Experiment 1Concrete Mix DesignObjective: To design concrete mix to a strength of 25N/ at 28 days and to test the cube strength of the mix design by mixing, casting and crushing a test cube.Equipment: Cement, Shovel, Trowel, Tamping Bar, Cement Mixer, Moulds ( 150mm 150mm 150mm ), Compression Machine, Water Tank.Procedure:Step 1: Determining the free water / cement ratio1. Given the required characteristics strength at a specified age, use Equation 1.0 to obtain the target mean strength at that age, which is the compressive strength to be used in the mix design.

Target mean strength = characteristic strength + 1.64

Where is the standard deviation of the strength of the strength tests. Typical values can be obtained from Table 1.0.Suppose the target mean strength to be obtained is 43 N/ at 28 days.

2. Given the type of cement and aggregate, use Table 1.1 to obtain the compressive strength, at the specified age, that corresponds to a free water / cement (w/c) ratio of 0.5.

Suppose ordinary Portland cement and uncrushed aggregate are used. From the table 1.1 of the compressive strength of 40 N/ at 28 days (and 27N/ at 7 days and so on). The pair of data (40 N/, w/c ratio curve in Figure 2.0.9 method as explained below).

3. In figure 2.0, follow the starting line to locate the curve which passes through the point (40 N/, w/c ratio 0.5) in this particular case, it is the fourth curve from the top of the figure. This curve shows that, to obtain our target mean strength of 43 N/, we need a w/c ratio 0.47.

4. If the w/c ratio obtained in step 3 exceeds the max. w/c ratio specified for durability ( see table 1.2), then adopt the lower value resulting in a concrete having a higher strength than required.

Step 2: Determining the water content1. Given the slump or VB time, determine the water content from Table 2.0. Using Table 2.0, when coarse and fine aggregates of different types are used, the water content W is estimated as follow :

W = 2/3 + 1/3 Where ; water content appropriate to the type of fine aggregate. = water content appropriate to the type of coarse aggregate.2. The aggregate type in Table 2.0 refers to all types of aggregates. Indeed, the fine aggregate has a considerably greater specific surface area and hence has a greater influence on the workability : this explains the greater weight, 2/3 assigned to in Equation 2.1Step 3: Determining the cement content1. The value given by Equation 3.1 should be checked against any maximum or minimum cement contents that may have been specified, for durability. Refer Table 1.2.

Cement content (kg/) =

If the cement content calculated from Eqn 3.1 is below a specified minimum, this minimum must be used resulting in a reduced w/c ratio and hence has a higher strength than the target mean strength. If the calculated cement content is higher than a specified maximum, the specified strength and workability cannot be simultaneously met with the selected materials, try to change the type and maximum size of aggregate. Step 4: Determining the aggregate content1. Having calculated the water content and the cement content, the total aggregate content in practice is obtained from the chart in the DoE document (35). The value can also be calculated from basic principles. For each cubic meter of fully compacted fresh concrete;

Volume occupied by the aggregate = 1- - (Equation 4.1)Where 3150 kg/ is the density of the cement particles = 1000 kg/ is the density of water.

Therefore total aggregate content (kg/) = (volume occupied by aggregates) (Equation 4.2)Where is the density of the aggregate particles. The DoE (35) recommends that if no information is available, should be taken 2600 kg/ for uncrushed aggregates and 2700 kg/ for crushed aggregates.Step 5: Determination of the fine and coarse aggregate contents1. How much of the total aggregate content should consist of fine aggregate depends on the grading zones 1, 2, 3, and 4. The general principle in mix design is : the finer the grading of the fine aggregate i.e. the larger its structure area per unit weight, the lower will be the proportion, expressed as a percentage of the total aggregate required to produce a concrete of otherwise similar properties.2. For a given slump and w/c ratio, the proportion of fine aggregate can be determined from Figure 3.0 in which the grading zones are those of Table 3.0. For example, for a specified slump of 10-30mm, w/c ratio 0.47 and fine aggregate in grading zone 3, Figure 3.0 will give the proportion of fine aggregate as between 32 and 38% by weight, say 35%. Therefore, for this particular example: Fine aggregate content = 35% of total aggregate content Coarse aggregate content = 100 35% of total aggregate contentSolution:The solution will follow the data listed aboveStep 1: Determination of required w/c ratioFrom Table 1.1, at the standard water cement ratio of 0.5, the 28 days strength is 40N, then the w/c ratio required to obtain target mean strength (43 N/ is approximately 0.47. This is within permitted maximum w/c ratio of 0.60. Therefore, adopt a w/c ratio of 0.47.Step 2: Determination of required water contentFrom table 2.0, water content for 10mm nominal maximum size uncrushed aggregate with 10-30mm slump is 180 kg/.Step 3: Determination of cement ratioFrom Equation 3.1Cement ratio = = 385 kg/ > 300 kg/Step 4: Determination of total aggregate contentFrom Equation 4.1 and Equation 4.2, total aggregate content = 2600 (1-- ) = 1815kg/

Step 5: Determination of fine and coarse aggregate contentFrom figure 3.0, for a slump of 10-30mm, w/c ratio of 0.47 and fine aggregate in grading zone 4, the proportion of the fine aggregate is 27.5 to 32% by weight, say 30%. Fine aggregate content = 0.30 x 1815 = 545 kg/ Coarse aggregate content = (1-0.30) x 1815 = 1270 kg/Answer:The required mix proportions are:Cement: 385 kg/Water content: 180 kg/Fine aggregate content: 545 kg/Coarse aggregate content: 1270 kg/Calculation1. Determining the target mean strength:Target mean strength = 25 + (1.6*4)= 31.4 N/2. Determining the water/cement ratioFrom table 1.1, at the standard water cement ratio of 0.58, the 28 days strength is 40 N/. Figure 1.0 shows that if a w/c ratio of 0.58 give 40 N/, then the w/c ratio required to obtain the target mean strength ( 31.4 N/) is approximately 0.58. This is within the permitted maximum w/c ratio of 0.65. Therefore, adopt a w/c ratio of 0.58.3. Determining the water and cement contentFrom table 2.0, water cement for this experiment is 180 kg/ Therefore, 0.58= Cement content= = 310.345 kg/ (acceptable because >275 (minimum cement content))4. Determining fine and coarse aggregate content + + = 1 + + = 1Aggregate content = 1875.842 kg/.From figure 2, fine aggregate (%) vs water cement ratio, the percentage of the fine aggregate is 38%

Therefore, total fine aggregate content = 1875.842 * = 721.947 kg/.Therefore, total coarse aggregate content = 1875.842 * = 1163.022 kg/.Cube Test1. Fill mould in 3 layers, tamper each layer for 35 times with tamping bar.2. Finish top of mould with trowel. Mark date and numbers.3. Strike formwork in 24 hours, during which damp socking is used over cube.4. Cure in water for 7 days, 14 days, 21 days and 28 days.5. Test 2 cubes for compressive strength on the 7th, 14th, 21st, and 28th day.Precautions1. Coat mould with mould oil black oil.2. Bolt tightly.3. User scoop and not shovel so that larges stones drop off.4. Leave mix proud out of mould and allow for water separation.5. Take mould apart completely when removing cube.6. Clean mould as soon as it is stripped.7. Dont scoop top; dig hold and trowel flat.Calculation1. The concrete strength, = 25/ is design by calculated the water, cement, coarse and fine aggregate content.2. The water content; 180 kg/ * 0.02835 = 5.13 kg3. The cement content; 310.345 kg/ * 0.02835 = 8.845 kg4. The fine aggregate content; 721.847 kg/ * 0.02835 = 20.573 kg5. The coarse aggregate content; 1163.022 kg/ * 0.02835 = 33.146 kgTotal volume = 8* * 5% = 0.02836ResultsDay7th14th21st28th

Compression strength (kN)170.00209.15270.00183.50

Durability of concrete The durability of concrete refers to its ability to withstand the environmental conditions to which it is exposed. There is need to emphasize durability in the design and construction of concrete structures. The I.Struc.E.Manual has summed up the main requirements for durability such as :a) An upper limit to the w/c ratio (see table 1.2)b) A lower limit to the cement content (see table 1.2)c) A lower limit to the concrete cover to reinforcement (see table 1.2)d) Good compaction e) Adequate curing The durability of concrete is intimately to its permeability, which terms refers to The ease with which water can pass through the concrete. A low permeability makes the concrete better able to withstand the effects of weathering, inculding the effects of driving rain and the distruptive action of freezing and thawing, in connection with the strength of concrete, evaporable water and the bubbles of trapped air occupy spaces called voids. As expected, therefore the permeability increase rapidly with the w/c ratio and broadly speaking, factors which increase the strength of the concrete would reduce its permeability and improve its durability : low w/c ratio, good compaction and adequate curing. Indeed, the authors own experience with permeability is what when the absorption of water in a sample of concrete exceeeds 7%, there is bound to be corrosion of the reinforcement.Corrosion of reinforcement can seriously affect the service life of a concrete structure. The mechanisms of reinforcement corrosion are explained in an ACI Committee report, which also gives guidance on the protective measure for new concrete construction, on the procedures for identifying corrosive environments and active corrosion in concrete, and on remedial measures. Broadly speaking, the factors which reduce the permeability of the concrete (and protect the reinforcement from the ingress of external moisture) will help to inhibit reinforcement corrosion : low w/c ratio, good compaction, adequate curing and an adequate concrete cover. The chloride contents of the concrete must be held down, marine aggregate must be washed and sea-water should not be used in concrete making.Experience has shown that too low a cement content makes it more difficulty to obtain durable concrete. Hence is often considered desirable in practice to specify a minimum cement content (see table 1.2). It should also be pointed out what, while a higher cement content usually improves the quality of the concrete, this is no longer so if the cement content exceeds a certain limit. Too high a cement content increase the risk of cracking due to drying shrinkage in thin section sections or to thermal stresses in thicker sections. It is therefore necessary to specify maximum cement content: BS 8110 recommends that the cement should never exceed 550 kg/

Table 1.0 : Standard deviations under different conditionsConditionsStandard Deviation (N/mm2)

Good control with weight batching, use of graded aggregates ,etc. Constant supervision4-5

Fair control with weight batching.Use of two sizes of aggregates.Occasional supervision5-7

Poor control.Inaccurate volume batching of all in aggregates. No supervision.7-8 and above

Table 1.1 : Standard deviations under different conditionsType of CementType of coarse AggregateCompressive strength (N/mm2)Age (Days)3 7 28 91

Ordinary PortlandUncrushed Crushed18 27 40 4823 33 47 55

Rapid-Hardening PortlandUncrushedCrushed25 34 46 5330 40 53 60

Table 1.2 : Durability requirementsExposure conditionNominal Cover (mm)

Mild20 20 20 20

Moderate- 35 30 20

Severe- - 40 25

Very severe- - 50 30

Maximum free w/c ratio0.65 0.60 0.55 0.45

Minimum cement content (kg/m3)275 300 325 400

Concrete fcu (N/mm2)30 35 40 50

Table 2.0 : Approximate free-water contents (kg/m3) required to give various Level of workability (after DoE[35]Slump(mm)VB time (seconds)0-10>1210-3012-630-606-360-1803-0

Maximum Type ofSize of aggregateAggregate(mm)

10 Uncrushed Crushed150180180205205230225250

20 Uncrushed Crushed135170160190180210195225

40 Uncrushed Crushed115155140175160190175205

Table 3.0 : Grading limits for DoE mix design procedureStandard Sieve sizesPercentage by weight passing standard sieves

Grading Zone1Grading Zone2Grading Zone3

Grading Zone4

10mm100100100100

5mm90-10090-10090-10095-100

No.7(2.36mm)60-9575-10085-10095-100

No.14(1.18mm)30-7055-9075-10090-100

No.25(600 micro m)15-3435-5960-7980-100

No.52(300 micro m)5-208-3012-4015-50

No.100(150 micro m)0-100-100-100-15