Overcoming slump flow loss in Self Compacting Concrete
Slump loss of concrete at construction sites is a common phenomenon associated with the formation of hydration products such as ettringite and the calcium silicate hydrate. A study was undertaken to observe the effectiveness of overdosing the chemical admixture which was used to provide suitable self compacting concrete (SCC) for different transportation times.
A critical aspect of SCC is the retention of flowability of the mix at the point of discharge at the jobsite. The reduction in flowability is frequently encountered in the ready-mix concrete owing to the long distance transport and delays associated with placement and subsequent operations at the job site. Overdosing the admixture amount in attaining the target slump at the job site or retempering with admixture instead of water are the preferred methods in remediation of the slump loss, since the use of extra water in retempering or in making a higher initial slump can induce side effects on the properties and serviceability of the hardened concrete (i.e. decrease in strength and durability, increase in permeability and drying shrinkage, etc.) 3,4. In the case of SCC, full capacity mixer truck load may not be feasible with very high flowability due to potential spillage. In such situations it is prudent to transport SCC at lower flowability and adjust the mixture with HRWR (High-Range Water Reducer) admixtures at the jobsite.
The study was intended to overcome the adverse effect on the delay in transportation times on freshly-mixed self-compacting concretes made by an overdosing method. The method consisted of using sufficient initial admixture dosages to obtain the target fresh properties of the trial mixtures at various transportation times. Five different transportation times, namely: 10, 30, 60, 90 and 120 minutes, were used. The slump flow and the passing abilities of the remediated SCCs were evaluated at the end of each transportation time, and compared to the equivalent fresh properties obtained at the control transportation time of 10 minutes.
The applied Ordinary Portland cement (similar to ASTM Type I) and the classified fly ash meet the requirements mentioned in IS:12269 (53 grade) and ASTM C618 (Class F) respectively. In addition metakaolin was also used as mineral additive satisfying the criteria of ASTM C 618.
Crushed granite with nominal grain size of 12mm and good quality well-graded river sand of maximum size 4.75mm were used as coarse and fine aggregates, respectively. The specific gravities of aggregates were determined experimentally. The coarse aggregates with 12 and 6mm fractions had specific gravities of 2.89, 2.87, whereas the fine aggregate had specific gravity of 2.65, respectively. HRWR used in this study was a novel, commercially available polycarboxylate ether (PCE) complying with the ASTM C 494 Type F requirements.
All the mixes were prepared with a constant water-to cementitious materials ratio of 0.30, a uniform cement content of 356kg/m3, fly ash content of 171kg/m3 and metakaolin content of 43kg/m3. In proportioning the aggregate contents, particular attention was given to the coarse-to-fine aggregate ratio due to its critical role in generating a sufficient amount of mortar for the selected self compacting concretes. The optimum volumetric coarse to-fine aggregate ratio, was found at 0.52/0.48. This ratio was obtained by combined aggregate grading as recommended by the DIN 1045 standard. The aggregates were combined in such a way, so that it meets nearly the combined grading specification of DIN ‘B’ curve. The different size fractions of coarse aggregates (12mm down graded and 6 mm downgraded) were taken in order to get a dense concrete.
The quantities of coarse and fine aggregates used in the mixtures were 901 (47% for 12mm & 5% for 6mm) and 765kg/m3, respectively. The optimum (minimum) dosages of HRWR used at the control transportation time of 10 minutes was 1% of the total cementitious content respectively. These dosages were obtained by evaluating the consistency and stability of concrete using different trial batches until a satisfactory slump flow of 650±25mm was attained.
“When SCC is sensibly utilized, the reduction of costs by better productivity, shorter construction time and improved working conditions compensate higher material costs. Moreover, in densely reinforced sections, such columns and beams in moment-resisting frames in seismic areas and in some repair sections, it is quite difficult to cast normal concrete and to ensure that all spaces in the formwork are filled with concrete. The use of SCC is necessary in such cases.”
Mixing, Sampling & Testing
Ready mix concrete plant central mixer was used with capacity of 1.5m3. The concrete was batched twice so as to make the whole capacity to 3m3. The truck mixer was then loaded to the minimum required capacity of 3m3 and kept aside to simulate the different delay in transportation times on the fresh SCCs. The initial mixing speed of the transit mixer (14.5rpm) was changed to an agitating speed of (7.25rpm) until the desired delay in transportation time was achieved. The concrete mixtures at the end of each transportation time were used to determine the slump flow and J-ring passing ability in accordance with the ASTM C 1611 and C 1621.
The delay in transportation time affected the fresh performance of self-consolidating concretes in the form of decrease in workability especially the slump flow. The dynamic stabilities of the fresh concretes remained unchanged.
The fundamental mechanism of slump flow loss of concrete during its transportation involves mainly:
- The additional fines brought to the concrete mortar by the grinding of aggregates and cement particles
- The growth of the cement hydration products
- The competitive adsorption between the superplasticizer and the sulfate ions (SO4 2- ) on the cement hydrated products throughout the transportation time.
Since the fluidity of concrete is mostly controlled by the fluidity of the mortar portion, the slump flow losses can be explained through the increase in specific surface area of concrete mortar and the change in the adsorption amount of chemical admixtures. In order to overcome the above mentioned adverse effects, an overdosing remediation method was used. This technique consisted of using sufficient initial admixtures amount to attain the target fresh properties at the end of the selected transportation times.
The percentage decrease in slump loss was used to determine the extra amount of HRWR required to overcome the slump losses of the SCCs. Similarly, the extra amount of HRWR required for the other transportation times were also evaluated. The extra dosage of HRWR over and above optimum in attaining the required workability increased as transportation time increased. It should be reminded that use of admixture could be logical as long as its effects are beneficial with respect to the fresh properties of SCC. Otherwise, particularly for economical aspects and possible adverse effects on concrete such as excessive amount of air entrainment, bleeding, and segregation; an engineering justification has to be made in terms of effective use of admixture.
Also, the selected self-compacting concretes did not require any viscosity modifying agent (VMA) neither in the control SCC nor in the remediated SCCs at different transportation times. The metakaolin used in the mixture served the purpose of VMA. The test results showed that all remediated self-compacting concretes were within the target slump flows±25mm, and J -Ring values between 0 and 50mm, indicating that the overdosing method was effective in obtaining the workability and passing ability similar to those of the control transportation time (10 minutes).
The fresh performance of self compacting concrete affected by the delay in the transportation times in the truck mixer manifested in the form of loss in flow ability and increase in the viscosity. These changes in the fresh properties were overcomed by way of admixtures overdosing which produced self-compacting concretes with similar fresh properties to those obtained at the control transportation time. The additional amount of admixtures generated supplementary repulsive electrostatic and steric hindrance forces between the cement particles to assist in dispersing the cement agglomerations generated by the grinding and hydration of cement particles during transportation times.Dr. P. Dinakar
School of Infrastructure
Indian Institute of Technology