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INVESTIGATIONS FOR STRUCTURAL DEFECTS

1. What is a failure?

A failure can be considered as occurring in a component when that component can no longer be relied upon to fulfill its principal functions. Limited deflection in a floor which caused a certain amount of cracking/distortion in partitions could reasonably be considered as a defect but not a failure. While excessive deflection resulting in serious damage to partitions, ceiling and floor finishes could be classed as a failure.

2. Introduction

This section considers the situation if the initial inspection/investigation detailed in Chapter 4 indicated that some parts of the structure may require strengthening. This can arise for three basic reasons:

a. serious deterioration of some of the structural members;

b. serious overloading of members.

c. proposed change of use involving substantial increase in floor loading.

3. Indications of structural defects

What are the likely signs of structural distress? No precise answer can be given to this question, but the following brief notes are relevant:

a. Diagonal cracks in beams and walls usually denote high shear stress and should be investigated.

b. Excessive deflexion in beams and floor slabs indicates that the members are over-loaded. This is also likely to show as cracking in the soffit at right angles to the main reinforcement (flexural cracking).

c. Bowing in columns and load-bearing walls is likely to cause cracking parallel to the main reinforcement.

4. Bowing in wall panels may be due to differential shrinkage/thermal effects between one face and the other.

5. Errors in the location, design and/or execution of movement joints, isolation joints, stress relief joints and sliding joints can result in cracking, spalling and distortion. This type of defect can be very difficult to rectify.

4. Investigation procedure

It will be seen from the previous section that visible cracking plays an important part in indicating that the structure or parts of the structure are suffering from structural distress. In other words, the members affected were unable to carry the loads imposed on them with an adequate factor of safety.

Such a state of affairs may be brought about by:

a. error(s) in design;

b. errors in construction (workmanship and/or materials);

c. actual loading significantly in excess of the design load;

d. physical damage, impact, explosion fire etc.

e. serious corrosion of reinforcement, which may be the result of many factors.

The engineer should make every effort to obtain copies of the structural calculations and as-built drawings. Unfortunately, this important information is often not available, in which case a ‘structural appraisal’ would be needed and this is time-consuming and expensive.

Assuming that adequate background information is available the general procedure is basically the same but with the emphasis on obtaining information for a practical diagnosis of the structural shortcomings. Additional methods of investigation may include:

a. Ultrasonic Pulse Velocity (UPV) survey

b. an impulse radar survey

c. core testing for strength

d. load tests (seldom used)

It should be noted that the above methods are supplementary to normal investigation techniques, and often used in combination.

4.2 Impulse radar survey

The author is indebted to GB Geotechnics for the information which follows. A transducer containing the transmit and receive antennae is drawn over the surface under investigation at a constant speed. Pulses of energy are transmitted into the material and are reflected from internal surfaces and objects, e.g. changes in density, voids, reinforcing steel. The data is recorded graphically or digitally thus enabling a preliminary assessment on site, followed, if considered necessary, by detailed processing in the laboratory.

Radar responds to changes; it can identify boundaries between layers, measure thicknesses and assess voids and relative moisture content. The radar profile is effectively continuous, radio pulses are transmitted at around 50 000 pulses per second.

Transducers can be hand-held or mounted below survey vehicles, and can be operated up to 200 m from the recording station.

4.3 Core testing for strength

The location of the cores should be carefully selected to provide the information required and for checking the results of UPV and radar surveys.

The cores should be cut, prepared and tested in accordance with the appropriate National Standard; in the UK this is BS 1881: Part 120.

Reference should also be made to BS 6089: Assessment of Concrete Strength in existing Structures, and to Concrete Society Technical Report No. 11.

Misunderstandings sometimes arise over the interpretation of the test results. The actual test on the core will give the compressive strength of the concrete in the core. BS 6089 refers to the ‘estimated in situ cube strength’ which is defined as ‘The strength of concrete at a location in a structural member estimated from indirect means and expressed in terms of specimens of cubic shape’. The Concrete Society Report refers to two types of strength; firstly: ‘Estimated Potential Strength’ which is defined as:

The strength of concrete sampled from an element and tested in accordance with this procedure, such that the result is an estimate of the strength of the concrete provided for manufacture of the element, expressed as the 28 day BS.1881 cube strength, allowance being made for differences in curing, history, age, and degree of compaction between core and BS.1881 cube.

The report also provides for a correction for the influence of included steel. When all these corrections have been made, the result is intended to give the 28-day cube strength of the concrete if cubes had been made and tested in accordance with BS 1881, at the time the member was cast. The intention is to provide an acceptable answer to the questions which arise in new construction when cubes fail. Many experienced engineers feel that with so many corrections only limited reliance can be placed on the results.

The second ‘type of strength’ referred to in the CS Report is ‘The Estimated Actual Strength’; this is defined as:

The strength of concrete sampled from an element and tested in accordance with this procedure, such that the result, expressed as an equivalent cube strength, is an estimate of the concrete strength as it exists at the sampling location, without correction for the effect of curing, history, age or degree of compaction.

The majority of investigations involving existing buildings are concerned with a reasonable assessment of actual strength as defined above, of the concrete in the load-bearing members.

4.4 Load tests

The testing described above should provide information on the general quality of the concrete and condition of the reinforcement. For the engineer to be able to predict with reasonable accuracy the load-carrying capacity of the various structural elements—beams, columns, floor slabs etc.—the following information would also be required:

a. original or, preferably, the as-built drawings of the structure;

b. similar information on any alterations made subsequently;

c. assessment of existing dead and live loads based on the present use;

d. assessment of dead and live loads which will arise from any proposed alterations.

When there is serious doubt about the value of the information available, consideration may have to be given to a load test on selected structural elements. It is accepted that design assumptions do not exactly match the as-built conditions; this is due mainly to the effects of composite action and load sharing. A load test on a beam or floor slab, if correctly carried out, will show how the element under test will react to the applied load under working conditions. During the test it is necessary to record deflections, recovery on removal of load, and details of any crack development.

Load tests must be carried out with great care by an experienced firm with an experienced engineer on site during the test. Provision must be made to deal with any unexpected collapse. All necessary safety precautions must be observed.

Load tests are time-consuming and expensive and should only be carried out after careful consideration of practical value of the results.

Portland cement consists mainly of compounds of calcium silicate and calcium aluminate, the calcium silicates are predominate being between 55% and 85%. There is also tricalcium aluminate, 7% to 12% and ferrites 6% to 10%.

It is made by burning at high temperature a mixture of chalk and clay in a rotary kiln. The clinker is ground, and gypsum is added to control the set. BS 12 limits the amount of sulphur (expressed as SO3) to 3.5%. The fact that Portland cement contains sulphate is important when investigating the possibility of sulphate attack on the concrete or mortar.

The hydration of the cement (the addition of water), results in a complex chemical reaction accompanied by the evolution of heat.

Revised British Standards for cement were published in 1991 and included BS 12: Portland cement and BS 4027: Sulphate-resisting Portland cement.

The new designations for Portland cements likely to be used for repair are as follows:

• Portland cement-class 42.5; to BS 12:1991 (CEM 1)

• Portland cement-class 52.5; to BS 12:1991

• Portland cement-class 42.5R; to BS 12:1991

• Sulphate-resisting Portland cement-class 42.5; to BS 4027:1991

• Masonry Cement: BS 5224:1995—ENV 413–1

The letter R denotes high early strength.

The revisions were mainly concerned with methods of test and terminology and were intended to agree with the European Standard for cement (ENV 197–1). Minor changes in composition were also introduced.

If a cement equivalent to ‘ordinary Portland is required, then this should be ordered as ‘Portland cement—class 42.5, to BS12:1991”. If a rapid hardening Portland cement is required, equivalent to ‘rapid hardening Portland cement’, then a Portland cement-class 52.5 to BS 12:1991, or

Portland cement-class 42.5R to BS 12:1991 should be ordered. The above listed cements are the ones used almost exclusively for repair work.

In 1990 and 1991, a completely revised edition of BS 5328:1991: Concrete, was issued in four Parts.

Blended cements consisting of mixtures of Portland cement and pulverized fuel ash (pfa) and Portland cement and ground granulated blast furnace slag (ggbs) are used in concrete for special purposes, but I have not come across their use in repair mortars and normal repair concrete.

The principal characteristics of Portland cement are as follows.

1. A very fine powder, particle size 1–50 microns.

2. The paste (cement and water) is highly alkaline, having a pH of about 13.5. This high alkalinity is relevant to the occurrence of alkali aggregate reaction.

3. The setting time (initial and final) is in the range of 45 minutes to 10

hours.

4. Both setting, and hardening (rate of gain of strength) are affected by temperature; an increase in temperature speeds up the chemical reaction between the cement and the mixing water.

5. Portland cement provides a comparatively high compressive strength to concrete and mortar. Tensile strength is only about 10% of the compressive strength.

6. The compounds which are responsible for the cementing action of the cement paste are mainly the calcium silicates (the C2S and the C3S).

7. It is the hydration products of the cement which, other things being equal, determine the strength of the concrete/mortar. The hydration products are very complex chemical compounds, the principal

compounds are calcium silicate gel, calcium hydroxide (about 20%) and tricalcium aluminate hydrate. Calcium hydroxide (Ca(OH)2) is liberated by the hydrolysis of the calcium silicates. The various hydration products hydrate at different rates, but the hydration is rapid to start with and then slows down.

8. The two major factors which influence the rate of gain of strength are its chemical composition and its fineness. With modern cements the increase in strength after the first 28 days is likely to be very small and should generally be ignored.

9. The amount of water in the mix (usually referred to as the water/ cement ratio) is a vital factor in determining the strength, permeability and absorption of the concrete/mortar. For higher strength and durability the w/c should not exceed 0.50, and for special purposes, 0.40–0.45; this is the free w/c which means the aggregates are saturated but surface dry.

The action of acids on Portland cement

Solutions of sulphates and their effect on Portland cement

1. INTRODUCTION

Quality control in concrete technology continues to rely mainly on results of tests on hardened concrete, particularly on the assessment of compressive strength. The non-compliance of the properties of hardened concrete with the required specifications gives rise to difficult and costly remedial actions.

Such actions often require a removal of substantial quantities of hardened concrete. The over-reliance on the assessment of hardened concrete can be reduced significantly and the concrete construction process can be made simpler if a fast, uncomplicated, reliable and inexpensive method for direct checking of the composition of a fresh mix were available. The knowledge that the fresh concrete placed consists of correct constituents in the required proportions and that this composition is within acceptable tolerances will improve greatly the quality of concrete.

A modern batching plant can provide a continuous record of what type of material has been discharged into the mixer and its quantity. To some extent the records can be used as evidence of the composition of a fresh mix. An independent, direct check, however, which will verify the composition and assess the within-batch homogeneity of the freshly mixed and transported concrete remains very desirable.

The potential advantages of such a system have been appreciated for a long time and several methods for analysis of fresh mixes have been proposed. These methods were based mainly on the following principles:

(a) chemical / mechanical separation of one of the basic constituents of the mix, particularly cement,

(b) electrical tests for resistance and capacitance,

(c) nuclear methods, x-ray, γ-ray or neutron activation.

Practical methods of analysis currently used are almost entirely based on principles listed under (a) above. The other principles have been used for research and laboratory work rather than for a practical assessment on sites.

Most of the methods proposed were not at all comprehensive. The methods were normally based on a direct determination of the content of only one constituent alone. The proportions of the other constituents had to be obtained from additional tests carried out independently. Despite the very considerable progress which has been achieved recently in the analysis of the fresh mixes, a method which would be truly comprehensive and entirely satisfactory has not yet been developed.

Although a completely satisfactory method has not developed as yet, considerable progress has been achieved. The accuracy of the analytical methods currently available has improved and the analysis is now capable of coping with mixes other than simple cement-water-aggregate ones. The time required for the completion of the analysis has shortened to approximately 10 - 15 minutes for one complete test. However, the apparatus required for the shortest tests which also offer acceptable accuracy has remained expensive and its use has been therefore limited to large projects only.

The origins, principles, apparatus and test procedures for significant methods currently in use are described below.

2. RAPID ANALYSIS (RAM) TEST

Fig. 1: Rapid Analysis Machine type 300 (Photograph courtesy of the BCA Special Projects group).

Origin and principle:

The Rapid Analysis Machine (RAM) has been developed in Great Britain during the late 1970s and early 1980s. It is sometimes known as the C&CA Rapid Analysis Test or as the Constant Volume method.

The method is based on a separation of cement and similar fines by their flocculation from water within a container of constant volume.

Application:

The test method can be used for analysis of fresh mixes with a maximum size of aggregate up to 40 mm, including mixes containing some types of cement substitutes (ground granulated slag) and an air-entrained concrete.

The main application of the RAM test is for an assessment of compliance with specifications of minimum cement contents of concrete mixes, such as are required in BS 8110 : 1985 (ref.l), BS 8007 : 1987 (ref.2), etc.

Description:

The Rapid Analysis machine (RAM) is a floor mounted unit as shown on Fig. 1. It consists of a frame which supports the Elutriation column and the Conditioning vessel. At the top of the Elutriation column is a Sampling head and the bottom of the column is fitted with a Dump valve. At the top of the Conditioning vessel is a Vibrating sieve and a Constant Volume vessel is attached to its lower end.

Basic control of the RAM is by using the panel on its front. The testing cycle itself is carried out automatically.

The most recent version of the RAM (type 300) has a total mass of approx. 145 kg when empty and dimensions of approx. 66 cm x 78 cm x150 cm.

The machine requires a supply of electricity and uses about 80 litres of water per test (ref.l) . The main parts are identified on Fig. 2.

Fig. 2: Main parts of the Rapid Analysis Machine

Equipment required:

- RAM testing machine,

- Laboratory balances capable of weighing 10 kg, accurate to 1.0 g, and 3000g, accurate to 0.10 g,

- Spare nozzles and a supply of chemicals,

- (Optional), a transformer to convert the available voltage to the 110 V used by the RAM.

Size of the sample:

The standard sample has a mass of 8 kg, equal to approx. 3.5 l of a concrete mix of normal density. It is possible to analyse samples of a smaller size, down to approx. 6 kg.

Operating Instructions:

Calibration:

A calibration chart is prepared from test results obtained from three calibration mixes. The mixes consist of approx. 7 kg of clean aggregate which is combined with 0 g, 750 g and 1500 g of cement and enough water to make the mixes workable. The mixes are tested using the standard procedure and a calibration chart such as is shown on Fig. 3 is prepared. The RAM should be re-calibrated in regular intervals, depending on the amount of usage, and after each replacement of the siphon nozzle.

Fig. 3: A calibration chart for Rapid Analysis Machine

Testing Procedure:

1. The sampling head is filled with approx. 8 kg of fresh concrete.

2. The 'start' button is pressed. The signal initiates the pumping of water which washes the sample at an accurately controlled rate. The cement particles are washed up and over the top of the Elutriation column where the 10% of the sample is removed into three sampling channels. The remainder of the sample is dumped to waste.

3. The proportion (10%) of the original sample which has been retained then passes through a 150 μm vibratory sieve and into the Conditioning vessel.

4. Chemical agents are stirred into the particle suspension in the Conditioning vessel. The chemicals cause the cement particles to agglomerate and sink to the bottom of the Constant Volume vessel.

5. After all the cement particles have been collected in the Constant Volume vessel the excess water is removed by coarse and fine syphoning.

6. The Constant Volume vessel is removed from the RAM and weighed on a balance accurate to 0.1g.

7. The cement content of the concrete sample is determined from a calibration diagram and adjusted for silt content.

8. Cement content per 1 m³ of fresh concrete mix is calculated.

Interpretation of the test results:

The RAM test determines primarily the cement content. It is therefore normally supplemented by three other tests to determine:

- the water content,

- aggregate content / grading of the mix

- the amount of ground slag in case it has been used as a cement substitute.

The test for water content is based on drying out of a 2.5 kg sample of the fresh concrete, eg. by using an industrial microwave oven and a suitable container. The process takes approximately 15 minutes (ref.3) and can be carried out simultaneously with the RAM analysis. The precision of the determination of the water content depends on the advance of the hydration of the cement in the mix tested. It has been suggested that the faster drying out in a microwave as compared with an ordinary fan-assisted oven, causes much more of the mixing water to be evaporated before it becomes tied in the hydration products.

The aggregate content is determined from the aggregate retained in the Elutriation column and which is dumped, and from the 150 μm sieve through which approx. 10% of the sample has passed. Aggregate from both the sources is dried out separately and the proportions of coarse and fine aggregate are established by appropriate factoring, sieving through 5 mm sieve. The silt content obtained from separate RAM tests is also noted.

On large projects the delivery of batches of fresh concrete and their placing is not normally held up until the results of the analysis are known. Instead, regular consecutive testing is carried out and the results shown as diagrams.

The trends indicated are constantly monitored and corrective actions taken when required. The test for the slag content is based on the determination of the sulphide content of the cementitious material, including any silt, which accumulates in the Constant Volume vessel at the end of the RAM test. A test set for detection of hydrogen sulphide is required. The material is dried and a small sample (1 g) is mixed with hydrochloric acid. The hydrogen gas produced is drawn by a vacuum pump into a disposable detection tube. The slag content is then determined by comparing the reading on the detection tube with a calibration chart obtained from testing of laboratory samples of the slag-cement mix used (ref.3).

Precision:

The test results are subject to a total error which is made up principally of the sampling error, batching error, machine error and testing error due to the silt content of the concrete. The magnitude of the silt error is approximately equal to the other errors combined. This means that the total error is greatly reduced in cases of concretes with very low silt content or when the silt content is known and an appropriate correction can be made to the result of the analysis. The total standard deviation of the cement content obtained by the RAM varies from approx. 7 to 13 kg/m³ , depending largely on the accuracy of the silt correction (ref.4).

Duplicate samples should be always tested and the results compared. If the difference between the cement contents obtained is not greater than 20 kg/m³ the mean of the two test results is recorded as the final result. If the difference between the two test results is greater, the test is abandoned and new samples have to be obtained and tested.

Delay between the mixing of the concrete and the RAM test has an effect on the cement content determined; the longer the delay, the lower is the cement content measured. A three-hour delay reduced the value of the cement content measured to 92% of the actual content. Such reductions however have been shown to be reasonably predictable (ref.5).

Air entrainment and coarse aggregate with maximum size greater than 25 mm increase the variabiity of the RAM test results. In case of air-entrainment it is recommended to remove the entrained air by mixing the fresh concrete with a suitable de-training agent such as tri-n-butyl phosphate.

An extensive comparative assessment carried out by Dhir et al. (ref.5) indicated that the precision and repeatability of the RAM test results were much better than that of the Buoyancy method which used to be standardised in Great Britain (ref.6).

Advantages:

- Speed of testing. The latest model of the RAM uses a 6-minute automatic operating cycle. The total time from loading of the RAM with a sample of concrete to reading-off of the test result from the graph is not more than 10 minutes. The test is faster than any of the other tests based on separation of concrete constituents. A fully comprehensive analysis of the composition of the fresh mix could be completed within approx. one hour, depending on the number of operators involved.

- Convenience. The RAM test is highly automated. It is therefore relatively easy to carry out, compared to other methods for analysis of fresh concrete.

Disadvantages:

- Cost. The apparatus is expensive. As an alternative to an outright purchase, the RAM can be hired for specific tests or for the duration of a particular project.

- Calibrations. Each testing machine is calibrated when manufactured and the calibration should remain essentially the same in use. However, it is recommended to re-calibrate the machine in regular intervals. The silt correction is based on results of RAM tests carried out on mixes using the same aggregate with a known cement content or by testing the aggregate directly and obtaining an average silt correction amount. The silt correction is essential and it should be checked regularly.

Standardization:

The RAM test method is not yet covered by a standard.

3. OTHER METHODS

3.1 Buoyancy method

The Buoyancy method uses the principle of displacement of water by different basic materials which are separated from the fresh concrete. The test was standardized in Britain until 1983 (ref.6). The test method is based on an assumption that solid particles having size lesser than 0.150 mm are almost entirely those of cement. The method is suitable for all types of concrete. In case of air-entrained mixes, the air has to be removed from the sample by a de-entraining, air-releasing admixture prior to the test.

Fig. 4: Basic apparatus for the buoyancy test

The equipment for the test requires a special balance for weighing of materials in water and in air. This can be either a special balance, as shown on Fig. 4 or an electronic balance placed on a suitable frame above a water tank from which a special copper bucket can be suspended. Sieves with 5.0 mm and 0.15 mm apertures and washing out facilities are also required.

The concrete sample is weighed in air and water using the special copper sample bucket. The mix is then washed over 5 mm and 150 μm sieves until each sieve contains clean material.

The coarse aggregate retained on the 5 mm sieve and the fine aggregate retained on the 5 μm sieve are then weighed in water using the same equipment as for the fresh mix. The cement content is then calculated using the following equation:

mass of cement (g) = (c_w - c_a ) x 3120 / (3120 - 1000)

where:
c_w = mass of concrete under water (g)

c_a = mass of concrete in air (g)

The precision of the method depends on the content of non-cementitious material consisting of particles with sizes < 150 μm, such as the silt or clay and on a correct assumption for the density of the cement tested.

The test also indicates the content of the fine and coarse aggregate. The water content of the mix should, however, be preferably determined by a separate test.

3.2 Pressure filter method

The pressure filter method is also sometimes known as the Sandberg method. It is based on the separation of solid particles from the fresh mix by pressure filtering.

A concrete sample of a known mass is stirred with added wash water and then wet-sieved through sieves of sizes from the maximum down to 0.150 mm. The wash water and all the fine particles are then passed through a filter paper assisted by compressed air. All the water which was filtered is collected and its mass determined. The filter paper with the fines are weighed and then dried in an oven to a constant weight.

The mass of cement is corrected for the silt content and the mass of the filter paper used. Further correction is required if the cement used showed some water solubility. The cement content of the sample is then converted into the cement content per 1 m³ of the mix.

3.3 Chemical method

Chemical methods are mostly based on the determination of calcium content of the fresh mix. Provided the amount of the calcium which can be brought into the mix as part of the aggregate and the type of cement used are known, the results in the form of the calcium content can be converted into cement content with the aid of a conversion chart established for each particular case.

Typical examples of the chemical methods are the two methods (A & B) described in the ASTM C 1078 - 87 (ref. 7) or the ' GLC' method developed in Great Britain in early 1970's (ref.8).

Fig. 5: Main parts of the apparatus for the rapid filter method

In the 'GLC method a sample of concrete is washed through a 300 μm sieve and a sub-sample of the material which has passed the sieve is treated with nitric acid. The concentration of the calcium in the sub-sample is then determined by flame photometry. The results do not have to be adjusted for the content of fines, eg. the silt in the mix such as in the case of the RAM method. The method is, however, considered unsuitable for concretes containing fine calcareous aggregate.

The ASTM methods are also based on the separation of a mixture of fines and cement and on the treatment of a sub-sample by nitric acid. The method A determines the concentration of the calcium ion in the sub-sample by titration using a solution of di-sodium ethylenediamine tetraacetate in the presence of a colour indicator (Eriochrome Black T). The cement content is determined by converting the result of the titration using a calibration diagram. The ASTM method B is based on a fluorometric determination of the calcium ion concentration in a sub-sample also treated with the nitric acid. The titration is based on ethylene glycol-bis-tetra-acetic acid solution and a Calcein indicator is used. The process is carried out in an automatic titration apparatus. The calcium content measured is converted again into the cement content using a calibration diagram.

In all cases supplementary tests are required to determine the content of other concrete constituents, particularly water, in cases were w/c ratios have been specified.

A test procedure used by the 'GLC test, similar to the ASTM C 1079 - 87 (ref.9) standard test for water content is based on the measurement of the dilution of a chloride added to the fresh mix. Sodium chloride is normally used.

The determination of the chloride concentration is either carried out by titration or by using a suitably sensitive proprietary chloride meter.

The chemical methods offer precision better than that achieved by the separation methods such as the RAM test. The assessment of the precision of the chemical test methods has been largely confined to research rather than site laboratories, thus the practical advantage may be less than claimed. The presence of calcium in the aggregate affects strongly the results and if the calcium content is variable, regular re-calibrations have to be carried out.

Disadvantages of all the chemical methods include the use of analytical equipment and operators with skills which are above those normally required for operators carrying out other tests on fresh and hardened concrete in a site-laboratory which provides continuous quality control of manufacture of concrete. Potentially toxic materials have to be used in some of the tests.

The chemical tests are more appropriate to situations in which samples of fresh concrete which is known or strongly suspected of non-conformity have to be analysed with greater precision.

3.4 Physical separation method

The test method known sometimes as the 'Laing' test is based on the principle of separating cement from aggregate by the use of a liquid separating medium.

The 'Laing' method uses bromoform which is a liquid of approx. density of 2900 kg/m³ . This density is between the density of ordinary aggregate (approx. 2600 to 2700 kg/m³ ) and cement (approx. 3150 kg/m³ ).

The fresh mix is washed out through sieves and the suspension of cement and fines passing 212 μm sieve is retained. Bromoform is then added to a sub-sample of this suspension and the mixture is centrifuged to determine the cement content (ref.8).

4. REFERENCES

1. BS 8110: 1985, Structural use of Concrete, British Standards Institution, London, Great Britain.

2. BS 8007: 1987, Code of Practice for Design of Concrete Liquid retaining Structures , British Standards Institution, London.

3. D.M.Weatherill, Developments in the Analysis of Fresh Concrete, BCA Bulletin, British Cement Association, Slough, 1988, 5.
4. M.R.Hollington, Development of Compliance Rules for the Analysis of Fresh Concrete, RILEM Symp. Quality Control of Concrete Structures, Stockholm, June 1979.

5. R.K.Dhir, J.G.L.Munday, Nyok Young Ho, Analysis of Fresh Concrete: Determination of Cement Content by the Rapid Analysis Machine, Mag. of Cone. Res.,June 1982 (34) 59 - 73 .

6. BS 1881: Part 2: 1970 (withdrawn) Methods for Testing Fresh Concrete, British Standard Institution, London, Great Britain.

7. ASTM C 1078 - 87, Standard Test Methods for determining the Cement Content of Freshly Mixed Concrete, American Society for Testing and Materials, Philadelphia, U.S.A.

8. P.M.Barber, Analysis of Fresh Concrete, Concrete, June 1983 (17) 12-13.
9. ASTM C 1079 - 87, Standard Test Methods for Determining the water Content of Freshly Mixed Concrete, American Society for Testing and Materials, Philadelphia, U.S.A.

A lot can be done with site visits and visual inspections. However, it is often laboratory work that tells the tale of the tape. 

LABORATORY WORK

Petrographic exams use a branch of geology that deals with the descriptions and classifications of rocks. Hardened concrete is considered a synthetic sedimentary rock. Petrographic exams check for the following:

■ Aggregate condition

■ Pronounced cement-aggregate reactions

■ Deterioration of aggregate particles in place

■ Denseness of cement paste

■ Homogeneity of concrete

■ Settlement and bleeding of fresh concrete

■ Depth and extent of carbonation

■ Occurrence and distribution of fractures

■ Characteristics and distribution of voids

■ Presence of contaminating substances

CHEMICAL ANALYSIS

Chemical analysis of hardened concrete can be used to estimate the cement content, original water –cement ratio, and the presence and amount of chloride and other admixtures. This is another form of testing that is part of the larger puzzle in determining the qualities of concrete.

PHYSICAL ANALYSIS

Physical analysis is often done on core samples. This testing looks for nine elements:

■ Density

■ Compressive strength

■ Modulus of elasticity

■ Poisson’s ratio

■ Pulse velocity

■ Direct shear strength of concrete bonded to foundation rock

■ Friction sliding of concrete on foundation rock

■ Resistance of concrete to deterioration caused by freezing and thawing

■ Air content and parameters of the air-void system

NONDESTRUCTIVE TESTING

Nondestructive testing (NDT) is used to determine various relative properties of concrete. Strength, modulus of elasticity, homogeneity, and integrity of concrete can be calculated with NDT. There are many approaches to NDT, and they require inspectors to have expertise in the given approach to arrive at accurate data.

Rebound hammers

Earlier, we talked a little about rebound hammers. This is a form of NDT and a fast and simple way to test concrete. However, the test is imprecise and cannot accurately predict the strength of concrete. Some factors that can skew a test with a rebound hammer include the following:

■ Smoothness of a concrete surface

■ Moisture content

■ Type of course aggregate

■ Size, shape, and rigidity of specimen

Carbonation of concrete surface probes

Probes can be used to do NDT. The probe may use a powder cartridge to insert a high-strength steel probe into a section of concrete. The results of probe measurements can be converted to compressive strength values. There are reports, however, that probes can sometimes supply inaccurate data.

A probe is normally used to test density. A probe will embed deeper in concrete that is suffering from failure in density, subsurface hardness, and as the strength of concrete weakens. This type of testing is fine for on-site, general tests, but it is limited.

Precise measurements are not available from probe testing. The act of probing concrete will leave a hole in the concrete surface that must be repaired.

Ultrasonic pulse-velocity testing

Ultrasonic pulse-velocity testing is probably the most frequently used means of NDT. The results of this testing can be calculated. High velocities indicate good concrete, while low velocities reveal weak concrete. The system for this testing is portable and can penetrate about 35 linear feet of concrete. Testing of this type is fast. However, an inspector must have access to opposite sides of the section being tested, and this can present a problem.

Acoustic mapping

Acoustic mapping provides comprehensive evaluation of the top surface wear of concrete in such structures as aprons, sills, lock chamber floors, and so forth. Fast, accurate evaluations of horizontal sections below water can be done with acoustic mapping. Dewatering is not needed. Accuracy falls off at depths greater than 30 feet.

Ultrasonic pulse-echo testing

Ultrasonic pulse-echo testing is good for fl at surfaces. It can detect steel and plastic pipe that is embedded in concrete. Resolution is good with this type of testing equipment. Improvements in this form of testing continue to develop.

Radar

Radar is an NDT. It does not require contact with concrete. Resolution and penetration is somewhat limited. Some opinions favor signal testing over radar, but radar is a growing element in concrete evaluation.

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☞Rebound hammer test

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Underwater inspections are usually conducted by scuba divers. When a very deep or long dive is required, a diver with surface-supplied air is a better option. Flexibility and speed is an advantage for the scuba diver. In clear water, a visual inspection can be done. Many types of structures are located in water that is not clear enough to perform a visual examination.

Fortunately, many types of test devices that are used above water have been adopted for use below water. Rebound hammers work underwater. Both direct and indirect ultrasonic pulse-velocity systems can be used below the water surface. These tools can give a diver a good reading on the general condition of concrete that is surrounded by water.

Underwater vehicles

Underwater vehicles are commonly used to inspect submerged concrete structures. These vehicles come in five different categories of manned units: untethered, tethered, diver lockout, observation or work bells, and atmospheric diving suits. All are operated by a person inside, who has viewing ports, dry conditions, and some degree of mobility.

Unmanned vehicles are another option for underwater inspections. These include tethered, free swimming, towed, towed midwater, bottom-reliant, bottomcrawling, structurally reliant, and untethered. Unmanned vehicles are known as remotely operated vehicles (ROVs). Television cameras are mounted on the vehicles.

Control of the vehicle is done from the water surface with a navigation system, such as a joystick. These vehicles can be fitted up to perform inspections and maintenance.

ROVs offer the advantage of being operated at extreme depths. They can remain underwater for long periods of time. Repeated tasks can be completed accurately with ROVs. Another advantage is that ROVs can be operated in harsh conditions that would hamper general diving operations.

When ROVs are compared to manned vehicles, there are pros and cons. For example, manned vehicles are big, bulky, and expensive to operate. ROVs are small,flexible, and relatively inexpensive. An ROV provides a two-dimensional view, while a manned unit can provide three-dimensional assessments. Both types of vehicles have their place in underwater inspections.

HIGH-RESOLUTION ACOUSTIC MAPPING SYSTEM

High -resolution acoustic mapping systems can be used to check for erosion and faulting. These systems consist of three basic components: the positioning subsystem, the acoustic subsystem, and the compute-and-record subsystem. An acoustic subsystem is made up of a boat-mounted transducer array and signal-processing electronics. This type of system sends output back to a computer. The computer calculates the elevation of the bottom surface from the information supplied.

A lateral positioning subsystem has a sonic transmitter on a boat and two or more transponders in the water at a known or surveyed location. The transponders receive a sonic pulse from the transmitter. This information is radioed to the survey vessel. A time and location is determined by the survey vessel. Compute -and-record subsystems provide computer-controlled operation of the system and for processing, display, and storage of data. Real-time mapping is done in a computerized manner.

While high-resolution systems are extremely accurate, they do have limitations. These systems typically work in depths ranging from 5 to 40 feet. Another drawback is that a high-resolution system works best in calm water. If there is wave activity that exceeds 5 degrees, a hi-res system will shut down. When removing concrete forms, workers must be sure to maintain all safety and serviceability of the concrete structure.

SIDE SCANNER

A side scanner sonar requires two transducers mounted in a waterproof housing. When a signal is sent from the scanner, it is called a sonograph. Darkened areas and shadows are used for evaluation. The width of shadows and the position of objects can be used to calculate height. Newer versions of scanners have far fewer limitations than the earlier models did. Side scanners have been proven useful in breakwaters, jetties, groins, port structures, and inland waterway facilities, such as locks and dams.

OTHER MEANS OF UNDERWATER TESTING

Other means of underwater testing include radar, ultrasonic pulse velocity, ultrasonic pulse-echo systems, and sonic pulse-echo techniques for piles. All of these methods have their advantages. Let’s look at some of them.

Advantages of radar systems

■ The electromagnetic signal emitted from radar travels very quickly.

■ Conductivity controls the loss of energy and, therefore, the penetration depth.

■ Dielectric constant determines the propagation velocity.

Advantages of ultrasonic pulse velocity

■ Provides a nondestructive method for evaluating structures.

■ Measures the time of travel of acoustic pulses of energy through a material of known thickness.

■ Piezoelectric transducers are housed in metal casings and are excited by high impulse voltages as they transmit and receive acoustic pulses.

■ An oscilloscope in the system measures time and displays acoustic waves.

■Reliable in situ delineations of the extent and severity of cracks, areas of deterioration, and general assessments.

■ Capable of penetrating up to 300 feet of continuous concrete with the aid of amplifiers.

■ Can be transported easily.

■ Has a high data acquisition to cost ratio.

■ Can be converted for underwater use.

Advantages of ultrasonic pulse-echo systems

■Uses piezoelectric crystals to generate and detect signals and the accurate time base of an oscilloscope to measure the time of arrival of a longitudinal ultrasonic pulse in concrete.

■These systems can delineate sound concrete, concrete of questionable quality, deteriorated concrete, delaminations, voids, reinforcing steel, and other objects within concrete.

■ Can determine the thickness of concrete up to about 1.5 feet.

■ Can be adapted to water environments.

Advantages of pulse-echo techniques for piles

■ Can determine the length of concrete piles, in tens of feet, in dry soil or underwater.

■ Uses a round-trip echo time in the pile to measure an accurate time base of an oscilloscope.

■ Can be used to calculate the reference between length and diameter ratios.

Why Concrete Cracks?

Generally, it is assumed that cracks are due to some problems in the foundation, whereas it is not always correct and should not be considered failure of structure or improper design or bad quality work. Generally, 1/16 to 1/4-inch-wide cracks is acceptable limits.

The American Concrete Institute as per ACI 302.1-04 addresses this issue, even the best construction & concreting cannot prevent cracking in concrete, and 0% cracks is an unrealistic thing.

Causes of Cracks in Concrete

Causes of cracks in concrete can be many summarized as:

• Concrete expands and shrinks due to temperature differences
• Settlement of structure
• Due to heavy load applied or
• Due to loss of water from concrete surface shrinkage occurs
• Insufficient vibration at the time of laying the concrete
• Improper cover provided during concreting
• High water cement ratio to make the concrete workable
• Due to corrosion of reinforcement steel
• Many mixtures with rapid setting and strength gain performance have an increased shrinkage potential.

Types of Cracks in Concrete

The following figure shows types of cracks in concrete:

How to Prevent Cracks in Concrete Structures?

Preventive measures to avoid creation of cracks:

Preventive measures must be taken at the time of concreting and later to reduce cracks after concrete formation. Main factors are:

Reduce Water Content in Concrete:

A low water cement ratio will affect the quality of concrete. W/C ratio is weight of water to the weight of cement used. A lower w/c ratio leads to high strength in concrete and lesser cracks.

W/C ratio shall not exceed 0.5 in concreting, which reduces the workability of concrete which can be covered by use of plasticizer or superplasticizer. Less water content increases the durability of concrete.

Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink. Shrinkage is the main cause of cracks, when concrete hardens it evaporates the excess water and thus shrinks, so lesser the water content, lesser is the shrinkage.

Cracking shrinkage in slabs is ½ inch per 100 ft. The shrinkage of concrete pulls the slab apart showing it as cracks on surface.

Proper Concrete Mix Design and use of Quality Materials

The concrete itself must be properly proportioned, and properly mixed. If you use too little cement, you can almost guarantee cracks. Using too much water will make the concrete weak, leading to cracking.

Use good quality aggregates so will produce lower shrinkage concrete. Hard, dense aggregate, using a large top size aggregate and optimizing the gradation of the aggregate is able to reduce the shrinkage of the concrete.

If the aggregate is of poor quality, maximizing the size, gradation, and content may have little effect on the concrete shrinkage. Mixing large aggregate with poor qualities to a mid-size aggregate with good properties may increase the shrinkage of the concrete.

Avoid the use of shrinkage-promoting admixtures (such as accelerators, dirty aggregate which increases water demand and using a cement with high shrinkage characteristics.

Finishing of Concrete Surface

Use proper finishing techniques and proper timing during and between finishing operations. Flat floating and flat troweling are often recommended.

Avoid overworking the concrete, especially with vibrating screeds. Overworking causes aggregate to settle and bleed water and excess fines to rise.

Don’t finish the concrete when there is bleed water on the surface, finishing leads the water back to concrete instead of evaporating thus leading to cracks.

Proper Curing of Concrete

Stop rapid loss of water from surface or drying of concrete due to hydration (liquid concrete converts to plastic and then to solid state) causes drying of the slab, so it’s recommended to cure the slab for several days.

As soon as the concrete on slab sets its general practice to make boundary with mortar on the slab and keep it filled with water. Cover slab with cotton mats soaked with water or spray on a curing compound also prevents loss of water.

The concrete should not be subjected to load during the curing period, which can last up to one month.

Proper Placement and Vibration of Concrete

Properly placed, vibrated, finished concrete reduces the chances of producing cracks. Properly vibrate to release entrapped air which later leads to cracks.

Proper Compaction of Soil to Prevent Settlement Cracks in Concrete

The area below the concrete slab has to be compacted properly and in layers so as to ensure against settlement of soil later. If the soil is left loose it will settle over time and create cracks on surface. This applies in the home as well as constructions on highways.

Providing Control Joints in Concrete

Control joints should be located at regular intervals so as to adjust the shrinkage of concrete. Generally, for 4-inch depth of slab joints are provided 8 to 12 ft. apart. Control joints are pre-planted cracks. An engineer should have an idea that concrete will crack at control joints instead of cracking any other location.

Some Other Preventive Control Measures for Cracks in Concrete:

• Applying good acrylic silicone sealer yearly to concrete works
• Avoid calcium chloride admixtures
• Prevent extreme changes in temperature
• Consider using a shrinkage-reducing admixture
• Warm the subgrade before placing concrete on it during cold weather
• Consider using synthetic fibers to help control plastic shrinkage cracks.
• Repairing Methods of Cracks in Concrete

Various types of Concrete Crack Repair Methodologies:

• Stitching
• Muting and sealing
• Resin injection
• Dry packing
• Polymer impregnation
• Vacuum impregnation
• Autogenously healing
• Flexible sealing
• Drilling and plugging
• Bandaging

To summarize, always prevention is better than cure. Prevention of concrete cracks give good quality, saves time, money and peace of mind to the owner.

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