Influence of mixing water on the setting time of concrete

The International Journal of Cement Composites and Lightweight Concrete, Volume 3, Number 1

February 1981

I n f l u e n c e of mixing w a t e r on the setting t i m e of c o n c r e t e

SYNOPSIS This paper reports a study of the setting time of concrete as a function of mixing water and the results are presented diagrammatically. To establish the setting time, a set of experiments, in which four concrete mixes were used, were designed, the only difference being in the quantity of the mixing water in each mix. The object was to determine the start and the end of the setting time for each mix.

G. D. Stefanou* and Ch. Larsinos

KI=YWORD8 Cements, setting time, testing, concretes, penetration tests, aggregates, portland cements, water-cement ratios. INTRODUCTION The setting of concrete depends directly on the setting of cement, which is a complex physical and chemical phenomenon. During setting a gradual lessening of the gel plasticity is observed and a continual increase of the mechanical stress which finally reaches reasonable values. Gradually, the gel integrally hardens. The investigation of this change in favourable conditions of ambient temperature aqd humidity takes many years. The structure which is formed under these conditions as well as the mechanical resistance are the result of bond forces between the separate products formed from the cement and the cement grains. The bond helps in the formation of one hard monolithic body. The complex development of bond necessitates a careful study of the setting time. The quantity of mixing water, which is one of the factors governing the setting time of concrete, is examined in this study. EXPERIM ENTAl. W O R K The experimental work performed in this study was carried out according to ASTM specifications C-403-70. Ten tonnes of aggregates were taken from Larsinos Co. quarry. From this quantity, representative specimens were taken by quartering, and particle size analysis was performed. The grading of aggregate is shown in Figure 1.

Table 1 Water analysis

Solvent

*University of Patras, Patras, Greece. © The Construction Press Ltd., 1981

1. pH (18°C) Electrical resistance 2. Salts 3. Alkalinity (CaCo3) 4. Chlorides (CI) 5. Chlorides (NaCI) 6. Theosulphate (SO4) 7. Theosulphates (Na2SO4) 8. Organics 9. Grease and sugar

Quantity 7.6 914.00/~.mhos/cm 595.67 mg/I 297.50mg/I 67.02mg/I 110.49mg/I 12.00mg/I 17.75 mg/I Negative Negative

0142-095X/81/03160045/$02.00

45

Stefanou and Larsinos

Influence of mixing water on the setting time of concrete

G R A I N 'Standard curves ~Aggregate

curves

S I Z E

for a g g r e g a t e s

D I A G R A M

( M i n i s t y of P u b l i c W o r k s R e s e a r c h C e n t e r )

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100

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Figure 1 Grain size

The materials used for the experiments are: aggregate from Larsinos Co. quarry, portland cement Greek type plain TITAN, and drinkable water. The water and cement analysis are given in Tables 1 and 2 respectively, Table 3 shows all the conditions under which the experiment was performed. Four mix designs were made. In all four mixes all quantities were kept constant except for water, which was gradually increased in steps of 15kg except in the last mix, where it was increased by 20kg. The aggregates were dry and subjected to the same treatment.

EXPERIMENTAL RESULTS The results obtained from the experimental work are presented diagrammatically in Figures 2 and 3, in which the start and the end of setting for each mix is plotted against the resistance to penetration, according to ASTM C-403-70.

46

DISCUSSION AND CONCLUSIONS The diagrams show that increase in water causes lengthening of the setting time. Also, it is shown that for the start and for the end of the setting there is a constant difference of 20-30 rain for every 15 kg of water added. As the quantity of mixing water increased, the differences in setting time also reduced. Table 2 Cement analysis Element 1. 2. 3, 4. 5. 6. 7. 8.

SiQ 2 AI2Q s Fe203 CaO MgO SO3 Na2Q K20

Quantity (%) 18.31 4.65 3.47 55.00 3.06 2.32 2.68 1,56

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PENETRATION

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Figure 3

Influence of mixing water on the setting time of concrete

SETTING TIME DELAY BY WATER INCREASE i

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MATERIALS

USED

Stefanou and Larsinos

CAUSED

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CONDITIONS:

t,-17"

C

tx=

~Concrete temperature , tx=ambient temperature

48

gl" C

Sv=7Oql&

Sv=Relatlve humidity

-/u

Table 1 Conditions under which the experiment was performed