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The Effect of Different Curing Conditions on Compressive Strength of Concrete.

Introduction

Concrete work is a major part of civil constructions. In order to obtain good quality concrete, properties of aggregate used, types of cement, environment of work execution, age of concrete as well as curing of concrete plays an important role. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability and resistance to freezing and thawing. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.

Portland cement consists of five major compounds i.e., tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite and gypsum. When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength. Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times.

Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved. The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. The evolution of heat is then dramatically increased. Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive.

The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well but these reactions do not contribute significantly to strength.

The durability of concrete structures is greatly influenced by curing of the concrete. Inadequate curing can result in a very weak and porous material near the surface of the concrete that is vulnerable to ingress of various harmful substances from the environment (Gowripalan, 1990). If concrete is to perform its unmitigated function over design life of structure, curing is necessity (Soroka et al., 1978). Curing involves procedures for controlling temperature during cement hydration and moisture movement from and into concrete during early stages of hardening (Hameed, 2009). In case of low water-cement ratio concrete, curing with excessive water in early ages is more beneficial than in case of high water-cement ratio (ACI, 1990). Drying before completion of curing shows larger reduction in compressive strength for high strength concrete than normal strength concrete (Carrasquillo et al., 1981). Method of curing also affects the hardened silica fume concrete. Steam curing enhance the properties of silica fume concrete whereas air curing exhibit adverse effects as compared to moist curing (Toutanji and Bayasi, 1999).

Temperature of curing water also affects the compressive strength of concrete specimens. Concrete samples cured at higher temperature than the normal temperature (25[degrees]C) of curing water shows an increase in compressive strength. This increase in compressive strength varies with the type of cement used (Cebeci, 1987). There is a possibility of dramatic reduction in strength at early ages if the curing conditions of ASTM C-31 are not followed. This situation might cause the rejection of acceptable concrete (Obla et al., 2005).

An investigation was carried out earlier to check the performance of slag, fly ash, and silica fume concretes under four different curing regimes. The water-cementitious materials ratio of all the concrete mixtures was kept constant at 0.50, except for the high-volume fly ash concrete mixture, for which the ratio was 0.35. The concrete specimens were subjected to moist curing, curing at room temperature after demoulding and curing at room temperature after two days of moist curing. The results indicate that the reduction in the moist-curing period results in lower strengths, higher porosity and more permeable concretes. The strength of the concretes containing fly ash or slag appears to be more sensitive to poor curing (Ramezanianpour, 1995).

The effects of curing conditions on properties of slag cement concrete have been studied by other researchers. Four different concrete mix designs with the same mix proportions and different cement replacements were used. The effects of slag replacement and curing conditions upon concrete properties were examined. The properties examined included mechanical properties (compressive and tensile strength), transport properties (chloride permeability and chloride penetration), and micro structural properties (pore structure and phase composition). There is little effect of slag replacement up to 50% upon strength, whereas higher replacement results in a drop in compressive strength. Steam curing reduces the compressive strength compared to the other curing types (Aldea et al., 2000). Poor curing conditions are more adversely effective on the strength of concretes made by pozzolanic cements than that of ordinary portland cement (OPC), and it is necessary to apply water curing to the former concretes at least for the initial 7 days to expose the pozzolanic activity. However, when the pozzolanic cement concretes have sufficient initial curing, they can reach the strength of OPC concretes in reasonable periods of time (Ozer and Ozkul, 2004).

Effect of curing method on the properties of plain and blended cement concretes was investigated. Concrete specimens were cured either by covering with wet burlap or by applying two types of curing compounds, namely water-based and acrylic-based. Results indicated that the strength development in the concrete specimens cured by covering with wet burlap was more than that in the specimens cured by applying water and acrylic-based curing compounds. Concrete specimens cured by applying curing compounds exhibited higher efficiency in decreasing plastic and drying shrinkage strain than specimens cured by covering with wet burlap. The performance of acrylic-based curing compound was better than that of water-based curing compound. Curing compounds could be utilized in situations where curing with water is difficult (Al-Gahtani, 2010).

In this research work, an attempt was made to analyse the effect of curing on compressive strength of concrete.

Materials and Methods

Materials used in this study include crushed rock and natural sand in addition to the drinking water, plasticizer and ordinary portland cement. Crushed aggregate was taken from locally available and widely distributed Sargodha quarry while natural sand belongs to easily accessible Lawrencepur area. For this purpose, 54 test specimens were casted and tested at the age of 3, 7, 10, 14, 21 and 28 days. These specimens were placed in different curing conditions for required age days to analyse the effect of curing on compressive strength.

Coarse aggregate. Crushed gravel from Sargodha quarry was used as coarse aggregate. Petrographic analysis was carried out according to the guidelines of ASTM C-295 (ASTM, 1998a). The aggregate was found to be non-deleterious having no Alkali Silica Reaction (ASR) potential. The major rock types identified through petrographic analysis are given below.
Dark grey meta-dolerite      78.7%
Greenish grey meta-dolerite  20.5%
Rhyolite                      0.8%


Other properties of coarse aggregate and respective ASTM Standards are given in Table 1.

Fine aggregate. Natural sand from Lawrencepur area was used as fine aggregate. The petrographic model analysis of fine aggregate is given in Table 2. The other properties of fine aggregate are given in Table 3.

Ordinary portland cement. Ordinary portland cement of grade 53 confirming to British Standard (BS, 2011) and Pakistan Standard (PS) 232-2008 was used in this study. The physical and chemical properties of cement are given in Table 4.

Water. Potable water confirming to the requirements of BS 3148 was used for both mixing and curing. The properties of water are given in Table 5.

Admixture. Naphtha-Plast G-808UL, high range water reducer complying with ASTM C-494 (ASTM, 1999a) was used in this study at the rate of 1% of cement to control the slump of concrete mix. It was obtained from PAGEL Pakistan (Pvt.) Ltd. The properties of the admixture are given in Table 6.

Curing compound. Rock Cure O2P water based concrete curing compound complying with ASTM C-309 (ASTM, 1999b), Type 2, Class A was used in this study.

Experimental work. The concrete mix was designed as per ACI (1991). The mix proportioning was adopted as cement: sand: coarse aggregate/water-cement ratio respectively 1: 1.5: 2.5/0.45. Total 54 test specimens of standard cylindrical shape (L =12 inch, D = 6 inch, L/D = 2) were prepared from this mix. These specimens were demoulded at the end of 24[+ or -]2 h and tested to analyse the effect of curing on the compressive strength of concrete.

To analyse the effect of curing on strength of concrete, 3 specimens were placed in open environment as experienced by the actual structure without curing, 3 were placed in water at 23 [+ or -] 2[degrees]C for conventional curing according to ASTM C-31 (ASTM, 1998b) and 3 were cured with curing compound for each age days of 3, 7, 10, 14, 21 and 28 days.

Results and Discussion

After the completion of age days, compressive strength testing was carried out by1500 KN capacity compression machine according to the guidelines of ASTM C-39 (ASTM, 1999c). Each result of compressive strength obtained was the average of three specimens. The results of compressive strength testing are shown in Table 7. The graphical representation of strength in different curing conditions for different age days is shown in Fig. 1.

For nondestructive testing of concrete samples, Schmidt rebound hammer testing was applied according to the guidelines of ASTM C-805 (ASTM, 1997). Average of ten readings was taken for each sample in different curing conditions for 3, 7, 10, 14, 21 and 28 age days. The compressive strength of concrete can be estimated from correlation chart provided with the instrument.

This correlation chart has rebound number on horizontal axis and compressive strength on vertical axis. The inner part of chart consists of different correlation curves with respect to the angle of hammer impact on the surface of sample. The results of Schmidt rebound hammer testing is shown in Table 8. The graphical representation of the same is shown in Fig. 2.

From Fig. 1, it is clearly shown that the compressive strength of concrete samples cured with water according to the guidelines of ASTM C-31 (ASTM, 1998) is relatively high at all age days as compared to the other samples. The compressive strength of concrete samples without any type of curing placed in open environment as the structure is relatively low for all age days. Up to the age of 10 days, concrete samples cured with compound shows higher strengths as compared to the samples placed in open environment. After 10 days, the difference between strength of concrete samples without curing and with curing compound becomes relatively low which indicates that the curing compound remains effective in early days of hardening.

In non-destructive testing, Schmidt rebound hammer testing, the results are not effective. The Schmidt rebound hammer number for samples cured with water at 7 days is less than the 3 days. At the age of 10 days, Schmidt rebound hammer number for samples without curing is high as compared to the samples cured with curing compound. Similar behaviour of Schmidt rebound hammer number is for the age of 28 days. At the age of 14 days, Schmidt rebound hammer number for samples without curing and for samples with curing compound is equal. At the age of 21 days, Schmidt rebound hammer number for samples cured with curing compound and samples cured with water shows a little difference.

Conclusion

From the above discussion, it is concluded that curing plays an important role in gaining strength of civil structures, especially the water curing. Curing with curing compound remains effective in early days of hardening. After the early hardening, the effect of curing compound becomes less effective, while the water curing plays an important role in later stages of hardening. Concrete samples cured with water always shows higher strengths as compared to the samples without curing and samples cured with curing compound. The abnormalities in Schmidt rebound hammer testing might be due to the fact that Schmidt hammer testing gives only surface hardness. Where the plunger of the Schmidt hammer strikes the surface of an aggregate, a higher number achieved without any effect of curing. If the plunger of Schmidt hammer strikes a surface formed with paste, a lower number will be achieved. Schmidt rebound hammer testing is less reliable in estimating the compressive strength of concrete samples. From the above discussion and the results obtained, conventional water curing as ASTM C-31 is strongly recommended for concrete structures. Moreover, there is need to explore the effect of daily change in temperature and humidity on the strength of concrete structures.

Acknowledgement

The authors heartedly acknowledges the cooperation of Mr. Muhammad Zaki, Resident Engineer, National Engineering Services of Pakistan (Pvt.) Ltd., who provided his time and space for the completion of this work. The assistance of Mrs. Maria Yaqub, Assistant Director, Geological Survey of Pakistan is also appreciated for provision of required samples for petrographic analysis.

References

ACI, 1991. Standard Practice for Selecting Proportions for Normal, Heavy weight and Mass Concrete. American Concrete Institute Committee, 211.1.

ACI, 1990. State of the Art Report on High Strength Conerete. ACI Manual of conerete Prectice. Part 1, 48 pp., ACI Committee363, American Conerete Institute, Detroit, MI, USA.

Aldea, C. M., Young, F., Wang, K., Shah, S. P. 2000. Effects of curing conditions on properties of concrete using slag replacement. Cement and Concrete Research, 30: 465-472.

Al-Gahtani, A. S. 2010. Effect of curing methods on the properties of plain and blended cement concretes. Construction and Building Materials, 24: 308-314.

ASTM, 1999a. Standard Specification for Chemical Admixtures for Concrete. American Society for Testing and Materials Standard, C 494, 04.02.

ASTM, 1999b. Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete. American Society for Testing and Materials Standard, C 309, 04.02.

ASTM, 1999c. Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. American Society for Testing and Materials Standard, C 39, 04.02.

ASTM, 1998a. Standard Guide for Petrographic Examin-ation of Aggregates for Concrete. American Society for Testing and Materials Standard, C-295, 04.02.

ASTM, 1998b. Standard Practice for Making and Curing Concrete Test Specimens in the Field. American Society for Testing and Materials Standard, C31, 4.01.

ASTM, 1997. Standard Test Method for Rebound Number of Hardened Concrete. American Society for Testing and Materials Standard, C 805, 04.02.

BS, 2011. Composition, Specifications and Conformity Criteria for Common Cements. British Standards Institution, EN 197-1.

Carrasquillo, R. L., Nilson, A. H., Slate, F.O.1981. Properties of high strength concrete subject to short term loads. Proceedings of Journal of American Concrete Institute, pp. 171-178.

Cebeci, O. Z. 1987. Strength of concrete in warm and dry environment. Materials and Structures, 20: 270-272.

Gowripalan, N. 1990. Effect of curing on durability. Concrete International, 12: 47-54.

Hameed, A. 2009. The effect of curing condition on compressive strength in high strength concrete. Diyala Journal of Engineering Sciences, 2: 35-48.

Obla, K., Rodriguez, F., Barka, S. 2005. Effects of Non-Standard curing on Strength of Concrete. A Research Project at the NRMCA Research Laboratory, pp. 57-59.

Ozer, B., Ozkul, M. H. 2004. The influence of initial water curing on the strength development of ordinary portland and pozzolanic cement concretes, Cement and Concrete Research, 34: 13-18.

Ramezanianpour, A. A. 1995. Effect of curing on the compressive strength, resistance to chloride-ion penetration and porosity of concretes incorporating slag, fly ash or silica fume. Cement and Concrete Composites, 17: 125-133.

Soroka, I., Jaegermann, C. H., Bentur, A. 1978. Short-term steam-curing and concrete later-age strength. Materials and Structures, 11: 93-96.

Toutanji, H. A., Bayasi, Z. 1999. Effect of curing procedures on properties of silica fume concrete. Cement and Concrete Research, 29: 497-501.

Muhammad Arslan (a*), Muhammad Asif Saleem (b), Maria Yaqub (c) and Muhammad Saleem Khan (d)

(a) National Engineering Services of Pakistan (Pvt) Ltd, Lahore, Pakistan

(b) National Logistics Cell, Lahore, Pakistan

(c) Geological Survey of Pakistan, Lahore, Pakistan

(d) University of Engineering and Technology, Lahore, Pakistan

(*) Author for correspondence; E-mail: arslan_uos@yahoo.com

(received May 05, 2016; revised June 23, 2017; accepted July 07, 2017)
Table 1. Properties of coarse aggregate from Sargodha Quarry

Test                               Result  Standard

Sodium sulphate soundness          0.43%    ASTM C-88
Clay lumps and friable particles   0.66%   ASTM C-142
Los angeles abrasion              15%      ASTM C-131
Specific gravity                   2.834   ASTM C-127
Water absorption                   0.45%   ASTM C-127

Table 2. Petrographic analysis of fine aggregate from Lawrancepur Quarry

Properties  (%)                        Properties (%)

Quartz      38.4  Carbonate            9.4
Amphibole    8.1  Quartz mica Schist   7.8
Granite      7.1  Biotite              6.3
Feldspar     4.9  Epidote              2.1
Magnetite    1.9  Muscovite            1.1
Chert        0.5  Lithic arenite       0.3

Table 3. Properties of fine aggregate from Lawrancepur Quarry

Test                        Result  Standard

Sodium sulphate soundness   0.30%     ASTM C-88
Organic impurities          Nil       ASTM C-40
Sand equivalent            85%      ASTM D-2419
Specific gravity            2.72     ASTM C-128
Water absorption            1.00%    ASTM C-128

Table 4. Physical and chemical properties of OPC

Test                         Result

Physical Properties
  Standard consistency(%)      25
  Initial setting time(min)   135
  Final setting time(min)     185
  Fineness(C[m.sup.2]/g)     3140
  Autoclave expansion(%)        0.127
Chemical Properties
  Los on Ignition(%)            2.12
  Insoluble residue(%)          0.74
  S[O.sup.3](%)                 2.61
  MgO(%)                        1.67

Table 5. Properties of water used for mixing and curing

Parameter                                  Value found

pH                                           6.7
Total dissolved solids(ppm)                  -
Conductivity at 25[degrees]C([micro]s/cm)  515.5
Chlorides as [Cl.sup.-1](ppm)               24.5
Sulphate(ppm)                               59.9
Magnesium(mg/L)                             14

Table 6. Properties of admixture

Parameter                              Value found

pH                                      4.79
Solid contents(%)                      34.63
Specific gravity at 25[degrees]C        1.163
Chlorides as [Cl.sup.-1](%)             0.08
Density at 25[degrees]C(g/[cm.sup.3])   1.159
Air content(%)                         <1

Table 7. The results of compressive strength testing in different
curing conditions

Condition of specimen              Sample  Dial reading
                                   No.     (KN)


                                     3 Days compressive
                                     strength of concrete samples
Test samples without curing         1      280
                                    2      298
                                    3      287
Test samples with curing compound   4      299
                                    5      307
                                    6      310
Test samples cured in water         7      366
                                    8      371
                                    9      358
                                     7 Days compressive strength
                                     of concrete samples
Test samples without curing        10      350
                                   11      363
                                   12      367
Test samples with curing compound  13      393
                                   14      389
                                   15      403
Test samples cured in water        16      427
                                   17      439
                                   18      442
                                     10 Days compressive strength
                                     of concrete samples
Test samples without curing        19      401
                                   20      412
                                   21      406
Test samples with curing compound  22      421
                                   23      417
                                   24      423
Test samples cured in water        25      485
                                   26      471
                                   27      479
                                     14 Days compressive strength
                                     of concrete samples
                                   28      452
                                   29      457
                                   30      443
Test samples with curing compound  31      463
                                   32      458
                                   33      453
Test samples cured in water        34      537
                                   35      543
                                   36      526
                                     21 Days compressive strength
                                     of concrete samples
Test samples without curing        37      487
                                   38      492
                                   39      500
Test samples with curing compound  40      497
                                   41      506
                                   42      511
Test samples cured in water        43      551
                                   44      557
                                   45      565
                                     28 Days compressive strength
                                     of concrete samples
Test samples without curing        46      537
                                   47      528
                                   48      521
Test samples with curing compound  49      531
                                   50      547
                                   51      539
Test samples cured in water        52      587
                                   53      580
                                   54      571

Condition of specimen              Load   Surface area
                                   (kg)   ([cm.sup.2])


                                     3 Days compressive
                                     strength of concrete samples
Test samples without curing        28552  182.4
                                   30387  ?
                                   29266  ?
Test samples with curing compound  30489  ?
                                   31305  ?
                                   31611  ?
Test samples cured in water        37321  ?
                                   37831  ?
                                   36506  ?
                                     7 Days compressive strength
                                     of concrete samples
Test samples without curing        35599  182.4
                                   36921  ?
                                   37328  ?
Test samples with curing compound  39972  ?
                                   39565  ?
                                   40989  ?
Test samples cured in water        43430  ?
                                   44651  ?
                                   44956  ?
                                     10 Days compressive strength
                                     of concrete samples
Test samples without curing        40890  182.4
                                   42012  ?
                                   41400  ?
Test samples with curing compound  42930  ?
                                   42522  ?
                                   43134  ?
Test samples cured in water        49456  ?
                                   48028  ?
                                   48844  ?
                                     14 Days compressive strength
                                     of concrete samples
                                   46091  182.4
                                   46601  ?
                                   45173  ?
Test samples with curing compound  47213  ?
                                   46703  ?
                                   46193  ?
Test samples cured in water        54758  ?
                                   55370  ?
                                   53637  ?
                                     21 Days compressive strength
                                     of concrete samples
Test samples without curing        49660  182.4
                                   50170  ?
                                   50986  ?
Test samples with curing compound  50680  ?
                                   51597  ?
                                   52107  ?
Test samples cured in water        56186  ?
                                   56798  ?
                                   57614  ?
                                     28 Days compressive strength
                                     of concrete samples
Test samples without curing        54758  182.4
                                   53841  ?
                                   53127  ?
Test samples with curing compound  54147  ?
                                   55778  ?
                                   54962  ?
Test samples cured in water        59857  ?
                                   59143  ?
                                   58225  ?

Condition of specimen              Compressive       Average
                                   strength
                                   (kg/[cm.sup.2])

                                     3 Days compressive
                                     strength of concrete samples
Test samples without curing        157               161
                                   167
                                   160
Test samples with curing compound  167               171
                                   172
                                   173
Test samples cured in water        205               204
                                   207
                                   200
                                     7 Days compressive strength
                                     of concrete samples
Test samples without curing        195               201
                                   202
                                   205
Test samples with curing compound  219               220
                                   217
                                   225
Test samples cured in water        238               243
                                   245
                                   246
                                     10 Days compressive strength
                                     of concrete samples
Test samples without curing        224               227
                                   230
                                   227
Test samples with curing compound  235               235
                                   233
                                   236
Test samples cured in water        271               267
                                   263
                                   268
                                     14 Days compressive strength
                                     of concrete samples
                                   253               252
                                   255
                                   248
Test samples with curing compound  259               256
                                   256
                                   253
Test samples cured in water        300               299
                                   304
                                   294
                                     21 Days compressive strength
                                     of concrete samples
Test samples without curing        272               276
                                   275
                                   280
Test samples with curing compound  278               282
                                   283
                                   286
Test samples cured in water        308               312
                                   311
                                   316
                                     28 Days compressive strength
                                     of concrete samples
Test samples without curing        300               296
                                   295
                                   291
Test samples with curing compound  297               301
                                   306
                                   301
Test samples cured in water        328               324
                                   324
                                   319

Table 8. Schmidt rebound hammer number for concrete samples in
different curing conditions for different age days

Condition of specimen                       Age
                                   3 days  7 days  10 days

Test samples without curing        12      15      20
Test samples with curing compound  14      16      19
Test samples cured with water      19      18      29

Condition of specimen                       Age
                                   14 days  21 days  28 days

Test samples without curing        21       25       28
Test samples with curing compound  21       32       27
Test samples cured with water      30       35       40
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Author:Arslan, Muhammad; Saleem, Muhammad Asif; Yaqub, Maria; Khan, Muhammad Saleem
Publication:Pakistan Journal of Scientific and Industrial Research Series A: Physical Sciences
Date:Sep 1, 2017
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