Generally the factors that affect the strength of concrete are: 1. Age of Concrete 2. Coarse Aggregate 3. Richness of the Mix 4. Curing of Concrete 5. Temperature of Concrete.
Factor # 1. Age of Concrete:
The relation between the water/cement ratio and strength of concrete applies to one type of cement at one age only. Different types of cements require different length of time to produce the same quantity of gel. It is assumed that 90% hydration is complete within 28 days.
Actually there are many variables which influence the intrinsic rate of hardening of commercial cements mixes with low water/cement ratio, gain strength more rapidly than with higher water/cement ratio. It is because in the first case the cement grains are closer to one another and a continuous system of gel is formed more rapidly.
For this reason a general rule connecting 7 day strength to 28 days strength is not easy. However when no specific data on the materials used is available, the 7 days strength may be assumed as 60-67% of the 28 days strength as per Indian code of practice. In general it is taken 2/3 at 7 days and 75 to 80% at 14 days of 28 days strength. Fig. 8.9 shows the development of strength of moist cured concrete with age. From the Fig. 8.9 it will be seen that hydration is far more rapid at early ages than at later ages.
Factor # 2. Coarse Aggregate:
The shape and surface texture of coarse aggregate have been found to affect the strength of concrete to a great extent. The influence of the type of coarse aggregate on the strength of concrete varies in magnitude and depends upon water/cement ratio of the mix. For water/ratio less than 0.40, the use of crushed aggregate has been found to give higher strengths by more than 38% than when gravel is used.
With the increase in water/cement ratio, the influence of aggregate has been found to decrease, presumably because the strength of paste itself becomes paramount (superior) and at a water/cement ratio of 0.65, the influence of type of aggregate does not exists, and no difference is found in strengths made with crushed stone aggregate and gravel.
Factor # 3. Richness of the Mix:
Mixes with a very low water/cement ‘ratio and an extremely high cement content (470 to 530 kg/m3) exhibit retrogression of strength, particularly when large size aggregate is used. But the aggregate/cement ratio has been found to affect the strength of all medium and high strength concrete i.e., concrete having strength 350 kg/cm2 and above. However aggregate/cement ratio is a secondary factor in the strength of concrete, but it has been found that for a constant water/cement ratio a leaner mix leads to a higher strength.
This behaviour is probably due to the absorption of water by aggregate as larger quantity of aggregate absorbs greater quantity of water, reducing the effective water/cement ratio. Thus total water content per cubic metre of concrete is lower in a leaner mix than in a rich mix. As a consequence in a leaner mix the voids form a smaller fraction of the total volume of concrete, resulting in higher strength.
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Fig. 8.10 shows the influence of maximum size of aggregate on compressive strength of concrete made with different w/c ratio.
Factor # 4. Effect of Curing on Strength of Concrete:
Curing is the name given to the procedure used for promoting the hydration of cement. It controls the temperature of concrete and moisture movement from and into the concrete.
The object of curing is to keep the concrete saturated or as nearly saturated as possible, till the originally water filled space in the fresh cement paste has been filled to the desired extent by the products of hydration of cement. In case of site concrete, it has been observed that active curing stops long before the maximum possible hydration could take place. The study of Fig 8.11 will show that longer the period of moist storage, greater the strength.
Exposure to air, with consequent drying, arrests hydration. The rate and extent of drying depends on the mass of the concrete relative to the exposed surface area as well as on the humidity of the surrounding air. During testing it was observed that specimens exposed to air and tested in air dry condition gave 25% to 33% higher strengths than corresponding specimens exposed to air for the same period, but saturated just before the test. Resumption of moist curing after a period of air drying results in resumption of hydration, though at a slower rate than that in progress when drying was begun. The loss of strength due to inadequate curing is more pronounced in thinner sections.
The necessity of curing arises due to the fact that hydration of cement can take place only in water filled capillaries. That is why a loss of water by evaporation from capillaries should be prevented. Further, water lost internally by self-drying up (desiccation) has to be replaced by water from outside i.e., the ingress of water in concrete should be made possible.
In case of sealed specimen, hydration can proceed only if the amount of water present in the paste is at least twice that of the water already combined. Thus self-desiccation is of importance in mixes with water/cement ratio below 0.5. For higher water/cement ratios the rate of hydration of a sealed specimen is equal to that of a saturated specimen. Further it should be remembered that only half the water present in the paste can be used for chemical combination.
This is true even if the total amount of water present is less than the water required for combination. The recent studies have shown that a hydration can take place only when the vapour pressure in the capillaries is sufficiently high about 0.8 times of the saturation pressure. Below 0.8 times of saturation pressure is low and below 0.3 it is negligible. Thus hydration at a maximum rate can proceed only under saturation Conditions.
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Recent studies also showed that for a satisfactory development of strength, the hydration of total cement is not necessary, nor it is achieved in practice at site. The quality of concrete depends mainly on the gel/space ratio of the paste. However if the water filled space in fresh concrete is greater than the volume that can be filled by the products of hydration, greater hydration will take place resulting in higher strength and lower permeability of concrete. The evaporation of water from concrete just after its placement depends on the temperature, relative humidity of the surrounding air and the velocity of wind.
Factor # 5. Effect of Temperature:
The influence of temperature of most curing on concrete strength depends on the time-temperature history. The strength of concrete is a function of ∑ (time interval x temperature). This summation is known as maturity. Thus a rise in temperature accelerates the chemical reactions of hydration and affects the early strength of concrete. A higher temperature during placing and setting though increases the early strength of concrete, but affects adversely later strengths from about 7 days on wards.
The reason of this is that a rapid initial hydration appears to form products of a poor physical structure, probably more porous, such that a large proportion of pores always remain unfilled. Helmuth on the basis of his experimental results has suggested that the rapid initial rate of hydration at higher temperatures retards the subsequent hydration and produces a non-uniform distribution of the products of hydration in the paste.
The probable reason for this is that at high initial rate of hydration there is insufficient time available for the diffusion of the products of hydration away from the cement grain and for a uniform precipitation in the interstitial space resulting building up a high concentration of the products of hydration in the vicinity of hydrating grain.
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This retards subsequent hydration and adversely affects the long term strength of concrete. Fig. 8.12 shows the ratio of strength of concrete cured at different temperatures as shown on the Fig. to the 28 days strength of concrete cursed at 76°F for a particular concrete.
The effect of temperature on the strength of concrete is shown in Fig. 8.13 (a). Fig. 8.13 (a) shows the strength of concrete cast and cured at the, constant indicated temperatures on the curve. From the Fig. it will be seen that higher the temperature within limits, the more rapid the hydration and higher the resulting strength at ages upto 28 days. At later ages, higher the temperature, lower the strength obtained. The decrease in strength is found to vary from 25% to 35%.
Fig. 8.13 (b) shows the effect of temperature during the first two hours after mixing on the development of strength of concrete. The temperature was varied from 4°C to 46°C (40°F to 115°F). The specimens cast and maintained for two hours at the indicated temperature and after two hours they all were cured at 21 °C (70°F. From the Fig. it will be seen that higher the initial temperature, lower the strength at 28 days. However upto the age of 7 days higher temperature results higher strength. Further cylinders moist cured during the first 24 hours at 2°C (36°F) and 18°C (64°F) and after 24 hours all cylinders were cured at 18°C (64°F). Test results have shown that cylinders cured at 2°C (36°F) gave 10% higher strength.
Fig. 8.13 (c) shows the effect of temperature on strength when concrete cast and maintained at 70°F for 6 hours and then cured at temperature indicated on the diagram. From the Fig. it will be seen that lower the curing temperature within limits, lower the strength. At a temperature of 33°F the strength is found only 47% of the continuously cured concrete at 70°F and at 16°F (below freezing point) only 9% strength was obtained.
The temperature at the time of placement also affects the strength of concrete. Some field tests have confirmed that an increase of 5°C (9°F) in temperature decreases the strength of concrete by 19 kg/cm2.
Fig. 8.14 shows the influence of curing temperature on the strength of concrete tested after 1 and 28 days curing. From the curve it will be seen that strength after 1 day curing increases upto the maximum range of temperature, while the strength at 28 days curing decreases considerably at high temperature at 50°C (122°F).
At 122°F the strength decreases from 400 kg/cm2 to 300 kg/cm2 i.e., a decrease of about 25% occurred.