and other polymers are mixed into the concrete to reinforce it after drying. The fibers help absorb the tension that leads to cracking, adding to the durability of the concrete.

Availability

Not all of these concrete mixes or their additions are available for purchase at your local hardware. Nano-concrete is not available until the product has undergone further testing. Post tensioning is a technique that has been used for quite some time, however. You can purchase steel rebar and cable materials at any large hardware store. The fiber additives are also available as well as concrete pre-mixed with fibers. You can find these with retailers who specialize in concrete.

Concrete as Construction Materials

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Concrete is the most widely used man made construction material in the world, and is second only water as the most utilized substance on planet. It is obtained by mixing cementitious materials, water and aggregates (and sometimes admixtures) in required proportions. The mixture when placed in forms and allowed to cure, hardens into a rock-like mass know as concrete. The hardening is caused by chemical reaction between water and cement and it continues for a long time, and consequently concrete grows stronger with age. The hardened concrete may also be considered  as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles ( fine aggregate) and the voids of fine aggregate are filled with cement. In a concrete mix the cementitious material and water form a paste called cement-water paste which in addition to filling the voids of fine aggregates , coast the surface of fine and coarse aggregates and binds them together as it cures, thereby cementing the particles of the aggregates together in a compact mass.

The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on proportion of mix, the method of compaction and other control during placing, compaction and curing. The popularity of concrete is due to the fact that from the common ingredients, it is possible to tailor the properties of concrete to meet demands of any particular situation. The advance in concrete technology  have paved the way  to make the best use of local availability materials by judicious mix proportioning and proper workmanship, so as the produce concrete satisfying performance requirement.

The key to producing a strong , durable and uniform concrete, i.e. high performance concrete  lies in the careful control of its basic and process components .

Why is the Amount of Water so Important for Concrete?

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An important item to understand in concrete work is the water-cement ratio. A minimum amount of water, approximately 25% of the weight of the cement, must be included to chemically hydrate the concrete batch. In the actual mixing process, though, it takes about 35% to 40% water to work through the mixing process, get to the actual cement, and cause effective hydration.

In practice, though, much more water gets added to increase the workability of the concrete. So why does it matter if there is lots of water in the concrete mix? Any water above the theoretical ideal of 25% doesn’t get used in the chemical hydration process. Therefore, the excess water remains in the concrete while the concrete cures. Over time, this excess water evaporates out of the concrete and voids remain. These voids weaken the concrete, causing less strength and more cracking.

The water-cement ratio matters to the engineer, but why does the Construction Supervisor care? Anyone who has placed concrete knows how much easier a flowing, more liquid concrete is to place than a drier concrete. There is a tendency to add water to the mix, as it is ready to be placed, to make the concrete flow better. In fact, if the concrete doesn’t flow well, it may not properly surround the rebar (causing a poor bond with the rebar) or it may not flow properly against the forms (causing voids and areas needing patching). Insert photo.

So, a conflict often exists on the jobsite:

1. Add water to the concrete mix to make it flow better, but weaken the quality of the concrete (both strength and crack resistance)

or

2. Don’t add water to the concrete mix to keep the proper water-cement ratio but work harder to place the concrete and possibly have significant voids.

The easy answer is never add water on the site to concrete, but that answer ignores the reality of the placement dilemma. This is often a complicated decision, with Structural Engineers, Building Officials, Specifications, Concrete Foreman and others all having input. It’s important the Construction Supervisor at least be aware of this issue for every concrete placement and understand how the decision to add water will be handled.

Basics of Concrete

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Concrete is made up of five primary constituents.

• Water
• Cement
• Air
• Fine Aggregate (FA)
• Coarse Aggregate (CA)

The water and cement form a paste, which binds the aggregate into a rock-like mass as the water and cement combine through a chemical reaction called hydration. The paste also includes entrapped air introduced by mechanical mixing and entrained air introduced by the addition of chemical admixtures. The paste constitutes between 25 and 40 percent of the volume. The aggregate makes up the remaining 60 to 75 percent. Air in concrete varies from about ½ to 2 percent in non-air-entrained concrete to about 4 to 8 percent entrained air in concrete containing air-entraining admixtures.
FA, sometimes called “sand”, is composed of particles that pass the 4.75 mm (No.4) sieve. CA, or gravel, consists of particles retained on or above 4.75 mm (No.4) sieve. Well-graded aggregate, consisting of a wide range of FA and CA sizes, provides for efficient use of the water/cement paste. Since aggregate makes up most of the mix volume, it should consist of particles with adequate strength and resistance to exposure conditions.
Concrete
Concrete is a construction material that consists, in its most common form, of Portland cement, aggregate (generally gravel and sand) and water.
Concrete solidifies and hardens after mixing and placement due to a chemical process known as hydration. The water reacts with the cement, which hardens, bonding the other components together and eventually creating a stone-like material.
Concrete is used more than any other man made material on the planet. It is used to make pavements, architectural structures, foundations, motorways/roads, overpasses, parking structures, brick/block walls and footings for gates, fences, and poles.
Concrete powers over a $35 billion industry which employs over two million workers in the United States alone. Over 55,000 miles of freeways and highways in America are made of this material. The People’s Republic of China currently consumes approximately 40% of world cement production.

The vulnerability of porous concrete

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Though it is believed that porous concrete to protect not need all the same should pay attention to the presence of a set of tiny cavities, making it vulnerable to external factors (wind, water). Therefore, additional protection such as plastering, tiling, painting – cannot hurt. And facing materials offered today by various construction companies, encouraged by a wide selection. However, each material has its pluses and minuses. Aerated concrete – lightweight material with a lot of advantages. But we should remember that in no case be obkladyvat wall of aerated concrete bricks on the front side. This is one of his weaknesses.
WARNING: brickwork, laid up against the cellular-concrete wall, will reflect the resulting vapor in the house and send it back to the room that faces the consequence of an increase in moisture content and the appearance syrostnyh phenomena on the walls of the house.
CONCLUSION: Pay close attention to the selection of materials and use them exactly as prescribed.

Concrete: To determine the name

At Mastou Ready Mix, we understand that our product, concrete, is one of the most unique and versatile building materials in the Saudi Arabia.

Building full of questions. When are you going to buy a faucet at once confronted with the questions: “What a crane to buy, and where?” “Is not it better to buy a faucet?” When it comes to concrete, issues no less. Many people do not quite clearly understand what is the difference between an aerated concrete, foam and aerated concrete. Just do not understand their accompanying values “autoclave” and” “non-autoclaved concrete. So what, five of these titles represent a whole, or each exist separately? Answer, for starters, on this issue. Overriding and fundamental in this series is a cellular concrete. Under aerated concrete involve materials for which a single, the same composition (having equidistant cells, strengthening of physical and mechanical properties of concrete). Due to the porosity of the material is much reduced, its density and weight. These enhanced qualities distinguish the concrete from the resulting product, produced from the usual mixture of sand, water and cement. By the term “cellular concrete” is often added the definition of “light”. Lightweight cellular concrete is divided into two types: foam and aerated concrete. The difference is in the process of production. Hence, too, came the concept of autoclave and non-autoclave. Autoclaved aerated – aerated and non-autoclaved aerated – this is aerated concrete.

Crushed stone and gravel of solid rocks

After many years’ research, Mastour company has developed an advanced vertical shaft impact crusher, which can be used together with other machinery as a complete sand making flow. This design racks a leading role in the same industry

Fields of mining granite and other dense rock myriad of the former Soviet space. Those who are still not closed, or just under construction, capable of supplying companies for the production of building materials for many years. One of the most essential elements for building work can be considered as crushed stone and gravel of solid rocks.
Based on the fact that there are standards for road metal and gravel, rocks, those instances that have a relative density of the particles from 2 to 3 grams per cubic centimeter, are used exclusively as a filler for concrete mixtures and as material for road and other works building character. What concerns rubble and gravel, which does not apply the standard, its application is limited to use as ballast for railway tracks and of course, as a decorative gravel.
Granite rubble, as well as other rocks, starting with mine explosions solid granite slabs. After that huge chunks of granite transported to the crushing plant, where with the help of special equipment all parts and sorted into fractions. Thus, the smallest fraction of a granite screenings, which is used exclusively for decorative purposes. All subsequent fractions used in the manufacture and filling of building materials. The largest is usually used very rarely. Their use is limited to decorative features, such as decorating the fountains, pools and artificial lakes.
Before the rubble and gravel get into the building materials, conducted a series of studies, which determine the structure of rocks, their strength, flakiness, frost resistance, bulk density, and of course radionuclide diagnosis to identify the class of ionizing radiation. Only after all results are known, rubble and gravel can be sorted by application. To date, rubble and gravel can be bought in DIY stores, or directly at the mining complex.

What Are the Correct Proportions for Cement Mix?

Mastour ReadyMix provide end-to-end Ready mix concrete and cement products solutions

Concrete is typically made of four components mixed together and poured or placed in a stiff, slurry form. This material goes through the chemical process of hydration, which hardens the concrete into a rock-like material. The proper proportions of the four components of the concrete mix are important in producing a workable material that is durable and looks good. The common proportions are one part cement to two parts sand and two parts gravel, based on volume.

Cement

Cement is a calcium product processed through a kiln in to a fine powder. Cement makes up the smallest part — 12 percent — of a standard concrete mix. The cement is the active ingredient that generates the chemical process that causes the hardening of the concrete.

Sand

Sand is fine particle inorganic material. For masonry mortar it is the principle fill material; gravel is omitted due to the size of the organics. Sand can be screened to remove larger stones or aggregate pieces. The sand should not include any organic materials. The sand component of the concrete mix is commonly about 34 percent.

Gravel

Gravel, also inorganic, is larger pieces of material. Think of it as small stones or crushed pieces of rock. The gravel increases the volume of the cement and provides strength. Gravel or crushed rock is sold in various sizes of stone separated by a screening process. Masonry mortar contains smaller pieces of gravel, if any, while cement for walkways or driveways could include stones up to 3/4 inch.

Water

Water reacts with the cement to cause the hydration or hardening process. Water makes up about 6 percent of the weight of the concrete although the amount is more commonly determined by slump tests rather than measurements. Cut out the end of a Styrofoam cup and fill with the concrete mix. Place the cup upside down on a dry surface and remove the cup. The concrete should “slump” to about half the height of the cup if it has the proper water quantity. If it slumps more than half it is too wet. If it doesn’t slump it has too little water.

Ready Mix Concrete Composed Of

Mastour ReadyMix provides its customers with high-quality branded Ready mix concrete and cement products for their construction needs

Ready mix concrete is composed of standard concrete ingredients and additives, according to the intended use of the concrete. “Ready mix” is not a special type of concrete. Instead, the term describes the way the concrete is delivered to a job site–already mixed. Ready mix concrete is the solution to the problem of having to mix concrete components in large quantities while maintaining consistently precise properties.

Features

By volume, ready mix concrete is compose of 60 to 75 percent aggregate including sand, gravel and stone. Any clean, hard, non-reactive, non-porous rock or sand may be a suitable aggregate. Size and shape of aggregate are selected according to the desired strength and texture of the concrete, how the ready mix concrete will be placed, and also cost.

Function

Cement is the binder in ready mix concrete, and it represents 10 to 15 percent of ready mix concrete volume. The most commonly used cement, Portland cement may contain limestone, marl, shale, blast furnace slag, silica sand and iron ore. Ingredients may vary; it’s the chemical constituents of these–calcium, silicon, aluminum, iron and gypsum–that are important in ready mix concrete.

Effects

The components of ready mix concrete cannot become solid without water. Water molecules, when combined with calcium silicate, react in a chemical process called hydration. The hydrated compounds of water and calcium silicate form a dense crystaline structure that binds the aggregate into a single solid form. So technically concrete does not dry. Instead, it cures chemically.

Significance

The ratio of water to cement in ready mix concrete is the critical determining factor in concrete strength. Water not used up by the hydration reaction remains a part of the concrete, causing it to be less strong. Ideally, only enough water would be added to the mix to react with the cement, and no more. In practice though, this makes for concrete that is difficult to work with, so more water is added than necessary for hydration. Depending on the intended use, ready mix concrete is composed of 15 to 20 percent water by volume.

Potential

Ready mix concrete may also contain fly ash, silica fume, blast furnace slag or metakaolin as a substitute for a percentage of the Portland cement. These components, like Portland cement, react with water to form a monolithic solid. The advantage of their use is the improvement of concrete strength and durability. Fly ash, silica fume and furnace slag are byproducts of power generation or industrial processes, so their use benefits the environment by reducing waste.

About Ready Mix Concrete

Mastour ReadyMix is a Ready mix concrete and cement products company Jeddah, Khamis Mushyat, Dammam, Abaha Kingdom of Saudi Arabia

Ready-mix concrete is made at a batching plant and delivered by a cement truck to a work site. This type of concrete speeds up the construction process by having the concrete already mixed and ready to pour once it reaches the site. Ready-mix concrete saves builders time and money.

History

Ready-mix concrete was first mixed in a factory in the 1930s. It wasn’t until the 1960s that ready-mix concrete became more in demand. Since that time, the ready-mix concrete business has continuously grown and is now a major industry.

How Ready-Mix Concrete is Made

Ready-mix concrete is made out of 10 to 15 percent cement, approximately 60 to 75 percent aggregates, and about 20 percent water. Cement is made out of limestone, sand or shale, and clay. Aggregates are sand, gravel and rock.

Function

Ready-mix concrete is used for small and large construction projects. It is used to pave driveways and for foundations on homes and large commercial buildings. It is also used for bridges, roads and sidewalks.

Benefits

There are several benefits to the use of ready-mix concrete. Ready-mix concrete helps speed up the building process. Construction companies save time and money by using concrete that is already mixed. It eliminates the need for a crew to mix concrete at the work site. Ready-mix concrete also helps to reduce construction-site pollution. A lot of dust is created when concrete is mixed on site.

Limitations

Ready-mix concrete cannot be transported over long distances. When concrete is mixed at a batching plant, it must be poured within two hours to hold up to industry standards. This creates the need for central mixing plants to serve the construction industry.

Speculation

The advantages of ready-mix concrete far outweigh all limitations. The building and construction industry would be greatly inhibited without it. What used to take several man-hours to do can now be accomplished in a very short period of time.

What Materials Are Used to Make Concrete?

Mastour ReadyMix is a Ready mix concrete and cement products company Jeddah, Khamis Mushyat, Dammam, Abaha Kingdom of Saudi Arabia

Concrete is an integral part of modern society. From roads and bridges to the buildings in which we live and work, everywhere around you are structures and infrastructures comprised of concrete. Considering concrete is used to make vital parts of our everyday lives, you should know the science behind concrete that makes it so strong and reliable.

Composition of Concrete

Concrete is made from more than one type of material, making it a composite material. The composite material is formed with a filler, which is an aggregate used to make the texture of the concrete, and a binder or paste used to “glue” the filler together. Cement and water make up the binder. The mixture of aggregates, cement and water creates the useful composite we know as concrete.

Cement

Cement is made from clay, sand, iron ore and limestone burned together at extremely high temperatures. The limestone must be quarried and crushed into small pieces. A mixer combines the small pieces of limestone, iron ore, sand and clay to form a powder of all four components. A rotating cylinder-shaped kiln then burns this composition of materials for up to two hours.

Water

When you mix water with cement, it forms a paste that binds all the components together. The process of hydration is a chemical reaction between the water and the components of the concrete that causes the concrete to harden. You must use pure water to ensure the chemicals react correctly and create strong cement. The ratio between the cement and the water is crucial to making useful cement. Too little or too much water could make the cement too weak or unworkable. Useful cement requires the perfect balance between cement and water.

Aggregates

Aggregates are another component used in making concrete. Aggregates are materials such as sand or rocks added to the cement and water mixture. Since cement is the most expensive part of making concrete, adding cheaper aggregates gives you more concrete for less price. The final product is about 70 to 80 percent aggregates. Choosing a type of aggregate depends on the type of concrete you want to produce. You can create dense, strong concrete by using dense aggregate. Likewise, using soft and porous aggregates like sand creates weaker concrete with less resistance to wear. Like water, good aggregates should be pure. Any impurities in the aggregate could interfere with the chemical reaction required to make concrete or make the concrete weaker.

Concrete mix Design

Concrete Mix Design

Basic considerations 
1-  The proportioning of concrete mixes consists of determination of quantities of different concrete-making materials necessary to produce concrete having the desired workability and 28-days compressive strength of concrete for a particular grade of concrete and durability requirements. Emphasis is laid on making the most economical use of available materials so as to produce concrete of required attributes at the minimum cost.
2-  Concrete has to be satisfactory both in fresh and hardened states.  The proportioning of concrete mixes is accomplished by the use of certain established relationships from experimental data which provides reasonably accurate guidance for selecting the best combination of ingredients so as to achieve the desired properties of the fresh and hardened concrete. Out of all the physical
The following basic assumptions are made in design of concrete mixes of medium strength:
a) For given aggregate characteristics, the workability of concrete is dependent on its water content.
b) The compressive strength of concrete is related to its water-cement ratio.
3-  For high strength concrete mixes, considerable interaction   occurs between these two criteria and validity   of. such   assumptions may become limited. Moreover, there are various other factors which affect the properties of concrete e.g. the quality and quantity of cement, water, aggregates and admixtures (if used);  procedures of batching, mixing, placing, compaction and curing etc.

Is there a difference between cement and concrete?

Difference between cement and concrete

Yes, there is a difference.  Cement, or Portland cement, is a finely ground powder made from limestone and other raw materials.  The materials are blended together and fired in a kiln at extreme temperatures.  The resulting stone sized pieces, called “clinker,” are then ground into a fine powder that we call Portland cement.  Portland cement is the active ingredient in concrete.
Concrete is the product used in buildings, bridges, road paving, sidewalks, patios, etc.  Concrete is a mixture of Portland cement, water, aggregate (sand and stone), and miscellaneous chemical admixtures.  Concrete is mixed at a concrete plant facility and delivered to customers in revolving drum truck mixers, also referred to as concrete trucks or mixer trucks.  Concrete is sometimes generically referred to as “ready-mix” because it is ready to use or already mixed upon arrival at the project location.  Many concrete producers have incorporated various forms of this terminology into their company name.  It’s common to see names such as Ready Mixed Concrete Company, Hank’s Ready Mix, Smith Ready-Mix, etc.

Many Uses of Ready-Mix Concrete

Mastou Ready Mix, we understand that our ready mix concrete and cement products, are the most unique and versatile building materials.

Concrete is a popular building material. You can see it in home building sites, commercial building sites and government projects such as bridges and highways. It is easier to complete big projects like these by using ready-mix concrete. Completing projects on a tight deadline is possible and achieved in less than half the time it would when not using this product.
Ready-mix concrete comes from manufacturers and plants ready for delivery and mixing at work or project sites. With the use of a concrete mixer, one can get a precise mixture of concrete for the project. Using this type of cement material eliminates too much confusion at the work site. It also saves considerable amount of time because it requires less time to prepare than concrete prepared from scratch.
Cements, sands, aggregates (gravels) and water are the main contents of the mixture. It has the same ingredients as the ones that are not except that instead of carrying and mixing the raw materials at the site, the ready-mix arrives at the site on mixer trucks already pre-mixed and ready to use. Adding different additives and aggregates offsite and then delivering onsite for a different project based upon the specification of the customer is another feature of the ready-mix concrete. Different textures, finishes and colors are also available in ready-mix form.
Using ready-mix concrete eliminates having to carry and mixing the materials on site, which is a painstaking process. It eliminates errors that come with wrong measurement of water and the concrete materials. Using ready-mix saves time and effort. Big projects take less time to complete after pouring in the mixture using the transit mixer. The quality of the product is also much better than those that come in a non-ready mixed form.
Ready-mix concrete has a big potential in a lot of building projects. Aside from using it on big infrastructure projects such as building bridges, highways and huge buildings, we see this type of concrete as the choice of homeowners when building driveways, walkways made of concrete. Some homeowners choose concrete for their kitchen and bathroom countertops and as floorings. Stained concrete floors and counters provide a rustic look and patina adding more character to the interior of the home.
When using ready-mix concrete one must work with caution. One must ensure that there is enough space for the transit mixer. In addition, the location should be strong enough to handle the weight of the transit mixer and the concrete. The workers practice caution when working with the concrete mixer, by avoiding standing near the way of the mixer, especially when operating and pouring concrete. Accidents can and do happen which can endanger the well-being of the workers.

Concrete : ready mix, precast, barrier and block

Mastour ReadyMix provide end-to-end high quality ready mix concrete and cement products

Effective quality control of construction materials is key to the production of long lasting structures that satisfy precise design and safety requirements.
We have established associations with a wide range of organizations involved in the production and use of ready mix concrete, precast concrete and concrete pipes and blocks.
In each case, it is essential that existing and new quality control facilities have the correct equipment to meet current and future testing requirements.
Mastour ReadyMix has a longstanding record of providing dedicated testing solutions for concrete producers.  Our solutions range from a single item of test equipment to a fully fitted laboratory, based either on our standard systems for materials quality control and testing centers or a custom built facility.
In addition, we offer comprehensive product support:
• Dedicated Service Department for fast response by trained service engineers.
• National service contracts that offer single supplier cost savings, consistent performance and Health and Safety compliance.

Why Mastour ReadyMix ?

Mastour Ready Mix is one of the major companies operating in the field of construction and reconstruction depends in its work

we are helping people build their dream home, the tallest buildings, the longest bridges and the cities of the future. At the same time, we are solving problems like waste management, and using alternative sources of fuel and energy.
People depend on infrastructure to enable them to move from one place to another, to have a place to work, and to make a home for themselves and their families. At Mastour ReadyMix, we are passionate about delivering effective building solutions that improve people’s lives, their communities, and the world.
Mastour ReaddyMix provides building materials and building solutions to our customers for all kinds of building projects, large and small. From sourcing aggregates and producing cement products to marketing, selling, and distributing cement and ready-mix concrete, we are deeply involved in ensuring the success of our customers’ building projects.

Concrete Durability

Mastour ReadyMix is a Ready mix Concrete, cement and related construction materials and services company Jeddah Saudi Arabia

Durability Means Longer Lasting, More Efficient Structures 
Durability is a significant sustainable attribute of concrete because it will not rust, rot, or burn, requiring less energy and resources over time to repair or replace. Concrete builds durable, long-lasting structures including sidewalks, building foundations and envelopes, as well as roadways and bridges. As the most widely used building material in the world, concrete structures have withstood the test of time for more than 2,000 years. Because of its longevity, it can be a viable solution for environmentally responsible design.
 Durability
Durability is the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and the properties desired. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of the concrete.

About Ready Mix Concrete

Mastour ReadyMix provide end-to-end high quality ready mix concrete and cement products .

Ready-mix concrete is made at a batching plant and delivered by a cement truck to a work site. This type of concrete speeds up the construction process by having the concrete already mixed and ready to pour once it reaches the site. Ready-mix concrete saves builders time and money.

History

•    Ready-mix concrete was first mixed in a factory in the 1930s. It wasn’t until the 1960s that ready-mix concrete became more in demand. Since that time, the ready-mix concrete business has continuously grown and is now a major industry.

How Ready-Mix Concrete is Made

•    Ready-mix concrete is made out of 10 to 15 percent cement, approximately 60 to 75 percent aggregates, and about 20 percent water. Cement is made out of limestone, sand or shale, and clay. Aggregates are sand, gravel and rock.

Function

•    Ready-mix concrete is used for small and large construction projects. It is used to pave driveways and for foundations on homes and large commercial buildings. It is also used for bridges, roads and sidewalks.

Benefits

•    There are several benefits to the use of ready-mix concrete. Ready-mix concrete helps speed up the building process. Construction companies save time and money by using concrete that is already mixed. It eliminates the need for a crew to mix concrete at the work site. Ready-mix concrete also helps to reduce construction-site pollution. A lot of dust is created when concrete is mixed on site.

Limitations

•    Ready-mix concrete cannot be transported over long distances. When concrete is mixed at a batching plant, it must be poured within two hours to hold up to industry standards. This creates the need for central mixing plants to serve the construction industry.

Speculation

•    The advantages of ready-mix concrete far outweigh all limitations. The building and construction industry would be greatly inhibited without it. What used to take several man-hours to do can now be accomplished in a very short period of time.

Concrete Facts About Sustainability

Concrete Facts About Sustainability

The ready mixed concrete industry is dedicated to upholding the principles of sustainable development-development that meets the needs of the present without compromising the ability of future generations to meet their own needs-by attempting to balance social, economic and environmental impacts.

Sustainability has become part of the fabric of society. Corporations in every industry are shaped by their customers’ desire to be more environmentally responsible. Companies that adopt sustainable practices will become preferred suppliers. While environmental performance, including greenhouse gas emissions, will be increasingly monitored and regulated, voluntary initiatives such as the one presented here will help achieve ambitious sustainability goals.

Construction industry stakeholders-including project owners, designers, contractors and product manufacturers-are especially affected by the challenges of sustainable development since the built environment has significant environmental, social and economic impact on our lives and planet. On one hand, our built environment provides us with places to live and work and contributes to a robust economy and societal needs. On the other, operating our buildings, houses and infrastructure consumes enormous amounts of energy and valuable resources. Building products require natural resources and energy to produce and transport. New construction projects can burden natural habitats.

The concrete industry is uniquely positioned to meet the challenges of sustainable development. Its products help improve the overall environmental footprint of the built environment. For example, high performance concrete wall and floor systems help improve energy performance of buildings. Light colored pavements reduce urban heat islands and minimize lighting requirements. Previous concrete pavements reduce and treat storm water runoff. Concrete is extremely durable and provides for long service life.  And the industry continues to develop new sustainable products through research and development.

The concrete industry is dedicated to continuous improvement through product and process improvements. The industry continues to increase the use of recycled materials, including industrial by-products, thus conserving valuable natural resources and reducing process energy required to manufacture concrete. The industry continues to explore new ways to further reduce the carbon footprint through the development of innovative cements and concrete mixtures. Concrete companies also strive to improve manufacturing processes, including the use of alternative energy sources, to minimize the energy of production and the associated greenhouse gas emissions. Finally, the industry continues to enhance transportation efficiency and delivery methods to reduce the environmental impact of the construction process.

What is Ready Mix Concrete ?

Mastour Ready Mix is a well-established and market leading ready mix concrete producer in Jeddah Kingdom of Saudi Arabia

In its simplest form, concrete is a mixture of paste and aggregates (sand & rock). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete.
Within this process lies the key to a remarkable trait of concrete: it’s plastic and can be molded or formed into any shape when newly mixed, strong and durable when hardened. These qualities explain why one material, concrete, can build skyscrapers, bridges, sidewalks, and superhighways, houses and dams.

Proportioning

The key to achieving a strong, durable concrete rests on the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete will be more likely to crack and be uneconomical.
A properly proportioned concrete mixture will possess the desired work-ability for the fresh concrete and the required durability and strength for the hardened concrete.
The character of concrete is determined by the quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the work-ability of fresh concrete. Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated and cured.

Hydration Begins

Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All Portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.
The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff.
During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets. For slabs, concrete is left to stand until the surface moisture film disappears. After the film disappears from the surface, a wood or metal hand float is used to smooth off the concrete. Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel toweling.
Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Curing ensures the continued hydration of the cement and the strength gain of the concrete. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or special sprays (curing compounds).
Special techniques are used for curing concrete during extremely cold or hot weather to protect the concrete. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends upon the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions. Most of the hydration and strength gain take place within the first month of concrete’s life cycle, but hydration continues at a slower rate for many years. Concrete continues to get stronger as it gets older.