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1 Introduction. Ready-mixed concrete has been popularized and applied in China for many years, and up to now, not only in economically developed large and medium-sized cities, ready-mixed concrete has been widely used in many counties and towns. However, in the process of use, due to insufficient preparation, mold explosion, equipment failure, pipe blockage, etc., the concrete waits for a long time before unloading after arriving at the construction site, and the situation of unloading after waiting for 5 or 6 hours at the construction site occurs from time to time. Coupled with the lack of management and quality awareness, the overtime concrete is unwilling to be scrapped, and the on-site workers add a large amount of clear water to the concrete mixer truck to pour the concrete onto the main structure. This seriously affects the quality of the building. In this paper, the slump of ready-mixed concrete with time is analyzed by experiment, the amount of water added to the initial slump and the influence of the slump on the concrete strength.
2. Raw materials and mix ratio.
2.1. The cement test adopts the Kaicheng brand P. produced by Huizhou Everbright Cement Enterpriseo42.5R grade cement. The physical properties of cement are shown in Table 1, and the chemical composition of cement clinker is shown in Table 2. 
2.2 admixtures.
The admixture adopts the Pegasus brand retarding high-efficiency superplasticizer (naphthalene system) produced by Huizhou Jianke Industry. The solid content is 295%, the water reduction rate is 182%, the difference in setting time (initial setting) is 215 minutes, and the pH value is 102. The chloride ion content is 0025%。
2.3. The aggregate coarse aggregate is the 5 25mm continuous graded gravel of Zengcheng Yongheyuan Fengshi Quarry. The apparent density is 2610kg m3, and the crushing index is 76% and 69%, the mud content is 04%。The fine aggregate is Dongjiang river sand, and the fineness modulus is 27. The mud content is 10%。2.4. Admixture: The admixture is made of Shadian grade II pulverized coal of Dongguan Humen Shajiao Power Plant. The fineness (sieve residue) is 19%, the water requirement ratio is 101%, and the loss on ignition is 21%。2.5. Mix ratio: C30 pumped concrete, which has the largest usual production capacity, is selected for this test, and the design slump is 180mm. For example, see Table 3. 
3. Test process.
3.1. During the slump, slump loss and water addition adjustment test, we produce a car of concrete from the production line. After all the concrete is put into the mixer truck, the first test is carried out, and the slump test is carried out every hour, and the compressive strength test block is done (a group of 7 days and 28 days), and the same batch of concrete samples is added to adjust to the initial slump (record the amount of water added each time) immediately after the slump, and then form two groups of compressive test blocks. The variation between the strength of the test block formed at different times and the strength of the test block formed with and without water concrete was compared. During the test, we produced a truck of concrete (6 squares) according to the mix ratio in Table 3, which was parked in the station, and the test results were shown in Table 4 every hour from 9:00 to 15:00. 
3.2. The concrete strength test is done in two groups of test blocks when the concrete is out of the machine, and then two groups are done every hour before the water adjustment, and two groups of test blocks are also done after the water adjustment is raised to 7 days and 28 days for pressure testing. A total of 13 sets of standard test blocks were performed. The compressive strength values of the 7-day-old and 28-day-old test blocks are shown in Table 5. 
The variation of concrete strength can be seen from Table 5. Without adding water, the strength of concrete gradually decreases over time. The initial 7-day strength of the formed test block is 267MPa, 6 hours molding test block 7-day strength of 235mpa；The initial 28-day strength of the formed test block is 404MPa, 6 hours molding test block 28 days strength of 322mpa。The 7-day strength of the specimen formed after the adjustment of hourly water is 238MPa, 6 hours of water molding test block 7-day strength is 148mpa, a difference of 90mpa；The strength of the specimen formed after 1 hour of water adjustment is 34 in 28 days8MPa, 6 hours of water molding test block 28 days strength of 206MPa, a difference of 142mpa。The difference in strength between the test block formed at the beginning and the test block formed after 6 hours of water is 119mpa；The difference in strength between the test block formed at the beginning and the test block formed after 6 hours of water was increased in 28 days was 198mpa。It can be seen that the longer the time and the more water is added, the lower the strength of the concrete. In addition, the increase in strength of the test block formed after adding water from 7 to 28 days is less than that of the test block without water. 4 Reasons that affect the strength of concrete.
4.1. The influence of time on the strength of concrete, the concrete will start the hydration reaction after adding water, and the reaction is relatively slow at the beginning because of the action of the retarder. With the extension of time, there will be initial and final coagulation. Over time (the continuation of the hydration reaction), the cement slurry will gradually ossify, coagulate, and finally harden. The hydration of cement is a very complex physicochemical reaction process, which includes: the dissolution of certain components of cement, chemical reactions and the interaction between various electrolyte ionsFormation of hydration products and other precipitates;Deposition of hydration products and precipitates on solid surfaces;Generation of permeable membranes;The cement components continue to dissolve through the permeable membrane and the sediment is continuously deposited;Formation, growth and morphological changes of hydration product crystals;The crystals of the hydration products are filled and bridgled between the voids of solid particles. After 5 or 6 hours, the concrete has no slump and fluidity, and it can be judged that it is close to the initial setting at this time. At this time, part of the cement has been hydrated and a crystal structure has been formed. At this time, by adding a large amount of water and stirring before pouring, the early microscopic crystal structure that has been produced will be destroyed under the action of external forces. Some crystals are recombined after vibration, and the force between the hydration products is weakened, so the strength is significantly reduced. In addition, due to the poor workability of concrete for a long time, the fluidity and filling performance are also poor, which is not conducive to vibration and compaction, and will also lead to the reduction of strength. 4.2. Analysis of the effect of water addition on the strength of concreteThere are many factors influencing the strength of concrete, but the most important is the water-glue ratio [obtained from a·fb (fcu,0+ a· b·fb)=w b, fcu,0= a·fb (w b) a· b·fb]. All else being equal, the greater the amount of water used (the greater the water-to-glue ratio), the lower the strength. After the concrete is mixed and transported to the construction site, the amount of cementitious materials and other materials can not be changed, and the water-glue ratio becomes larger after adding water, and the strength will inevitably become lower. There are also many peer experts who have conducted a large number of experiments on this, and studies have shown that the strength will decrease by 1 by 10 kg per cubic concrete water consumption5～2.0mpa。If the mixing is uneven and the vibration is not compacted after adding water, the strength of the concrete will be more affected. 5. The impact of adding water at will on the construction site on the quality of the project.
We have seen some cases of indiscriminate water addition at some construction sites. There are two main reasons for adding water, one is that the workers are for the convenience of pouring, because the slump becomes larger and the fluidity becomes better after adding water, and it is easier to do the work;Second, because of the long waiting time and large collapse, it had to be poured with water. Through subsequent observations, it can be found that the concrete components that have been poured with water have obvious characteristics: loose surface, more floating slurry, yellowish color, peeling, exposed sand, etc. Some parts of the dense steel bar will also appear honeycomb and cavity due to poor vibration, which seriously affects the quality of the structure. If you further test its strength, you will find that it is very different from the normally poured concrete. There is a construction site in the process of pouring four trucks of concrete during the construction of the floor and wall columns, the pump truck failed, and another pump truck was rented in the case of poor on-site repair. As a result, three truckloads of concrete waited at the construction site for 5 to 6 hours before unloading (it was 23:30 at that time, the management was not in the current strong, and the workers added a lot of water to the mixer truck). When the mold was removed, it was found that the appearance of the parts of the three cars of concrete pouring was obviously different, and the strength was also very low. Party A and the supervisor requested to stop work after discovering it. After 28 days, the strength of these parts is 11MPa, 15MPa, and 14MPa (the design strength is C30), and the strength of other parts that are not poured with water within two hours are more than 35MPa (the concrete components poured after a project is overtimed and added water are shown in Figure 1). Later, it had to be reinforced. 
It can be seen that the strength of the concrete that has been waiting for a long time on the construction site without unloading, and then adding a large amount of clean water to pour it at will will be greatly reduced, leaving a serious quality hazard. 6 Summary.
The concrete gradually undergoes a chemical reaction after adding water to mix, and on the one hand, the extended slump of the concrete used for overtime is less and less, which is not conducive to construction, and on the other hand, the strength is gradually reduced. Therefore, it should be poured in the shortest possible time. GB T14902 "Ready Mixed Concrete" also stipulates that it takes no more than 90 minutes for concrete to be transported to the construction site from the mixer truck. This is evidenced by the trial process and engineering examples. Without the addition of water, the intensity of the extension gradually decreases over time, and the magnitude of the decrease is not very large in 3-4 hours. However, after a long time, the collapse is large, and the normal construction cannot be guaranteed. After adding water, the strength of the concrete is significantly reduced, and the more water is added, the more the strength decreases, and the increase in strength is also less. Therefore, in the construction process, the concrete that has arrived at the site should be properly disposed of immediately, and the mixing plant can also be transferred to other construction sites nearby for use in negotiation. If it exceeds four hours and the slump is very small and there is no fluidity, it is strictly forbidden to add a large amount of water to the mixer truck and pour it forcibly, otherwise it will seriously affect the quality of the project and the safety of the structure.