I. Preface In recent years, with the rapid development of China's construction industry and the gradual improvement of construction laws and regulations, the requirements for building materials testing are getting higher and higher. The quality control of building engineering construction depends not only on the technical management and quality management of the construction process, but also on the necessary testing of building materials for construction. The detection of building materials is the basis to ensure the quality of engineering construction. Second, the existing problems At present, most construction projects in China are subcontracted. After winning the bid, the construction company divided the whole project into several parts and subcontracted them to various companies. This makes the construction quality of building projects difficult to control, and some companies do not fully realize the importance of building materials during the construction process. In addition, the construction management personnel did not operate according to the regulations, so the test results could not fully reflect the actual situation of the project. All these will make unqualified building materials used in the project for various reasons, which will inevitably leave hidden dangers for the quality of the project. This paper briefly introduces several important building materials. 1. Detection of reinforcement in building materials (1) Strength detection mainly detects the yield strength and tensile strength of reinforcement through tensile test: 1) Adjust the pointer of dynamometer dial of testing machine to zero point, and pull the auxiliary pointer to coincide with the main pointer. 2) Fix the sample on the chuck of the testing machine, and start the testing machine for stretching. 3) When stretching, the constant load when the dynamometer pointer stops rotating, or the minimum load without considering the initial instantaneous effect, is used to calculate the yield point load. 4) continuously loading the sample until it breaks. The maximum load read by the dynamometer disc is the ultimate tensile load. (2) Ductility detection Evaluate the ductility of steel bars by testing the elongation through tensile test: 1) Align both ends of the tensile specimen with the fracture point, and try to make its axis in a straight line. If a gap is formed at the tensile fracture for various reasons, the gap should be included in the length of the standard size part of the specimen after tensile fracture. 2) If the distance from the fracture point to the end point of the adjacent gauge is greater than 1/3, the gauge length (mm) after stretching can be measured directly with calipers. 3) If the sample breaks at the end of the measuring distance or at the measuring distance, the test result is invalid and the test should be repeated. (3) Bending performance testing The bending performance of steel bars is mainly tested by bending test. The cold bending test is to bend the steel bar sample by 90? Or 180? , and then check whether the sample has cracks, scales, fractures and other phenomena. It can not only detect the quality of raw materials of steel bars, but also the quality of welded joints of steel bars. Bending test of reinforcement shall be conducted on a press or universal testing machine, and the test shall generally be conducted in the temperature range of 10-35℃. For tests with strict temperature requirements, the test temperature shall be (23 5)℃. Repeated bending test is a method of cold bending test of steel wire on special bending testing machine. (4) Weight deviation detection When measuring the weight deviation of steel bars, samples should be intercepted on different steel bars, the number should be no less than 5, and the length of each sample should be no less than 500 mm Length should be measured one by one, and should be accurate to 65438 0 mm.. When measuring the total weight of the sample, the' corridor' is accurate to no more than 65438 0% of the total weight. 2. The sampling of cement (1) is not representative. According to the sampling method of cement GB 12573- 1990, the cement from the same cement plant, the same variety, the same strength grade, the same batch number and continuously entering the site should be regarded as a testing sampling unit. A batch of bagged products is 200 t, and a batch of bulk products is no more than 500 t, and each batch is sampled at least once. Sampling should be representative, and the same amount of samples should be taken from 20 different parts, and the total amount should not be less than 12 kg. In fact, due to the unstable purchase channels of cement in some construction units, in order to save the cost and convenience of the test, it is often the case that a certain variety takes half a bag or the whole bag as a sample at one time, which leads to the distortion of the samples sent. (2) It will be used in the construction site before the stability test results come out. According to the cement standard, the cement stability test must be qualified, otherwise it will be rejected and banned from use. Generally, the stability inspection of cement requires 2~ 3 inspection reports, but some construction units transport cement to the construction site for immediate use in order to seize the construction period, and then send samples to the testing unit for testing. When the cement stability test results are unqualified, the construction site has been used for concrete, mortar or some concealed works, resulting in quality accidents and economic losses. For example, a factory in Lvliang, which uses artificial foundation in a residential area of a county, produces ps32.5 cement. After the materials enter the site, the construction unit immediately carries out the construction of dry jet grouting pile, and then detects that the feedback stability of the unit is unqualified and the cement is wasted. Finally, reinforcement measures must be taken, such as manual excavation, waste stacking and re-stacking. Moreover, (3) the cement is not stacked in the required environment. There is often water in the construction site, especially in rainy season, which affects the storage and preservation of materials on the construction site, and this factor also directly affects the strength of cement. (4) Bagged or bulk cement is not measured strictly according to the requirements. The weight of bagged cement should be randomly checked when it enters the site, because there are many negative deviations in the weight of bagged cement at present, and the phenomenon of out-of-tolerance is serious. This phenomenon mainly occurs in the products of some small cement manufacturers. For example, a construction site found that the cement bags sent were in bulk and randomly selected. The weight standard of luggage is (50 1) kg, and the minimum deviation of actual weight is 4kg. If the concrete or brick masonry mortar is prepared according to the construction mixture ratio, it will directly affect the strength of the engineering entity and cannot guarantee the engineering quality. 3. The water content of sand (1) was not adjusted in time when it was used. It is medium sand instead of fine sand that is sent for inspection, and the field sampling is distorted. According to the Quality Standard and Inspection Method of Sand for Ordinary Concrete (JGJ52- 1992), coarse sand and medium sand should be preferred when preparing concrete. If fine sand is used, the mixture ratio issued by the testing unit shall be adjusted in time. Because the sand used in the construction site is generally piled in the open air, the water content is high. (2) Strictly control the stone powder content of the sand entering the site. Our area is mainly natural sand produced by aeolian fossils, but the dust in natural sand cannot exceed the standard, which will greatly affect the quality and structure of concrete and mortar. When the construction unit puts in materials, it sometimes puts in sand with excessive dust, and its content of stone powder and organic matter exceeds the standard requirements, which directly affects the strength of structural engineering concrete and mortar. For example, when a complex building in a residential area is built to the third floor, it is found through supervision and inspection that the strength of masonry mortar is low. According to the investigation by the supervisor, the stone powder content of sand temporarily purchased by the construction unit at night greatly exceeds the standard requirements, and it is used without inspection. After mortar strength appraisal, the design strength is m5? 0 mortar, the actual strength barely reached m2? 0, the design department needs to strengthen the structure after checking. 4. The particle gradation of crushed stone does not meet the specification requirements, and the content of stone powder and impurities is high, and the gradation is not adjusted and washed in time, so the Quality Standard and Inspection Method of Crushed Stone or Pebble for Ordinary Concrete (JGJ53- 1992) is directly adopted. 0? Article 1 stipulates that it is not appropriate to use single particle gradation to prepare concrete. If it is really necessary to use it, technical analysis should be carried out, and it is proved by experiments that segregation will not occur or affect the quality of concrete. Three. Preventive measures and suggestions, if the quality of building materials cannot be strictly controlled, will inevitably lead to the use of unqualified building materials in the project, which will affect the quality of the project to varying degrees and even lead to structural hidden dangers. In order to eliminate the hidden dangers caused by building materials, it is suggested that it is necessary to: 1 when strictly checking building materials. Control the quality of products produced by some small factories with poor production equipment and weak technical force from the source. It is suggested that the building materials factories should be centralized and rectified to cut off the sources of inferior building materials. 2. Strictly implement the product production license. For steel, cement and other major building materials with product production license requirements, the product production license must be strictly implemented. Strictly control the production license issued by the national technical supervision department in the process of material procurement, entry, re-inspection and specific use. The construction market and steel and cement manufacturers in a certain area should be inspected many times a year. At the same time, increase the frequency of supervision and spot checks at the construction site, and spot check 3 ~ 4 times a year by time. Confiscate the unqualified materials found, impose administrative penalties on the responsible units, and resolutely rework the parts that use unqualified materials. 3. Strictly control the purchase channels. When purchasing building materials, the construction unit should first choose regular manufacturers, purchase directly, reduce transit links, and ask for product quality guarantee and ex-factory product inspection report. 4. Strengthen on-site management. All building materials entering the construction site must be as follows: 1) The incoming materials must be visually inspected, and the appearance, packaging and identification of the materials should be checked and accepted against the factory certificate. According to the specification, unqualified products (such as steel) can be judged as unqualified products, and should be returned immediately to prevent unqualified materials from entering the site. 2) Strictly implement witness sampling principle and inspection system to ensure the authenticity of samples. The inspection unit refused to inspect the samples that did not meet the witness sampling, so that the building materials were under control. Building materials with special requirements should be well protected and kept (such as cement moisture-proof, steel corrosion prevention, oil pollution prevention, etc.). 3) Strengthen the training and management of relevant personnel on the construction site, especially the management of building materials managers, sampling personnel, witnesses and supervisors on the construction site, make them familiar with the professional knowledge of building materials performance, acceptance batch division, sampling, trial delivery, sample delivery and storage, strengthen the awareness of quality acceptance, strengthen the management of inspection units and strengthen the engineering quality inspection. Take timely measures to prevent unqualified building materials from being used in the project.
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