How to do if the strength test of concrete test blocks fails

1. What are the effects of failing the strength test of concrete test blocks

The failure rate of testing reflects the production management level of the enterprise. The excessive number of failed test blocks, from an economic perspective, is also relatively high in cost. If the test results do not meet the design requirements, other testing methods need to be used for retesting, which increases the cost of the enterprise. The excessive pressure test value has led to higher cement consumption, increased costs, and reduced economic benefits in the enterprise, resulting in the entire process not being in a reasonable state. If the pressure test value is too low, it indicates that the mixture ratio given in the test has not been strictly followed, and the test block has not been made strictly according to industry standards and specifications.

Concrete engineering is a systematic project, and concrete is only one part of many engineering materials, requiring the joint cooperation of various relevant units and departments to complete the production and testing of concrete test blocks. Commercial concrete is also a process product, and there are multiple factors that affect the pressure test value of the test block.

2. Analysis of the reasons for unqualified

The main reasons for the occurrence of unqualified commercial concrete test blocks are summarized as follows:

A. The production of commercial concrete test blocks is not standardized.

Mainly reflected in the sampling and specimen production process of concrete. The construction unit often fails to take timely and random samples, and the supervision unit also fails to supervise the construction unit to make test blocks as required. The production of concrete test blocks is also very non-standard. In order to save project costs, the construction unit cannot strictly follow the proportion of cement, sand, stone, and water usage specified in the mix ratio for construction.

B. Concrete test block molds are not standardized

Long term use of concrete trial molds can lead to issues such as non-standard verticality and flatness, and the size of the concrete specimens produced does not meet the standard requirements. The main reason for the non-standard concrete test mold is that the test block produced did not undergo self inspection during deformation, resulting in the test block not meeting the requirements of the specifications. If the test mold has already deformed, it can still be used. The shape of the produced test block becomes a parallelogram, and if the compression surface and adjacent surfaces are not perpendicular, it will cause errors in the compression of the concrete test block. Before the production of the test piece, not rubbing lubricant inside the test mold can cause difficulty in removing the concrete mold, damage, and affect the strength evaluation of the concrete.

C. Insufficient curing of concrete test blocks

The gradual hardening of concrete test blocks after production is due to the hydration of cement, which requires appropriate temperature and humidity to continuously increase their strength. The maintenance of early concrete test blocks is crucial for their strength growth. The production and maintenance of concrete test blocks require that every three test blocks be grouped together, and the mixture used for each group of test blocks should be taken out of the concrete mixed on the same plate or transported by the same vehicle according to the different requirements of the parts. However, the test block production personnel of the construction unit often use strength as the standard in actual production, and when multiple sets of test blocks are produced at once, they need to send qualified test blocks with that strength. Some test blocks have not been cured under standard conditions for 28 days.

3. Several suggestions for solving problems

A. Strengthen regulatory efforts

Establish a dedicated management department to increase supervision over concrete production enterprises and construction sites, including concrete production, labeling, maintenance, and inspection. Regularly inspect concrete manufacturers to promptly identify problems that arise during the production process, and urge concrete production enterprises to strengthen their own management capabilities and improve the quality of commercial concrete. Orderly guide and eliminate Cut throat competition between concrete production enterprises.

B. Sampling of concrete test blocks

Concrete test blocks should be taken at the pouring site, and the supervising unit should play the role of a witness to prevent construction units from directly taking test blocks from concrete production enterprises and sending them to the testing center for testing.

C. Selection of concrete trial molds

The standard specifications for pressure and mold testing of commonly used concrete test blocks in buildings are: 75 ×75× 75、100 ×100× 100、150 ×150 × 150. The test mold used for making the test piece is made of HT200 cast iron or steel with a performance index not lower than Q235. The internal surface should be machined flat and smooth, without any sand holes, cracks, or scratches. It should have sufficient stiffness and be easy to disassemble. During vibration molding, it should not be loose or leak. After the trial mold is assembled, the included angle between its adjacent sides and between each side and the upper surface of the bottom plate shall be a right angle. The unevenness of the side plate, end plate and internal surface of the trial mold shall not exceed 0.05mm every 100mm. After the trial mold of cube and Rectangular cuboid is assembled, each adjacent surface must be vertical and the edge length shall not exceed ± 1%. Before making the test piece, the test mold should be cleaned and a layer of lubricating grease or release agent should be applied to its inner wall. When making test blocks, at least three of them are usually made simultaneously in each group, and corresponding size test molds are selected based on the size of the aggregate particles. There must be a slight margin during mold installation. At the same time, it should be noted that the pouring of concrete into the trial mold must be completed within half an hour after the concrete is made. After the concrete trial mold is used, it should be immediately wiped clean and promptly coated with anti rust oil to prevent corrosion.

D. Forming of concrete test blocks

The forming method of concrete specimens is similar to the method of on-site concrete block formation. In general, the methods for forming concrete test blocks are as follows:

(1) Vibration table method. The vibration table method is applicable to concrete specimens with a slump of less than 70mm. The main operating steps of the vibration table method are to put the concrete mixture into the customized trial mold at once, and then carry out the loading operation. During the loading process, it is necessary to fully use a trowel, which is tightly inserted along the wall of the trial mold. The purpose is to make the concrete mixture higher than the mouth wall of the trial mold, and then vibrate after the concrete mixture is higher than the mouth wall of the trial mold, During vibration, the trial mold must be tightly attached to the vibration table until the surface of the concrete mixture is grouted, and the vibration can stop.

(2) Manual insertion method with a tamping rod. The manual insertion method of the tamping rod is suitable for concrete specimens with a slump of over 70mm. The main operating steps of the manual insertion method of the tamping rod are to first divide the concrete mixture into two layers, and then load them into customized trial molds. The thickness of the concrete mixture loaded in the two layers should not differ too much. Then start inserting the screw, starting from the edge of the trial mold and gradually moving towards the center of the trial mold, ensuring a uniform speed during this process. And when inserting and tamping the bottom concrete of the trial mold, the tamping rod must touch the bottom of the trial mold. When inserting and tamping the upper layer of concrete in the trial mold, the tamping rod must penetrate the entire upper layer and be inserted into the lower layer about 20mm~30mm.

(3) Use the immersion vibrator method. Load the concrete mixture into the trial mold at once, and use a trowel to insert and vibrate along the trial mold wall during loading, making the concrete mixture higher than the trial mold mouth. Use a diameter of Φ 25mm plug-in vibrating rod. When inserting the test mold for vibration, the vibrating rod should be 10mm to 20mm away from the test mold bottom plate and should not touch the test mold bottom plate. The vibration should continue until the surface is grouted, and should not be over vibrated to prevent concrete segregation. The general vibration time is 20S. The vibrating rod should be pulled out slowly, and there should be no voids left when pulled out. All three methods can achieve the purpose of forming concrete test blocks, but special attention should be paid to avoiding excessive vibration during the forming process, which may affect the quality of the concrete test blocks.

E. Curing of concrete test blocks

The main purpose of curing is to ensure the normal hardening of concrete under certain temperature and humidity conditions. The maintenance methods are generally divided into standard maintenance and same condition maintenance. Standard maintenance refers to maintenance carried out in a standard maintenance box or room, usually at a temperature of 20 ± 2 ℃ and a relative humidity of over 95%. When there is no standard curing room, concrete specimens can be cured in non flowing water at a temperature of (20 ± 2) ℃. Curing under the same conditions reflects the actual strength of concrete on site. Same condition maintenance refers to the maintenance of test blocks and components under the same temperature and humidity environment, which serves as the basis for component removal, continued construction, and structural acceptance.