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.