【Daily Topic】【Civil Engineering Version 20160616】

Water-cement ratio, cement strength, air content, mud content in aggregates, aggregate strength grading, curing method, additives

The main factors affecting concrete strength include: the method of preparing the concrete, the curing regime, and the various materials that make up the concrete. The water-cement ratio, mixing method, and vibration technique used in concrete preparation all have a significant impact on the strength of the concrete. While ensuring workability, reducing the water-cement ratio as much as possible can improve the strength of concrete. For dry and hard concrete, a forced-action mixer is suitable, while for plastic concrete, a gravity-fed mixer should be used. Compacting concrete using vibration methods can increase its strength; for air-dry concrete, this can boost early-strength properties by 20% to 30%. Other methods such as vacuum water absorption, rolling, pressure vibration, and centrifuging can also make the concrete more compact, thereby enhancing its strength. The effects of curing conditions on strength mainly include three aspects: curing temperature, curing age, and humidity during curing. The higher the curing temperature, the longer the curing period, and the greater the humidity during curing, the higher the strength of the concrete. Among the various materials that make up concrete, there is an almost linear relationship between cement strength and concrete strength; in other words, the higher the cement strength, the higher the concrete strength. Aggregates have a certain impact on the strength of concrete. Broken rocks, being rough and angular in most cases, possess good mechanical cohesion, which can enhance the strength of concrete. However, this effect gradually diminishes as the water-cement ratio increases. The higher the content of silt and organic matter in the aggregates, the greater the decrease in the strength of the concrete

Answer: The cement strength grade and water-cement ratio are the main factors determining the strength of concrete.

Composition materials and mix ratio. The influence of cement strength grade and water-cement ratio. Effect of aggregates. Admixtures and admixtures. Maintenance conditions. Test Method

What I’ve learned: The most important factor is the water-cement ratio. If there are problems with the strength of concrete on site, it’s usually due to an improper control of this ratio. Of course, if the mixing plant doesn’t measure accurately, it can lead to changes in the proportions of various materials used, which also affects the strength of the concrete. Of course, there are also factors such as cement strength, air content, mud content in the aggregates, strength grade distribution of the aggregates, curing methods, and testing conditions

There are various factors that affect the strength of concrete; generally, they can be categorized into four main aspects: the composition of the materials, the method of preparation, the curing conditions, and the testing conditions.

Factors affecting concrete strength: 1. Cement strength and water-cement ratio. The strength of the cement and the water-cement ratio are the most important factors determining the strength of concrete. Cement is the binding component in concrete, and its strength directly affects the strength of the concrete. Under the same mix ratio, the higher the strength of the cement, the higher the strength of the concrete as well. When the same cement is used (with the same type and strength), the strength of concrete is primarily determined by the water-cement ratio ； When the concrete can be fully compacted, a higher water-cement ratio results in more pores in the cement paste, lower strength, and weaker bonding with the aggregates, thereby reducing the overall strength of the concrete. Conversely, the lower the water-cement ratio, the higher the strength of the concrete. The compressive strength of concrete is related to the water-cement ratio and the strength of cement by the following approximate formula: fcu=αafce(C/W—αb), where C represents the amount of cement used per cubic meter of concrete, in kg ； W—Water consumption per cubic meter of concrete, kg ； fcu—compressive strength of concrete at 28 days, MPa ； fce—actual strength of cement, MPa ； αa, αb—are empirical coefficients that are related to factors such as the type of aggregate; their values must be determined through testing. Typically, the following values are used: for crushed stone: αa=0.46, αb=0.07. For pebbles: αa=0.48, αb=0.33. The FCE shall be determined through testing. When the actual strength value of the cement is not available, the following formula can be used to estimate fce=γc·fce,k, where fce,k is the strength grade value of the cement, in MPa ； γc—is the safety factor for the cement strength grade value (usually taken as 1.13). II. Influence of aggregates: The surface condition of aggregates affects the bond between the cement paste and the aggregates, thereby influencing the strength of concrete. The surface of crushed stones is rough, resulting in strong adhesion ； The surface of the pebbles is smooth, resulting in low adhesion. Therefore, under the same mix ratio conditions, the strength of crushed stone concrete is higher than that of pebble concrete. The maximum particle size of the aggregate also affects the strength of concrete; the larger the maximum particle size of the aggregate, the lower the strength of the concrete. The lower the sand ratio, the higher the compressive strength of the concrete, and vice versa. III. Admixtures and admixtures in concrete: The addition of admixtures to concrete enables it to achieve early-strength and high-strength properties; the inclusion of early-strength agents in concrete can significantly enhance its early strength ； The incorporation of water reducers can significantly reduce the amount of water used in mixing; even at lower cement-to-water ratios, the concrete can still be properly shaped and compacted, resulting in high 28-day strength. Adding admixtures to concrete can increase the density of the cement paste, improve the interfacial bonding strength between the cement paste and the aggregates, and enhance the long-term strength of the concrete. Therefore, the incorporation of high-efficiency water reducers and admixtures into concrete is an essential technical measure for producing high-strength and high-performance concrete. IV. Temperature and humidity for curing The hardening of concrete is the result of cement hydration and setting. The curing temperature has a significant impact on the hydration rate of cement; higher curing temperatures result in a faster initial hydration rate of cement, leading to higher early strength of concrete. High humidity ensures the availability of the water required for proper cement hydration, which is beneficial for strength development. Below 20°C, the lower the curing temperature, the lower the compressive strength of the concrete; however, within the range of 20°C to 30°C, the curing temperature has little effect on the compressive strength of the concrete. The higher the curing humidity, the greater the compressive strength of the concrete; conversely, the lower the compressive strength. V. Age: Under normal curing conditions, the strength of concrete increases as it ages. During the first 7–14 days, the strength increases rapidly; after 28 days, the increase slows down, but the concrete continues to gain strength over an extended period of time. The effect of temperature on concrete properties: The temperature of concrete is determined by the heat energy stored within it. Since there is a difference between the concrete’s temperature and the surrounding air temperature, heat exchange occurs between the concrete and its environment. In fresh concrete, apart from the heat generated by the hydration of cement, any changes in temperature are due to heat exchange with the surrounding environment. When the ambient temperature is very low, this heat exchange causes the concrete’s temperature to drop rapidly. For freshly mixed concrete, the speed at which the temperature drops determines the extent of hydration; in other words, the faster the temperature drops, the slower the increase in strength. When concrete freezes prematurely, its strength ceases to increase. The amount of free water remaining within the concrete also increases, which leads to greater frost heave stress. As a result, the concrete is more prone to damage. The reasons for the decrease in concrete strength can be summarized in the following 3 aspects: ① When water freezes, its volume increases by 9%. The more free water there is in the concrete, the greater the frost heave stress. The expanded volume does not return to its original size after thawing, but remains as it is. Therefore, the porosity of freshly mixed concrete increases significantly after freezing. If the porosity increases to 15. The strength will decrease by 10. When the frost heave stress becomes so great as to cause cracks, the concrete structure is damaged, and its strength no longer increases. ②Around the aggregates, there is a layer of water film or cement paste film; once frozen, its bonding strength is severely damaged and cannot be restored even after thawing. Experiments have shown that if this bonding strength is completely lost, the strength will decrease by 13%. ③During the freezing and thawing process, water transfer occurs; when freezing takes place, the low temperature of the concrete surface causes freezing to occur first, generating frost heave pressure that pushes the water inward into the concrete. During the dissolution process, the outer layers dissolve first while internal stresses remain high; this forces water to move toward the surface, resulting in reverse migration of water. Due to this change in water volume, the relative positions of the various components in the concrete change, which can easily cause structural cracks in concrete that still has low strength. The first few hours after concrete pouring are the most critical period, as the durability of the concrete can be severely damaged by just one or two freeze-thaw cycles. Observations have shown that as long as the freshly mixed concrete is allowed to warm up for a certain period of time until it reaches a certain strength, it can withstand freezing damage; this led to the concept of the critical strength against freezing damage. The concept of critical strength is defined as the initial strength required when fresh concrete, after being frozen and then subjected to thawing and curing, can continue to gain strength and reach a value above 95% of the design strength. By the time the critical strength is reached, a considerable amount of the mixing water in the concrete has been trapped within the already formed hydrates. At this point, not only is there less water available for freezing, but the concrete also possesses a certain degree of strength, thereby giving it some resistance to freezing. Currently, the concept of critical strength has been **accepted by many** and is used in standards. In fact, the main issues to address in winter concrete construction are the following two. ①It is to prevent concrete from freezing. ②, to improve concrete strength, especially early-stage strength. Analysis of the causes of concrete slump loss. Concrete slump loss is a common problem. The factors affecting the loss of concrete slump are multiple and interrelated. It mainly includes four aspects: First, regarding cement, these include the types of mineral components in cement, the content of different mineral components, the appropriate level of alkalinity, as well as factors such as fineness and particle size distribution. Second, concerning chemical admixtures, this involves the chemical composition of high-efficiency water reducers, their molecular weight, degree of cross-linking, degree of sulfonation, balanced ion concentration, as well as the types and amounts of retarding agents used. Third, environmental conditions such as temperature, humidity, and transportation time play a role. Fourth, factors related to the concrete itself include the water-cement ratio, the timing of adding water reducers, the types of supplementary materials used, and their proportions.

The strength of cement and the water-cement ratio are the most important factors determining the strength of concrete. Cement is the binding component in concrete, and its strength directly affects the strength of the concrete. Under the same mix ratio, the higher the strength of the cement, the higher the strength of the concrete as well. When the same cement is used (with the same type and strength), the strength of concrete is primarily determined by the water-cement ratio ； When the concrete can be fully compacted, the higher the water-cement ratio, the more pores there are in the cement paste, the lower its strength becomes, and its bond with the aggregates also weakens, resulting in lower strength of the concrete. Conversely, the lower the water-cement ratio, the higher the strength of the concrete.