Influence of Steam Curing Cycle on Compressive strength of Concrete

Pratik Deogekar, Ashwini Jain, Sudhanshu Mishra, Prakash Nanthagopalan

Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India

Abstract

Steam curing at atmospheric pressure is one of the techniques for obtaining high early strengths

in concrete especially in precast concrete production. This technique enables early removal of

shuttering and facilitates early vacating of the pre-stressing bed in precast industry providing a

major economic advantage as well. It also aids in faster and safer construction as sufficient

strength is attained in short period and maintained without any other form of curing. The strength

enhancement depends on steam curing cycle. The parameters involved in a steam curing cycle

include a delay period, a gradual increase to a temperature where it is to be maintained for a

specific curing period followed by gradual cooling. The object of this study is to optimise the

steam curing cycle for conventional concrete. For this purpose, a concrete mixture of M 40 grade

with water/cement ratio 0.4 was designed and subjected to 3 variable parameters (i.e.) delay

period (2 and 4 hours), curing period (6 and 8 hours) and temperature (50, 60 and 70C). The

compressive strength of these cubes was measured at 1, 7 and 28 days and compared with cubes

subjected to the conventional water curing method. A comparative study between steam curing

and conventional curing was carried out. Also, the effects of each of the parameters involved in

the steam curing were analysed. From the results, it is observed that the optimum cycle is having

a delay period of 4 hours, curing period of 8 hours at a temperature of 60 C. An attempt was

also made to understand the vital parameters which affect the compressive strength significantly

using Robust Design Engineering methodology.

1. INTRODUCTION:

In recent times, the increased usage of the high strength concrete elements coupled with demand

for high productivity of the projects has resulted in adequate curing time not being followed

which has affected the strength and durability of the concrete structures all over India. This

problem can be tackled to some extent through the application of steam curing to the concrete

mix. Steam curing is the process by which concrete is cured at high temperatures at atmospheric

pressure in steam. The hydration rate of cement increases with temperature and hence the gain of

strength can be speeded up. The process can be regarded as a special case of moist curing in

vapour saturated atmosphere which ensures a supply of water [1, 2]. Application of steam curing

to concrete specimens makes them develop sufficient compressive strength with the advantage

that they can mature properly without application of any further form of curing. At the same

time, with regards to the precast industry, moulds can be removed earlier and less curing storage

is required which gives an economic advantage. Also, the concrete can start to perform its

function soon after casting which significantly saves time.

The typical parameters of this steam curing process are curing time, maximum steam

temperature and duration at the maximum temperature [3]. The basic process consists of a delay

period of 2 to 5 hours, followed by heating at a rate of 22 to 44ºC per hour up to a maximum of

50 to 85 ºC, maintaining it at that temperature for a period of up to 10 hours followed by a

cooling period with the total cycle lasting not more than 18 hours [14]. The difference in thermal

expansion coefficients of the concrete ingredients can lead to micro-cracking and increased

porosity. However, delaying the steam curing cycle by a period equal to its initial setting time

greatly reduces such deleterious effects. It also facilitates the reaction of gypsum with Tri-

Calcium-Aluminate which gets reduced to a decrease in solubility of gypsum at higher

temperatures [15]. Although early application of steam curing is a common practice, many

researchers here indicated that this is quite detrimental and that some delay prior to steam

exposure is beneficial to concrete properties, such as strength and durability [4, 5]. Mironov [4]

concluded that delay period should be determined in such a way that the steam curing operation

should not cause expansion. According to Alexanderson [3], lower quality of concrete due to

shorter delay period is the result of increased porosity and cracks caused by the tensile stresses

formed by internal pressure in the pores. It was established that concrete should have a critical

tensile strength before start of steam curing operation. Erkdem [6] stated that if steam application

starts before the initial setting time of concrete, outer position (or the faces) of a concrete

specimen harden earlier while inner concrete is still plastic because the concrete temperature lags

behind that of the curing chamber during temperature rise. The inner plastic concrete can expand

and induce tensile stress in the exterior rigid shell. Hence, initial setting time was proposed as a

quantifiable criterion for the delay period before application of steam curing.

Oztekin [7] showed that for compressive strength development, duration of steam curing is also

an important parameter as well as temperature. The curing period and temperature is adjusted

according to the targeted 1-day strength level. A rise in curing temperature speeds up the

chemical reactions of hydration. It also reduces the length of the dormant period and hence the

overall structure of the hydrated cement paste gets established very early. However, rapid initial

hydration provides less time for diffusion of products which leads to formation of local areas of

lower strength. When the curing temperature of concrete changes, the composition of the hydrate

changes. Accordingly, this change influences the diffusion and penetration of the hydrate related

to the medium- or long-term strength of the concrete [12]. It is obvious that heat treatment

application at a lower temperature is more economical and energy saving [8]. Saul [9] evolved

the cumulative effect of curing period and curing temperature on steam curing by introducing the

term “maturity” and defining it as product of age and average temperature above freezing. It was

found that the samples of concrete from the same mixture will have equal strength if they have

equal maturity values, although their temperature histories may differ. The proposed model was

found to fail for relative humidity below 70% and corrections to take into account the effect of

humidity were proposed by others [10].Curing of concrete at high temperatures for prolonged

period increases the rate of hydration which results in higher strength. However, the expansion

coefficient of air bubbles being twice that of solid concrete induces tensile stress that has an

adverse effect on the initial compressive strength [5]. Further, a gradual cooling period was seen

to be necessary to prevent sudden contraction which can cause cracking. After performing the

steam curing cycle, delayed expansion in the concrete was attributed to the transformation of

metastable mono-sulphate to ettringite when steam curing was followed by normal temperature

moist curing at later ages [11]. Under such conditions, concrete may show abnormal expansion

and associated micro-cracking, which may lead to failure in the long term. Hence, it was

suggested that concrete specimens must be exposed to air after steam curing cycle.

1.1. Research Significance

From the literature review, it was observed that many studies were performed to understand the

effect of delay period, curing period and curing temperature individually, however, very few

studies were done to understand the combined effect of these parameters on the compressive

strength of concrete. Hence, in the present experimental investigation, the cumulative effect of

these parameters on the early (10-14 hrs) and later (28 days) compressive strength were

evaluated. M40 grade concrete with a water cement ration of 0.4 was used. The study aims to

provide a reference for achieving the desired compressive strength by performing an appropriate

steam curing cycle. The analysis of each of the parameters involved provides the flexibility to

optimise them to achieve varied requirements. Based on the results obtained, a suitable curing

cycle for M40 grade concrete was specified.

2. EXPERIMENTAL INVESTIGATIONS

2.1. Materials

Ordinary Portland cement of 53 grade conforming to IS 12269 [16] was used for the study. Fine

aggregates and coarse aggregates conforming to IS 383 [20] were used. The nominal maximum

size of coarse aggregate used was 12.5 mm. The physical properties of both coarse and fine

aggregates have been given in Table 1. The gradation of the fine and coarse aggregates are

shown in Fig. 1. Potable water was used for mixing of concrete.

Fig 1: Particle size distribution of Aggregates

Table 1 Physical property of aggregates

Property Fine aggregate

Coarse Aggregate

Fineness Modulus 2.61 2.84

Water Absorption 2.82% 0.74%

Specific gravity 2.81 3.05

The M40 grade Concrete mixture was proportioned based on ACI 211 and the proportions are

given in Table 2.

Table 2 Mixture proportions of concrete

Ingredients Quantity (kg/m3)

Cement 566

Coarse aggregate 846

Fine Aggregate 638

Water 226

2.2. Experimental procedure

Initially, Penetration resistance tests were carried out as per IS 8142 [17] in concrete to

determine the setting time. The initial setting time was found to be 2 hours 30 minutes and final

setting time was 4 hours 30 minutes. The parameters varied in the steam curing cycle were as

follows:

2.2.1. Delay period:

Delay periods of 2 hours and 4 hours were chosen for investigation such that the effect of delay

period before and after initial setting time can be determined.

2.2.2. Curing temperature:

Based on the literature [14], the experimental investigations were carried out for temperatures of

50, 60 and 70ºC.

2.2.3. Curing period:

Based on the literature [14], it was decided to have a curing period (period during which the

temperature was sustained constant) of 6 hours and 8 hours, followed by a cooling period of 2

hours such that the total steam curing cycle was less than 18 hours.

These parameters were varied to perform 12 steam curing cycles which have been tabulated

below (Refer Table 3). A set of 3 cubes were tested at the immediately at the end of curing cycle.

The compressive strength obtained is referred to henceforth as the “early compressive strength”

and was obtained to understand the effect of steam curing on early (refers to 10 - 14 hours)

strength of concrete. After the steam curing cycle, they were exposed to air and tested at 7 and 28

days to estimate the effect of steam curing on ultimate compressive strength. These results were

compared with cubes subjected to water curing for compressive strength at corresponding time.

3. RESULTS AND DISCUSSIONS

The slump of the concrete of all 12 combinations as per IS 1199 [18] was observed to be 25 mm.

The average compressive strength of 3 cubes tested (as per IS 516 [19]) after steam curing cycle ,

at 7 and 28 days for the 12 curing cycles have been tabulated in Table 3 along with

corresponding strength. The average compressive strength of concrete cubes at 12 hours, 7 and

28 days by normal water curing was found to be 10.42 MPa, 38.44 MPa and 51.09 MPa

respectively. The influence of three parameters (i.e.) delay period, curing temperature and curing

period on compressive strength is discussed in this section.

Table 3 Effect of steam curing cycle on Compressive strength at different age

Cycle

Name

Curing

temperature

Delay

period

Curing

period

Early

str.(MPa)

7 day

str.(MPa)

28 day

str.(MPa)

Percentage of 28

day strength obtained as

early str.

APS 1 50ºC 2 hrs 6 hrs 16.01 35.25 43.27 37.00%

APS 2 50 ºC 4 hrs 6 hrs 19.15 37.67 45.95 41.68%

APS 3 50 ºC 2 hrs 8 hrs 20.75 41.52 53.29 38.94%

APS 4 50 ºC 4 hrs 8 hrs 23.67 43.13 48.98 48.33%

APS 5 60 ºC 2 hrs 6 hrs 21.64 41.46 49.77 43.48%

APS 6 60 ºC 4 hrs 6 hrs 21.72 42.64 56.12 38.70%

APS 7 60 ºC 2 hrs 8 hrs 22.95 42.92 54.65 41.99%

APS 8 60 ºC 4 hrs 8 hrs 28.77 46.37 58.05 49.56%

APS 9 70 ºC 2 hrs 6 hrs 22.71 40.03 50.39 45.07%

APS 10 70 ºC 4 hrs 6 hrs 23.15 41.91 49.44 46.82%

APS 11 70 ºC 2 hrs 8 hrs 22.57 40.16 46.91 48.11%

APS 12 70 ºC 4 hrs 8 hrs 25.67 41.56 48.16 53.30%

3.1. Influence of steam curing on compressive strength

From the results, it was observed that the steam curing cycles conducted during the experimental

investigations resulted in maximum increase of 176% at the early stage over the normal cured

concrete. The increase in early compressive strength for the steam cured concrete over the

normal cured concrete (measured after 12 hours) has been tabulated in table 4. With the progress

of time, the difference in compressive strength between steam cured concrete and normal cured

concrete was seen to reduce implying that a greater percentage of the 28 day compressive

strength was attained in case of steam curing after 0.5 days than in case of normal cured

concrete. The last column of Table 3 represents the percentage of 28 day strength the concrete

specimens obtain as early compressive strength when subjected to steam curing. For, normal

cured concrete the compressive strength attained after 12 hours was 20.39 % of the compressive

strength it achieves after 28 days. This is because the increase in strength obtained by steam

curing cycle over water curing decreases with the progress of time as indicated by last 3 columns

of Table 4. Researchers have attributed this trend to the presence of very fine cracks caused by

the expansion of air bubbles in the cement paste and the uneven distribution of hydrated products

which leads to areas of local weakness [5]. From Table 3, it is observed that the experiment APS

12 results in highest percentage (53.5%) of early strength gain over 28 day strength, however, the

early strength gain by APS 8 is maximum among the steam curing cycles performed. This could

be attributed to fact that APS 12 has relatively lower 28 day compressive strength increment.

Table 4:- Comparison between the compressive strength of concrete specimens subjected to

steam curing and normal curing at different ages

Cycle Name

Percentage increase in Early compressive

strength of steam cured concrete over 12 hour strength of normal cured concrete

Difference between compressive

strength of steam cured concrete and water cured concrete (MPa)

after 12

hours

after 7

days

after 28

days

APS 1 54% 5.59 -3.19* -7.82*

APS 2 84% 8.73 -0.77* -5.14*

APS 3 99% 10.33 3.08 2.20

APS 4 127% 13.25 4.69 -2.11*

APS 5 108% 11.22 3.02 -1.32*

APS 6 108% 11.30 4.20 5.03

APS 7 121% 12.53 4.48 3.56

APS 8 176% 18.35 7.93 6.96

APS 9 118% 12.29 1.59 -0.70*

APS 10 122% 12.73 3.47 -1.65*

APS 11 117% 12.15 1.72 -4.18*

APS 12 146% 15.25 3.12 -2.93*

* Negative sign signifies that the strength of steam cured concrete was relatively lower than

water cured concrete at corresponding age

3.2. Effect of curing temperature on compressive strength

In the experimental investigations, it was observed that initial strength was optimized at a

particular temperature for every curing period. For a curing period of 6 hours, the initial

compressive strength (after 0.5 days) was seen to increase up to 70ºC. On the other hand, for a

curing period of 8 hours, initial strength of concrete was seen to increase to an optimum value at

60ºC after which a drop in compressive strength was noticed on increasing the curing

temperature. The 28 day compressive strength results indicated that irrespective of the curing

period, the strength was observed to be maximum at 60ºC. This is in agreement with the report

[13] by researchers that the optimum maximum temperature of steam curing was near 60ºC with

higher temperatures of 70ºC and 80ºC showing a distinct reduction in strength after 28 days [22].

The variation of compressive strength with temperature for the same delay and curing period for

1, 7 and 28 days has been plotted below in Fig.2.

3.3. Effect of delay period in compressive strength

Experimental investigations indicate that higher delay period gives rise to a higher initial

strength of up to 25% (60 ºC, 8 hrs curing). With the progress of time, the difference between the

strength of concrete achieved through the two different delay periods was seen to decrease and

lie within 10% of each other. It was also observed that the advantage obtained by delaying the

cycle in terms of early compressive strength depends on the curing period and curing

temperature. At higher temperatures of 60ºC and 70ºC increase in early compressive strength

obtained by delaying the steam curing cycle for 4 hours against delaying it for 2 hours for a

curing period of 8 hours lies between 3 and 6 MPa.

2 hrs delay, 6 hrs steam curing 4 hrs delay, 6 hrs steam curing

2 hrs delay, 8 hrs steam curing 4 hrs delay, 8 hrs steam curing

Fig.2: Effect of Temperature on early and final compressive strength

6 hrs steam curing period 8 hrs steam curing period

Fig.3: Effect of delay period on early compressive strength

The corresponding increase for a 6 hours steam curing cycle was seen to be less than 0.5 MPa.

The observation could be attributed to the fact that a delay period is introduced to counter the

deleterious effects of rapid hydration and these effects become significantly more prominent

when the steam curing period was increased from 6 to 8 hours. With an increase in temperature,

the initial strength of concrete for the two delay periods converges to a common value. This may

attributed to the fact that a delay period allows for initial hydration and provides a barrier against

a sudden increase in hydration at higher temperatures and this effect is predominant at lower

temperatures where increase in rate of hydration is gradual than at higher temperatures. This

phenomenon has been shown above graphically in Fig.3.

3.4. Effect of curing period on initial compressive strength

Experimental investigations indicate that increasing the curing period up to an optimum limit

increased the initial compressive strength before the adverse effects set in. This optimum limit

was a function of the curing temperature. Up to a temperature of 60ºC, increasing the curing

period from 6 hours to 8 hours had a beneficial effect on the initial compressive strength.

However, at temperature of 70 ºC, increasing the curing period from 6 hours to 8 hours had a

detrimental effect (Refer table 4 also). This phenomenon has been represented graphically in Fig.

4.

2 hrs delay period 4 hrs delay period

Fig. 4: Effect of curing period on early compressive strength

3.5 Identification of parameter affecting the early compressive strength significantly:

Robust Design Engineering methodology [21] was used as a tool to identify the parameter which

influences the compressive strength significantly relative to the other parameters considered in

the study. The percentage contribution of each of these parameters to change in the early

compressive strength by varying these parameters is obtained by following method. From Table

3, the average of the early compressive strength of the steam cured samples was determined to be

22.39 MPa.

By taking factor effect of every variable:

1) fa1(50⁰C) = (16.01+ 20.75+ 19.15+ 23.67)/4 =19.895 MPa

Where fa1 represent the factor effect of temperature corresponding to 50⁰C and is calculated

as the average strength of cubes having steam curing temperature 50⁰C by varying other 2

parameters.

Similarly for 60⁰C and 70⁰C

fa2 (60⁰C) = 23.77 MPa

fa3 (70⁰C) =23.525 Mpa

2) fb1(2 hrs. for delay period) =(16.01+ 20.75+ 21.64+ 22.95+ 2.71+ 22.57)/6 = 21.105 MPa

Where fb1 represent the factor effect of delay period corresponding to 2 hrs. and is calculated as

the average strength of cubes having delay period 2 hrs. by varying other 2 parameters.

Similarly, fb2 (4 hrs. delay period) = 23.6883 MPa

3) fc1(6 hrs. steam curing period) =(16.01+ 19.15+ 21.64+ 21.72+ 22.71+ 23.15)/6 = 20.73

MPa

Where fc1 represent the factor effect of steam curing period corresponding to 6 hrs. and is

calculated as the average strength of cubes having steam curing period 6 hrs. by varying

other 2 parameters.

Similarly, fc2 (8 hrs. steam curing period) = 24.0633 MPa

By picking the highest factor effect value from each category gives best result at: (60⁰C, 4hrs.

delay period, and 8 hrs. steam curing period).

Now by considering difference between the factor effect for each parameter value and the

average strength of 12 cycles:

Table 5: Strength deviation with the calculated average early compressive strength

Name Difference

a1 fa1-µ 19.895-22.3967 -2.5017

a2 fa2-µ 23.77-22.3967 1.3733

a3 fa3-µ 23.525-22.3967 1.1283

b1 fb1-µ 21.105-22.3967 -1.2917

b2 fb2-µ 23.6883-22.3967 1.2916

c1 fc1-µ 20.73-22.3967 -1.6667

c2 fc2-µ 24.0633-22.3967 1.6667

By using ANOVA (analysis of variance) method:

A1= a1²+ a2²+ a3²= 9.4175

B1= b1²+ b2² = 3.3367

C1= c1²+ c2² = 5.5554

Here A1 is representative of the deviation in early compressive strength obtained by changing the

parameter value of temperature by 10⁰ C.

So, after all of these:

F ratio for temperature (% temperature effect):

(4*9.4175/(4*9.4175+6*3.3367+6*5.5554))*100=41.38%

Where F ratio is the weighted percentage of effect of temperature on compressive strength

Similarly for delay period: 22%

And for steam curing period: 36.62%

Based on the results obtained using the Robust Design Engineering Methodology, it is

understood that effect of temperature (41.38%) is significant when compared with delay period

(22 %) and curing period (36.62%).

4. CONCLUSIONS:

The results obtained from experimental studies can be summarized as follows:-

1) All concrete specimens subjected to Steam curing developed higher compressive strength

than those subjected to water curing at the age of 0.5 days.

2) Taking into consideration both initial and final compressive strength an optimum

temperature of 60ºC was observed.

3) For temperatures up to 60ºC increasing the curing period has a beneficial effect on the

initial compressive strength without any deleterious effects with the age.

4) Delay period of the steam curing cycle has a significant effect on initial compressive

strength at lower temperatures.

5) By using Robust Design Engineering methodology it has been found that the effect of

temperature is more significant in comparison of steam curing period and delay period.

Hence for the concrete mix designed a steam curing cycle having 4 hours delay period, 8 hours

curing period at a temperature of 60ºC is deemed optimum for this given combinations of

materials and grade of concrete. Considering the results, it is evident that durability part is

essential which will be done in the near future. Further, influence of Pozzolana on the steam

curing will also be evaluated with some microstructural studies.

6. ACKNOWLEDGEMENT: The Authors are thankful to Prof. Prakash R. Apte, Department

of Electrical Engineering, Indian Institute of Technology Bombay (IIT Bombay), Mumbai for

providing his inputs in terms of Robust Design Engineering Methodology.

7. REFERENCE:

1) Neville A. M. ( 1997), Properties of Concrete, Pitman publishing, London.

2) Hanson J. A. (1963), “Optimum steam curing procedure in pre-casting plants”, ACI J. Proc.

60, 75-100.

3) Ho DWS, Chua CW, Tam CT. (2003) “Steam-cured concrete incorporating mineral

admixtures” Cem. Concr. Res.33(4), 595–601.

4) Mironov S. A. (1964), “Some generalizations in theory and technology of acceleration of

concrete hardening”, Rilem International Conference on the Problems of Accelerated Hardening

of Concrete in Manufacturing Precast Reinforced Concrete Units, Moscow.

5) Alexanderson J. (1972), “Strength losses in heat cured concrete”, Swed. Cem. Concr. Res.

Inst. Proc. 43 (Stockholm).

6) Erdem T. K., Turanli L., Erdogan T. Y. (2004), “Setting time: An important criterion to

determine the length of the delay period before steam curing of concrete”, Cem. Concr. Res. 33,

741-745.

7) Oztekin E. (1984), “Determination of heat treatment cycle for cements”, Turkish cement

manufacturers’ association, Cem. Bull. 206 (3), 24-26.

8) Ergodu S., Kurbetci S. (1998), “Optimum heat treatment cycle for cements of different type

and composition”, Cem. Concr. Res. 28, 1595-1604.

9) Saul A.G.A (1951), “Principles underlying the steam curing of concrete at atmospheric

pressure”, Mag. Concr. Res. 2, 127-140.

10) Wei-Chong Liao, Lee B.J., Kang C.W. (2008), “A humidity-adjusted maturity function for

the early age strength prediction of concrete”, Cem. & Concr. Composites 30, 515–523.

11) Fu Y., Xie P., Gu P., Beaudoin J. J. (1994), “Significance of pre-existing cracks on

nucleation of secondary ettringite formation in steam cured cement paste”, Cem. Concr. Res. 24

(6), 1015-1024.

12) Seong-Tae Yi, Young-Ho Moon, Jin-Keun Kim (2005), “Long-term strength prediction of concrete with curing temperature”, Cem. and Concr. Res. 35, 1961 – 1969.

13) Yang QB, Yang QR, Zhu PR (2003) “Scaling and corrosion resistance of steam-cured

concrete”, Cem. Concr. Res. 33(7), 1057–61.

14) ACI 517.2 R-87 (1992), “Accelerated Curing of Concrete at Atmospheric Pressure- State of the Art”, ACI Manual of Concrete, Revised.

15) Dodson V. (1990), Concrete Admixtures, Van Nostrand Reinhold, New York.

16) IS: 12269- “Specification for 53 grade ordinary portland cement”, Bureau Indian Standards,

2004.

17) IS: 8142 – “Method of test for determining setting time of concrete by penetration resistance”, Bureau Indian Standards, 1976.

18) IS: 1199 – “Method of sampling and Analysis of concrete”, Bureau Indian Standards,1959.

19) IS: 516 – “Method of Tests for Strength of Concrete”, Bureau Indian Standards,1959.

20) IS: 383 - “Specification for Coarse and fine Aggregates from Natural sources for Concrete”, Bureau Indian Standards, 1970.

21) Phadke M.S. (2008), Quality Engineering using ROBUST design, Dorling Kindersley Pvt. Ltd., India.

22) Selcuk Turkel, Volkan Alabas (2005) “The effect of excessive steam curing on Portland

composite cement concrete”, Cem. Concr. Res. 35, 405–411.