THE EFFECT OF CURING CONDITION ON COMPRESSIVE STRENGTH IN HIGH STRENGTH CONCRETE

The paper shows the effect of curing condition on compressive strength in high strength concrete in three cases (Group A(moist curing in water for 7 days followed by air curing ) ,Group B(curing until the age test in water) and Group C(curing at high temperature 60oC±2oC for six days ) and two types of specimen of cubes (150 x150 and 100 x 100) used in the test age (7,28and 90 day) respectively in four mix proportion (Mix No.1(40 Mpa ,Mix No. 2(fcu 60 Mpa) ,Mix No. 3 (fcu 70 Mpa) and Mix No. 4 (fcu 80 Mpa) ). Results demonstrate that, in general, concrete specimens moist cured until testing ages (Group B) give compressive strength greater than specimens moist cured for 7 days in water then followed by air – drying (Group A). The percentage of increase in strength is (5 and12%) for mix No.3 and 6% for mix No.4, as compared with 3% for mix No.1 and (2 and4%) for mix No.2. When the curing temperature (group C ) increases, the compressive strength increases at different ratios ,the percentage of increase in compressive strength at 7,28 and 90 days for mix No.1 , mixes No.2 and 3 are (20,15 and 14% ), ( 7,11 and 5% ) and (13,12 and 5% ) respectively, while mix No4. shows an increase of 4 and 10% in compressive strength at 7 and 28 days where there is a reduction in the strength at 90 days by about 2%. Generally, as the size of specimen decreases, the effect of temperature curing (group C)on the compressive strength increases.


INTRODUCTION
Curing is the name given to procedures used for promoting the cement hydration, and

Diyala Journal of Engineering Sciences
consists of temperature control and moisture movement from and into the concrete .The effect of curing condition on strength ,In order to obtain good concrete, the place of an appropriate mix must be followed by curing in a suitable environment during the early stages of hardening.Klieger (cited by ref. 1) reported that for low watercement ratio concrete, it is more advantageous to supply additional water during curing than in the case with higher water-cement ratio concrete (1) .
Carrasquillo, Nilson, and Slate (2) studied in their work the effect of two different drying conditions on concrete compressive strength: moist curing for 7 days followed by drying at 50 percent relative humidity until testing at 28 days, and moist curing for 28 days followed by drying at 50 percent relative humidity until testing at 95 days.These specimens were tested compared with the control condition of continuous moist curing until testing.
They found that high strength concrete shows a larger reduction in compressive strength than normal strength concrete when allowed drying before completion of curing.HSC showed an average reduction of 10 percent relative to continuous moist curing when moist cured at 7 days and then allowed drying until testing at 28 days, while under similar conditions, normal strength concrete showed 4 percent strength reduction.If moist curing for 28 days is followed by drying at 50 percent relative humidity until testing at 95 days, HSC showed a 4 percent lose in strength, while no appreciable reduction occurs on the normal strength concrete.
While other investigators (3) compared the strengths obtained from concrete specimens subjected to different curing conditions.They concluded that 7 days of moist curing period (including the first 24-h in molds) is sufficient to make the high strength concrete impervious.
Further moist curing beyond this period is not needed to substantially enhance the compressive strength and elastic modules of concrete.They found that the difference in compressive strength between the specimens subjected to 7 days moist curing followed by 21 days air curing and those subjected to 28days moist curing followed by 28 days air curing is only 6%, which is not significant.
Neville (4) reported that moist curing for 28 days thereafter in air is highly beneficial in securing HSC at 90 days.
Aitcin and Riad (5) observed that the 28 days compressive strength of specimens cured under standard conditions gives a fair representation of the actual strength when the water / cementitious ratio is below 0.3.
Klieger (6) concluded that temperature increases strength during the first few days after casting, but after one to four weeks, the strength reduces.
Eliverly and Evans (7) confirmed that specimens mixed in normal temperature (17˚C) and cured under high temperature (40˚C) have a higher crushing strength than those mixed and cured under normal temperature by about 19% and 16% at 7 and 49 days, respectively.
Hester (8) reported that sealed specimens tested at 43˚C have measure strength up to 10%lower than 21˚C specimens, and specimens at 71˚C have lower strength up to 20%.
Cebeci (9) concluded that concrete cured in water within (37˚C), has higher compressive strength up to age of 90 days and lower ultimate compressive strength (360 days) compared with concrete cured within (17˚C).
Selman (10) found, that the compressive strength of concrete mixed and cast at temperature not exceeding (29˚C) and moist cured under hot weather for 7days, increases as the curing temperature is increased (up to 90 days).The increase ranges between (4-22%) with respect to mixes cured at normal weather conditions.
Konstantin, and Isaak (11) concluded that the compressive strength at 30 C 0 increases with time much faster(compared to 20 C 0 curing )developing mostly during the first week of curing .

Material
The materials used in this study are locally available and widely distributed over large areas in Iraq .These materials include crushed gravel and natural silica sand, in addition to the drinking water and Lebanon cement .

Aggregate
Aggregate were used in this study include fine aggregate.The grading and particle shapes of fine aggregate are significant factors in the production of HSC.Fine aggregate with rounded particle shape and smooth textures requires less mixing water in concrete and for this reason is preferable in HSC (1,4) .Sand with fineness modulus of about 3.0 or more is preferable to obtain a workable concrete mix with limited amount of water, and for enough fine material (cement) in the mix to obtain the required consistency Natural Sand from Al-sadoor region with fineness modulus, specific gravity and absorption 3.18, 2.7% and 1.5% respectively is used in this work.In Table (1) shows its grading and the limits of BS882-92.
Al-hassani (12) have shown that the smaller size aggregate produces higher strength values.Therefore, the maximum coarse aggregate size is chosen to be 14mm.
Crushed gravel from Al-sadoor region with specific gravity and absorption 2.64 and 0.57%, respectively is used.In Table (2) shows the grading of this aggregate .This table also gives the limits specified by BS 882-92.

Admixtures
For HSC production, the water content of the mix is needed to be reduced, which can be achieved by using superplasticizers and to compensate for the associated reduction in water content and workability of the concrete mix.A superplasticizer (SP) of melamine formaldehyde condensate, known as (Melment L-10) is used in this work, its properties are listed in In Table (3).According to ASTM-C 494, this superplasticizer is classified as type (F), because it has the capability of more than 12% water reduction for a given consistency.
The optimum dosage is found to be 4 % by weight of cement and the reduction in water for this dosage is about 25%, Superplasticizer is used as a white powder by dissolving 1 part solid (SP) in 4 parts of water, a liquid of 20 % concentration (SP) is prepared, five minutes wellstirred in water by small glass rod and left in laboratory for at least (2 hours) before added to the wet concrete mixes (13) .2-100mm cubes which are prepared and tested according to the same specifications.

Curing and Testing Age
Water curing of high-strength concrete is very essential due to the low water-cement ratios employed.
Three types of curing are simulated: -Group A-Moist curing in water for 7 days followed by air curing inside the laboratory at (26-30ºC) until testing age.

Group B-Moist curing in water until testing age.
Group C-High temperature curing: by placing the specimens in a water-curing tank placed in a controlled-temperature room.The curing temperature is 60ºC±2ºC for six days then these specimens are air cured in the laboratory at temperature range between (26-30ºC) until testing age.The test ages are 7,28 and 90 days.

Effect of Curing Condition on Strengths
The results of compressive strength for the concrete mixes are shown in Table ( (٢) .These Figures also show that the small specimens are more affected by drying than large ones.Since the difference in the compressive strength between the two groups of concrete is slight, so a 7-day initial moist curing period is sufficient for the development of potential strength.This result is in agreement with Aitcin et al (5) .
Where : r is the correlation factor

Effect of Curing Temperature on Strengths
Figures (1) to (5) show the relationships between compressive strength and age of concrete specimens cured at 60 0 C (group C) and those cured at 25 0 C (group A).It is clear from these figures that as the curing temperature increases, the compressive strength increases at different ratios.
These results show that there is a significant increase in compressive strength for concrete specimens cured at 60 0 C with respect to those cured at 25 0 C,for mixes No. Similar observations were reported by Cebeci (9) who stated that concrete cured in water within (37 0 C) has higher compressive strength than that cured in water with (17 0 C).
Selman (10) also showed that the specimens cured at temperature of (60 0 C) have an increase in The rise in curing temperature speeds up the chemical reaction of hydration and, thus, affects the early strength of concrete.The rapid initial rate of hydration at higher temperature retards the subsequent hydration and produces a non-uniform distribution of the products of hydration within the paste.This is due to the fact that at the high initial rate of hydration, there is insufficient time available for the products of hydration away from the cement particle and for a uniform precipitation of the products of hydration space.As a result, a high concentration of the products of hydration is built in the vicinity of the hydration particles, and this retards the subsequent hydration and adversely affects long-term strength (13) .

there 7 ,
28, and 90 days compressive strength of about 22,18 and 9% respectively in comparison with those cured at normal temperature (25 0 C).Mixes No.3 and No. 4 show slighter increase in compressive strength than mixes No.1 and No. 2. The rates of increase in their 7 and 28 days compressive strength are 10 and 8 %for mix No.3, and 6 and 11% for mix No. 4 respectively, but at 90 days they give 0 and 9% reduction in their strength.

Table (
1): Grading of Fine aggregate.(#)Propertiesobtainedfromproductcatalogue2.2.Concrete MixesFour concrete strength levels are investigated in this work, namely 40, 60, 70 and, 80 MPa which are expressed as Mix No.1, 2,3 and 4, respectively.British Standard BS 5328: part 2:1991 mix design method is used because it yields mixes with strength range higher than the compressive strength ACI 211 method.The details of four groups mix proportions are shown in Table(4).2.3.1.Compressive Strength MeasurementCompressive strength is carried out by 2000 KN capacity compression machine.Each result of compressive strength obtained is the average of three specimens -2.3.

2. Two types of moulds are used 1
-150mm cubes which are prepared according to BS 1881:Part 108:1983 and tested according to BS 1881:Part 116:1983.

Table ( 5
): The result of Compressive Strength with Different Specimens, Curing Types and Mix No.
This behavior is for large specimens since small specimens show a higher increase in their compressive strength, as compared with large specimens.The rates of increase in compressive strength for mix No.1 at 7,28 and 90 days are 20,15 and 14%, for mixes No.2 and 3 and the rates of increase are 7,11 and 5% and 13,12 and 5% respectively, while mix No.4.shows an increase of 4 and 10% in compressive strength at 7 and 28 days where there is a reduction in the strength at 90 days by about 2%.Generally, as the volume of specimen decreases, the effect of hot weather on compressive strength increases, and the rate of increase in compressive strength increases.C, and moist curing for 7 days, play a significant role in the reduction of the harmful effect of hot weather on compressive strength especially for high strength concrete.
0 Fig. (1): Compressive Strength Development of 150mm and 100mm Cube with Age for Mixes Cured at Varying Conditions, mix No.1