WHAT IS CONCRETE

Concrete is the product of mixing, aggregate, cement and water.

The setting of concrete is a chemical reaction between the cement and the water, not a drying process.

This reaction is called hydration, it evolves heat as does any chemical reaction, and the process is irreversible.

There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to 24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves the strength required.

Differing mix proportions and cement types will achieve required strengths in differing time spans.

CONSTITUENTS OF CONCRETE

Cement, Aggregate and Water, (and sometimes additives).

Aggregate

Aggregates are usually distinguished between fine and coarse aggregate.

Aggregates are classed as inert materials, such as washed natural sand (fine); and natural gravel, which can be crushed to produce the appropriate size and grading of aggregate, and similarly crushed, quarried stone (coarse).

The aggregate must have a minimum inherent strength requirement for structural concrete, the coarse aggregate must not be weaker than the concrete paste.

All aggregate must be "clean", i.e. not contaminated with organic matter or clay/silty soils and overburden during extraction and storage.

Cement

Basically a material made by heating limestone and a suitable clay to produce a clinker rich in calcium silicates.

This clinker is ground to produce a fine powder, this is cement.

By using different clinkers, grinding them to differing degrees of fineness and the use of additives many different types of cement are produced with varied properties in their use, e.g. rapid hardening cement, sulphate resisting cement, etc..

Generally speaking the more cement in a mix the stronger more durable the concrete produced will be, but this does have to be related to other factors, primarily the amount of water used in the mix, i.e. water/cement ratio.

Water

Water is an extremely important part of concrete, and drinking quality water is usually required, or water from an approved source free from impurities.

Additives

The most commonly used additive is a "foaming" agent to produce air entrained concrete, mainly for carriageway concrete, but also other exposed situations.

Another common use of an additive is to increase the workability of concrete without adding extra water and thus increasing the water/cement ratio and decreasing the strength of the concrete.

TIME TAKEN TO PLACE CONCRETE AFTER BATCHING

From the time of adding water to the cement the chemical reaction has begun and you only have a limited amount of time to place and compact the concrete, this is usually specified as 90 minutes.

The delivery ticket of the load of concrete will be stamped with the time of batching.

ADDITION OF WATER

Given a set amount of cement and aggregate there is an optimum amount of water to be added to produce a chemical reaction to give the maximum obtainable strength, too little or too much water will produce a weaker concrete.

Unfortunately as in all things, life is not that simple, and the workability of the concrete has to be considered when placing concrete, especially in difficult situations.

These situations can be areas of high density of reinforcing bars, complicated formwork design, or where the concrete needs to be suitable for pumping.

In these situations water content is increased to make the concrete more workable, BUT this increase in water content is calculated at the design stage and the cement content is increased accordingly to retain the strength of the mix.

For every designed concrete mix with a specified strength there is a set WATER:CEMENT RATIO which must be retained in order to achieve the designed strength.

WATER SHOULD NEVER BE ADDED TO CONCRETE, ESPECIALLY ON SITE, WITHOUT THE CONSIDERATION OF ALL THE ABOVE POINTS,

AND THE APPROVAL OF THE ENGINEER

CONCRETE WORKABILITY

An on site simple test for determining workability is the SLUMP TEST.

This consists of a conical mould 300mm. high, with an opening at the top of 100mm. diam., and at the bottom of 200mm. diam..

The mould is filled with concrete in 4 layers and rodded to remove air voids, with the smaller orifice uppermost.

The "slump" is the difference in height between the height of the mould and the height of the concrete column with the mould removed.

The workability of the concrete will depend upon the situation into which the concrete is being placed.

Low workability, i.e. stiff concrete, is needed for carriageway concrete which is laid by a "paving train".

High workability concrete is needed in situations of high density of reinforcing steel to enable the concrete to flow around all the reinforcing without leaving any voids.

SPECIFYING CONCRETE STRENGTH

The strength/grade of concrete is specified and measured in newtons/sq. mm., meganewtons/sq. metre or even megapascals, in fact the numerical figure will be the same in each case.

E.g. a strength of 20 newtons/sq.mm. is the same as 20 meganewtons/sq.metre.

The strength/grade of concrete is normally specified by stating the strength you wish the concrete to achieve after a period of 28 days.

EASURING CONCRETE STRENGTH

The strength is measured by crushing concrete cubes to failure and recording this strength.

Concrete cubes are made from fresh concrete sampled at the time of pouring by placing correctly sampled concrete into a steel mould and compacting to remove air voids.

The concrete is allowed an initial "set" period of 24 hours, the mould is then stripped and the cube is cured in water at a temperature of 20 deg.c for 28 days prior to crushing.

If you wish to strike shuttering before 28 days, extra cubes will be required to determine that the in-situ concrete has achieved the appropriate strength at the time you wish to strike the shuttering. 

This is usually an arrangement agreed by the contractor, the concrete supplier and the engineer.

Concrete strength

Many factors influence the rate at which the strength of concrete increases after mixing. Some of these are discussed below. First, though a couple of definitions will be useful:

The process of strength growth is called 'hardening.' This is often confused with 'setting' but setting and hardening are not the same.

Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it, for example. Setting is due to early-stage calcium silicate hydrate formation and to ettringite formation. The terms 'initial set' and 'final set' are arbitrary definitions of early and later set; there are laboratory procedures for determining these using weighted needles penetrating into cement paste.

Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.

The rate at which concrete sets is independent of the rate at which it hardens. Rapid-hardening cement may have similar setting times to ordinary Portland cement. 

Factors affecting concrete strength

There are many relevant factors; some of the more important follow:

Concrete porosity: voids in concrete can be filled with air or with water. Air voids are an obvious and easily-visible example of pores in concrete. Broadly speaking, the more porous the concrete, the weaker it will be. Probably the most important source of porosity in concrete is the ratio of water to cement in the mix, known as the 'water to cement' ratio. This parameter is so important it will be discussed separately below.

Water/cement ratio: this is defined as the mass of water divided by the mass of cement in a mix. For example, a concrete mix containing 400 kg cement and 240 litres (=240 kg) of water will have a water/cement ratio of 240/400=0.6. The water/cement ratio may be abbreviated to 'w/c ratio' or just 'w/c'. In mixes where the w/c is greater than approximately 0.4, all the cement can, in theory, react with water to form cement hydration products. At higher w/c ratios it follows that the space occupied by the additional water above w/c=0.4 will remain as pore space filled with water, or with air if the concrete dries out.

Consequently, as the w/c ratio increases, the porosity of the cement paste in the concrete also increases. As the porosity increases, the compressive strength of the concrete will decrease.

Soundness of aggregate: it will be obvious that if the aggregate in concrete is weak, the concrete will also be weak. Rocks with low intrinsic strength, such as chalk, are clearly unsuitable for use as aggregate.

Aggregate-paste bond: the integrity of the bond between the paste and the aggregate is critical. If there is no bond, the aggregate effectively represents a void; as discussed above, voids are a source of weakness in concrete.

Cement-related parameters: many parameters relating to the composition of the individual cement minerals and their proportions in the cement can affect the rate of strength growth and the final strength achieved. These include:

alite content

alite and belite reactivity

cement sulfate content

Since alite is the most reactive cement mineral that contributes significantly to concrete strength, more alite should give better early strengths ('early' in this context means up to about 7 days). However, this statement needs to be heavily qualified as much depends on burning conditions in the kiln. It is possible that lighter burning of a particular clinker could result in higher early strength due the formation of more reactive alite, even if there is a little less of it. Not all alite is created equal!

For a particular cement, there will be what is called an 'optimum sulfate content,' or 'optimum gypsum content.' Sulfate in cement, both the clinker sulfate and added gypsum, retards the hydration of the aluminate phase. If there is insufficient sulfate, a flash set may occur; conversely, too much sulfate can cause false-setting.

A balance is therefore required between the ability of the main clinker minerals, particularly the aluminate phase, to react with sulfates in the early stages after mixing and the ability of the cement to supply the sulfate. The optimum sulfate content will be affected by many factors, including aluminate content, aluminate crystal size, aluminate reactivity, solubilities of the different sources of sulfate, sulfate particle sizes and whether admixtures are used.

If this were not already complicated enough, the amount of sulfate necessary to optimize one property, strength for example, may not be the same as that required to optimize other properties such as drying shrinkage. Concrete and mortar may also have different optimum sulfate contents.

In addition to the compositional parameters considered above, physical parameters are also important, particularly cement surface area and particle size distribution.

The fineness to which the cement is ground will evidently affect the rate at which the cement hydrates; grinding the cement more finely will result in a faster reaction. Fineness is often expressed in terms of total particle surface area, eg: 400 square metres per kilogram. However, of as much, if not more, importance is the particle size distribution of the cement; relying simply on surface area measurements can be misleading. Some minerals, gypsum for example, can grind preferentially producing a cement with a high surface area. Such a cement may contain very finely-ground gypsum but also relatively coarse clinker particles resulting in slower hydration.

Some basic definitions used in cement and concrete

A few useful basic definitions follow, since the meanings of the words 'cement' and 'concrete' are rather blurred in general use. 

Portland Cement: Material made by heating a mixture of limestone and clay in a kiln at about 1450 C, then grinding to a fine powder with a small addition of gypsum. Portland Cement, the main subject of this site, is the most common type of cement - 'basic cement', if you like. In particular, ordinary Portland cement is the normal, grey, cement with which most people are familiar. Other types of Portland cement include White Portland Cement and Sulfate Resisting Portland Cement (SRPC).

Clinker: Portland cement is made by grinding clinker and a little added gypsum. Clinker is a nodular material before it is ground up. The nodules can be anything from 1mm to 25mm in diameter.

Cement: Usually taken to mean Portland Cement, but could mean any other type of cement, depending on the context. 

Difference between Cement (left) and clinker (right). The coin is a UK

one-pound coin about 23mm across. 

Aggregate: Cobbles, pebbles, gravel, sand and silt - the 'rock' component of all particle sizes in concrete.

Concrete: Synthetic rock made using cement (usually, but not necessarily, Portland cement) mixed with aggregate and water.

Mortar: Mixture of cement and fine aggregate, mainly sand. Used typically to bond bricks and building stone.

Grout: Mixture of cement (possibly of various types) and other fine material such as fine sand. Used in a wide range of applications from filling the gaps between bathroom tiles to oil wells.

Composite cements: Some types of cement are mixtures of Portland cement with other material, such as blastfurnace slag from iron production and pulverised fuel ash from coal-fired electricity power stations. These widely-used mixtures are called 'composite' cements.

Non-Portland cements

Of course, there are other types of cement apart from Portland cement. Important examples include:

• Calcium aluminate cements

• Lime concrete/mortar

• Expansive cements

Calcium aluminate cements (CACs)These may also be termed 'Ciment Fondu' and used to be called 'high alumina cements.' They are made from lime or limestone mixed with bauxite (aluminium ore) or other high-alumina material.

Concretes made with CACs develop strength quickly and are resistant to chemical attack. CACs have a wide range of compositions, mainly with different ratios of lime to alumina; strictly, ‘Ciment Fondu’ is only one part of this compositional range. CACs are generally brown or grey-black, but can be white if made from pure alumina.

As well as being used in concrete, CACs are also used in grouts and other specialised applications, often mixed with Portland cement and other materials such as gypsum. 

Lime concrete and mortar

Lime mortar and concrete have been used for thousands of years (see history of cement) so, historically, lime is probably the most basic cementitious material of all. Today, lime mortar and concrete are used mainly in the rebuilding or repair of historic or ancient buildings, although in the UK there has been some recent use of lime mortar in the construction of new buildings. There are several advantages in using lime mortar:

• Cracks that develop in lime mortar tend to heal themselves, unlike conventional mortar made with Portland Cement.

• Lime mortar is usually weaker than mortars made with Portland Cement and so can be removed from the brick or stone at the end of the useful life of the building. Particularly in the case of bricks, this means that they can be recycled, saving a lot of energy otherwise needed to make newbricks. Bricks used with mortar made with Portland Cement generally can’t be re-used as it is difficult to detach the mortar from the brick without damaging it.

• Lime is produced at a lower temperature than Portland Cement, so other things being equal, it takes less energy to produce a lime mortar compared with a mortar containing Portland Cement.

Expansive cementsThese are special cements designed to exert an expansive force on their surroundings after the cement has set. (With most cements, manufacturers go to a lot of trouble to make sure the cement is not expansive). Expansive cements are used mainly in demolition and also in mining.

What is Flexural Strength?

It is the ability of a beam or slab to resist failure in bending. It is measured by loading un-reinforced 6x6 inch concrete beams with a span three times the depth (usually 18 in.). The flexural strength is expressed as “Modulus of Rupture” (MR) in psi.

Flexural MR is about 12 to 20 percent of compressive strength. However, the best correlation for specific materials is obtained by laboratory tests.

Why Test Flexural Strength?

Designers of pavements use a theory based on flexural strength. Therefore, laboratory mix design based on flexure may be required, or a cement content may be selected from past experience to yield the needed design MR. Some also use MR for field control and acceptance of pavements. Very few use flexural testing for structural concrete. Agencies not using flexural strength for field control generally find the use of compressive strength convenient and reliable to judge the quality of the concrete as delivered.¹

How to Use Flexural Strength

Beam specimens must be properly made in the field. Pavement concretes are stiff (1/2 to 2 ½ inch slump). Consolidate by vibration in accordance with ASTM C 31 and tap side to release bubbles. For higher slump, after rodding, tap the molds to release bubbles and spade along the sides to consolidate. Never allow the beam surfaces to dry at any time. Immerse in saturated lime water for at least 20 hours before testing.

Specifications and investigation of apparent low strengths should take into account the higher variability of flexural strength results. Standard deviation for projects with good control range from about 40 to 80 psi. Values over 100 psi indicate testing problems, and there is high likelihood that testing problems, or moisture differences within a beam, will cause low strength.

Where a correlation between flexural and compressive strength has been established, core strengths by ASTM C 42 can be used for compressive strength to check it against the desired value using the ACI 318 85 percent criteria. It is impractical to saw beams from a slab for flexural testing. Sawing beams will greatly reduce measured flexural strength and should not be done. Some use has been made of measuring indirect tensile strength of cores by ASTM C 496, but experience is lacking on how to apply the data.

Another procedure for in-place strength investigation uses compressive strength of cores calibrated by comparison with acceptable placements on either side of the concrete in question:

What are the Problems with Flexure?

Flexural tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beam specimens are very heavy, and allowing a beam to dry will yield lower strengths. Beams must be cured in a standard manner, and tested while wet.²A short period of drying can produce a sharp drop in flexural strength.

Many state highway agencies have used flexural strength but are now changing to compressive strength for job control on concrete paving. Cylinder strengths are also used for concrete structures.

“The data points to a need for a review of current testing procedures. They suggest also that, while the flexural strength test is a useful tool in research and in a laboratory evaluation of concrete ingredients and proportions, it is too sensitive to testing variations to be usable as a basis for the acceptance or rejections of concrete in the field.”³ The CSI Spec-Data Sheet by NRMCA, the Municipal Concrete Pavement Manual by ACPA, ACI 325, and ACI 330 on Concrete Pavements, all point to the use of compressive strength as more convenient and reliable. The Pennsylvania DOT uses compressive strength of cylinders; 3750 psi is specified with 3000 psi for opening a pavement to traffic.

The concrete industry and inspection agencies are much more familiar with traditional cylinder compression tests for control and acceptance of concrete. Flexure can be used for design purposed, but the corresponding compressive strength should be used to order and accept the concrete. Any time trial batches are made, both flexural and compressive tests should be made so that a correlation can be developed for field control.

What are Admixtures?

Admixtures are natural or manufactured chemicals which are added to the concrete before or during mixing. The most often used admixtures are air-entraining agents, water reducers, retarders and accelerators. Why Use Admixtures? Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the durability, workability or strength characteristics of a given concrete mixture. Admixtures are used to overcome difficult construction situations such as hot or cold weather placements, pumping requirements, early strength requirements or very low water-cement ratio specifications.

How to Use Admixtures

Consult Cadman Lab Systems about which admixtures may be appropriate for your application. Admixtures should be evaluated for compatibility with cement(s), construction practices, job specifications, and economic advantage before being used.

Follow this Guide to Use Admixtures

1. AIR-ENTRAINGING AGENTS – are liquid chemicals added during mixing to produce microscopic air bubbles in concrete. These bubbles improve the concrete’s durability and increase its resistance to damage from freezing and thawing and deicing salts. Air entraining admixtures improve workability and may reduce bleeding and segregation. For exterior flatwork (parking lots, driveways, sidewalks, pool decks, patios) that is subject to freezing and thawing weather cycles, or in areas where deicer salts are used, specify an air content of 5 to 7%. Air-entrainment is not necessary for interior structural concrete since it is not subject to freezing and thawing. In high cement content concretes adding air will reduce strength by about 5% for each 1% of air added; but in low cement content concretes adding air has less effect and may increase strength slightly.
    
2. WATER-REDUCERS – are used for two different purposes: (1) to lower the water content and increase the strength; (2) to obtain higher slump using the same water content. Water-reducers will generally reduce the required water content for a given slump by about 10%. This increases strength or allows the cement content to be reduced and maintain the same water-cement ratio. Water-reducers are used to increase slump for pumping concrete and are used in hot weather to offset the increased water demand. Water-reducers may aggravate slump loss problems. Water-reducers tend to retard concrete and sometimes have accelerators blended in to offset retardation. Water-reducers are Type A Chemical Admixtures in ASTM C 494.2.
    
3. RETARDERS – are chemicals which delay the initial set of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. For large jobs, or in hot weather, specify concrete with retarder to allow more time for placing and finishing. Most retarders also act as water reducers. Retarders are covered by ASTM C 494.2 Types B and D.
    
4. ACCELERATORS – reduce the initial set time of concrete. Liquid accelerators are added to the concrete at the plant. Accelerators are recommended in cold weather to get high-early strength. Accelerators do not act as antifreeze; rather, they speed up the strength gain and make the concrete stronger to resist damage from freezing. Accelerators are sometimes used to allow finishing operations to begin early. Calcium chloride is the most commonly used accelerator, although non-chloride (non-corrosive) accelerators are available. Calcium chloride is specified at not more than 2% by the weight of the cement. Pre-stressed concrete and concrete with embedded aluminum or galvanized metal should not contain any calcium chloride because of the potential for corrosion. See NRMCA Publication No. 173.3. Accelerators are covered by ASTM C 494.2 Types C and E.
    
5. HIGH RANGE WATER-REDUCERS (HRWR) – are a special class of after-reducers. Often called superplasticizers, HRWR’s reduce the water content of a given concrete from 12 to 25%, which increases strength. HRWR’s can also greatly increase the slump to produce “flowing” concrete. For example, adding a normal dosage of HRWR to a concrete with a slump of 3 to 4 inches will produce a concrete with a slump of about 8 inches. Within 30 to 60 minutes the concrete will return to its original slump. HRWR’s are covered by ASTM Specification C 494.2 Types F and G, and C 10175 Types 1 and 2.
6. 
   

Concrete Maintenance Proper maintenance and care for your concrete will ensure you get the best looking concrete for its maximum life. Concrete should be regularly sealed with a concrete sealer when the previous seal has worn down. Sealers can last anywhere from one to five years, if your contractor applied a seal, he should explain to you how long the seal will last. Seals are available in both matte and high gloss finishes, and are recommended to be applied by contractors but can be applied by homeowners with proper care. Sealed concrete can be slippery, if concerned, it is recommended you use (or ask your contractor to use) a texturing agent such as Increte System's Shur-Grip. Concrete that has been poorly cared for can be cleaned with a variety of cleaning products, or even colored with Increte Systems Stain-Sealer, which works like a concrete paint to both color and seal your concrete. Chips and divots can be patched with a concrete patcher. In extreme cases a concrete overlay can be applied, although it is recommended that only trained professionals do this.  A concrete overlay is a new surface for your pre-existing concrete, and can be finished like normal concrete.

What is Corrosion of Steel?

ASTM terminology (G 15) defines corrosion as “the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.” For steel embedded in concrete, corrosion results in the formation of rust which has two to four times the volume of the original steel and none of the good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.

Why is Corrosion of Steel a Concern?

Reinforced concrete uses steel to provide the tensile properties that are needed in structural concrete. It prevents the failure of concrete structures which are subjected to tensile and flexural stresses due to traffic, winds, dead loads, and thermal cycling. However, when reinforcement corrodes, the formation of rust leads to a loss of bond between the steel and the concrete and subsequently delamination and spalling. If left unchecked, the integrity of the structure can be affected. Reduction in the cross sectional area of steel reduces its strength capacity. This is especially detrimental to the performance of tensioned strands in pre-stressed concrete.

Why Does Steel in Concrete Corrode?

Steel in concrete is usually in a non-corroding, passive condition. However, steel reinforced concrete is often used in severe environments where sea water or deicing salts are present. When chloride moves into the concrete, it disrupts the passive layer protecting the steel, causing it to rust and pit.

Carbonation of concrete is another cause of steel corrosion. When concrete carbonates to the level of the steel rebar the normally alkaline environment, which protects steel from corrosion, is replaced by a more neutral environment. Under these conditions the steel is not passive and rapid corrosion begins. The rate of corrosion due to carbonated concrete cover is slower than chloride-induced corrosion.

Occasionally, a lack of oxygen surrounding the steel rebar will cause the metal to dissolve, leaving a low pH liquid.

How to Prevent Corrosion.

Quality Concrete and Concrete Practices The first defense against corrosion of steel in concrete is quality concrete and sufficient concrete cover over the reinforcing bars. Quality concrete has a water-to-cementitious material ratio (w/c) that is low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow the rate of carbonation and less than 0.40 to minimize chloride penetration. Concretes with low w/c ratios can be produced by (1) increasing the cement content; (2) reducing the water content by using water reducers and superplasticizers; or (3) by using larger amounts of fly ash, slag, or other cementitious materials. Additionally, the use of concrete ingredients containing chlorides should be limited. The AI 318 Building Code provides limits on the maximum amount of soluble chlorides in the concrete mix.

Another ingredient for good quality concrete is air entrainment. It is necessary to protect the concrete from freezing and thawing damage. Air entrainment also reduces bleeding and the corresponding increased permeability due to the bleed channels. Spalling and scaling can accelerate corrosion damage of the embedded reinforcing bars. Proper scheduling of finishing operations is needed to ensure that the concrete does not scale, spall, or crack excessively.

The correct amount of steel will help keep cracks tight. ACI 224 helps the design engineer to minimize the formation of cracks that could be detrimental to embedded steel. In general, the maximum allowable crack widths are 0.007 inch in deicing salt environments and 0.006 inch in marine environments.

Adequate cover over reinforcing steel is also an important factor. Chloride penetration and carbonation will occur in the outer surface of even low permeability concretes. Increasing the cover will delay the onset of corrosion. For example, the time for chloride ions to reach a steel rebar at 2 inches from the surface is four times that with a 1 inch cover. ACI 318 recommends a minimum of 1.5 inches of cover for most structures, and increases it to 2 inches of cover for protection from deicing salts. ACI 357 recommends 2.5 inches of minimum cover in marine environments. Larger aggregates require more cover. For aggregates greater than ¾ inch, a rule of thumb is to add to the nominal maximum aggregate size ¾ inch of cover for deicing salt exposure, or 1 – ¾ inch of cover for marine exposure. For example, concrete with 1 inch aggregate in a marine exposure should have a 2 – ¾ inch minimum cover.

The concrete must be adequately consolidated and cured. Moist curing for a minimum of seven days to 70°F is needed for concrete with a 0.40 w/c ratio, whereas six months is needed for a 0.60 w/c ratio. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved.

Modified Concretes and Corrosion Protection Systems – Increased corrosion resistance can also come about by the use of concrete additives. Silica fume, fly ash, and blast-furnace slag reduce the permeability of the concrete to the penetration of chloride ions. Corrosion inhibitors, such as calcium nitrite, act to prevent corrosion in the presence of chloride ions. In all cases, they are added to quality concrete at w/c less than or equal to 0.45.

Water repellents may reduce the ingress of moisture and chlorides to a limited extent. However, ACI 222 indicates that these are not effective in providing long-term protection. Since good quality concrete already has a low permeability, the additional benefits of water repellents are not as significant.

Other protection techniques include protective membranes, cathodic protection, epoxy-coated reinforcing bars, and concrete sealers (if reapplied every four to five years).

How To Limit Corrosion

1. Use good quality concrete air-entrained with a w/c of 0.40 or less.
2. Use a minimum concrete cover of 1.5 inches and at least 0.75 inch larger than the nominal maximum size of the coarse aggregate.
3. Increase the minimum cover to 2 inches for deicing salt exposure and to 2.5 inches for marine exposure.
4. Ensure that the concrete is adequately cured.
5. Use fly ash, blast-furnace, slag, or silica fume and/or a proven corrosion inhibitor.

What are Some Forms of Cracks?

Concrete, like other construction materials, contracts and expands with changes in moisture content and temperature and deflects depending on load and support conditions. When provisions for these movements are not made in design and construction, then cracks can occur. Some forms of common cracks are:

Figure A - Plastic Shrink Cracking (See page 9) Figure B – Cracks Due to Improper Jointing (See page 11) Figure C – Cracks Due to Continuous External Restraint (Example-Cast in place wall Restrained along bottom edge of footing) Figure D – Basement Floor Cracks (See page 11) Figure E – D-Cracks from Freezing and Thawing Figure F – Craze Cracks (See page 5) Figure G – Settlement Cracks

Cracks rarely affect structural integrity. Most random individual cracks look bad and although they permit entrance of water they do not lead to progressive deterioration. They are simply unsightly. Closely spaced pattern cracks or D-cracks due to freezing and thawing are an exception and may lead to ultimate deterioration.

Why Do Concrete Surfaces Crack?

The majority of concrete cracks usually occur due to improper design and construction practices, such as:

1. Omission of isolation and control joints and improper jointing practices.
2. Improper subgrade preparation.
3. The use of high slump concrete or addition of water on the job.
4. Improper finishing.
5. Inadequate or no curing.

How to Prevent or Minimize Cracking

All concrete has a tendency to crack and it is not possible to consistently produce completely crack free concrete. However, cracking can be reduced and controlled if the following basic safeguards are observed:

1. Subgrade and Formwork. All top soil and soft spots should be removed. Regardless of its type, the soil beneath the slab should be compacted soil or granular fill, well compacted by rolling, vibrating or tamping. The slab and, therefore, the subgrade should be sloped for proper drainage. Smooth, level subgrades help prevent cracking. All formwork must be constructed and braced so that it can withstand the pressure of the concrete without movement. Polyethylene vapor barriers increase bleeding and greatly increase cracking of high slump concrete. Cover the vapor barrier with 1 to 2 inches of damp sand to reduce bleeding. Immediately prior to concrete placement, dampen the subgrade, formwork, and the reinforcement.
2. Concrete. In general, use concrete with a moderate slump (not over 5 inches). Avoid retempering. If higher slump, up to 7 inches, is to be used, proportions will have to be changed and special mixtures developed to avoid excessive bleeding, segregation and low strength. Specify air-entrained concrete for outdoor slabs subjected to freezing weather. (See page 3)
3. Finishing. DO NOT perform finishing operations with water present on the surface. Initial screeding must be promptly followed by bullfloating. For better traction on exterior surfaces us a broom finish. If evaporation is excessive reduce it by some means to avoid plastic shrinkage cracking. Cover the concrete with wet burlap or polyethylene sheets in between finishing operations if conditions are severe.
4. Curing. Start curing as soon as possible. Spray the surface with liquid membrane curing compound or cover it with damp burlap and keep it moist for at least 3 days. A second application of curing compound the next day is a good quality assurance step.
5. Joints. Provisions for contraction or expansion movements due to temperature and/or moisture change should be provided with construction of control joints by sawing, forming or tooling a groove about ¼ the thickness of the slab, no further apart than 30 times the thickness. Often closer spacing of control joints will be necessary to avoid long thin areas. The length of an area should not exceed about 1.5 times the width. Isolation joints should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated; such as where floors meet walls, column, or footings. These are full-depth joints and are constructed by inserting a barrier of some type to prevent bond between the slab and the other elements.
6. Cover Over Reinforcement. Cracks in reinforced concrete caused by expansion of rust on reinforcing steel should be prevented by providing sufficient concrete cover (at least 2 inches) to keep salt and moisture from contacting the steel.

Follow These Rules to Minimize Cracking

1. Design the members to handle all anticipated loads.
2. Provide proper control and isolation joints.
3. In slab-on-grade work, prepare a stable subgrade.
4. Place and finish according to established rules.
5. Project and cure the concrete properly.

What is Curling?

Curling is the distortion of a slab into a curved shape by upward or downward bending of the edges. This distortion can lift the edges of the slab from the base leaving an unsupported edge or corner which can crack when heavy loads are applied. Sometimes, curling is evident at any early age. In other cases, slabs may curl over an extended period.

Why do Concrete Slabs Curl?

Typically, upward curling of the edges of a slab is caused by shrinkage or contraction of the top relative to the bottom. When one surface of the slag changes size more than the other, the slab will warp its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners.

Changes in slab dimensions which lead to curling are most often related to moisture and temperature gradients in the slab. One primary characteristic of concrete which affects curling is drying shrinkage. The most common occurrence of curling is when the top part of the slab dries and shrinks with respect to the bottom.

The slab edges curl upward (Figure 1A). Immediate curling of a slab is most likely related to poor curing and rapid surface drying; and anything that increases drying shrinkage, such as an admixture, will tend to increase curling.

In slabs, bleeding and poor curing both tend to produce surface concrete with higher drying shrinkage potential than the concrete in the bottom of the slab. Bleeding is accentuated in slabs on polyethylene or topping mixtures placed on concrete slabs; and shrinkage differences from top to bottom in these cases are larger than for slabs on an absorptive subgrade.

Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing.

In industrial floors, close joint spacing may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints. The other factor that can cause curling is temperature differences between the top and bottom of the slab. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing a downward curling of the edges (Figure 1B). Alternately, during a cold night when the top cools and contracts with respect to a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials.

How to Minimize Slab Curling

The primary factors controlling dimensional changes of concrete which lead to curling are drying shrinkage, construction practices, moist or wet subgrades, and day-might temperature cycles. The following practices will help to minimize the potential for curling:

1. Use the lowest practical slump and avoid adding retempering water, particularly in hot weather.
    
2. Use the larges practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage.
    
3. Take precautions to avoid excessive bleeding. Use a damp, but absorptive, subgrade so that all the bleed water is not forced to the top of the slab.
    
4. Avoid using polyethylene vapor barriers unless covered with at least two inches of damp sand.
    
5. Avoid a higher than necessary cement content if the subgrade is wet in service. Dense, impermeable concrete will produce larger top to bottom moisture differentials and curl more. Use of fly ash is preferable to very high cement content, and consideration should be given to specifying strength at 56 to 90 days.
    
6. Cure the concrete thoroughly, including joints and edges. If membrane curing compounds are used, apply at twice the recommended rate in two applications at right angles to each other.
    
7. For floor areas where curling tends to be a problem, cure the concrete with a heavy wax floor sealing compound of the type used on terrazzo. (Note: Tile adhesives will not stick to these materials.)
    
8. Use a joint spacing in feet equal to two times the slab thickness in inches (PCA recommendation for maximum size aggregate less than ¾ inch).
    
9. For thin toppings, clean the base slab to ensure bond and consider use of studs and wire around the edges and particularly in the slab corners.
    
10. Use a thicker slab.
     
11. The use of properly designed and placed slab reinforcement may help reduce curling.

How to Calculate PSI Strength of Concrete

Concrete is a unique material in that it is very strong in compression, but extremely weak in tension. Cracks in some concrete surfaces occur not directly from compressive forces, but tensile forces that occur when the surface deforms slightly. However, concrete structures depend on concrete's compressive strength, and it is necessary to experimentally test the strength of any particular concrete mixture. The most common test of compressive strength is the cylinder test.Instructions

1. • 1
     Note the characteristics of your concrete mixture. You should note what percentage of the mixture is course aggregate, fine aggregate and cement. You may also want to perform a slump test, which involves compacting wet concrete into a cone and measuring how many inches it slumps when removed. This indicates how firm or runny the mixture is.
   • 2
     Create a cylinder out of the wet concrete using a mold. The cylinder should be twice the height of its diameter, preferably with dimensions around 12 and 6 inches. Small cylinders can produce aberrant results because they can exaggerate the stress factors produced by the large aggregate. Allow the concrete to sit for 28 days to reach its full strength.
   • 3
     Apply neoprene or sulfur mortar pad caps to the cylinder one to two days before testing. These caps essentially merge the rough, eccentric surface of the cylinder with a softer, more formative material. This allows a compressive force applied atop the cylinder to be evenly distributed, rather than concentrating on a few bumps that jut out of the surface.
   • 4
     Load the cylinder into a compression machine. A compression machine is an industrial strength tool for applying thousands of pounds of pressure to a test specimen. The compression machine should display the force it applies and the displacement of its plunger.
   • 5
     Increase the pressure on the specimen at a rate no greater than 0.05 inches per minute. Try to capture the machine's reading of compressive force at the moment the concrete first cracks. Continue applying pressure until the concrete "fails." Concrete may still have some structural integrity after the point when it first cracks, so it is helpful to also identify the point when it loses its overall shape; a large chunk may break off or it may crumble throughout. This is known as "catastrophic failure."
   • 6
     Divide the machine's compressive force by the surface area of the cylinder to obtain its strength in pounds per square inch. The surface area is simply pi times the square of its radius. You may wish to perform a calculation of strength for both initial failure and catastrophic failure.

How to Mix High Strength Concrete

Concrete continues to be an important product for use in the construction of many different types of buildings, wall structures, floors and other items. However, with today's complicated high rise structures, the need to strengthen the product has become of utmost importance. According to the American Concrete Institute (ACI), high strength concrete must meet very specific requirements of at least 6,000 psi. To accomplish this, the concrete is generally manipulated within its basic cement and aggregate mixtures and mixed with compound additives like calcinated shale, fly ash, granulated blast furnace slag, metakaolin or silica fume. An explanation of these mixtures is noted below.

Instructions

1. • 1
     Outline why high strength concrete is required. For example, it could be due to a reduced drying time allowed for the job. It could be to reduce the size of the support columns while still maintaining their strength. It might be to build extended structures like dams or bridges. Whatever the use, it will have an impact on how the concrete will be mixed. Additional considerations include, but are not necessarily limited to, the item's permeability, the estimated amount of material shrinkage and the needed workability within the product. Beyond that, one also has to consider ornamental considerations, such as the mixture's ability to be stamped and whether or not it will be stained.
   • 2
     Decide on the appropriate ratio between the dry and wet compounds of the mixture. In many instances, this ratio may change drastically, while in others it may alter only slightly. Much of this is dependent upon the way the concrete will be used as well as the additives that will be introduced into the basic mixture. If necessary, refer to the ACI website for guidance.
   • 3
     Calculate the type of aggregate product to be used and the appropriate ratio. Aggregate includes materials like gravel, limestone, granite and sand. The chosen aggregate will depend upon whether the product finish may be coarse or must be more fine in nature.
   • 4
     Figure out which additives, such as calcinated shale, fly ash, granulated blast furnace slag, metakaolin or silica fume, should be used. This decision will be somewhat dependent upon how the concrete is to be used. Considerations will include the product's need for strength and durability, its ultimate appearance, density requirements, and lastly where the concrete will be placed (like near standing water, in a damp, moist environment or in high humidity or other similar considerations). Fly ash tends to improve the concrete's finished look and allows for an extended drying time. Metakaolin and slag tend to lighten the concrete's final appearance. This can be important if the concrete will be stained.
   • 5
     Calculate the proportions of cement, water, aggregate and additives to be used. Depending upon how the concrete will be used and the qualities it requires, additives can replace as much as 40 percent of the cement within the completed mixture. Again, you can refer to the ACI website to help determine the proper additives and to check for formula suggestions. Another option is to purchase a software program that can calculate the formula for you based upon the information that you provide concerning the concrete's requirements.
   • 6
     Mix the basic cement mixture reducing the amount of water as determined in Step 2 above. Continue mixing for approximately two to three minutes.
   • 7
     Cease mixing for a couple of minutes as you put the additives identified from Step 3 above into the mixture. Continue to mix for three to four minutes or until you get the desired mixture.
   • 8
     Check the concrete for proper consistency. The mixture is too dry if it crumbles and flakes as you try to mix it with a mixing tool. Add additional water in small quantities until the right texture is achieved. The mixture is too wet if it looks runny and doesn't appear to "set up." In this case, you may have to go back to Step 2 and begin all over again.
   • 9
     Pour the concrete for testing (if time allots). Decide how long to allow the mixture to sit before performing any actual tests. The standard waiting time is generally between 20 and 60 days.
   • 10
     Test the concrete for the desired results with regard to strength, durability, density, look and feel. If it meets the test guidelines, you are set to proceed with the final mixing of the product. If it does not, you will need to back up to Step 2 and recalculate the formulas and products chosen for the mixture.