Concrete

INTRODUCTION

Most structures have concrete as their principal materials, used in one of the following forms:     

-  reinforced concrete

-  prestressed concrete

-  mass concrete

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What is concrete?

This following video shows us "WHAT IS CONCRETE" ENJOY~

A mixture of cement, water and aggregates(coarse and fine). The cement and water combined to form a paste and when hardened, binds the aggregates particles together to form a monolithic whole. The cement and water hardened by a chemical reaction, called hydration.


Basic desired properties of concrete
(a) good workability when the concrete is fresh (newly mixed); i.e, the concrete can be transported from the mixer, handled, placed in the moulds (or formwork) with cover of oilly surface and compacted satisfactorily;
(b)  high strength and hardness;
(c)  adequate durability.

Mix proportions of concrete
The proportions of the constituent materials, by volume, in freshly mixed concrete:
   (a)  6 -16% cement
   (b) 12-20% water
   (c) 20-30% fine aggregate
   (d) 40-55% coarse aggregate
The properties of concrete are affected by the amounts of the constituents, i.e. the mix proportions. Measuring exact volumes of the materials is difficult. Therefore the mix proportions are usually expressed as the weight of each material required in a unit volume of the concrete production., usually in kg/m3.
         The mix proportion by weight are:

            Cement                              150-600kg/m3
            Water                                110-250kg/m3
            Aggregates(coarse+fine)     1600-2000kg/m3
           
The water/cement ratio is an important factor influencing many of the concrete properties.

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CEMENT

Production of cement (Portland cement)

Portland cement usually refers to the Ordinary Portland cement(OPC) is the most common type of cement being used. The important ingredients in the manufacture of cement are calcium oxide (CaO) and  silicon dioxide (SiO₂ ) or silica. Calcium oxide occurs naturally in various form of calcareous calcium carbonate (CaCO₃), eg. chalk and limestone, while silica is found in argillaceous clay or shale.. As huge quantities of these materials are required in the manufacture of cement, cement plants are normally sited close to a suitable source of one or both of these. The OPC is the cheapest cement among all, hence it have been widely use to reduce the cost.

The raw materials all contain some other components, and in particular clays contain oxides of aluminum, iron, magnesium, sodium and potassium. Aluminum and iron oxides have a significant effect on the manufacture and composition of the resulting cement.

The production process

Chalk & clay reduced to particle sizes ≤ 75 µm

│

Mixed in the required proportion

│

Mixture either mixed with water to

form a slurry or dried as powder

│

                            WET  PROCESS                                    DRY  PROCESS

                                       ┌──────────────────────┐

                       Slurry fed into higher                               Dried powder preheated

                     end of inclined rotary kiln                            and some calcining   

                                        │                                                    in heat exchangers

                                        │                                                              │      

                    Heated as it passes down the                       Mixture fed into higher

                    kiln under combined action of                   end of inclined rotary kiln

                         rotation and gravity                                                 │

                                        │                                             Mixture under increasing     

                Mixture under increasing temperature            temperature undergoes

                 as it passes down the kiln undergoes              2 chemical changes

                    4 physical & chemical changes                                 │

                                        │                                                     1. Calcining

                                 1. Drying                                                           │

                                        │                                                     2. Clinkering

                                2. Preheating

                                        │

                                3. Calcining

                                        │

                                4.  Clinkering (or burning)                     (see Fig 13.1)

FIG 13.1

                 Calcining –  base materials are heated to form oxides;

                                      CaO (lime) = C

                                      SiO₂ (silica) = S

                                      Al₂O₃ (alumina) = A

                                      Fe₂O₃ (iron oxide) = F

                                      

                 Clinkering – oxides at high temperatures combine to form compounds mainly

                                      calcium silicates, calcium aluminates and calcium aluminoferrites

The final products of the above processes, in the form of clinkers, are chiefly the calcium silicates and aluminates and smaller amounts of other compounds;

                                    Tricalcium silicate = C₃S

                                    Dicalcium silicate = C₂S

                                    Tricalcium aluminate = C₃A

                                    Tetracalcium aluminoferrite = C₄AF

Each grain of cement consists of a mixture of the above compounds.

After cooling a small amount of gypsum (calcium sulfate dehydrate, CaSO₄.2H₂O) is added to the clinker before the mixture is ground to a fine powder. The purpose of gypsum is to retard the curing process so as to prevent immediate stiffening of the cement paste during hydration.

The composition of the clinker determines the types of Portland cement.. The principal compounds (derived from their oxides) are C₃S and C₂S and these together normally amount to ¾ of the content of the cement (see Table 13.1).

           

            Cement A – ordinary Portland cement

            Cement B -  early strength cement

            Cement C – low heat cement

            Cement D – sulphate resisting cement

Hydration

What is hydration?

- the chemical combination of cement and water

Initial period after mixing	Between 2 – 4 hrs after mixing	Between 3-10 hrs after mixing	Between 7-28 days after mixing	28 days till many months
fluidity of cement and water paste constant	initial set – paste starts to stiffen but no strength	final set – paste starts to harden and gain strength	rate of strength gain rapid	hardened cement continues to gain strength but at a slower rate

See Fig 13.3

Heat is given off during the setting and early hardening of cement paste (concrete).

Fig 13.2 shows a typical graph on the heat given off during hydration of cement. Immediately after mixing, there is a high but short peak (A) of heat released, lasting only a few minutes. This quickly declines to a low constant for the dormant period when the cement is relatively inactive; this may last for up to 2 or 3 hours. The rate of heat released start to  increase rapidly at a time corresponding to the initial set. This reaches a peak (B) sometime after the final set. The reactions then gradually slow down, with sometimes a short spurt of heat release after 1 or 2 days giving a narrow peak (C).

The behavior of the aluminates and silicates is particularly important in the early stages of hydration;

1)  C₃A reacts violently with water, resulting in immediate stiffening of the paste and giving off great amounts of heat.  That is why gypsum is added to slow down the reaction.

C₃A gives the cement (concrete) the early initial strength.

2)  Between C₃S and C₂S, the former is the faster to react giving high early strength to the cement (concrete).

3)  C₂S reacts much more slowly and it contributes importantly to the long-term strength of hardened cement.

As C₃S and C₂S form the bulk of the unhydrated cement, their hydration products give the hardened  cement most of its significant engineering properties such as strength and stiffness. See Fig13.4.

Water/cement ratio

Water/cement ratio is the ratio of the weight of the water to the weight of the cement. Lower water ratio lead to the increase of strength and durability of the concrete.

Modifications of Portland cement.

The rate of heat output and timescales of setting and strength gain of cement govern some of the critical operations in concrete practice;

-                    the transport of fresh concrete

-                    the placing of fresh concrete

-                    the removal of formwork

Different types of Portland cement are produced to meet the above requirements

Types of Portland Cement

Ordinary Portland Cement

This is the most widely used in construction site as it is cheaper.

Rapid-hardening Portland Cement

Rapid-hardening properties can be achieved by;

1. Having higher C₃S and lower C₂S content in the cement.

    By having higher C₃S which develops earlier strength compared to C₂S(as shown in

    Fig 1.4), the cement paste (concrete) gain strength more rapidly than ordinary Portland cement.

FIG 1.4

2. Grinding the cement clinkers to finer particles.

     Being more finely ground, rapid-hardening cement has greater surface area for hydration , thus it develops strength more rapidly. 

In cold weather, the high rate of heat output helps to prevent damage by frosts. However,  the higher rate of heat output in the early stages of hydration will increase the risk of thermal cracking in large pours.

Ultra-high early strength Portland cement

In this cement, the cement clinkers are ground to extremely fine particles. Concrete made with this cement achieves the 3-day strength of rapid-hardening cement in 16 hours and its 7-day strength in 24 hours. Even higher early strength can be achieved by steam curing.

Low-heat Portland cement

This cement is required for thick concrete work, where the heat generated by ordinary cements will be excessive and lead to serious cracking. It is manufactured by either;

1. Less finer grinding of cement clinkers, or

2. Lower C₃S and higher C₂S content in cement.

Sulphate resisting Portland cement

This cement is used where there is presence of sulphates (sulfates) from external sources, such as in industrial wastes, sulphate bearing soils and in sewage wastes.

Sulpahtes react chemically with the hydration products of calcium aluminates, causing cracking and loss of strength in the hcp. The solution is to reduce tricalcium aluminate (C₃A) in the cement during manufacture (not exceeding 3.5%).

White Portland cement

The gray color of Portland cement is due to ferrite (C₄AF) in the limestone or clay.  White cement is made from non- ferrite containing material – white china clay. White cement is more expensive than normal Portland cements due to higher raw materials cost and greater care during manufacturing.

Portland blast-furnace cement

This is made by adding about 30-35% by weight blast furnace slag to ordinary Portland cement (opc) clinker before grinding. The rate of hardening of this cement in the first 28 days and the heat evolved is less, so the cement is not suitable for use at low curing temperatures.

However, the strength of mature concrete is the same with concrete made from opc. It has good resistance to dilute acids and sulphates and can be used for construction in sea water.

Pozzolanic cement

This cement is made from pulverized fuel ash and opc. Pozzolanic cement is a low-heat cement – initially slower in hardening but attains strength equal to that of opc after 3 months. Good resistance to sea water and sulphates (sulfates).

Other types of cement (non Portland cement)

High alumina cement

Bauxite and limestone are fused in reverberatory furnaces. These are cooled and the extremely hard product is broken and ground to cement.

High alumina cement is grey-black (different from Portland cement). This cement is resistant to sugar, oils, fertilizers, beer, acids and sulphates, making it suitable to be used in places where such materials are present. However its resistant to caustic alkalis (sodium and potassium hydroxides) is low.

Supersulphated cement

It is made by grinding together 85-90 % of blast furnace slag, 10-15% sulphate and 1-5% Portland cement clinker.

Resistance to acids is high and unlike high alumina cement it is also resistant to caustic alkalis. It is a low-heat cement suitable for mass (thick) concrete and for work in hot climates. As it is low-heat, early strength development is slow, particularly in cold weather, but at later ages strengths are at least equal to those of Portland cement mixes.

Admixtures for Portland cement.

Admixtures - chemicals that are added to the concrete before or during mixing.  They significantly change its fresh, early age or hardened state of the concrete. The popularity in their use has increased considerably in recent years.

There are 5 distinct types of admixtures;

-                    plasticizers

-                    superplasticisers

-                    accelerators

-                    retarders

-                    air entraining agents

Plasticizers

These are workability aids which increase the fluidity (or workability) of a concrete (cement) paste. Plasticizers are  polymers, which are absorbed onto the surface of cement grains with an ionic group pointing outwards.  The negatively charged surfaces cause the mutual repulsion of the cement particles, thereby dispersing the particles and releasing entrapped water to give greater fluidity (see Fig 14.1).

Plasticizers are also known as water-reducers, since they can produce a concrete (or hcp) with the same workability at lower water/cement ratio. The lower water/cement ratio increases the strength of the concrete with the same cement  amount. Plasticizers are also often used when pozzolanic ash is added to concrete to improve strength. This method of mix proportioning is especially popular when producing high-strength concrete and fiber-reinforced concrete

Undesirable secondary effects of  plasticizers;

-                    delaying (retarding) the set

-                    decreasing early strength gain

-                    entrain (trap) air in the form of air bubbles

-                    contain impurities affecting strength of hardened concrete.

Superplasticisers

Also known as high-range water reducers.

Superplasticisers are used to achieve increases in fluidity and workability greater than those obtainable with plasticizers. They are manufactured to higher standards of purity – can achieve substantially higher primary effects without significant undesirable side-effects.

Accelerators

Used to increase the rate of hardening of cement paste, thus;

-                    enhancing early strength gain

-                    reducing the curing time for concrete placed in cold weather

-                    reducing setting time

Since accelerators increase rate of setting, resulting in high rate of heat evolution, they should only be used in cold weather.

Calcium chloride (CaCl₂) was the most commonly used accelerator as it was effective and readily available. It accelerates both the initial and final setting of cement. It is no longer recommended to be used in reinforced concrete (rc) as it lead to corrosion of embedded steel reinforcement. A number of alternative chloride-free accelerators are now used based on calcium format, sodium aluminate or triethanolamine.

Retarders

Retarders delay the setting time of  a cement mix. Their uses include;

-                    counteracting the accelerating effect of hot weather

-                    controlling the set in large pours, where concreting may take several hours

-                    delaying the set where concrete has to be transported over long distances

Admixtures based on sugars (sucrose), starches, citric acid, zinc oxide or boric oxide are used to retard the setting of cement.

Air entraining agents

Air entraining agents (AEAs), when added, entrain a controlled quantity of air in the form of microscopic, discontinuous and uniformly distributed bubbles in the cement paste.

The primary effect of entrained air provides resistance to frost (freeze-thaw in temperate climate), which will otherwise lead to progressive deterioration of the concrete (cement).

Air entrainment has 2 important secondary effects;

-                    increase in workability of mix; the air bubbles act like small ball-bearings

-                    increase in porosity results in drop in strength, but the improvement in workability means the loss can be partially offset by reducing the water/cement ratio.

Air entrainment can be achieved by adding vinsol resins,  alkylsulfonates and alkylsulfates.

Damp-proofing admixtures

Damp-proofing admixtures prevent water movement by capillary action. Technically they should not be called waterproofing admixtures

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AGGREGATES

Aggregates are added to cement with water to form concrete.  Usually they occupy about 65-80% of the total concrete volume.        

Reasons for using aggregates in concrete;

-                    they reduce the costing of concrete

-                    they reduce the heat output, hence, reduce thermal stress

-                    they reduce shrinkage of concrete

-                    they help to produce a concrete (when fresh) with satisfactory plastic properties

Desirable properties of aggregates;

-                    they must be sufficiently strong

-                    clean, free from constituents which can react harmfully with cement

-                    have small or no moisture movement

-                    be well graded
-                    right shape and texture so as not to adversely affect the properties of fresh and hardened concrete

-                    low thermal conductivity

Strength

Normal concrete strengths are lower than those of the natural aggregates. Most aggregates are stronger than the concrete strength designed or specified. In fact, aggregates of moderate and low strength reduce the stress in the cement paste and increase the durability of concrete.

Cleanliness

Aggregates should be free from significant quantities of substances which;

-                    are chemically incompatible with cement, eg sulfates and organic matter

-                    reduce bond with aggregate, eg clay and oil

-                    expand, eg bituminous coal,

-                    decompose, eg organic matter

-                    attract moisture, eg salt

-                    cause staining, eg pyrites

Porosity and absorption of water

Aggregates contain pores which can absorb and hold water. This is acceptable as long as there is no or little moisture movement.

Before concrete mixing, aggregates can be in one of the 4 moisture conditions as shown in Fig 16.4;

1. completely dry

2. air dry, pores partially filled with water

3. saturated with water and surface dry

4. wet with excess water on surface

What will happen when aggregates in the above conditions are mixed in concrete?

Which moisture condition is the most desirable?

Grading of aggregates

What is grading of an aggregate?

The proportions of the different sizes of particles, expressed as  percentages by weight passing various sieves.

Aggregates which are retained on a 5mm BS sieve and bigger are termed coarse aggregates, while those passing 5mm sieve are termed fine aggregates.

Aggregates are described by their maximum size, graded down, eg 14mm, 20mm or 40mm.

It is important to use well graded aggregates in a concrete mix to achieve the following:

-                    the various sizes of particles interlock, leaving the minimum volume of voids to be filled with cement

-                    the particles flow together readily, ie the mix is workable

-                    a lower water/cement ratio resulting in higher strength of  hardened concrete

-                    maximum density for good strength and durability

BS 882: 1992, Specification for Aggregates from natural sources for concrete, British Standard, gives grading limits for the various particle sizes (see Table 16.1). Aggregates for use in concrete must fall within the limits of the grading curves for coarse and fine aggregate. Fig 16.2  shows the both coarse and fine aggregates.

The coarse aggregate can be either;

-                    single size where nearly all of the particles are within 2 successive sizes, eg 5-10mm, 10-20mm or 20-40mm

-                    graded where the smallest size is 5mm, e.g. 5-14mm, 5- 20mm or 5-40mm.

The range for fine aggregate is wide. The BS standard subdivides this into 3 divisions of;

-                    fine

-                    medium

-                    coarse

Sieve analysis does not take into account particle shape, but this does influence the void content of aggregate. More rounded particles will pack more efficiently and will therefore have a lower void content.

Is there an ideal grading which is applicable to all aggregates?

No, for important work, tests need to be carried out to determine the grading (for the particular type of aggregate) which gives maximum workability, economy, density, strength and durability in the concrete.

As a guide, a ratio of  1 fine aggregate : 1½ to 3 coarse aggregate is satisfactory.

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CONCRETE

The study of concrete concerns with;

-                    Properties of fresh concrete

-                    Early age properties of concrete

-                    Properties of hardened concrete

-                    Concrete mix design

Properties of fresh concrete

Properties of concrete when freshly mixed, between placing and setting, and during early stages of hydration.  These properties have significant effects on the long term performance of the hardened concrete.

The main properties of interest:

1. Fluidity – being capable of being handled and of flowing into the formwork and  around reinforcement;

2. Compatibility – the air entrapped during mixing, transporting and handling should be

     capable of being removed;

3. Stability or cohesiveness – the concrete should remain as a homogeneous uniform mass.

Properties (1) & (2) – fluidity and compatibility – are combined into the property called workability.

What is the definition of workability?   

‘The property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, transported, placed, compacted and finished’

In general, higher workability concretes are easier to mix, transport (especially, place and compact. Higher workability of concrete can be achieved by one or a combination of the following:

1. Use of a well graded aggregate.

2. Use of smooth and well rounded, rather than irregularly shaped aggregate.

3. Use of air-entraining admixtures.

4. Use of plasticizers and superplasticisers.

5. Higher water/cement ratio.

The undesirable side-effects from the use of some of the above methods;

-                    Use of smooth and well rounded aggregate lead to lower strength, but this can be offset by lower water/cement ratio.

-                    Higher porosity arising from the use of air-entraining admixtures results in lower strength. However, this loss is partially offset by the gain in strength by lower water/cement ratio.

-                    Use of higher water/cement ratio is the simplest way to achieve higher workability but there is the risk of segregation of aggregate which results in lower strength.

Workability Tests

The most common tests are;

1. Slump test (see Fig 17.2)

       The video above shows the process of slump test

2. Compacting factor test (see Fig 17.3)

Procedure to determine workability of fresh concrete by compacting factor test.

i) The sample of concrete is placed in the upper hopper up to the brim.
ii) The trap-door is opened so that the concrete falls into the lower hopper.
iii) The trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.
iv) The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades.
v) The concrete in the cylinder is weighed. This is known as weight of partially compacted concrete.
vi) The cylinder is filled with a fresh sample of concrete and vibrated to obtain full compaction. The concrete in the cylinder is weighed again. This weight is known as the weight of fully compacted concrete.

3. Vebe test

4. Flow table test (see Fig 3.4 – fig 17.5)

Early Age Properties of Concrete

4 inter-related phenomena are important during the period after placing and setting of concrete;

- bleeding

- segregation

- plastic movement

- plastic shrinkage

Segregation and bleeding

Segregation – the denser constituents, larger aggregate and cement particles falling to the lower part of the pour.

Bleeding – upward displacement of water after placing, appearing as a layer of water on

the top surface of concrete.

Problem associated with segregation;

-                      the upper part of the placed concrete will have more fines, resulting in the    upper part of the hardened concrete being weaker than the lower part (see Fig 18.2)

-                       

Problems associated with bleeding;

-                      the layer of water at the top either evaporates or is re-absorbed into the concrete with hydration, resulting in a net reduction of the original concrete volume.

-                      The cement paste just below the surface of the placed concrete becomes water rich and therefore hydrates to a weak layer.

-                      The upward migrating water can be trapped under aggregate particles, causing local weakening of the zone between the cement paste and aggregate, resulting in overall loss of concrete strength.

Plastic movement/settlement

Movement increases with the richness of a mix and with higher water/cement ratio. Greater movement in the fresh concrete occurs near the surface of a pour. If there is any local restraint to this movement (e.g. presence of horizontal reinforcing bars, then plastic settlement cracking can occur. This appear as vertical cracks form along the line of the bars (see Fig 18.3)

Plastic shrinkage

This is due to quick loss of water from the surface of concrete while the concrete is still plastic. The restraint of the lower mass of concrete on the shrinkage at the surface cause tensile strains to be set up in the surface region, resulting in plastic shrinkage cracking (see Fig 3.7 – fig 18.3). Tendency to plastic shrinkage cracking will be encouraged by greater evaporation rates of surface water due to;

-                    higher concrete temperatures

-                    higher ambient temperatures

-                    concrete being exposed to wind

Curing

What is curing?

The protection of concrete from moisture loss after placing  and for the first few days of hardening.

Purpose of curing?

-                    Reduce or eliminate plastic shrinkage cracking

-                    Ensure adequate supply of water for continued hydration and strength gain.

Curing methods

o        Spraying or ponding surface of concrete with water.

o        Protecting exposed surfaces from wind and sun by windbreaks and sunshades

o        Covering surface with wet Hessian or polythene sheets

o        Applying a curing membrane; spray applied resin seal, to exposed surface of concrete

Longer periods of curing are required for mixes that gain strength slowly, such as in conditions of low ambient temperatures. In such a case, steam curing can be applied to speed up the curing process

Properties of Hardened Concrete

The properties of hardened concrete that are of  interests are:

-                    permeability

-                    frost resistance

-                    resistance to abrasion

-                    strength

Among the above properties, strength is the most important single property, since the first (and most important) consideration in structural design is that the structural members must be capable of carrying the imposed loads.

Permeability

Concrete which is made with low water/cement ratio and well-compacted has good resistance to the absorption of water, i.e. low permeability. A concrete that is low in permeability has high durability.

Frost resistance

Concrete can be damaged by expansion of ice crystals (in cold weather) form in capillary pores or cracks. Air-entrainment admixtures form discontinuous minute bubbles which improve resistance to frost.

Resistance to abrasion

Depends upon the hardness of the aggregate particles and the ability of the mortar matrix to retain them (i.e. minimal bleeding).

Strength of hardened concrete

As mentioned earlier, strength is the most important single property. Strength of concrete can be compressive or tensile.

Cracking (failure) pattern in normal strength concrete (see Fig 20.5)

Necessary to understand the cracking pattern

Factors influencing strength of concrete

What are the factors influencing strength of concrete?

-                    Water/cement ratio

-                    Effect of age

-                    Temperature during hydration

-                    Humidity during curing

-                    Aggregate properties

Water/cement ratio

For a fully compacted concrete, the strength of concrete is inversely related to the water/cement ratio (see Fig 20.7).

Limitations of the above relationship;

-                    at low water/cement ratio, the concrete becomes less workable and increasingly more difficult to compact

-                    the difficulty in compacting leads to increasing air being entrapped in concrete

-                    the entrapped air will reduce the concrete strength; each 1 % of entrapped air by volume will reduce the strength by 6% (see Fig 3.8 – shown by dashed lines)

How can the difficulty in compaction and poor workability be overcome, without increasing the water/cement?

-                    use of more efficient method,

-                    use of plasticizers or super plasticizers

-                    Use of a well-graded aggregate

-                    Use of air-entraining admixture

Effect of age

The degree of hydration increases with age; the older the concrete (in the presence of moisture) the greater the strength . The strength at 28 days is often used to characterize the concrete for specification and compliance purposes.

However, it is important to note that the rate of gain of strength varies with;

-                    water/cement ratio

-                    rate of hydration

-                    cement fineness

Humidity

For adequate curing (strength gain) it is necessary to have a humid environment. Concrete cured with the presence of water will achieve greater strength than if cured in air for some or all its life 

Aggregate properties

For normal concrete, the strength of the cement paste/aggregate bond (transition zone) has the dominant effect on strength of concrete.  This bond is influenced by;

-                    aggregate mineralogy

-                    aggregate surface

Increase surface roughness can improve the bond, due to greater mechanical interlocking. Therefore concretes made with crushed rocks are typically 15% stronger than those made with uncrushed gravels (provided all other mix proportions are the same).The use of larger maximum aggregate size reduces the concrete strength due to lower overall surface area with a weaker transition zone (bond).

Strength tests

Types of strength properties in hardened concrete;

-                    compressive

-                    tensile

-                    torsional

-                    fatigue

-                    impact strength

-                    multiracial loading

Different test methods/techniques are used in different countries. Two main objectives of strength testing are;

1. Quality control

2. Compliance with specifications.

Tests can be:

-                    mechanical tests to destruction

-                    Non-destructive tests, which allow repeated testing of the same specimens and thus make possible a study of the change of properties of the concrete with time.

Compressive strength tests

Is the most common tests on hardened concrete. 2 types of compression test specimens are used;

-                    cubes; used in Great Britain, Germany and many countries in Europe,

-                    cylinders; used in the US, France, Canada, Australia and New Zealand

Cube test

the upper cube will be success cube, but the cube below... failure!!!

The specimens are cast in steel or cast-iron moulds, usually of 150mm cubes.

The procedure for preparation of cubes;

1. Fill the mould in 3 layers; each layer of concrete to be compacted by a vibrating hammer, vibrating table or by a 25mm ramming rod with no fewer than 35 strokes,

2.  Trowel smooth’s the top surface of the cube,

3.  Store the cube undisturbed for 24±4 hours,

4.  At the end of the period, the mould is stripped and the cube is further cured in water.

Specimens are usually tested for 7, 14 and 28-day strengths.

Testing procedure;

1.  The cube is taken out from the curing tank and allows drying for 30 minutes,

2.  It is then placed with the cast faces in contact with the platens of the testing machine,

3.  The load on the cube is applied at a constant rate of stress of 0.2-0.4 MPa/sec, until failure of cube.

Cylinder test

The standard cylinder is 150mm diameter x 300mm high. Cylinders are cast in moulds generally of steel or cast-iron. Making and testing of cylinders are similar to those for cubes, with the difference that treatment need to be made to the trowelled end of the cylinder to ensure that the surface will be in good contact with the platen of the testing machine. This is to ensure even distribution of the load over the surface area. Treatment is usually done by capping, with a suitable material like high-strength gypsum plaster or high early strength cement paste.

Standard cylinders have height/diameter (h/d) ratio of 2, but sometimes specimens could be of other ratios, particularly the case where samples are cored from in situ concrete, where the height of the core varies with the thickness of the slab or concrete member. With cylinders where the h/d ratio is less than 2, it is necessary to determine the strength of the concrete by multiplying the test readings with the correction factors ( see Table 12.1).

As a rule of thumb, cube strength is generally 25% higher than cylinder strength.

Tensile strength tests

Direct testing of concrete in uniaxial tension (Fig20.3a) is difficult;

Reasons:

-        relatively large sections are required to be representative of the concrete

-        concrete is brittle, therefore making it difficult to grip and align

Fig 20.3a 1

For the above reasons, tensile strength testing of concrete is done by indirect tests;

-        Splitting test

-        Flexural test




Splitting test

A concrete cylinder of normally 300 or 200mm long  by 150mm or 100mm diameter, is placed on its side in a compression testing machine and loaded across its vertical diameter (see Fig 20.3b). Load (P) is then applied on the specimen until failure occurs by a split or crack along the vertical plane. The cylinder splitting strength is given by;

                        ƒs = 2P/πld      (see Fig 20.3b)

However it should be noted that ƒs is higher than the uniaxial tensile strength.

                                                                               Fig 20.3b

The video above shows splitting tensile strength test for concrete

Flexural test

A rectangle length of concrete of x-section usually 100 or 150mm is simply supported over a span L (usually 400 or 600mm). Load is applied at the third points (see Fig20.3c) and failure occurs when a flexural tensile crack at the bottom of the beam propagates upwards through the beam. Maximum tensile stress, ƒb (known as modulus of rupture) is calculated as;

                                                ƒb = PL/bd²

The modulus of rupture is greater than the uniaxial tensile strength.  See Fig 3.11 – fig 20.4 for a comparison of the direct and indirect tensile strength measurements.

The video above shows flexural strength test for concrete

Non-destructive testing of hardened concrete

Non-destructive (or sometimes known as in situ) testing is used for 2 main purposes;

·         in laboratory studies, where it is useful for repeated testing of the same specimen to determine the change of properties over time

·         to assess the properties of concrete in an existing structure, e.g. uniformity of concrete or after damage by fire or when a change in use is proposed.

It is important to note that a single non-destructive test rarely gives a single definitive answer, and engineering judgment is required in interpreting the results.

Limitations to non-destructive tests;

Rebound (Schmidt) hammer test

This is a simple apparatus contained in a hand-held cylindrical tube (see Fig 3.12 – fig 22.1), and consists of a spring-loaded mass which is fired with a constant energy against a plunger held against the concrete surface. Procedure in testing;

·         the  surface to be tested first need to be smoothened by polishing

·         the plunger is pressed (at right angle) against the concrete face

·         the spring is released by pressing the release button

·         the rebound number on the scale is recorded

A higher rebound number indicates a harder surface. However, there is considerable local variation due to the presence of coarse aggregate particles or voids just below the surface. Therefore, a number of readings (usually 10 over an area of 150mm dia.) must be taken and averaged.

The test does not give a direct measure of the strength of the concrete being tested. Instead indication of the strength is provided by the correlation chart between rebound number and compressive cube strength (see Fig 22.2). This correlation depends on;

·         the aggregate type used in the concrete;

·         moisture condition of the surface.

Ultrasonic pulse velocity test (upv)

This is a versatile and popular test for both in situ and laboratory use. It involves measuring the time taken for an ultrasonic pulse to travel through a known distance in concrete. The ultrasonic pulse is generated by a piezo-electric crystal housed inside a transducer which is then detected by a second similar transducer.  The instrumentation measures and displays the time taken for the pulse to travel between the transducers.

Like rebound hammer test, upv test does not directly measure the strength of the concrete. Upv can be correlated empirically to strength, and this correlation only gives an indication of the actual strength of the concrete being tested (see Fig 22.6). Upv test, like the rebound hammer test, is limited by;

·         the constituent materials

·         moisture conditions

The velocity through a harder material is higher than that through a softer material. Hence, in in-situ testing, it is very important to ensure that measurements are taken where there is no presence of steel bar in the path of the pulse, which would otherwise result in a falsely low transit time.


Other non-destructive tests; like resonant frequency test, pull-off test, pull-out, penetration test, etc.