NB/T 20437-2017Specification for test of concrete in nuclear power engineering (English PDF)

Foreword

This document was issued on 1 April 2017 by the National Energy Administration of the PRC and takes effect on 1 October 2017.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.120.20, Chinese classification F65.

This standard was drafted in accordance with the rules given in GB/T 1.1-2009.

This standard was proposed by the Nuclear Power Standardization Technical Committee of the energy industry.

This standard is under the jurisdiction of the Nuclear Industry Standardization Institute.

Responsible drafting organizations: China Nuclear Industry 24 Construction Co., Ltd., China Nuclear Power Engineering Co., Ltd. (CGN), China Nuclear Industry Huaxing Construction Co., Ltd. and Sichuan Zhonghe Airuite Engineering Testing Co., Ltd.

Participating drafting organizations: China Nuclear Concrete Co., Ltd. and China Nuclear Huatai Construction Company.

1 Scope

NB/T 20437-2017 sets out how concrete is tested and inspected on nuclear power plant construction. Concrete in a nuclear plant carries more than load: the containment, the reactor building and the spent fuel structures must perform for sixty years under temperature, radiation and, in some cases, pressure, so the evidence that the concrete as placed matches the concrete as designed is itself a safety record. The specification sets the scope and applies to ordinary concrete, with self-compacting and radiation shielding concrete able to follow it as a reference. It gives the normative references and the terms, then the tests on fresh concrete: sampling, slump and slump flow, temperature, setting time, bleeding under normal conditions and under pressure, density, and air content with the calibration of the apparatus. Physical and mechanical testing follows - the making and curing of specimens, dry density, compressive strength on cubes and cylinders, splitting and flexural tensile strength, and the modulus of elasticity. Durability testing covers water penetration, chloride ion permeability, carbonation, freeze-thaw resistance and shrinkage; thermal testing covers adiabatic temperature rise, thermal conductivity, specific heat and thermal diffusivity; and the remaining inspection chapters cover the in-situ testing of the structure. For each test the apparatus, the procedure, the calculation and the reporting are specified. Note that the Chinese title covers inspection as well as testing, which the printed English omits.

This standard specifies the test methods for fresh concrete, the test methods for the mechanical properties of concrete, the test methods for the long-term performance and durability of concrete, and the test methods for the thermophysical parameters of concrete used during the construction of nuclear power projects.

This standard applies to the performance testing and inspection of ordinary concrete during the construction of nuclear power projects. The test and inspection methods for special concretes such as self-compacting concrete and radiation shielding concrete may be carried out with reference to this standard.

2 Normative references

The following documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

GB/T 176 Methods for chemical analysis of cement

GB/T 2611 General technical requirements for testing machines

GB/T 3159 Hydraulic universal testing machines

GB/T 8077 Methods for testing uniformity of concrete admixtures

GB/T 11896 Water quality — Determination of chloride — Silver nitrate titration method

GB/T 14684 Sand for construction

GB/T 50080 Standard for test method of performance on ordinary fresh concrete

JGJ 55 Specification for mix proportion design of ordinary concrete

JG 237 Moulds for concrete test specimens

JG/T 243 Freeze-thaw test apparatus for concrete

JG/T 245 Vibrating table for concrete tests

JG/T 247 Carbonation test chamber for concrete

JG/T 248 Slump cone for concrete

JG/T 249 Impermeability tester for concrete

JG/T 261 Chloride ion electric flux tester for concrete

JG/T 262 Chloride ion diffusion coefficient tester for concrete

3 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

3.1 air content: the percentage of air contained in a unit volume of fresh concrete.

3.2 bleeding: the phenomenon in which water seeps out at the upper surface of freshly mixed concrete in a static state.

3.3 pressure bleeding: the phenomenon in which mixing water seeps out of freshly mixed concrete under a certain pressure.

3.4 compressive modulus of elasticity: the stress required to produce a unit deformation when a concrete prism or cylinder specimen is subjected to a certain axial compressive load.

3.5 resistance to freezing and thawing: the ability of concrete to resist freeze-thaw cycles.

3.6 resistance to hydraulic pressure: the ability of concrete to resist a certain water pressure.

3.7 Poisson's ratio: the ratio of the transverse deformation to the longitudinal deformation of a concrete specimen under a certain pressure.

4 Tests on the properties of fresh concrete

4.1 Sampling and preparation of samples. 4.1.1 Sampling: samples of fresh concrete shall be representative.

4.1.1 a) Concrete samples should be taken from the truck mixer; when several tests are to be performed at the same time, the concrete required shall be taken from the same truck. Before sampling, not less than 20 L of concrete should first be discarded; the quantity sampled shall be more than 1.5 times the quantity required for the tests and should not be less than 20 L.

4.1.1 b) If the slump of the concrete is greater than 160 mm, the drum of the truck mixer shall be rotated rapidly for 5 to 10 revolutions before sampling; the whole sampling process shall be completed within 10 min.

4.1.1 c) The time from the completion of sampling to the start of the property tests should not exceed 5 min.

4.1.1 d) Concrete should be sampled at the placing location. When sampling is carried out elsewhere, it shall be justified by testing.

4.1.2 a) When fresh concrete is prepared in the laboratory, the laboratory temperature during mixing shall be kept at 20 degrees C plus or minus 5 degrees C, and the temperature of all materials shall be consistent with the laboratory temperature. When concrete used under simulated construction conditions is required, the temperature of the raw materials should be consistent with that at the construction site.

4.1.2 b) When concrete is mixed in the laboratory, the quantities of materials shall be measured by mass. Weighing accuracy: plus or minus 1 per cent for aggregates; plus or minus 0.5 per cent for water, cement, supplementary cementitious materials and admixtures.

4.1.2 c) The preparation of fresh concrete in the laboratory shall comply with the relevant provisions of JGJ 55.

4.1.2 d) The time from the completion of sample preparation to the start of the property tests should not exceed 5 min.

4.2.1 Slump and slump flow test: this method applies to the determination of the consistency of fresh concrete with a maximum aggregate size not greater than 40 mm and a slump not less than 10 mm.

4.2.2 The slump cone used for the slump and slump flow tests shall comply with the relevant technical requirements of JG/T 248.

4.2.3 The base plate used for the slump and slump flow tests should be made of hard stainless steel or another hard non-absorbent material, as shown in Figure 1 (a 1000 mm square plate marked with concentric circles of diameter 200 mm, 300 mm, 500 mm, 600 mm, 700 mm, 800 mm and 900 mm; unit: mm).

4.2.4 a) Moisten the slump cone and the base plate; there shall be no free water on the inner wall of the cone or on the plate. Place the base plate on a firm horizontal surface, place the cone at the centre of the plate and stand on the foot pedals on both sides, so that the cone remains in a fixed position while being filled.

4.2.4 b) Load the concrete sample into the cone evenly in three layers with a small scoop, so that each layer after rodding is about one third of the cone height; rod each layer 25 times. Rodding shall proceed spirally from the outside towards the centre and be distributed evenly over the cross-section. When rodding concrete at the edge of the cone, the rod may be slightly inclined; the bottom layer shall be rodded through its full depth; the second and top layers shall be rodded through the layer into the surface of the layer below. The top layer shall be filled above the rim; if the concrete settles below the rim during rodding, more shall be added. After rodding the top layer, strike off the excess concrete and smooth the surface with a trowel.

4.2.4 c) After removing the concrete from around the cone on the base plate, lift the slump cone vertically and steadily. Lifting shall be completed within 5 s to 10 s; the whole process from the start of filling to the lifting of the cone shall be carried out without interruption and completed within 150 s.

4.2.4 d) After lifting the cone, measure the difference in height between the cone and the highest point of the slumped concrete; this is the slump value of the fresh concrete. If the concrete collapses or shears off on one side after the cone is lifted, a new sample shall be taken and the test repeated; if the phenomenon occurs again in the second test, the workability of the concrete is poor and this shall be recorded.

4.2.4 e) Observe the cohesiveness and water retention of the slumped concrete. Cohesiveness is checked by lightly tapping the side of the slumped cone with the tamping rod: gradual sinking indicates good cohesiveness, while collapse, partial crumbling or segregation indicates poor cohesiveness. Water retention is assessed by the degree of thin paste bleeding from the mixture: if a large amount of thin paste flows out from the bottom after the cone is lifted and the aggregate in the cone is exposed through loss of paste, water retention is poor; if no or only a little thin paste flows out from the bottom, water retention is good.

4.2.4 f) When the slump flow is to be determined, measure with a steel rule the largest and smallest final diameters of the spread concrete; where the difference between the two diameters is less than 50 mm, their arithmetic mean is taken as the slump flow; otherwise the test is invalid. If coarse aggregate is found to accumulate at the centre or cement paste separates at the edge, the segregation resistance of the mixture is poor and this shall be recorded.

4.2.5 The slump and slump flow of fresh concrete are expressed in millimetres, measured to 1 mm, with the result rounded to 5 mm.

4.3.1 Temperature of fresh concrete: this method applies to the determination of the temperature of fresh concrete.

4.3.2 The test equipment may be a glass thermometer or an electronic thermometer with a required precision of 0.1 degrees C; the instrument shall have been verified by metrological calibration.

4.3.3 a) Sample the concrete in accordance with 4.1.1 of this specification and measure the temperature immediately after sampling.

4.3.3 b) Insert the thermometer at least 10 cm below the concrete surface and read the concrete temperature when the reading has stabilized.

4.3.3 c) Two measurements shall be taken at two different points, and the arithmetic mean of the two results is taken as the temperature of the concrete.

4.4.1 Setting time test: this method applies to the determination of the setting time of fresh concrete with a slump other than zero, using the penetration resistance method on mortar sieved from the concrete.

4.4.2 The penetration resistance apparatus shall consist of a loading device, penetration needles, mortar containers and a standard sieve; it may be manual or automatic. 4.4.2 a) Loading device: maximum measuring value not less than 1000 N, accuracy plus or minus 10 N.

4.4.2 b) Penetration needles: length 100 mm, with bearing areas of 100 square millimetres, 50 square millimetres and 20 square millimetres; each needle shall have a clear ring mark 25 mm from the penetrating end.

4.4.2 c) Mortar container: a rigid impermeable metal cylinder with an upper inner diameter of 160 mm, a lower inner diameter of 150 mm and a net height of 150 mm, fitted with a lid.

4.4.2 d) Standard sieve: a metal square-hole sieve with 5 mm apertures complying with GB/T 14684.

4.4.3 a) Sieve mortar from the sampled or prepared concrete with the 5 mm standard sieve, sieving clean each time, and then mix it uniformly. Place the mortar in one go into three containers for three tests. Mortar from concrete with a slump not greater than 70 mm should be compacted on a vibrating table; mortar from concrete with a slump greater than 70 mm should be rodded by hand. When vibrating, vibration shall continue until paste appears at the surface without over-vibration; when rodding, rod 25 times evenly in a spiral from the outside towards the centre and then tap the container wall lightly with a rubber hammer until the rod holes disappear. After compaction the mortar surface shall be about 10 mm below the rim, and the container shall be covered immediately.

4.4.3 b) After preparation, the specimens shall be numbered and placed in an environment at 20 degrees C plus or minus 2 degrees C or under the same conditions as the site, and the ambient temperature shall be kept at 20 degrees C plus or minus 2 degrees C throughout the test. For tests under site conditions, the conditions shall be kept consistent with those on site. Throughout the test the containers shall remain covered except when removing bleed water or performing penetration measurements.

4.4.3 c) Timing of the setting time starts at the moment the cement comes into contact with water. The time of the first needle test is determined according to the properties of the mixture; thereafter a test is made every 0.5 h, with additional tests near initial and final setting.

4.4.3 d) 2 min before each test, place a 20 mm thick block under one side of the container to tilt it, remove the bleed water from the surface with a pipette, and then gently return the container to level.

4.4.3 e) For the test, place the mortar container on the penetration apparatus with the needle tip in contact with the mortar surface, then make the needle penetrate uniformly to a depth of 25 mm plus or minus 2 mm within 10 s plus or minus 2 s; record the penetration force to 10 N, the test time to 1 min and the ambient temperature to 0.5 degrees C.

4.4.3 f) The spacing between test points shall be greater than twice the needle diameter and not less than 15 mm, and the distance between a test point and the container wall shall not be less than 25 mm.

4.4.3 g) At least 6 penetration resistance measurements shall be made between 0.2 MPa and 28 MPa, continuing until the penetration resistance exceeds 28 MPa.

4.4.3 h) During the test the needle shall be changed as appropriate according to the setting of the mortar; the needle should be selected in accordance with Table 1.

Table 1 Needle selection by penetration resistance: 0.2 MPa to 3.5 MPa, needle area 100 square millimetres; 3.5 MPa to 20 MPa, needle area 50 square millimetres; 20 MPa to 28 MPa, needle area 20 square millimetres.

4.4.4 a) The penetration resistance is calculated with formula (1): fPR = P/A, where fPR is the penetration resistance in MPa, P is the penetration force in N and A is the needle area in square millimetres. The calculation shall be accurate to 0.1 MPa.

4.4.4 b) The setting time should be determined by linear regression: take the natural logarithms ln(fPR) and ln(t) of the penetration resistance fPR and the time t, and perform a linear regression with ln(fPR) as the independent variable and ln(t) as the dependent variable to obtain formula (2): ln(t) = A + B ln(fPR), where t is the time in min and A, B are the linear regression coefficients. From formula (2), the initial setting time ts is obtained at a penetration resistance of 3.5 MPa, ts = e^(A + B ln(3.5)) (formula 3), and the final setting time tc at 28 MPa, tc = e^(A + B ln(28)) (formula 4), both in min.

4.4.4 b) (continued) The setting time may also be determined by graphical fitting: with the penetration resistance as ordinate and the elapsed time as abscissa (to 1 min), plot the curve of penetration resistance against time; draw two lines parallel to the abscissa at 3.5 MPa and 28 MPa; the abscissas of their intersections with the curve are the initial and final setting times of the fresh concrete.

4.4.4 c) The arithmetic mean of the initial and final setting times of the three tests is taken as the result. If the maximum or minimum of the three values differs from the middle value by more than 10 per cent, the middle value is taken as the result; if both the maximum and the minimum differ from the middle value by more than 10 per cent, the result is invalid. The setting time is expressed in h:min and rounded to 5 min.

4.5.1.1 Bleeding test: this method applies to the determination of bleeding of fresh concrete with a maximum aggregate size not greater than 40 mm.

4.5.1.2 a) Container: a metal cylinder with handles on both sides, 5 L in volume and fitted with a lid, with an inner diameter and inner height both of 186 mm plus or minus 2 mm and a wall thickness of 3 mm. The upper rim and inner wall shall be smooth, and the top and bottom faces shall be parallel to each other and perpendicular to the axis of the cylinder. The volume shall be calibrated: weigh a glass plate able to cover the top of the container together with the empty container, fill the container with clean water, and when the water nears the rim, keep adding water while sliding the glass plate over the opening so that no air bubbles remain under it; wipe the plate and the outside of the container dry and weigh them together; the difference between the two masses in kg is the volume of the container in L.

4.5.1.2 b) Platform scale: capacity 50 kg, sensitivity 5 g. c) Graduated cylinders of 10 mL, 50 mL and 100 mL and a pipette. d) Vibrating table complying with JG/T 245. e) Tamping rod complying with JG/T 248.

4.5.1.3 a) Moisten the inner wall of the container with a damp cloth and weigh it immediately, recording its mass. Then load the concrete into the container; there are two methods of filling and compaction.

4.5.1.3 a) 1) Method A, vibrating table: load the sample into the container in one go and start the vibrating table; vibration shall continue until paste appears at the surface, avoiding over-vibration. The surface of the mixture shall be 30 mm plus or minus 3 mm below the rim and smoothed with a trowel. Immediately after smoothing, start timing and weigh the container and sample together.

4.5.1.3 a) 2) Method B, rodding: load the mixture in two layers, rodding each layer 25 times evenly from the edge towards the centre; the bottom layer shall be rodded through its full depth and the second layer through into the surface of the layer below. After each layer, tap the outer wall of the container 5 to 10 times with a rubber hammer until the rod holes disappear and no large air bubbles are visible. The surface shall be 30 mm plus or minus 3 mm below the rim and smoothed with a trowel; start timing and weigh the container and sample together immediately.

4.5.1.3 b) Throughout the removal of bleed water, the container shall be kept level and free from vibration; the lid shall remain closed except during removal of water; the room temperature shall be kept at 20 degrees C plus or minus 2 degrees C.

4.5.1.3 c) Within the first 60 min after the start of timing, remove the water bleeding at the surface every 10 min; after 60 min remove it every 30 min until no further bleeding occurs. To facilitate removal, 2 min before each removal place a 35 mm thick block under one side of the container to tilt it, and return it gently to level afterwards. Put the water removed into a graduated cylinder, record the quantity of each removal and calculate the cumulative quantity to 1 mL.

4.5.1.4 a) Bleeding quantity is calculated with formula (5): Ba = V/A, where Ba is the bleeding quantity in mL per square millimetre, V is the cumulative bleed water after the last removal in mL and A is the exposed surface area of the sample in square millimetres. The calculation shall be accurate to 0.01 mL per square millimetre. The bleeding quantity is the mean of three samples; if the maximum or minimum of the three differs from the middle value by more than 15 per cent, the middle value is taken; if both differ from it by more than 15 per cent, the test is invalid.

4.5.1.4 b) Bleeding rate is calculated with formulas (6) and (7): B = Vw / ((W/G) Gw) x 100 and Gw = G1 - G0, where B is the bleeding rate in per cent, Vw the total bleed water in mL, Gw the mass of the sample in g, W the total water content of the mixture in mL, G the total mass of the mixture in g, G1 the mass of container and sample in g and G0 the mass of the container in g. The calculation shall be accurate to 1 per cent; the bleeding rate is the mean of three samples, with the same 15 per cent rule for the middle value and invalidity.

4.5.2.1 Pressure bleeding test: this method applies to the determination of pressure bleeding of fresh concrete with a maximum aggregate size not greater than 40 mm.

4.5.2.2 a) Pressure bleeding apparatus: its main components are a pressure gauge, cylinder, working piston and sieve screen (Figure 2). The pressure gauge has a maximum range of 6 MPa and a minimum graduation not greater than 0.1 MPa; the cylinder has an inner diameter of 125 mm plus or minus 0.02 mm and an inner height of 200 mm plus or minus 0.2 mm; the working piston pressure is 3.2 MPa with a nominal diameter of 125 mm; the sieve screen aperture is 0.315 mm. b) Tamping rod complying with JG/T 248. c) 200 mL graduated cylinder.

Figure 2 Concrete pressure bleeding apparatus. Key: 1 pressure gauge; 2 working piston; 3 cylinder; 4 sieve screen.

4.5.2.3 a) Load the fresh concrete into the cylinder of the pressure bleeding apparatus in two layers, rodding each layer 20 times evenly from the edge towards the centre; the bottom layer shall be rodded through its full depth and the second layer through into the surface of the layer below. After each layer tap the outer wall 5 to 10 times with a rubber hammer until the rod holes disappear and no large air bubbles are visible; the surface of the mixture shall be about 30 mm below the rim and smoothed with a trowel.

4.5.2.3 b) Wipe the outside of the container clean; after assembling the apparatus as specified, immediately apply a pressure of 3.2 MPa to the sample, open the bleed valve and start timing at the same time, maintaining constant pressure and collecting the water in the 200 mL graduated cylinder; read the bleed water V10 at 10 s and V140 at 140 s after pressurization.

4.5.2.4 The pressure bleeding rate is calculated with formula (8): BV = V10/V140 x 100, where BV is the pressure bleeding rate in per cent, V10 the bleed water at 10 s in mL and V140 the bleed water at 140 s in mL. The calculation shall be accurate to 1 per cent.

4.6.1 Apparent density test: this method applies to the determination of the mass per unit volume (apparent density) of fresh concrete after compaction.

4.6.2 The equipment for the apparent density test shall comply with the following: a) container complying with 4.5.1.2; b) platform scale with a capacity of 50 kg and a sensitivity of 5 g; c) vibrating table complying with JG/T 245; d) tamping rod complying with JG/T 248.

4.6.3 a) Wipe the inside and outside of the container clean with a damp cloth and weigh it to 5 g.

4.6.3 b) The method of filling and compaction depends on the consistency of the mixture: concrete with a slump not greater than 70 mm should be vibrated on a vibrating table, and concrete with a slump greater than 70 mm should be rodded. When rodding, load in two layers with 25 strokes per layer from the edge towards the centre, the bottom layer rodded through its full depth and the second layer through into the layer below; after each layer tap the outer wall 5 to 10 times with a rubber hammer until the rod holes disappear and no large bubbles are visible. When vibrating, fill the container above the rim in one go, rodding slightly during filling if needed; if the concrete falls below the rim during vibration add more, and vibrate until paste appears at the surface.

4.6.3 c) Strike off the excess concrete at the rim with a straightedge and fill any depressions; wipe the outside of the container clean and weigh the container with the concrete sample to 50 g.

4.6.4 The apparent density is calculated with formula (9): apparent density = (W2 - W1)/V x 1000, in kg per cubic metre, where W1 is the mass of the container in kg, W2 the mass of container and sample in kg and V the volume of the container in L. The result shall be calculated to 10 kg per cubic metre.

4.7.1 Air content test: this method applies to the determination of the air content of fresh concrete with a maximum aggregate size not greater than 40 mm. A direct-reading air content meter should be used; when other types of air meter are used, the test shall be carried out in accordance with GB/T 50080.

4.7.2 a) Air meter: as shown in Figure 3, consisting of a container and a cover. The container shall be made of hard metal not easily corroded by cement paste, with an inner surface roughness not greater than 3.2 µm, an inner diameter equal to its depth and a volume of 7 L. The cover shall be made of the same material as the container and shall include an air chamber, water level chamber, water inlet valve, drain valve, operating valve, air inlet valve, exhaust valve and pressure gauge; the gauge range shall be 0 to 10 per cent with a precision of 0.1 per cent. A sealing gasket shall be fitted between container and cover, which are joined by bolts with no trapped air and a tight seal. Tamping rod complying with JG/T 248.

4.7.2 b) Vibrating table complying with JG/T 245. c) Platform scale with a capacity of 50 kg and a sensitivity of 5 g. d) Rubber hammer with a head of about 250 g.

Figure 3 Air content meter. Key: 1 container; 2 cover; 3 water level chamber; 4 air chamber; 5 pressure gauge; 6 exhaust valve; 7 operating valve; 8 drain valve; 9 air inlet valve; 10 water inlet valve.

4.7.3 a) Before determining the air content of the mixture, the air content of the aggregates used shall first be determined. The masses of coarse and fine aggregate in each sample are calculated with formulas (10) and (11): mg = V/1000 x m'g and ms = V/1000 x m's, where mg and ms are the masses of coarse and fine aggregate in each sample in kg, m'g and m's the masses of coarse and fine aggregate per cubic metre of fresh concrete in kg, and V the volume of the meter container in L.

4.7.3 b) Fill the container with water to one third of its height, then slowly pour in the coarse and fine aggregate of masses mg and ms that pass the 40 mm sieve, mixed evenly. For every rise of about 25 mm in the water level, rod lightly 10 times and stir slightly to expel entrapped air; the water level shall always be kept above the top of the aggregate during filling. After all the aggregate has been added, soak for 5 min, then tap the outside of the container with a rubber hammer to expel bubbles, remove the foam from the water surface, top up with water, wipe the rim clean, fit the sealing gasket, and put on the cover and tighten the bolts.

4.7.3 c) Close the operating valve and the exhaust valve, open the drain valve and the water inlet valve and inject water into the container through the inlet valve; when the water flowing from the drain valve contains no bubbles, close the inlet valve and the drain valve simultaneously while water is still being injected.

4.7.3 d) Open the air inlet valve and pump air into the air chamber with the air pump until the pressure is slightly above the calibrated zero point; open the exhaust valve slightly to adjust the pressure to the calibrated zero point, then close the exhaust valve tightly.

4.7.3 e) Open the operating valve so that the compressed air in the air chamber enters the container; when the gauge reading has stabilized record the value Ag1, then open the exhaust valve so that the gauge returns to zero.

4.7.3 f) Repeat steps d) to e) above and measure the sample in the container again, recording the reading Ag2.

4.7.3 g) If the relative error between Ag1 and Ag2 is less than 0.2 per cent, take their arithmetic mean; otherwise carry out a third test to obtain Ag3. If the relative error between Ag3 and the closer of Ag1 and Ag2 is not greater than 0.2 per cent, take the arithmetic mean of those two values; if it is still greater than 0.2 per cent, the test is invalid and shall be repeated. Ag shall be accurate to 0.1 per cent.

4.7.4 a) Wipe the inner surfaces of the container and cover clean with a damp cloth and load the fresh concrete sample.

4.7.4 b) Compaction may be manual or mechanical. When the slump of the mixture is greater than 70 mm, manual rodding should be used; when it is not greater than 70 mm, mechanical vibration such as a vibrating table or immersion vibrator should be used. For rodding, load the mixture in three layers, each about one third of the container height after compaction; rod each layer 25 times evenly from the edge towards the centre, penetrating into the lower layer, then tap the outer wall 10 to 15 times with a wooden mallet to fill the rod holes; the last layer shall not be overfilled. For mechanical compaction, load in one go a quantity of mixture equal to the container volume after compaction, rodding slightly during filling; if the mixture falls below the rim during vibration add more; vibrate until the surface is flat and paste appears, without over-vibration. When an immersion vibrator is used, it shall not touch the inner wall or bottom of the container. When the air content is determined on site, a mechanical method with the same vibration frequency as used in construction should be adopted.

4.7.4 c) Immediately after compaction strike off with a straightedge and fill and smooth any depressions; if the apparent density of the mixture is also to be determined, weigh and calculate at this point. Then stick a small piece of plastic film on the concrete surface directly beneath the operating valve hole, wipe the rim of the container clean, fit the sealing gasket, put on the cover and tighten the bolts.

4.7.4 d) Close the operating valve and exhaust valve, open the drain valve and water inlet valve and inject water through the inlet valve; when the water from the drain valve contains no bubbles, close the inlet valve and drain valve simultaneously while water is still being injected.

4.7.4 e) Open the air inlet valve and pump air into the air chamber until the pressure is slightly above the calibrated zero point, open the exhaust valve slightly to adjust to the calibrated zero point, then close the exhaust valve tightly.

4.7.4 f) Open the operating valve; when the pressure reading is stable, record the air content value A01 (to 0.1 per cent).

4.7.4 g) Open the exhaust valve so that the gauge returns to zero; repeat steps d) to f) and measure the sample in the container again to obtain A02 (to 0.1 per cent).

4.7.4 h) If the relative error of A01 and A02 is less than 0.2 per cent, take their arithmetic mean; otherwise carry out a third test to obtain A03. If the relative error between A03 and the closer of A01 and A02 is not greater than 0.2 per cent, the arithmetic mean of those two values is taken as A0; if it is still greater than 0.2 per cent, the test is invalid.

4.7.5 The air content of fresh concrete is calculated with formula (12): A = A0 - Ag, where A is the air content of the fresh concrete in per cent, A0 the mean of the two air content determinations in per cent and Ag the air content of the aggregate in per cent. The calculation shall be accurate to 0.1 per cent.

4.7.6 The calibration of the container volume and the rating of the air content meter shall be carried out as follows. 4.7.6.1 a) Clean the container, assemble the whole meter and determine its total mass to 5 g.

4.7.6.1 b) Fill the container with water to the upper rim, fit the cover, close the operating valve and exhaust valve, open the drain valve and water inlet valve and inject water through the inlet valve; when the water flowing from the drain valve contains no bubbles, close the inlet valve and drain valve simultaneously while injecting, then determine the total mass to 5 g.

4.7.6.1 c) The container volume is calculated with formula (13): V = (m2 - m1)/(density of water) x 1000, where V is the volume of the meter in L, m1 the total mass of the dry meter in kg, m2 the total mass of meter and water in kg and the density of water is expressed in kg per cubic metre. The calculation shall be accurate to 0.01 L.

4.7.6.2 a) Rating of the air content meter: following steps d) to g) of 4.7.4, obtain the initial reading corresponding to an air content of 0.

4.7.6.2 b) Open the exhaust valve so that the gauge returns to zero; close the operating valve and exhaust valve, open the drain valve and collect the water discharged at the drain valve in a graduated cylinder; slowly pump air into the air chamber with the air pump, and when the water discharged equals exactly 1 per cent of the meter volume, obtain by the above steps the reading corresponding to an air content of 1 per cent.

4.7.6.2 c) Continue in the same way to obtain the readings for air contents of 2 per cent, 3 per cent, 4 per cent, 5 per cent, 6 per cent, 7 per cent, 8 per cent, 9 per cent and 10 per cent.

4.7.6.2 d) Each of the above tests shall be performed twice, with each pressure reading accurate to 0.1 per cent.

4.7.6.2 e) Each test above shall be checked; the relative error shall be less than 0.2 per cent, otherwise the meter shall be rated again.

5 Tests on the physical and mechanical properties of concrete

5.1 Preparation and curing of specimens: the preparation and curing of concrete specimens shall comply with the requirements of the design documents; where the design has no requirements, the following provisions apply.

5.1.1 a) Before casting, check the mould dimensions to ensure that the moulds meet the requirements of JG 237 and are within their three-month self-inspection period; the inner surfaces of the moulds shall be coated with a thin layer of mineral oil or another release agent that does not react with concrete.

5.1.1 b) When concrete is mixed in the laboratory, materials are measured by mass with a weighing accuracy of plus or minus 0.5 per cent for cement, supplementary cementitious materials, water and admixtures and plus or minus 1 per cent for aggregates; when concrete is mixed elsewhere, the accuracy is plus or minus 1 per cent for cement, supplementary cementitious materials, water and admixtures and plus or minus 2 per cent for aggregates.

5.1.1 c) Sampled or laboratory-mixed concrete shall be cast in the shortest possible time after mixing, generally not more than 15 min.

5.1.1 d) The sampled or mixed concrete shall be remixed with an iron shovel at least three times. The casting method is chosen according to the consistency: concrete with a slump not greater than 70 mm should be compacted on a vibrating table and concrete with a slump greater than 70 mm should be rodded by hand; for specimens used to check concrete in cast-in-place structures or precast members, the casting method should be the same as that actually used.

5.1.1 e) When specimens are compacted on a vibrating table, the concrete shall be placed in the mould in one go, trowelled along the mould walls during filling and heaped above the mould rim; the mould shall be attached or fixed to the table so that it does not jump during vibration, and vibration shall continue until paste appears at the surface, without over-vibration.

5.1.1 f) When specimens are rodded by hand, the concrete shall be placed in two layers of approximately equal thickness; rodding shall proceed spirally from the edge towards the centre, with not less than 12 strokes per 10000 square millimetres of cross-section per layer. The rod shall reach the bottom of the mould when rodding the bottom layer and penetrate 20 mm to 30 mm into the lower layer when rodding the upper layer; the rod shall be kept vertical. After rodding, tap the sides of the mould lightly with a rubber hammer until the holes left by the rod disappear, then insert a trowel along the inner walls of the mould several times.

5.1.1 g) When specimens are compacted with an immersion vibrator, the concrete shall be placed in one go, trowelled along the mould walls and heaped above the rim. An immersion vibrator of 25 mm diameter should be used, kept 10 mm to 20 mm above the bottom plate without touching it; vibration shall continue until paste appears at the surface, avoiding over-vibration to prevent segregation, generally for 20 s; the vibrator shall be withdrawn slowly and leave no holes.

5.1.1 h) Strike off the excess concrete at the top of the mould and smooth the surface with a trowel when the concrete approaches initial set.

5.1.2 a) Immediately after casting, the specimen surface shall be covered with an impermeable film.

5.1.2 b) Specimens under standard curing shall stand for one to two days and nights in an environment at 20 degrees C plus or minus 5 degrees C and then be numbered and demoulded. After demoulding they shall immediately be placed in a standard curing room at 20 degrees C plus or minus 2 degrees C and a relative humidity of 95 per cent or above, or in a still saturated calcium hydroxide solution at 20 degrees C plus or minus 2 degrees C. In the standard curing room the specimens shall be placed on racks spaced 10 mm to 20 mm apart, and their surfaces shall be kept moist without being directly sprayed with water.

5.1.2 c) Specimens cured under the same conditions as the structure may be demoulded at the same time as the actual members; after demoulding they shall still be cured under the same conditions.

5.1.2 d) The standard curing age is 28 d or the design age of the concrete mix proportion (counted from the addition of water during mixing).

5.2.1 Dry apparent density test: this method applies to the determination of the dry density of concrete cube specimens.

5.2.2 The specimens shall be 200 mm x 200 mm x 200 mm cubes or other cubes meeting the design requirements.

5.2.3 Sampling and casting of the specimens shall be carried out in accordance with 5.1.

5.2.4 The test equipment comprises: a) a forced-air drying oven able to control the temperature at 105 degrees C plus or minus 5 degrees C; b) a platform scale with a capacity of 50 kg and a sensitivity of 50 g; c) a vernier caliper with a maximum range greater than 300 mm and a precision of at least 0.1 mm.

5.2.5 a) Remove the specimen from the curing location, check its integrity, measure its dimensions to 0.1 mm and calculate the specimen volume V to 1 cubic millimetre.

5.2.5 b) Place the specimen in the electrically heated forced-air drying oven and dry it at 105 degrees C plus or minus 5 degrees C to constant mass (constant mass meaning that, for weighings at intervals of not less than 3 h, the difference between successive weighings is less than the weighing precision required for the test); weigh the mass m0 to 50 g.

5.2.5 c) The dry density is calculated with formula (14): r0 = m0/V, where r0 is the dry density in kg per cubic metre, m0 the mass of the specimen after drying in g and V the specimen volume in cubic millimetres. The final result is the mean of three specimens, rounded to 10 kg per cubic metre.

5.3.1 Compressive strength test: this method applies to the determination of the compressive strength of concrete cube specimens.

5.3.2 Specimen sizes include standard cubes with a side of 150 mm and non-standard cubes with sides of 100 mm and 200 mm; the specimen size shall be selected in accordance with Table 2, or in accordance with the design documents where these so require.

Table 2 Selection of specimen size: cross-section 100 mm x 100 mm for a maximum aggregate size of 31.5 mm; cross-section 150 mm x 150 mm for a maximum aggregate size of 40 mm.

5.3.3 a) The testing machine used for the cube compressive strength test shall comply with the technical requirements of GB/T 3159 and GB/T 2611, with a measuring accuracy of plus or minus 1 per cent, and shall be capable of applying load at a set constant rate.

5.3.3 b) When the concrete strength grade is C60 or above, an anti-burst mesh guard shall be provided around the specimen. The upper and lower platens of the testing machine shall have a flatness tolerance of 0.04 mm on the bearing surfaces, a surface hardness of not less than 55 HRC and a hardened layer about 5 mm thick.

5.3.4 a) Specimens shall be tested promptly after removal from the curing location, and the specimen surfaces and the upper and lower bearing platens shall be wiped clean.

5.3.4 b) Place the specimen on the lower platen or a bearing plate of the testing machine so that the loaded faces are perpendicular to the top face as cast. The centre of the specimen shall be aligned with the centre of the lower platen; start the machine and, when the upper platen approaches the specimen or steel bearing plate, adjust the spherical seat so that contact is even.