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Concrete Curing And Strength Testing Guide

With the rapid development of commercial concrete, both the supply and demand sides increasingly argue that structural concrete is too weak. Because the commercial concrete industry is a buyer’s market, to recover payments on time and continue cooperating with the demand side, most enterprises must make concessions and passively bear some economic responsibilities.

the main reasons for the low strength of structural concrete

Poor curing of structural concrete

Current status of concrete curing

To prevent early cracks, the cast-in-place slab is covered with plastic film, but most construction sites remove the film before the construction line is set out, and then switch to watering for curing; Most construction sites do not have dedicated personnel responsible for maintenance work, only arranging odd jobs to maintain a few times during their free time.
The method of wrapping concrete columns with plastic film is better, but there are also a few construction sites that have not adopted it.
After demoulding, most concrete shear walls are cured by watering, but there is no dedicated person responsible for it. The watering frequency is too low and the curing time is too short, which does not meet the specification requirements.
Especially in recent years, there have been more and more high-rise buildings. Once a building exceeds ten floors, water pressure may not reach the required level, so only water pumps can be installed for secondary pumping. Some water pipes are insufficient, and some water tanks cannot be raised in time, delaying maintenance. The higher the building, the more difficult it becomes, and the worse the concrete maintenance becomes.

Hidden dangers brought by poor maintenance to concrete

Insufficient curing in newly poured concrete can reduce concrete strength by 25% to 35%, creating hidden safety risks to the structure. In addition, the concrete surface is less dense, carbonation occurs faster, and the carbonation depth is greater. In some projects, when the structure rebounds, the carbonation depth of C30 concrete poured for less than half a year has already reached 6- 10 mm. This indicates that the area has not cured. Excessive carbonation depth reduces the concrete’s protective capacity for steel bars and shortens the durability of concrete structures.

The accuracy of the "Technical Specification for Rebound Testing of Concrete Compressive Strength" is not high.

When using the “Technical Specification for Rebound Testing of Concrete Compressive Strength” to test structural concrete, the estimated strength of C20~C40 concrete is about 10% lower; The estimated strength of C50 and above concrete is about 20% lower. The qualified range of the rebound gauge calibration is 80 ± 2, which is too wide and should be controlled within 80-82. When using a rebound tester with a higher fixed value for structural testing, the estimated compressive strength value of the structural concrete is more than 3MPa higher than the estimated strength value detected by a rebound tester with a fixed value of 78-80. This value is closer to the compressive strength value of the structural concrete detected by the drill core.

The drilling core detection is not standardized.

Some testing units cannot strictly follow the requirements of the “Core Drilling Regulations” during core drilling, cutting core samples without processing or conducting pressure tests, resulting in a 20% to 50% decrease in the core concrete’s compressive strength compared to the actual strength. Some units’ grinding machines do not meet regulatory requirements. The grinding machine lacks a device to clamp the core sample and requires two hands to hold the core sample while grinding it on the grinding wheel. How can such a grinding machine grind the core sample’s compressed surface?

Countermeasures to ensure the qualified strength of concrete

The Importance of Early Maintenance for Modern Concrete

The better the maintenance, the less likely concrete structural components are to crack. Impact on concrete carbonation: better curing slows the carbonation rate and reduces the carbonation depth.

New technology for concrete early curing

For the maintenance of cast-in-place concrete slabs: It is best to adopt a new process of covering plastic film with a flat surface as it is rubbed. That is, after pouring a section of concrete, first use a vibrating rod to compact the concrete, then find the flat surface and rub a small section with a wooden washboard while rolling the plastic roll from front to back. Press the beginning and end of the film roll with a wooden washboard to secure it to the concrete and prevent strong winds from blowing it away. Never lift all the plastic film when laying out the line. Use a paper-cutting knife to cut off parts that hinder laying out. Although it may be a bit troublesome, it saves a lot of maintenance water and labor in the future, which is still quite cost-effective.
For the maintenance of concrete columns: It is best to wrap the column surface with plastic film after the column formwork is removed. Before wrapping, sprinkle water, then wrap the column surface tightly to prevent internal moisture from evaporating and achieve good self-curing.
Curing of concrete shear walls: Apply a qualified concrete curing solution immediately after the removal of the shear wall formwork, especially during winter construction, which is the most suitable method. In addition to winter construction, you can also assign a dedicated person to sprinkle water for maintenance. Watering should be done hourly, with two people taking turns. Continuous maintenance should last at least 7 days, and ideally 14 days. When using the curing solution method, ensure an impermeable film forms on the concrete surface after application. If the first coat fails, apply a second coat. If a waterproof film cannot form, the curing solution is unqualified and cannot be used.
Maintenance of concrete pavement (ground): After the pavement indentation is completed, immediately cover it with plastic film. At the beginning, use soil or sand to press it down, then cover it well. Then press down on the upwind side and tail, and cover it in sequence. The next day, slightly lift one side of the plastic film, fill it with water, and cover it well. From now on, as long as you keep an eye on the thin film and prevent strong winds from blowing it away, everything will be fine. If strong winds blow it away, quickly sprinkle it with water and cover it again. Continue this for at least 14 days.

Guarantee measures for concrete curing

Enhance the maintenance awareness of supervisory personnel, so that each on-site supervisor attaches the same importance to supervising maintenance as supervising the construction of steel bars. If concrete curing is not in place, do not allow the next process to proceed. If excessive carbonation depth during structural rebound reduces concrete strength, the on-site supervisor should also be held responsible.

Determine the limit value of carbonation depth within one year: If newly poured concrete can meet the specification requirements in early curing, the carbonation depth of C40 and above concrete shall not exceed 1.5mm within one year, and the carbonation depth of standard curing (after 28 days of standard curing, it shall be transferred to outdoor natural curing) shall not exceed 1mm; The carbonation depth of C30 and C35 concrete shall not exceed 2.5mm, and the standard curing shall not exceed 1.5mm; The carbonation depth of C20 and C25 concrete shall not exceed 4mm, and the standard curing shall not exceed 2.5mm.

It is recommended that the country establish a one-year carbonation depth limit when revising relevant concrete specifications or regulations. When conducting rebound acceptance of the structure, if the concrete carbonation depth exceeds the limit, it indicates poor early curing, and construction and supervision personnel must be held responsible. Reduce mutual shirking and buck-passing between the demand side and the supply side due to inadequate structural strength.

Measures to ensure that structural concrete meets design requirements

Considering the unreasonable human factors at the construction site and the difficulty in ensuring that construction personnel can properly maintain the concrete in the near future, it is necessary to increase the surplus factor of the production mix ratio. The compressive strength of concrete after 28 days of standard curing should exceed 1.2 times the design strength.

In fact, the compressive strength of pre-mixed concrete after 28 days of standard curing accounts for only about 70% of its final strength, leaving a strength surplus of about 30% after 28 days. If early curing is poor, this 30% strength surplus will be wasted. Because ready-mix concrete is delivered to the demand side as a semi-finished product, the other half requires active cooperation from the demand side to produce qualified finished concrete.

Reasonable vibration and curing are important steps in producing qualified concrete. Because the supplier cannot control the demand side, the construction administrative department and supervision company must increase supervision over the construction units so they can carry out construction according to specifications and promote the development of ready-mixed concrete toward durability, environmental protection, and a green direction.

Construction personnel are prohibited from adding water to the concrete mixture and using admixtures to adjust the slump of the mixture. Because of the long transport time to the construction site, the concrete slump can drop significantly, making it difficult to pour. In addition, construction personnel often request an unnecessarily high slump. Even if the delivered slump meets the required demand, pump workers and pouring personnel still add water to the pump bucket. They focus only on pumping and ease of operation and never consider the concrete’s future strength and durability.

To prevent construction personnel from adding water to the mixture at will, concrete production enterprises should change their mindset and find problems in their own services: if the concrete we deliver to the construction site can be easily pumped and poured without rapid collapse; If our transport vehicle encounters special circumstances and waits for too long at the construction site, resulting in a small slump of the mixture, and our driver can actively add an appropriate amount of admixture to the mixing drum to increase the slump of the mixture, can the construction personnel still add water to the concrete mixture?

Therefore, do not blame only the construction personnel for adding water to the mixture; instead, replace them with concrete production enterprises that do not require construction personnel to add water to the concrete mixture. This requires our concrete production enterprises to increase training for transport vehicle drivers so that every driver can learn to add additives to the concrete mixture in a reasonable, moderate way to increase the mixture’s slump.