Admixtures for Self-Consolidating Concrete

Self-consolidating concrete (SCC) is a highly flowable non-segregating mixture that can be placed under its own weight, i.e. with little or no vibration. The need for SCC has grown over recent years to meet designers’ specifications for more heavily reinforced concrete elements and more complex formworks. The SCC technology offers the following advantages:

1. Faster casting rates with no mechanical vibration and less screeding.
2. Improved and more uniform architectural smooth surface finishes, with little to no surface remedial works required.
3. Ease of filling sections congested with reinforcements, hard-to-reach areas, and complex shapes, with little or no vibration.
4. Opportunities to create structural and architectural shapes and surface-finishes not otherwise achievable with conventional concrete.
5. Improved pumpability and uniformity of in-place concrete achieved by eliminating variable operator-related efforts of consolidation.
6. Labor savings, shorter construction periods, and resulting overall cost savings.
7. Reduction or elimination of vibrator noise, which may potentially lead to an increase in construction hours in urban areas.
8. Minimized movement of ready-mixed trucks and pumps during placement, thus enabling the producer to service the project more efficiently.

Requirements for mix proportions

High deformability, high passing ability, and adequate resistance to segregation are required to achieve successful placement and performance of SCC.

The assessment of free or non-restricted deformability involves the evaluation of the capacity to deform, indicating how far the concrete can spread into the formwork, as well as the rate of deformability. When using the slump flow test, the deformation capacity refers to the final spread diameter, regardless of the flow velocity. Concrete that achieves high deformation capacity (or relatively low yield value) but spreads slowly (i.e. at moderate viscosity levels) may require some mechanical consolidation to ensure complete filling of the formwork. In order to secure good deformability, it is important to reduce the friction among solid particles. Aggregate inter-particle contact can decrease by reducing the aggregate content and increasing the paste volume required to maintain high passing ability among closely spaced obstacles. For the powder materials, the increase of inter-particle distance through an increase in water content can lead to segregation. A high-range water-reducing (HRWR) admixture is used to maintain a relatively low water-to-cementitious based materials ratio and reduce internal friction of solid particles.

The required level of passing ability is a function of structural detailing and formwork shape. SCC with high deformability but insufficient cohesiveness may not achieve complete passing ability to spread fully among closely spaced obstacles. Low cohesiveness can promote segregation and blockage. The blockage mechanism partly results from the collision and instantaneous contact among aggregate particles at the vicinity of an opening. Therefore, a lack of cohesiveness can lead to local aggregate separation and segregation that can block the flow of the concrete across obstacles. Passing ability can be compromised when the nominal aggregate size is large or the content of the coarse aggregate is high. Nevertheless, it should be mentioned that the segregation between aggregates and the cement paste, especially in the vicinity of reinforcements, could accentuate the risk of blockage as it leads to local increases in the concentration of large solid particles.

The third criterion for producing SCC is to provide high resistance to segregation after casting to ensure homogenous distribution of the in-situ quality of the hardened concrete. Adequate cohesiveness can be secured by incorporating a viscosity-modifying admixture (VMA), along with HRWR, to control bleeding, segregation, and surface settlement. Another way to enhance the cohesiveness of SCC is to reduce the free water content and/or increase the volume of fines and cement paste. Fine materials, such as supplementary cementitious materials and fillers of high surface area can adsorb greater content of water compared to cement particles and can reduce the free water content. Incorporating a VMA, which increases the capacity to retain free water and the viscosity of the suspended liquid phase, can control segregation between water and solid phases. Other categories of segregation, namely blocking and non- uniformity, can be reduced by having a paste phase capable of ensuring better suspension of solid particles in fresh concrete.

Holderchem SCC range of products

Holderchem offers a comprehensive range of high-range water reducing (HRWR) products and viscosity modifying agents to achieve SCC performance with superior flowability and rheology. Mix design property values vary depending on the ambient mix conditions and the quality of materials used. For indication purposes, we provide herein below a typical starting mix design formulation with indicative concrete properties:

Material	Dosage, kg/m3
Type I Portland cement	450
Batimix Silica Fume 710	36
Water	204
Water-to-cementitious materials ratio	0.42
Fine Aggregates	815
Coarse Aggregates	915
Batimix HWR 1500 (or HWR 1400)	5.8 (or, 12.5)
Batimix VMA 510	2.4

Concrete Properties
Air content	2.5%
Slump flow	600 to 725 mm
Setting time	12 to 15 hrs
3-day compressive strength	24 - 26 MPa
7-day compressive strength	38 - 41 MPa
28-day compressive strength	54 - 56 MPa

Specifications and Testing Methods

ACI 237R-07 Self-Consolidating Concrete specifications provides various known practices and processes for producing self-consolidating concrete.

To assess self-compacting properties, several standard tests have been used including the following:

1. ASTM C1610, “Standard Test Method for Static Segregation of Self-Consolidating Concrete Using Column Technique,” ASTM International, PO Box C700, West Conshohocken, PA, USA.
2. ASTM C1611, “Standard Test Method for Slump Flow of Self-Consolidating Concrete,” ASTM International, PO Box C700, West Conshohocken, PA, USA.
3. ASTM C1621, “Standard Test Method for Passing Ability of Self-Consolidating Concrete by J-Ring,” ASTM International, PO Box C700, West Conshohocken, PA, USA.