There many factors influencing the compressive strength of concrete, such as the strength of cement, the bond strength between aggregate and cement, the quality of materials, the mixing ratio of materials, and the construction conditions. But the major factors are listed in the following:
(1) The Influence of Cement Strength Gradcs and Water-cement Ratio Cement strength grades and water-cement ratio are the main factors impacting concrete strength. The chemically combined water needed in cement hydration generally account for 23% of the mass of cement. But in the actual mixture of concrete, more water is needed to obtain greater mobility.
The space occupied by excessive water will turn into pores after hardening which will lower the density and strength of concrete (see Figure 5.10).
Vibrating
Manual Pounding I I
Ihc Complete Dense Concrete
0 Watercement Rntio ( W K )
Figure 5.10 The Relationship between Concrete Strength and Water-cement Ratio It is proved that the smaller the water-cement ratio is, the higher the strength of cement will be, the higher the cohesive power will be, and the strength of concrete will be, under the same condition.
A large number of tests have proved that: at the age of 28d, the relationship between the concrete strength ( xw,o), the actual strength of cement ( f, ) and the water-cement ratio ( W / C ) is in line with the following formula:
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In the formula: a, and a, are the regression coefficients. If crushed stones are used, a, =0.46 and a, =0.07; if gravels are used, a, =0.48 and a, =0.33.
In formula (5.3), if the actual strength of cement can be obtained, it can be calculated by the following formula:
f, = Yc * (5.4)
In the formula: & is the strength grade of cement (MPa);
y, is the safe coefficient of cement strength grade which should be determined by the actual statistics of various regions.
(2) The Influence of Aggregate
The strength of aggregate itself is generally higher than that of cement paste and the bonding strength between cement and aggregate (except lightweight aggregate), so it will not directly impact the strength of concrete. But if the strength of aggregate is lowered by weathering effect, it can reduce the concrete strength. The surface of aggregate is coarse so that its bonding strength with cement paste is huge. But more water is needed to achieve the same mobility, which will increase the water-cement ratio and reduce the strength. Thus, if the water-cement ratio is under 0.4, the strength of the concrete mixed with crushed stones is 38% higher than that of the concrete mixed with gravels. With the increase of the water-cement ratio, however, their difference will not be that obvious.
(3) The Relationship between Age and Strength
Under normal curing conditions, the strength of concrete increases with its age, shown in Figure 5.1 1. During the initial 3-7d, it grows fast and it can reach the numerical value of the regulated design strength. Afterwards, it grows gradually slowly, even unchangeable ever after.
Figure 5.11 shows that the growth of concrete strength is in direct proportion to the logarithm of age (which is more than 3d) under standard curing conditions, calculated as follows:
In the formula: f,, is the concrete compressive strength of 28d;
f , is the concrete compressive strength of nd ( n 3 3 ).
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The above formula applies to the concrete made of cement in media grades under standard curing conditions. The real situation, however, is complicated, so it is generally only taken as reference.
t
I \ g C
Figure 5.1 1 The Growth Curve of Concretc’s Strength (4) The Influence of Curing Temperature and Humidity
After casting and moulding of concrete, proper temperature and sufficient humidity should be maintained for the full hydration of cement to guarantee a better quality.
Under the guarantee of sufficient humidity, different curing temperature has great influence on the growth of concrete strength. If the temperature decreases, the hydration of cement will become slow and the increase of concrete strength will be slow, as well (see Figure 5.12). When the temperature falls below freezing point, most of the water in concrete freezes, and the concrete strength not only stop growing but also expand because the water in pores freezes, which will lead to the structural damage inside concrete and reduce the strength greatly. Thus, it is regulated that: if the average outdoor temperature is under 5°C for 5 days, the concrete construction should obey winter construction regulations. Generally, concrete aAer pouring and tamping can melt with the increase of temperature if it freezes, so its strength can keep growing. The earlier it freezes, the greater the loss will be; on the contrary, the loss will be smaller (see Figure 5.13). Therefore, the basic principle for the winter construction is to make concrete achieve certain strength (namely critical strength) before freeze, specifically referring to Code for Construction and Acceptance of Concrete Structures (GB50204-2002).
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110 100
% E 80
.- Y
E
2
vl
C" M I
40
20
0 1 3 5 7 14 21 28 d
Age
Figure 5.12 The Influence of Curing Temperature on Concrete Strength
% E
rn 40
30
fler growing for 1 Od e aAor growing for 7d
L a h 7 growing for 5d 20
10
0 4 8 12 16 20 24 28d Age
Figure 5.13 The Relationship bctween the Growth of Concrete Relative Strength and the Freeze Date
Environmental humidity has a significant impact on cement hydration.
Because the hydration can only happen in pores, the strength will stop growing with the evaporation of water in the dry environment. Thus sufficient humidity must be guaranteed during the maintenance period. Figure 5.14 is the relationship between humid curing and concrete strength. Generally, the watering maintenance time should not be less than 7d for Portland cement, ordinary cement and slag cement; the time should not be less than 14d for pozzolana cement and fly ash cement. In the hot season, watering should be paid attention to due to rapid evaporation.
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Mninlnining humid for 14d MuintuininR limnid fbr 7d Maintaining humid for 3d XO
‘-i t Maintaining humid for Id
G0lF
“‘y
20
of... . *
3 14 YO 365 d
7 28
AW
Figure 5.14 The Relationship between Concrete Strength and Humid Curing Time
5.3.3 The Deformability of Hardened Concrete
In addition to the external loads, hardened concrete can deform because of other factors, such as chemical shrinkage, temperature deformation, and dry shrinkage and wet swelling.