When stone or gravel, sand, cement and water mixed together, they form an easily workable plastic mixture which can be moulded into any desired shape. In a few hours after mixing, the cement and water begin to undergo a chemical reaction, resulting in solidification and gradual hardening. In a month’s time, the concrete almost attains its full strength.
The aggregates are considered as inert materials while the paste (cement + water) is the cementing medium which binds the aggregate particles into a solid mass. Therefore, the quality of the concrete is largely dependent on the quality of the paste and that the paste must have the strength, durability and water-tightness required by the job.
Common types of cement:
Ordinary Portland cement
Rapid-hardening Portland cement
High C3S content, high fineness
Low-heat Portland cement
High C2S content, low C3A and C3S
Sulphate-resistant Portland cement
Exceptionally low C3A content
High alumina cement
Portland cement is a closely controlled chemical combination of calcium, silicon, aluminum, iron and small amounts of other compounds, to which gypsum is added in the final grinding process to regulate the setting time of the concrete. Some of the raw materials used to manufacture cement are limestone, shells, and chalk or marl, combined with shale, clay, slate or blast furnace slag, silica sand, and iron ore. Lime and silica make up approximately 85 percent of the mass.
Type I is a general purpose Portland cement suitable for all uses where the special properties of other types are not required. It is used where cement or concrete is not subject to specific exposures, such as sulfate attack from soil or water, or to an objectionable temperature rise due to heat generated by hydration. Its uses include pavements and sidewalks, reinforced concrete buildings, bridges, railway structures, tanks, reservoirs, culverts, sewers, water pipes and masonry units.
Type II Portland cement is used where precaution against moderate sulfate attack is important, as in drainage structures where sulfate concentrations in groundwaters are higher than normal but not unusually severe (Table). Type II cement will usually generate less heat at a slower rate than Type I. With this moderate heat of hydration (an optional requirement), Type II cement can be used in structures of considerable mass, such as large piers, heavy abutments, and heavy retaining walls. Its use will reduce temperature rise, an important quality when the concrete is placed in warm weather.
Type III is a high-early strength Portland cement that provides high strengths at an early period, usually a week or less. It is used when forms are to be removed as soon as possible, or when the structure must be put into service quickly. In cold weather, its use permits a reduction in the controlled curing period. Although richer mixtures of Type I cement can be used to gain high early strength, Type III, high-early-strength Portland cement, may provide it more satisfactorily and more economically.
TYPE IA, IIA, IIIA
Specifications for three types of air-entraining Portland cement (Types IA, IIA, and IIIA) are given in ASTM C 150. They correspond in composition to ASTM Types I, II, and III, respectively, except that small quantities of air-entraining materials are interground with the clinker during manufacture to produce minute, well-distributed, and completely separated air bubbles. These cements produce concrete with improved resistance to freeze-thaw action.
Type IV is a low heat of hydration cement for use where the rate and amount of heat generated must be minimized. It develops strength at a slower rate than Type I cement. Type IV portland cement is intended for use in massive concrete structures, such as large gravity dams, where the temperature rise resulting from heat generated during curing is a critical factor.
Type V is a sulfate-resisting cement used only in concrete exposed to severe sulfate action -- principally where soils or groundwaters have a high sulfate content. The following Table describes sulfate concentrations requiring the use of Type V Portland cement. Low Tricalcium Aluminate (C3A) content, generally 5% or less, is required when high sulfate resistance is needed.
Relative Degree of Sulfate Attack Percentage Water-Soluble Sulfate (as SO4) in Soil Samples Sulfate (as SO4) in Water Samples, ppm Cement Type
Negligible 0.00 to 0.10 0 to 150 I
Positive 0.10 to 0.20 150 to 1500 II
Severe 0.20 to 2.00 1500 to 10,000 V
Very Severe 2.00 or more 10,000 or more V plus pozzolan
Raw materials for manufacture of Portland cements:
Found in calcareous rocks such as limestone or chalk
Silica, alumina and iron oxide
Found in argillaceous rocks such as clay or shale
Marl, which is a mixture of calcareous and argillaceous materials, can also be used.
Process of manufacture:
The raw constituents are crushed, ground and mixed in the proportions of approximately 2 parts of calcareous material to one part of argillaceous material.
The mixture is burnt at a very high temperature (about 1500。C) to form a clinker.
The clinker is cooled down and ground to the required fineness in ball mills. While grinding, 2-3% of gypsum is added (gypsum is added to control setting of the cement).
The cement is stored in silos, from which it is discharged into bulk-cement lorry or fed to packing plant.
During the burning process, a number of changes occur as the temperature increases progressively.
At 100。C, the water is driven off.
At 850。C, the carbon dioxide is given off.
At 1400。C, the incipient fusion takes place leading to the formation of calcium silicate and calcium aluminate.
The early stiffening, or setting, is mainly due to the reaction of C3A and it is controlled by the addition of a small amount of gypsum in the course of manufacture.
The hydration of Portland cements in general proceeds rapidly at first and slows down later. There is a corresponding rapid gain of strength at early ages and a slower, but still perceptible increase in strength in strength for as long as five years after the cement has been mixed with water and made into concrete.
It should be noted that while there can be large differences in the early strength of concrete made with different Portland cement, their final strengths will generally be very much the same.
Concrete gains its strength through the hydration of the cement paste which is a chemical combination of cement with water producing a very hard and strong binding medium.
The rate of hydration depends on:
Relative properties of silicate and aluminate compounds:
Time taken to achieve 80% hydration:
C3S 10 days
C2S 100 days
C3A 6 days
C4AF 50 days
Fineness of cement particle:
The greater the surface area of a given volume of cement, the quicker the rate of hydration
Temperature and moisture
Higher temperature leads to quicker rate of hydration
Hydration is accompanied by liberation of heat (exothermic reaction). Concrete is a poor conductor of heat and the heat generated during hydration can have undesirable effects on the properties of the hardened concrete as a result of microcracking of the binding medium.
Setting and hardening of the cement paste are the main physical characteristics associated with hydration of the cement. Hydration results in the formation of a gel around each of the cement particles and in time these layers of gel grow to the extent that they come into contact with each other. At this stage the cement paste begins to lose its fluidity. The beginning of a noticeable stiffening in the cement paste is known as the initial set. Further stiffening occurs as the volume of gel increases and the stage at which this is complete and the final hardening process commences is known as the final set.
cement Typical results British standard
Initial set Final set Initial set Final set
Ordinary 162min 214min >45min <10hrs>45min <10hrs>60min <10hrs>45min <10hrs>4000kg/cu.m)
Used to provide high density concrete for screening radioactive sources
Magnetite, haematite, limonite etc.
The grading of an aggregate defines the proportion of particles of different size in the aggregate.
The size of the aggregate particles normally used in the concrete varies from 37.5mm to 0.15mm
Fine aggregate – aggregate mainly passing 3/16’’ (5mm) sieve
Coarse aggregate – aggregate mainly retained on a 3/16’’ (5mm) sieve
All-in-aggregate – comprising both fine and coarse aggregates
The grading of an aggregate can have a considerable effect on the workability and stability of a concrete mix.
When 2 or more aggregates are available, it may be possible to combine them to give a grading approximating to the one required.
Bulking of sand:
When sand is moistened, films of water form on the particles and tend to hold them apart, causing an increase in volume of bulking.
In general, sand bulks rapidly to the extent of 20% or 30%, as the moisture content rises to about 4% to 6%. Further increases in moisture content result in a decrease in bulking, until when the sand is completely saturated, its volume is practically the same as it was in a dry condition.
When measuring aggregates by volume, it is necessary to make an allowance for bulking in order to obtain the correct proportion of sand particles required to produce good workable concrete.
Water used in concrete, in addition to reacting with cement and thus causing it to set and harden, also facilitates mixing, placing and compacting of the fresh concrete.
Water for making concrete should be clean and free from organic or inorganic matter in solution or in suspension in such amounts that may impair the strength or durability of the concrete.
Inorganic matter is all the parts of the biosphere that are not living things nor products of living things. Inorganic matter includes water, gasses, salts, acids, bases, as well as inorganic forms of nutrients such as nitrogen, potassium, and phosphorus. Inorganic matter moves through the ecosystem when plants and animals decompose, and by disturbances such as fire and flood.
In general, water fit for drinking, such as tap water., is acceptable for mixing concrete.
Seawater is not suitable for reinforced concrete works.
Admixtures are substances introduced into concrete mixes in order to alter or improve the properties of the fresh or hardened concrete or both.
Types of admixtures:
Entrainment of air in the form of very small bubbles
Disrupt the continuity of capillary pores and thus reduce the permeability of concrete
Reduce internal stresses caused by the expansion of water on freezing
Hence improve the durability of concrete, in particular its resistance to the effects of frost and de-icing salts
Reduce setting time
Particularly suitable for repair works involving water leakage
Adverse effect on subsequent strength development
Increase setting time
Used mainly in hot countries where high temperatures can reduce the normal setting time.
Water-reducers or plasticisers
Reduce viscosity of cement paste
Increase workability of fresh concrete
Thus for a given workability requirement, a lower water/cement ratio can be used which leads to higher strength and durability
Used for architectural purposes
Criteria for good quality concrete:
The quality of concrete is usually judged by its compressive strength, since it has been found that this bears an approximate relationship to most of the other properties of concrete and is moreover, relatively easy to determine. Increase in compressive strength of concrete usually indicates an all-round improvement in the quality of concrete.
The compressive strength of concrete depends on:
Water in a concrete mix has 2 functions
To enable hydration of the cement which causes setting and hardening of the cement paste
To lubricate the mixture of aggregates and cement in order to facilitate placing and compaction
For hydration of cement, the quantity of water required is only 1/5 to 1/4 of its own weight, i.e. theoretical minimum of W/C=0.2 to 0.25. The water which does not enter into chemical combination with the cement forms water voids which may subsequently dry out to form air voids.
If concrete is not fully compacted, numerous bubbles of air may be trapped, resulting in further voids.
Concrete with the minimum volume of total voids is the densest and strongest.
2% void – reduction of strength>10%
5% void – reduction of strength≈30%
The quantity of water in the mix is generally expressed in terms of the “free-water/cement ratio” (sometimes less exactly in terms of just “water/cement ratio”) which is calculated by dividing the total weight of water in the concrete by the weight of cement. The total weight of water includes the moisture on the surface of the aggregate as well as the water added at the mixer, but does not include the water absorbed by the aggregates.
The strength of concrete is primarily dependent on the W/C ratio. Other things being equal, the lower the W/C ratio the higher is the strength, provided the W/C ratio is not less than 0.23 – the minimum water content required for hydration of cement. Thus, theoretically, the optimum strength can be obtained at a W/C ratio of approximately 0.23, but with this low W/C ratio, the concrete mix would be extremely dry and virtually impossible to compact.
Type of cement
The influence of the type of cement on concrete strength, for given mix proportions, is determined by its fineness and chemical composition through the process of hydration. However, while there can be large differences in the early strength of concrete made with different Portland cement, their final strengths will generally be very much the same.
Strength & grading of aggregates
The strength of concrete is not significantly affected by the strength and grading of the aggregates provided the strength of the aggregates is higher than the design strength of the concrete. However, in the production of super-high strength concrete, the strength of the aggregates may limit the strength attainable by the concrete and in such cases, careful selection of the type of aggregates to be used should be made.
Most rock aggregates are sufficiently strong for the production of normal strength concrete.
Degree of compaction
Inadequate compaction may result in large volume of air voids which would significantly reduce the strength of the concrete. In the worst cases, even honecombing may occur. any honeycombs formed should be chipped off and re-concreted.
Method of curing
The hydration of the cement which requires certain quantity of water takes a fairly long time to complete. If, during the course of hydration, the water is lost due to evaporation, then incomplete hydration may occur and the strength of the concrete would be impaired. Thus adequate curing of the concrete is a pre-requisite for the production of good quality concrete.
Generally speaking, the longer the period during which the concrete is kept moist or kept in water, the greater its final strength.
Certain curing compounds have nowadays been developed which when sprayed onto the surface of the concrete, would inhibit the evaporation of the water required for the hydration of cement, thus dispensing with the need of constant watering of the concrete.
Age of concrete (increase with age)
The compressive strength of concrete increases with its age. The compressive strength at the 28 days is usually taken as the standard for design purposes.
At 7 days 70% of 28 day strength
At 28 days 100% of 28 day strength
At 90 days 120% of 28 day strength
At 1 year 130% of 28 day strength
The rate of strength development depends on the temperature and thus the above figures vary slightly from place to place.
Measure of strength
The compressive strength of concrete is measured
In the U.K. and Germany by cubes (150mm or 100mm size)
In the U.S.A., Canada and Australia by cylinders
The tendency nowadays, especially in research, is to use cylinders in preference to cubes.
In the U.K., the 28 day cube strength as measured by crushing 150mm cubes is used to signify the grad of the concrete.
e.g. concrete grade 30/20 means
28day cube strength=30MPa (N/mm2)
The cube strength obtained by crushing 100mm cubes may be slightly different from that of 150mm cubes.
The cylinder strength is usually only about 70 to 90% of the cube strength. The difference is due to the frictional forces which develop between the platen plates of the testing machine and the contact faces of the specimen. These end forces produce a multiaxial compressive stress state which increases the compressive strength of the concrete specimen. The multiaxial stress effects are significant throughout the cube; in the cylinders, however, the height/width ratio is sufficiently large for the mid-height region to be reasonably free from these effects. For practical purposes, the cylinder strength may be taken as the unaixial compressive strength of the concrete. The in-situ uniaxial compressive strength is taken as 0.67 x cube strength.
The tensile strength of concrete is usually measured indirectly.
Split cylinder test
Quite close to the real tensile strength
Flexural test – gives the modulus of rupture
Generally higher than actual tensile strength
Workability of concrete has never been precisely defined. For practical purposes, it generally implies the ease with which a concrete mix can be handled from the mixer to its final compacted shape. The 3 main characteristics of the property are:
Consistency measure of wetness or fluidity
Mobility ease with which a mix can flow into the completely fill the framework
Compactibility ease with which a mix can be fully compacted
Factors affecting workability:
The water content
Water/cement ratio and aggregate/cement ratio
The grading, shape of particle, surface texture, maximum size ect of the aggregates
The greatest factor affecting the workability is the amount of water (which act as lubricant) in the mix. The higher the water content is, the higher the workability will be.
As the water/cement ratio is normally fixed by the strength requirement, the only way of increasing the amount of water is to increase also the cement content in the mix, i.e. to reduce the aggregate/cement content.
If the aggregate consists of smooth, rounded particles, rather than rough, angular particles of similar grading, the area of contact between the paste and aggregate is smaller, and the concrete has greater workability. If the grading is varied so that the specific surface of the aggregate is decreased, the workability is increased e.g. increasing the maximum size of aggregate or decreasing the proportion of fine aggregate. However, the proportion of fine aggregate can be varied only within certain limits, because insufficient fine materials results in an uncohesive mix which is harsh and liable to segregate.
The workability of concrete may be significantly improved by the addition of workability admixtures, e.g. water-reduceing (plasticizing) or air-entraining agents.
The degree of workability required depends on the type of work, the degree of steel reinforcement congestion and the method of compaction.
Measurement of workability:
A 300mm high concrete cone, prepared under standard conditions is allowed to subside and the slump or reduction in height (in mm) of the cone is taken to be a measurement of workability. The test is very simple and useful at the site to serve as a control; however, it is not a good test, as the slumps of similar mixes may exhibit a wide range of variations.
Compaction factor test
The apparatus consists essentially of 2 conical hoppers and a cylindrical would mounted one above the other. The concrete is filled into the upper hopper and then allowed to first fall into the lower hopper and subsequently fall into the cylindrical mould which fills to overflowing. The excess concrete above the top of the cylinder is cut off by sliding 2 trowels across from the outside to the centre.
Compaction factor =
(wt.of partially compacted concrete)/(wt.of fully compacted concrete)
The compaction factor test gives more consistent results than the slump test and is thus a more reliable measure of workability. It is, however no so convenient as slump test.
Vebe consistometer test
The V-B time (in seconds) is the time taken to transform, by means of vibration, a standard cone of concrete to a compacted flat cylindrical mass. The V-B test is used mainly in laboratories.
Correlation between the 3 different measurement of workability:
Slump(mm) 75 25 5
Compaction factor 0.95 0.85 0.75
V.B (sec) 2-3 8-12 15-20
Durability of concrete includes:
Resistance to weathering ( especially frost action)
Resistance to abrasion
Resistance to chemical corrosion
Main factors promoting durability of concrete:
Low W/C ratio
Low A/C ratio
Clean, impervious and inert aggregate
Ample cover to reinforcement
For reinforced concrete works in which the nominal cover provided to the steel reinforcement is 25mm,
exposure Max. W/C ratio Min. cement content
mild 0.65 275kg
Moderate 0.50 350kg
severe 0.45 400kg
For concrete not containing embedded material (plain concrete), the corresponding requirements are:
exposure Max. W/C ratio Min. cement content
mild 0.80 180kg
moderate 0.65 275kg
Severe 0.60 300kg
Design of concrete mixes:
Basically, the problem of designing a concrete mix consists of selecting the correct proportions of cement, fine and coarse aggregate and water to produce concrete having the specified properties.
The properties usually specified are:
The compressive strength at a specific age
The workability of the fresh concrete
The durability, by means of specifying the minimum cement content or the maximum water/cement ratio, and in some cases restricting the type of materials to be used
Margin for mix design:
As a result of the variability of concrete in production, it is necessary to design the mix to have a mean strength greater than the specified strength by an amount termed the margin.
Fm = fs +kφ
Where fm=the target mean strength
Fs=the specified characteristic strength
φ=the standard deviation
k=a constant, equal to 1.64 for 5% defective
Compressive strength – W/C ratio relationship:
The relationship varies with the type of cement and the age of test, and to a smaller extent the type of aggregate.
Workability – free water content relationship
The workability (expressed in terms of slump of V-B time) increases with the free-water content. The relationship varies with the size and type of aggregates.
Maximum size of aggregate(mm) Type of aggregate Free water content (kg/m3)
V-B>12 sec Slump:10-30mm
V-B:6-12 sec Slump:30-60mm
V-B:3-6s ec Slump:60-180mm
10 Uncrushed 150 180 205 225
Crushed 180 205 230 250
20 Uncrushed 135 160 180 195
Crushed 170 190 210 225
40 Uncrushed 115 140 160 175
Crushed 155 175 190 105
When coarse and fine aggregates of different types are used, the free-water content is estimated by the expression
2/3 W_f+1/3 W_c
Where Wf = free-water content appropriate to type of fine aggregate
And Wc = free-water content appropriate to type of coarse aggregate
Procedure of mix design:
Selection of W/c ratio
Calculate the strength margin either from previous statistics or other data hence determine the target mean strength from the target mean strength determine the W/C ratio
Check max. W/C ratio for durability
Selection of free water content
Determination of cement content
cement content=(free water content)/(W/C ratio)
Check the min. cement content for durability
Determination of total aggregate content
Estimate the density of the fully compacted concrete
Total aggregate content = wet density of concrete - cement content - water content
Selection of coarse & fine aggregate content
The preceding design method determines a set of mix proportions for producing a concrete that has approximately the required properties of strength and workability. Trials are necessary to check if the specified properties are achieved or if any adjustment to the mix design is necessary.
During the trial mix, it is useful, initially, to withhold a small proportion of the mix water which is then added gradually in order to observe (visually) the change of workability with the water content.
After measuring the density of the fresh concrete the resultant value is compared with the density value used in the mix design. The mix proportions should then be corrected according to the measured density.
When results of the strength tests become available, they are compared with the target mean strength. If necessary an adjustment is made to the W/C ratio.
Minor adjustments may be made to the mix proportions for use in production mixes without the need to carry out further trials
Concrete mixing, compaction & curing processes:
Batching means the process of measuring the quantity of each of the materials in their correct proportions before they are placed in the mixer.
Methods of batching depends, to some extent, on the quantity of concrete required. For small jobs, batching is perhaps more commonly done by volume. On large jobs, usually all the materials, except the water, are weighted in large batching plants.
Batching of cement is always by weight or in whole number of bags. The size of batch should be such that is contains a whole number of bags of cement.
Batching of aggregates by weight eliminates errors due to variations in the proportions of voids contained in a specified volume.
Type of mixers:
Rotating on an inclined axis and comprising a single compartment drum having one opening
Non-tilting drum mixers
Rotating on a horizontal axis and comprising a single compartment drum having two openings
Comprising a pan and an eccentric mixing star rotating vertically (able to mix stiff concrete efficientl)
Used for ready mix concrete
The mix time is about 2 min. mixing should continue until the concrete is of uniform consistency and color. The complete content of the drum should be discharged in one operation.
The transportation of concrete from the mixing plant to the point at which it is to be placed must comply with the following requirements:
Transport must be rapid so that the concrete does not dry out or lose its workability during the time elapsed between mixing and placing.
Segregation must be reduced to a minimum in order to avoid non-uniform concrete.
A continuous supply of concrete should be maintained so as to avoid any undue delays which may result in the formation of cold joints.
Method of transporting concrete:
Wheelbarrow-for small quantity and short dist.
Dumpers-for roadwork & ground floor construction
Steel skips and buckets
Pumps-pump mixes must have high workability, and well graded without any tendency towards segregation; quite expensive
Good quality control
Avoidance of congested site
Truck mixers keep concrete agitated during transport
Placing & compacting:
Precautions during placing of concrete:
The concrete should be deposited as near as possible to its final position and should not be deposited in a large quantity at any point and allowed to flow along the form.
Concrete should be deposited in horizontal layers, and each layer should be compacted thoroughly before the succeeding layer is placed.
The concrete should be worked thoroughly into position around the reinforcement and into the corners of the formwork.
The concrete should be placed in its final position before the cement reaches its final set. The concrete should normally be compacted in its final position within 30 min. of leaving the mixer and, once compacted, it should not be disturbed.
The object of compacting the concrete is to eradicate air holes and to give maximum density. Compaction also ensures an intimate contact between the concrete and the surface of reinforcing steel.
External vibrators-vibrators attached to formwork
Surface vibrators-for road works
Mechanical hammers-for columns and walls
Vibrating tables-for precast concrete works
The chemical action which accompanies the setting of concrete is dependent on the presence of water, and although there is normally an adequate quantity for full hydration at the time of mixing, it is necessary to ensure that the water is either retained or replenished to enable the chemical action to continue until the concrete is fully hardened.
Method of curing:
Continuous watering to keep concrete moist
Application of sealing compound to concrete surface to minimize evaporation.
Cold weather and hot weather concreting:
Cold weather concreting
Concrete gains strength in proportion to its maturity which is a function of the temperature and the period of time after the concrete has set. Concrete dose not gain strength when frozen.
Unless some form of heat is applied and the concrete can be adequately protected, it is advisable to stop concreting when the temperature reaches about 4。C on a falling thermometer. With certain precautions, nevertheless, concrete can be continued at freezing temperatures.
Heat the water and aggregate (mix water and aggregate before adding cement).
Mix the concrete near its final position, so that concrete does not cool much during transporting.
Enclose the placed concrete and provide continuous heating-the temperature of the newly placed concrete should not be allowed to fall below 5。C.
Warm the forms and the reinforcement if they are frozen.
Extend the curing time and keep the formwork in position for longer time.
Hot weather concreting:
The following difficulties may arise because of higher concrete temperatures or more rapid drying out:
The workability of concrete with given mix proportions decreases as the temperature increases.
In concrete remains unprotected in hot weather, it is likely to crack while still green. Cracks may occur in hardened concrete at the joints between layers if the surface of the concrete in the lower layer dries out appreciably before the next is placed and the two do not bond properly.
Store cement in silos or sheds which are painted in a light color to reflect the heat.
Insulate the pipes and tanks containing water.
Protect the aggregates from direct sunshine.
Shade the concrete from direct sunshine and from drying wind.
Apply retarders to reduce the rate of setting.
Ensure efficient concreting operations.
Cure the concrete continuously (intermittent watering is no good, should spray water continuously, use sealing compound if necessary).
Durability of concrete & reinforced concrete:
Deterioration of concrete:
Deterioration of concrete mainly due to:
Certain aggregates (siliceous limestones, rhyolites and some colcanic rocks) contain active silica which can react with the alkaline content of the cement to form alkaline silica gel. The alkaline-silica gel absorbs moisture and swells and as a result internal pressures are built up leading eventually to cracking and disruption of the cement paste.
Differential volume changes
Principal factors for differential volume changes are:
Variation in temperature
Alternate wetting and drying
Porosity in concrete is due to:
Gel pores-hydrated cement gel contains many very fine pores (diameter around 0.015μ) and total some 28% of the cement paste.
Capillary pores-larger pores (diameter up to 5μ) and may form up to 40% by volume of the cement paste depending upon the W/C ratio and the extent of the hydration.
Voids-entrapped air, 1-2% by volume; large amount of voids lead to honeycombing.
The permeability of hardened concrete decreases with the W/C ratio, i.e. the lower the W/C ratio, the lower the permeability and therefore the higher the durability of the concrete.
Weathering, especially frost action
Frost action due to alternate freezing and thawing:
During freezing and thawing, either the paste or the aggregate or both may be damaged by dilation. Dilation and associated internal stresses are believed to be due to:
Expansion due to growth of capillary ice crystals
Hydraulic pressure generated when growing ice crystals displace unfrozen water and cause it to flow against resistance to unfrozen portions of the mass
Osmotic pressures brought about by local increase of alkali concentration caused by the separation of pure ice from the solution
Frost damage may be minimized by:
Using low W/C ratio to reduce porosity
Air entrainment-entrained air forms discrete air cavities in the cement paste so that no channels for the passage of water are formed.
CO_2+H_2 O(rainwater)→H_2 CO_3 (carbonic acid)
Carbonic acid has a solvent action on lime compounds present in the concrete.
〖Ca(OH)〗_2+H_2 CO_3→CaCO_3+2H_2 O
The depth to which the calcium hydroxide is changed to calcium carbonate depends on the porosity of concrete. The depth of carbonate increases with an increase in W/C ratio but seldom exceeds 15mm.
The protection of steel from corrosion by the alkaline condition of hydrated cement paste is neutralized by carbonation. The pH dropping from over 12 to about 8. Thus if the entire concrete cover to steel were carbonated, corrosion of steel would occur if moisture and oxygen could ingress.
Concrete is essentially an alkaline material. Common acid attacks of concrete are due to lactic and acetic acid in food-processing plants. These attacks are mild but persistent.
Sulphates in solution attack concrete by reacting with the hydrated tricalcium aluminate in the hardened cement paste. This causes expansion and disruption of the concrete. The rate of attack depends on:
Form in which the sulphate occurs. Easily soluble sulphates such as those of Na, Mg and NH4 react more rigorously than calcium sulphate.
Concentration-the higher the concentration, the more serious is the attack especially when a continuous supply of sulphate solution exists.
Permeability of concrete.
Formation of cracks.
The vulnerability of concrete to sulphate attack can be reduced by the use of cement low in C3A.
Corrosion is an electrolytic process whereby the depassivated area becomes anodic relative to the areas which remain protected by the sound alkaline concrete. Metallic ions are released from the anode and the resulting excess of electrons are attracted to the cathodic regions where they associate with oxygen and water to form hydroxyl ions.
Surface repair involves the process of conditioning the existing concrete to receive repair materials. Conditioning is required to remove deteriorated, contaminated, or damaged concrete to provide surfaces that will promote bonding of the repair materials. The surface preparation process is one of the most critical phases of site work. Without proper understanding and care, the necessary requirements will most likely not be met.
Many techniques are available to perform various aspects of concrete removal and cleaning. Each method has specific advantages and limitations. Much of the removal work is still done by small hand-held chipping hammers because of their mobility and versatility.
General surface preparation procedure
Locate area to be repaired. Hammer sounding or chain drag are used when locating delamination. Design and install temporary suppor system prior to any concrete removals.
Remove deteriorated concrete using acceptable methods. When embedded steel is encountered, follow recommendations on following pages. Under-cutting of exposed bars is critical to long term success of surface repairs. Bars which are damaged by the removal operation or have a significant section loss may require repair.
Prepare surface repair boundaries to prevent feather edged conditions Geometry of boundaries should minimize edge length. Shotcrete may require some modification to squared edges.
Clean the surface of the exposed reinforcing steel and concrete. Surface cleaning is critical to achieve an adequate bond between the repair and the existing concrete.
Reinforcing steel cleaning, repair & protection
Corroded or otherwise damaged reinforcing steel is usually found in conjunction with concrete deterioration. Heavy rust layers that build up on reinforcing steel during the corrosion process are the cause of concrete delamination and spalling. Removal oxide build-up is critical to the long term success of surface repairs. Many repairs have failed within a few years of completion because of insufficient cleaning.
The general procedure is as shown below:
Exposed corroded reinforcing steel encountered in the repair process requires concrete to be removed around the full circumference of the bar. This allows the reinforcing bar to be cleaned and allows a uniform material to be placed around it.
Heavy oxides or other bondinhibiting materials must be removed by any acceptable cleaning method. We can clean the reinforcing bar by needle scalars, high pressure water cleaning and abrasive blast cleaning.
Bars damaged during removal operations or with critical section loss may require repair or replacement.
In certain situations, special coatings may be applied to add additional protection to the reinforcing bars.
Selection of a surface repair placement method includes the following important steps:
Selection of a repair material that best reconstitutes the strength, integrity and performance required by the structure’s original design and current situation.
Selection of a method of placement that will successfully deliver the repair material onto the prepared concrete substrate.
Checking the constructability of the selected repair material and installation method.
Adjusting the material and installation methods to provide a constructible repair.
The placement technique must deliver the selected repair material to the prepared substrate with specified results. The repair material must achieve satisfactory bond to the existing substrate, must fill the prepared cavity without segregation, and fully encapsulate exposed reinforcing steel. Without achieving the above requirements, the surface repair may not perform its intended structural, protective, and aesthetic duties.
The placement method must consolidate the repair material and create intimate contact between the repair material and the substrate.
The placement method must also fully encapsulate any exposed reinforcing steel and produce a uniform cross section without segregation, sold joints or void.
Dry packing is a method of placing zero-slump, or near zero-slump, mortar or concrete, by ramming, into surface cavities. The consistency of dry pack mortar must be such that it can be molded into a ball without excessive bleeding.
Form and cast-in-place
One of the most common methods of surface repair of vertical and, in some cases, overhead locations is the placement of formwork and casting of repair material into the prepared cavity. Formwork facilitates the use of many different repair materials, selected on the basis of in-place performance vs. constructability. The repair material must be of low shrinkage and provide the necessary flowability. Placement of repair materials follows normal-placement practice. Rodding or internal vibration is necessary to remove air and provide intimate contact with the existing concrete substrate.
Hand-applied techniques are used to place non-sag repair material on vertical and overhead locations. Most hand-applied materials are special blends of cement, finely graded aggregates, non-sag fillers, shrinkage compensating systems, and water. The mixed material is applied to the prepared surface with either a towel or by hand. The applied pressure drives material into the pore structure of the exposed concrete. the repair material is designed to “hang” in place until subsequent layers are added.
Thursday, 15 January 2009