Reasons and solutions for insufficient concrete strength and poor homogeneity

1. Causes of occurrence

(1) The metering equipment of the mixer malfunctioned, the slump was out of control, and the concrete strength was highly dispersed.

(2) The cement has expired or become damp, and its activity has decreased. The gradation of sand and stone aggregates is not good, with large voids, high mud content, many impurities, improper use of admixtures, and inaccurate dosage.

(3) Improper concrete mix proportion and inaccurate measurement

(4) Water was added at will during construction, increasing the water-cement ratio.

(5) The sequence of adding concrete materials was reversed, the mixing time was insufficient, and the mixture was not uniform.

(6) During winter construction, formwork is removed too early or is frozen at an early stage.

(7) During the summer construction, the specimens were not covered and maintained in time.

(8) The concrete test blocks were not vibrated and compacted thoroughly, the curing management was poor, or the curing conditions did not meet the requirements. Under the same curing conditions, they were dehydrated prematurely or damaged by external forces.

2. Preventive measures

(1) Cement should have a factory certificate of conformity, be fresh and free of lumps. Expired cement should only be used after passing the test. The particle size, gradation and mud content of sand and gravel should meet the requirements. The concrete mix ratio should be strictly controlled to ensure accurate measurement. Concrete should be mixed in sequence, and the mixing time and uniformity should be guaranteed. To prevent concrete from freezing at an early stage, in Dongpeng construction, ordinary cement should be used to prepare concrete with a strength of over 30%, and slag cement should be used to prepare concrete with a strength of over 40% before it can be frozen. Concrete test blocks should be carefully made in accordance with the construction specifications, and the management and maintenance of the test blocks should be strengthened.

(2) Strengthen the maintenance of metering equipment to ensure accurate feeding and control the slump of the concrete mixture leaving the machine.

(3) Sampling at the construction site should be taken between 1/4 and 3/4 of the unloading process of the mixer truck. The quantity should be 1.5 times the required amount for the concrete quality inspection items and must not be less than 0.02 cubic meters. The test blocks formed by manual ramming should be loaded into the test mold in two layers, with the thickness of each layer being approximately equal. The number of ramming times for each layer should be determined based on the cross-sectional area of the test piece. Generally, it should be no less than 12 times for every 2500px ² cross-sectional area.

(4) Strengthen the curing of test blocks. After the standard curing specimens are formed, cover the surface to prevent water evaporation and dehydration. After demolding the next day, they should be placed in a standard curing room with a temperature of 20±1℃ and a humidity of over 95% for curing. When there is no standard curing room, concrete specimens can be cured in non-flowing water with a temperature of 20±3℃, and the pH value of the water should not be less than 7.