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Production and placement of self-compacting concrete
Production and placement of self-compacting concrete
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Alireza Kashania,b, Tuan Ngoa a Department of Infrastructure Engineering, University of Melbourne, Melbourne, VIC, Australia, bSchool of Civil and Environmental Engineering, University of New South Wales, Kensington, NSW, Australia
3.1
Production and quality control at batching plant
Production procedures in terms of raw materials storage and equipment are often similar between self-compacting concrete and traditional concrete. However, the mixture proportions and the methods of mixing, quality control, testing, delivery, placement and finishing can be very different. In terms of the mixture design, self-compacting concrete (SCC) normally has higher paste to aggregate and lower water to cement ratios compared to traditional concrete (Shi et al., 2015). On the other hand, SCC usually contains supplementary cementitious materials (SCM) such as fly ash and ground granulated blast furnace slag, and admixtures such as high-range water reducers (superplasticizers) and viscosity modifying agents. The enhanced rheological properties of SCC result in easily flowable concrete (as shown in Fig. 3.1), which can provide many benefits. For instance, SCC can quickly fill the gaps between congested reinforcements, and it substantially reduces the tedious surface levelling requirement of poured concrete. These unique properties are governed by an advanced mixture design and accurate quality control during production and placement. Therefore, the number of workers at the construction site is reduced. SCC is less forgiving in terms of the variability of its properties and proportion of constituents compared to traditional concrete. Very accurate raw material testing and batching are required for SCC mixtures during production at concrete plants, and any changes and adjustment at worksites should be limited. The effect of mixture design on fresh and hardened properties of self-compacting concrete was explained in Chapter 1. This section briefly explains methods of adding chemical admixtures and controlling the moisture content of aggregates, as well as mixing requirements and other quality control measures during the production of SCC.
3.1.1 Chemical admixtures Chemical admixtures such as high-range water reducers or superplasticizers, viscosity modifying agents, shrinkage reducing and air-entraining admixtures, accelerators, retardants and mineral admixtures have been used in SCC (Şahmaran et al., 2006; Self-Compacting Concrete: Materials, Properties, and Applications. https://doi.org/10.1016/B978-0-12-817369-5.00003-9 Copyright © 2020 Elsevier Inc. All rights reserved.
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Fig. 3.1 Smooth flow and self-levelling of SCC during casting. Courtesy of Ioannis Sfikas of Mott MacDonald.
Huang et al., 2018; Yu et al., 2014). Among the chemical admixtures, superplasticizers are essential ingredients for controlling the flowability or rheology of SCC. Polycarboxylate-based (PC) superplasticizers are the most common water reducing agents in SCC. Many different commercial grades of PC with different functionalities and polymer structures have been tailor-designed for SCC. The chemical structure of PC influences the workability retention and rheological performance of SCC (Felekog˘lu and Sarikahya, 2008). Superplasticizers can also change the rheology of SCC at varying temperatures (Schmidt et al., 2014). The quantity of chemical admixtures is often very low (<1 wt% of the total mass of concrete), but the increased performance at such a low concentration is phenomenal. Chemical admixtures are the most expensive ingredients for self-compacting concrete. Therefore, finding the minimum required chemical admixture for achieving specific properties is a common practice for concrete suppliers to produce cost-effective SCC. Small quantities of admixtures are often added to the concrete mixer manually whereas other ingredients such as cement and aggregates are normally transferred to the mixer via a conveyer belt. Chemical admixtures can also be dissolved in water and then pumped to the concrete mixer which can result in a more homogenous distribution of admixtures in concrete and a better performance. However, it is recommended that some admixtures are
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not to be added simultaneously. For instance, it is beneficial to add an accelerator before the addition of a superplasticizer to enhance strength development. Also, some admixtures are not effective when they are used together. For instance, using a calcium chloride-based accelerator in a concrete mixture, which has a sulphonate-based plasticizer and an air-entraining admixture, can increase autogenous shrinkage (Shanahan et al., 2016). It must be noted that the concrete mixture becomes saturated with admixtures at a certain ratio, meaning that adding more amounts of admixtures will not further enhance the properties of concrete. For instance, the adsorption isotherm of most superplasticizers on cement particles will be equilibrated at a certain concentration. Hence, no further adsorption (and plasticizing effect) will occur by increasing the concentration of superplasticizers (Yoshioka et al., 2002). The plasticising effect of water-reducing admixtures can be affected by the presence of supplementary cementitious materials (SCM) such as slag, resulting in a lower adsorption rate (Alonso et al., 2013). Also, the maximum benefits from the chemical admixtures for SCC are achieved by well-diluting and well-distributing them within the mixture. Sufficient mixing time and the dilution of admixtures within the water before mixing with concrete concrete mixture can help to achieve better performance at lower concentrations, thereby reducing costs associated with the use of expensive admixtures. Moreover, highly agitated mixing after adding the chemical admixture can result in excessive air entrainment, which can adversely affect the surface finish of SCC by the creation of bugholes during placement.
3.1.2 Moisture content Self-compacting concrete is highly susceptible to a small fluctuation of water content in the mixture design than traditional concrete. The changes in the moisture content of aggregates, especially in fine aggregates, can considerably affect the rheology, strength and durability of SCC (Shen et al., 2015; Poon et al., 2004). Excess water in an SCC mixture can cause bleeding and segregation. On the other hand, very dry aggregates can absorb water from the mixture and adversely affect the flowability of concrete. Therefore, controlling the moisture content of aggregates and an adequate method for the correction of water content are very important in SCC production. A study by Ortiz et al. (2009) showed the effects of the storage conditions for aggregates, i.e. the effects of the environmental temperature and the moisture content of aggregates on the workability of concrete. The moisture content of aggregates (especially if stored outside) must be monitored and controlled to produce highperformance SCC. In a hot and dry condition, entrapped water in aggregates evaporates quickly, thereby increasing the water absorption of aggregates. After the addition of aggregates to a mixer, a portion of water from the mixture will be absorbed by the aggregates. This loss of water, although it can be minimal, reduces the flowability of concrete (Poon et al., 2004). On the other hand, under high humidity and rainy conditions, aggregates are saturated with water so any excess water in the mixture can cause segregation and bleeding, as well as reductions in strength and durability. Segregation and reduction in the robustness of SCC are caused by a small increase in water content that comes from aggregates with saturated moisture content (Shen et al., 2015).
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Therefore, it is a routine quality control practice (especially for SCC) to measure the moisture content of aggregates daily or before production. Automatic moisture content measurements by sensors have also been used in some concrete batching plants. However, these sensors must be checked for accuracy and should be calibrated regularly. Ding and An (2017) developed a real-time method for estimating the aggregate moisture content of SCC mixtures, which can guide batch weight adjustment. In this method, the estimated moisture content, as determined by slump flow, shows the excess or lack of moisture in comparison with the moisture content in the reference mixture design. It must be noted that excess water can result from washing batching equipment or the truck-mixer drum, which can affect the properties of SCC. Therefore, it is recommended to include any water either from the aggregate moisture content, washing and so on in the final mixture design. The tolerance for changes in water content for SCC is considerably lower than that of normal concrete. Therefore, these quality control measures are essential to produce high-quality and durable SCC. Controlling the moisture content and water absorption of aggregates in the case of using recycled aggregates is more critical as these aggregates tend to absorb more water. Hence, the required flowability, strength and cracking-resistibility can be compromised (Poon et al., 2004; Ji et al., 2013; de Oliveira and Vazquez, 1996).
3.1.3 Concrete mixers The types of mixers, the sequence of adding mixture components and mixing time can affect the homogeneity and uniformity of SCC (Hemalatha et al., 2015). Selfcompacting concrete has lower yield stress and higher plastic viscosity compared to traditional concrete (Petit et al., 2007). Therefore, a very efficient mixer with an adequate mixing time is required to achieve high-quality SCC that can satisfy any standard requirements. It must be noted that concrete mixtures for SCC production are usually comprised of a higher amount of cement and fines which can adversely affect mixing quality compared to the mixtures with a higher portion of aggregates. One common problem stems from adhering and hardening a stiff paste of cement and fines to the drum or blade of a mixer, which can reduce the quality of SCC and affect the preparation of subsequent mixtures in the same mixer. Therefore, it is recommended that larger aggregates with a portion of water and superplasticizer are added to the drum before the addition of other mixture constituents to prevent the paste from sticking to the drum or blades. The mixing time is also critical and can increase the performance of SCC, which is attributed to a higher amount of fine materials in SCC that require more time and energy to be dispersed homogeneously in the mixture. A study by Ngo et al. (2016) showed that monitoring the mixing power evolution can be used as a tool to determine the amount of water and chemical admixtures needed for controlling the rheology, flowability and self-compactness of concrete in ready-mixed concrete plants. The particle size distribution and type of mixture components (e.g. the addition of supplementary cementitious materials) can also affect the rheology (yield stress and viscosity) of concrete (Kashani et al., 2014, 2019). Mixtures with a higher yield stress and viscosity (less flowable) require higher energy for mixing.
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Free-fall mixers, although typical for traditional concrete, may not provide enough energy and agitation for effectively mixing of SCC. As a result, some specifications do not allow the use of free-fall mixers to produce SCC. Mixers that can provide a higher shear rate to concrete by well-arranged blades and higher power can produce concrete with a very low yield stress, which is more suitable to achieve self-compacting properties (Hemalatha et al., 2015). Forced action mixers which blades and paddles that rotate for mixing are more effective in the production of a uniform SCC mixture over a shorter period (de Schutter et al., 2008). Truck-mixers, although useful for mixing, are not the most efficient method for preparing SCC. The added time of mixing at the back of the truck after production in concrete plants needs to be considered to optimize the total mixing time before placement. It must be noted that the flow properties of SCC can be adversely affected if there is a large gap between the production and placement of concrete.
3.1.4 Quality control at concrete batching plant A consistent and guaranteed availability of aggregates and other concrete constituents must be taken into consideration to produce SCC because a series of trials for mixture development, testing and optimization must take place to achieve the self-compacting properties. The introduction of a new concrete constituent with different properties often means that the mixture design procedure should be repeated. Therefore, it is vitally important to secure a consistent supply chain of materials for SCC production. The rheology and flow properties of concrete must be checked to avoid any loss of mechanical properties and durability and to prevent an improper surface finish. For instance, low viscosity and a high amount of water can cause bleeding, and subsequently cause segregation and surface imperfections such as scaling (de Schutter et al., 2008). An adequate flow rate is necessary for the filling capability of SCC. However, a very high flow rate induces additional pressure on formwork that may cause movement and displacement during the discharge of SCC. Both fresh and hardened properties of concrete must be checked and compared against specific standards of SCC based on the region and type of application. It must be noted that concrete properties achieved in smaller batches in the laboratory can be different from concrete batches prepared in larger mixers. The main reason is the difference in quality and time of mixing, as well as the type of mixer. Therefore, the amount of water and admixtures need to be revisited in upscaling the mixture design from the laboratory scale to a production plant. Barluenga et al. (2017) developed and tested a series of methods that permit real-time measurement during pumping namely, early age and hardened stages of SCC for quality control purposes. They measured the real-time pumping pressure and discharge rate and conducted non-destructive testing (NDT) on freshly casted concrete, such as ultrasonic pulse velocity (UPV), to assess the properties of SCC, which were compared with the results obtained from the laboratory. They found that pumpability can be affected by the discharge rate rather than the slump flow. Pumping resulted in up to 16% reduction in the compressive strength of SCC due to some level of air entrainment. Prolonged mixing during production can cause excessive air entrainment (especially for the mixtures with high amounts
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of admixtures), which subsequently results in bugholes on the surface of concrete that endanger the aesthetics and durability of concrete. The standard tests for SCC were discussed in Chapter 1. The required values in terms of flowability and mechanical properties can be different among many regional standards, guidelines and specifications. As discussed above, the moisture content of aggregates and the inclusion of any additional source of water (e.g. from washing) needs to be controlled, and the total water added to the mixture needs to be adjusted accordingly. The amount of bleeding water can be used as an index for quantifying the segregation resistance of SCC during quality control. Visual Stability Index (VSI) (AASHTO-T-351, 2014) is commonly used for the visual evaluation of segregation in SCC, and also in traditional concrete. This test determines an index for the visual distribution of coarse aggregates after spreading the concrete. Generally, after conducting slump flow test, a VSI can be evaluated and assigned to concrete. The VSI values are defined as follows and shown in Fig. 3.2: 0. Highly Stable: no evidence of segregation or bleeding. 1. Stable: no evidence of segregation and slight bleeding observed as a sheen on the concrete surface. 2. Unstable: a slight mortar halo (10 mm) or aggregate pile in the centre of the concrete spread. 3. Highly Unstable: segregated by an evident large mortar halo (10 mm) or a large aggregate pile in the centre of the concrete spread.
VSI 0
VSI 1
VSI 2
VSI 3
Fig. 3.2 Visual Stability Index (VSI) values of 0–3 for controlling the segregation of SCC (AASHTO-T-351, 2014).
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VSI values of 0 or 1 indicate an acceptable SCC mixture design without any segregation. A VSI value of 2 is an indication of unstable concrete that is not recommended for SCC applications. A VSI of 3 indicates highly unstable concrete with heavy segregation that must not be used as an SCC. The required power for mixing SCC (higher power can indicate a stiffer mixture that may not be self-compacting) can also be used for predicting the rheological properties and quality control of concrete during production. Testing frequencies are determined based on the specific guidelines for SCC production, which may be for every load, at the beginning of the day or every other load.
3.2
Transport, pumping, placing and finishing
3.2.1 Transport Self-compacting concrete has higher fluidity compared to traditional concrete. Hence, there is a higher chance of spillage during transport. Additional caution is advised by reducing the batch size on the back of a truck, as well as ensuring the water-tightness of the drum. Extreme weather conditions (very high or low temperature) can affect the self-compacting properties of concrete. Under such conditions, the transport duration must be minimized by choosing non-peak hours in congested areas and also by choosing the closest concrete production plant to the placement site. Overall, the average time that an SCC mixture can spend on the back of a truck before placement needs to be considered and the mixture design needs to be optimized accordingly. Otherwise, flowability properties might not be achieved. For example, a small percentage of retardants may be used to slow down the cement hydration and setting time for long distances of transport under hot conditions. Usually, fresh SCC can maintain its self-compacting behaviour for up to one hour and then start to show more semi-solid behaviour compared to the high fluidity required for SCC (de Schutter et al., 2008). However, the time-span that SCC can maintain self-compacting properties after mixing can be very different based on the mixture design, type and amount of admixture, temperature, humidity, the intensity of mixing and the amount of SCM versus Portland cement.
3.2.2 Pumping, placing and formwork An advantage of SCC is its excellent flow properties, which results in easier pumping and placing of SCC compared to traditional concrete. After discharging, selfcompacting concrete can flow up to 10 m in horizontal directions (de Schutter et al., 2008). The excellent flowability of SCC also results in a much higher filling capability compared to traditional concrete, i.e. it can quickly fill inaccessible voids between reinforcements and formwork. The placement rate of SCC can occur over a short amount of time. Therefore, it is essential that all formwork, linings, reinforcing steel and any other embedded objects are secured and tightened before placement. SCC can be placed with chutes, buckets and pumps. Pumping is the most common method of SCC placement because of excellent flowability without segregation
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Fig. 3.3 Pumping of SCC from a truck using a crane pump at a building site in Melbourne, Australia.
(Roussel et al., 2007). Fig. 3.3 shows the pumping of SCC from a truck using a crane pump at a building site in Melbourne, Australia. The diameter and total length of the pipelines, as well as the number of concrete pumps, need to be determined based on the rheological properties of SCC and job requirements on-site, i.e. the maximum permissible drop height, flow distance and structural requirements. The placement rate of SCC needs to be optimized based on its thixotropic behaviour, i.e. changes in rheological behaviour as time passes, which can be predicted and measured (Petit et al., 2007). The concrete setting time and reinforcement conditions are other factors for determining the placement rate. Increasing the pumping rate of SCC can result in a higher loss of pressure compared to traditional concrete. During the rapid pumping and placement of SCC, a fair amount of air entrainment into concrete occurs. Specifically, a long-distance free-fall can produce a large number of air bubbles during the placement of SCC. The escape of these bubbles to the surface is sometimes challenging. Therefore, such a practice must be eliminated if possible. In order to limit air entrainment and segregation, the pump-head should be placed under the concrete surface inside the formwork. For placement of very large vertical objects, the concrete flow should happen slowly to provide sufficient time and a short distance to the open surface for releasing the air bubbles. A good SCC mixture should push air out by its weight without any vibration. However, the level of air
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content in fresh SCC must be checked to ensure that the durability of concrete is not compromised. On the other hand, concrete can be pumped from the bottom of the formwork in order to reduce the chance of air entrainment compared to the free-fall method. The location of the injection point should be around the centre point of the bottom of the formwork in order to have an equalized horizontal flow of concrete. However, the application of this method can cause higher pressure at the injection point, i.e. the bottom of the formwork. Higher fluidity of SCC can also allow concrete to escape where the joints are not tight enough, thereby resulting in a considerable loss of materials and waste production, as well as imperfections on the surface after removing of the formwork. SCC requires a lower pumping pressure. Hence, concrete can be easily pumped over longer distances and heights compared to traditional concrete (de Schutter et al., 2008). A preferred placement process is a continuous one in which multiple concrete trucks feed into the hopper of a pump. This method maintains uniform SCC flow and limits surface marks and colour variations. Most concrete mixture designs for high-rise buildings are self-compacting or behave similarly to SCC. The highperformance concrete for tall buildings often consists of fibres and lower water content. Mixture designs of high-performance concrete to make it self-compacting allow much better pumpability in addition to easier compaction, placement and a better surface finish. The excellent flow properties and pumpability of SCC allow for bifurcated (branched) pipelines, which results in higher concrete output and faster placement of concrete, especially for high-rise structures. SCC can also be placed with a skip and crane similar to traditional concrete for low-volume applications. It must be noted that fresh SCC can quickly run down a slope. Therefore, the slope needs to be close to zero for foundation applications. Special grades of SCC can be designed to resist flow on a slope of <4%, but the filling capability will be compromised to some extent (de Schutter et al., 2008). In some SCC applications where steep or curved surfaces are required, formwork can be designed to be placed on top of the concrete while it is fresh (right after pouring) in order to manoeuvrer fresh concrete and achieve the final geometries required. Before and during the placement of SCC, the following quality control procedures need to be conducted on-site: (1) Evaluation of the requirements for placing SCC either via a chute, a bucket or a concrete pump. (2) Formwork design considerations to withstand vertical and lateral pressures. (3) Risk management strategies associated with the interrupted placement of SCC. (4) The maximum rate of concrete placement to avoid air entrainment and to limit the pressure on the formwork. (5) Visual checks on potential segregation inside the formwork. (6) Surface finishing and management of surface defects. (7) Curing requirement based on environmental conditions, including temperature and humidity.
Another advantage of SCC is reduced noise during placement as no vibration is required. Due to the elimination of formwork vibration for SCC compared to traditional concrete (vibration of SCC can cause segregation and bleeding), lighter
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formwork can be used provided that it can withstand the pressure of concrete. It must be noted that the higher flow rate of SCC compared to traditional concrete can cause a dynamic pressure, in addition to the hydrostatic pressure of placed concrete, and this must be taken into consideration for formwork design. Also, the longer setting time of SCC compared to traditional concrete (Bouzouba^a and Lachemi, 2001) causes higher lateral pressures over a more extended period.
3.2.3 Surface finish of SCC SCC is generally used for architectural concrete because the surface finish of SCC is of high quality, often more appealing with sharp edges compared to traditional concrete. The improved surface finish is attributed to the self-levelling and filling capabilities of SCC, which allows concrete to flow smoothly, and thereby fill holes. The surface finish of traditional concrete often has discolouration because of hydration by-products and segregation. Other imperfections such as sand textured areas, honeycombing (aggregate bridging), and some problems caused by mortar loss can also occur (de Schutter et al., 2008). Using SCC can increase the chance of eliminating these surface imperfections. However, a well-balanced concrete mixture with optimized rheological properties is required to achieve a high-quality surface finish for SCC, i.e. aesthetic appeal for exposed architectural use. Mixtures with a lower viscosity, i.e. higher slump flow allow for entrained air to escape more efficiently and thereby provide a better surface finish. The quality of formwork surfaces, type and amount of releasing agent, as well as production and placement methods also affect the surface finish of SCC. Three main types of surface imperfections of SCC are bugholes, honeycombing and surface cracking. These issues are explained below and some solutions to rectify these problems are provided. Bugholes are small cavities which result from air bubble entrapment between the concrete and formwork, or the trace of bubbles escaping from the free surface of concrete during the hardening stage, whereby the self-levelling property is no longer available. It was discussed previously that the method of placement, formwork and mixture design of concrete is essential to reduce air entrainment. There is a higher chance of air escape while the concrete is still plastic, which can recover its surface. The interrupted delivery of concrete, very long or very short flow length, very high viscosity, the rough internal surface of formwork, inappropriate application or choice of releasing agent are the main causes of bugholes. Image analysis has been developed for the quantitative evaluation of bugholes on the surface of concrete (Liu and Yang, 2017). Different grades of bugholes on the concrete surface, based on the Concrete Industry Board of American Concrete Institute, are shown in Fig. 3.4. Imperfections in the internal surface of the formwork can cause severe flaws in the surface finish of SCC compared to traditional concrete whereby these imperfections are less noticeable. In order to achieve a better surface finish, a permeable lining inside the formwork is typically used for escaping air bubbles and for limiting bugholes in the surface finish (Kothandaraman et al., 2016). However, the excellent flow properties of SCC can produce a good surface finish even in the case of using steel formwork
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Fig. 3.4 Different grades of bugholes on the concrete surface based on the Concrete Industry Board of American Concrete Institute. Courtesy of Liu, B., Yang, T., 2017. Image analysis for detection of bugholes on concrete surface. Constr. Build. Mater. 137, 432–440.
with an impermeable surface. Dry uncoated wooden formwork can excessively absorb water, which may result in staining discolouration or retarding of the surface of the concrete. The surface cleanliness of the formwork and use of an appropriate type and amount of releasing agent are also crucial in order to achieve a better surface finish. Dirty formwork can increase the chance of producing a rough surface finish. Also, using wax- or oil-based releasing agents, as well as the application of a thick layer of a releasing agent, increase the formation and retainment of air bubbles on the surface of concrete, which creates the bugholes. A thin layer of a water-based releasing agent, which is applied evenly on the surface of formwork, is highly recommended to achieve the highest quality surface finish. It must be noted that releasing agents should fully adhere to the internal surface of formwork. Detachment of the releasing agent and then blending it with SCC can cause severe reductions in mechanical properties and durability, as well as issues with the surface finish of concrete. The incompatibility of hydrophobic releasing agents (oil-based) with water can cause stabilization of air bubbles in concrete after casting, which can potentially increase the air content and porosity of the concrete. The amount of releasing agent should be adequate in order to easily remove the formwork and to create a perfect surface finish but should be controlled to prevent other issues. The excess amount of releasing agent can also run to the bottom of formwork due to gravity (especially for a sprayed releasing agent) and then blend with the concrete mixture. In this case, air-entrainment will be considerably higher because of the high pressure of SCC placement and bubble stabilization of releasing agents. Honeycombing happens in the concrete surface when the mortar between aggregates is missing, and voids between the aggregate are apparent. It is caused by leakage of the mortar from the formwork, especially at the joints. It can also result from aggregate bridging, which impedes the flow of mortar between aggregates. Insufficient mortar, using large aggregates and higher viscosity of concrete can cause honeycombing. A better gradation of aggregates and mixture design with lower viscosity and tight formwork joints can help to avoid honeycombing. An example of honeycombing on the surface of the concrete is shown in Fig. 3.5.
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Fig. 3.5 An example of honeycombing on the surface of concrete. Courtesy of V€olker, C., Shokouhi, P., 2015. Clustering based multi sensor data fusion for honeycomb detection in concrete. J. Nondestruct. Eval. 34, 32.
Drying and plastic shrinkage can cause surface cracking of SCC. Rapid surface drying and the lack of appropriate moist curing conditions, placement at higher temperatures which causes fast evaporation of water, and deep formwork with reinforcements close to the surface can also cause surface cracking. The mixture design of SCC is based on a higher volume of paste and lower water to cement ratio, which results in less water bleeding at the surface of SCC. Therefore, SCC is highly susceptible to moisture loss and surface cracking due to the fast drying of the surface. The addition of short fibres is a practical solution to control the surface cracking of concrete because of drying shrinkage (Barluenga, 2010). An example of surface cracking of concrete due to drying shrinkage is shown in Fig. 3.6.
Fig. 3.6 An example of surface cracking of concrete due to drying shrinkage. Courtesy of Barluenga, G., 2010. Fiber–matrix interaction at early ages of concrete with short fibers. Cem. Concr. Res. 40, 802–809.
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The effect of temperature must also be considered to achieve the highest quality surface finish of SCC and to avoid surface cracking. A higher temperature causes faster evaporation of water which results in the loss of adequate flow and filling capability of concrete. Therefore, the travel time of concrete on the back of a truck on a hot day must be taken into account in the production and mixture design of SCC. In most cases, a small increase of the water to solids ratio is recommended to compensate for water loss due to evaporation and in order to avoid the loss of flowability of SCC. Higher temperatures and lack of humidity can also cause rapid evaporation of water from the surface of concrete after placement. As a result, the concrete surface may crack, or bugholes may be created due to surface boiling (extremely hot temperature). In order to address these problems, the surface of concrete must be kept wet by regular water jetting or membranes that can hinder water evaporation from the surface of the concrete. Fog misting and evaporation retardants are other useful treatments to prevent surface cracking issues. On the other side, very low and freezing temperatures result in delayed hydration and setting of concrete. The delayed setting may cause segregation due to gravity forces or the freezing of water, which are required for the hydration of concrete. Therefore, the use of accelerators or warming up the formwork (if possible) may help to avoid problems caused by the placement of SCC at very low temperatures. The excellent flowability of SCC is attributed to a lubricant layer made of cement, fine aggregates, cementitious materials and water, which enclose the mass of concrete (Choi et al., 2013). During placement, this lubricant layer is moved to the top surface, or it is placed adjacent to the formwork. After removing the formwork, the hydrated lubricant layer is the surface finish of the concrete, which gives it an appealing and smooth appearance. However, there is a chance that this lubricant layer on the surface of concrete can contain a high amount of water, especially for mixtures with less superplasticizers. The higher water content in the top surface of concrete can cause problems such as drying shrinkage cracking (Barluenga et al., 2015). After the evaporation of water, the top layer of SCC with a high amount of water may create a porous and permeable surface which can cause higher chloride penetration, carbonation and efflorescence. It must be noted that surface defects not only affect appearance but can be signs of longer-term durability problems. The surface of concrete acts like a barrier against the intrusion of chemicals, carbon dioxide and salt-contaminated water (Kothandaraman et al., 2016). The penetration of harmful substances that adversely affect its durability properties is mainly controlled by the surface permeation of concrete (Basheer et al., 2001). For example, bugholes in the surface can be good entry points for the abovementioned chemicals to enter the structure of concrete and cause degradation and cracking of concrete after a few years. On the other hand, extensive bugholes on the surface of casted SCC can delay the production schedule as additional surface treatment is often required to cover these imperfections. Fig. 3.7 shows samples of SCC surface finish with imperfections caused by bugholes. A few techniques to classify the surface finish of SCC for quality control based on image analysis have been introduced (da Silva and Sˇtemberk, 2013; Liu and Yang, 2017). These methods are based on quantitative measurements of the size and numbers of bugholes per area of concrete by transforming natural images into a binary digital form (black and white), which can be suitable for quantitative computer processing.
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Fig. 3.7 The surface finish of SCC: (A) perfect surface finish (B) imperfect surface finish with many bugholes of various sizes. Courtesy of da Silva, W.R.L., Sˇtemberk, P., 2013. Expert system applied for classifying selfcompacting concrete surface finish. Adv. Eng. Softw. 64, 47–61.
3.3
Conclusions
Controlling the production, placement and finishing of self-compacting concrete are more complicated than traditional concrete. Many constructability and appearance issues, as well as inferior mechanical and durability properties, are attributed to
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neglecting standards and guidelines designed explicitly for SCC. The following factors are essential to achieve high-quality production and placement of SCC: l
l
l
l
l
l
l
l
a well-studied concrete mixture design; controlling moisture aggregates and the fluctuation of water content in the mixture; well-diluted and distributed chemical admixtures; effective mixing time with enough agitation; limiting water evaporation during delivery; choosing the right method of placement to limit air entrainment; well-designed formwork to resist the hydrostatic pressure in SCC with a proper lining and releasing agent to provide a perfect surface finish without bugholes; and proper curing to avoid surface cracking.
Therefore, robust quality control measures must be followed. The excellent flowability of SCC compared to traditional concrete makes pumping the best method of placement. However, there is a high chance of air entrainment due to a higher flow rate, which can result in surface defects and segregation. Normally SCC has fewer surface imperfections compared to traditional concrete, but it would be susceptible to bugholes, honeycombing and cracking.
References AASHTO-T-351, 2014. Standard Method of Test for Visual Stability Index (VSI) of SelfConsolidating Concrete (SCC). American Association of State Highway and Transportation Officials. Alonso, M.M., Palacios, M., Puertas, F., 2013. Compatibility between polycarboxylate-based admixtures and blended-cement pastes. Cem. Concr. Compos. 35, 151–162. Barluenga, G., 2010. Fiber–matrix interaction at early ages of concrete with short fibers. Cem. Concr. Res. 40, 802–809. Barluenga, G., Puentes, J., Palomar, I., 2015. Early age monitoring of self-compacting concrete with mineral additions. Constr. Build. Mater. 77, 66–73. Barluenga, G., Gimenez, M., Rodrı´guez, A., Rio, O., 2017. Quality control parameters for on-site evaluation of pumped self-compacting concrete. Constr. Build. Mater. 154, 1112–1120. Basheer, L., Kropp, J., Cleland, D.J., 2001. Assessment of the durability of concrete from its permeation properties: a review. Constr. Build. Mater. 15, 93–103. Bouzouba^a, N., Lachemi, M., 2001. Self-compacting concrete incorporating high volumes of class F fly ash: preliminary results. Cem. Concr. Res. 31, 413–420. Choi, M., Roussel, N., Kim, Y., Kim, J., 2013. Lubrication layer properties during concrete pumping. Cem. Concr. Res. 45, 69–78. da Silva, W.R.L., Sˇtemberk, P., 2013. Expert system applied for classifying self-compacting concrete surface finish. Adv. Eng. Softw. 64, 47–61. de Oliveira, M.B., Vazquez, E., 1996. The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete. Waste Manag. 16, 113–117. de Schutter, G., Bartos, P.J.M., Domone, P., Gibbs, J., Gibbs, R., 2008. Self-Compacting Concrete. Whittles Publishing, Dunbeath. Ding, Z., An, X., 2017. A method for real-time moisture estimation based on self-compacting concrete workability detected during the mixing process. Constr. Build. Mater. 139, 123–131.
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Felekog˘lu, B., Sarikahya, H., 2008. Effect of chemical structure of polycarboxylate-based superplasticizers on workability retention of self-compacting concrete. Constr. Build. Mater. 22, 1972–1980. Hemalatha, T., Ram Sundar, K.R., Murthy, A.R., Iyer, N.R., 2015. Influence of mixing protocol on fresh and hardened properties of self-compacting concrete. Constr. Build. Mater. 98, 119–127. Huang, F., Li, H., Yi, Z., Wang, Z., Xie, Y., 2018. The rheological properties of self-compacting concrete containing superplasticizer and air-entraining agent. Constr. Build. Mater. 166, 833–838. Ji, T., Chen, C.-Y., Chen, Y.-Y., Zhuang, Y.-Z., Chen, J.-F., Lin, X.-J., 2013. Effect of moisture state of recycled fine aggregate on the cracking resistibility of concrete. Constr. Build. Mater. 44, 726–733. Kashani, A., San Nicolas, R., Qiao, G., Van Deventer, J.S.J., Provis, J., 2014. Modelling the yield stress of ternary cement–slag–fly ash pastes based on particle size distribution. Powder Technol. 266, 203–209. Kashani, A., Ngo, T.D., Mendis, P., 2019. The effects of precursors on rheology and selfcompactness of geopolymer concrete. Mag. Concr. Res. 71 (11), 557–566. Kothandaraman, S., Kandasamy, S., Sivaraman, K., 2016. The effect of controlled permeable formwork liner on the mechanical and durability properties of self compacting concrete. Constr. Build. Mater. 118, 319–326. Liu, B., Yang, T., 2017. Image analysis for detection of bugholes on concrete surface. Constr. Build. Mater. 137, 432–440. Ngo, H.-T., Kadri, E.-H., Kaci, A., Ngo, T.-T., Trudel, A., Lecrux, S., 2016. Advanced online water content measurement for self-compacting concrete production in ready-mixed concrete plants. Constr. Build. Mater. 112, 570–580. Ortiz, J., Aguado, A., Agullo´, L., Garcı´a, T., Zermen˜o, M., 2009. Influence of environmental temperature and moisture content of aggregates on the workability of cement mortar. Constr. Build. Mater. 23, 1808–1814. Petit, J.-Y., Wirquin, E., Vanhove, Y., Khayat, K., 2007. Yield stress and viscosity equations for mortars and self-consolidating concrete. Cem. Concr. Res. 37, 655–670. Poon, C.S., Shui, Z.H., Lam, L., Fok, H., Kou, S.C., 2004. Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete. Cem. Concr. Res. 34, 31–36. Roussel, N., Geiker, M.R., Dufour, F., Thrane, L.N., Szabo, P., 2007. Computational modeling of concrete flow: general overview. Cem. Concr. Res. 37, 1298–1307. _ O., € 2006. The effect of chemical admixtures and Şahmaran, M., Christianto, H.A., Yaman, l. mineral additives on the properties of self-compacting mortars. Cem. Concr. Compos. 28, 432–440. Schmidt, W., Brouwers, H.J.H., K€uhne, H.-C., Meng, B., 2014. Influences of superplasticizer modification and mixture composition on the performance of self-compacting concrete at varied ambient temperatures. Cem. Concr. Compos. 49, 111–126. Shanahan, N., Buidens, D., Riding, K., Zayed, A., 2016. Effect of chloride-based accelerator in the presence of water-reducing and retarding admixture on autogenous shrinkage. J. Am. Ceram. Soc. 99, 2147–2158. Shen, L., Jovein, H.B., Shen, S., Li, M., 2015. Effects of aggregate properties and concrete rheology on stability robustness of self-consolidating concrete. J. Mater. Civ. Eng. 27 04014159.
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Shi, C., Wu, Z., Lv, K., Wu, L., 2015. A review on mixture design methods for self-compacting concrete. Constr. Build. Mater. 84, 387–398. Yoshioka, K., Tazawa, E.-I., Kawai, K., Enohata, T., 2002. Adsorption characteristics of superplasticizers on cement component minerals. Cem. Concr. Res. 32, 1507–1513. Yu, J.J., Yang, W.K., Zou, J.M., Qiu, R.W., 2014. Apparent quality control for the selfcompacting concrete. Appl. Mech. Mater. 584–586, 2137–2141.
Further reading V€ olker, C., Shokouhi, P., 2015. Clustering based multi sensor data fusion for honeycomb detection in concrete. J. Nondestruct. Eval. 34, 32.
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Alireza Kashania,b, Tuan Ngoa a Department of Infrastructure Engineering, University of Melbourne, Melbourne, VIC, Australia, bSchool of Civil and Environmental Engineering, University of New South Wales, Kensington, NSW, Australia
3.1
Production and quality control at batching plant
Production procedures in terms of raw materials storage and equipment are often similar between self-compacting concrete and traditional concrete. However, the mixture proportions and the methods of mixing, quality control, testing, delivery, placement and finishing can be very different. In terms of the mixture design, self-compacting concrete (SCC) normally has higher paste to aggregate and lower water to cement ratios compared to traditional concrete (Shi et al., 2015). On the other hand, SCC usually contains supplementary cementitious materials (SCM) such as fly ash and ground granulated blast furnace slag, and admixtures such as high-range water reducers (superplasticizers) and viscosity modifying agents. The enhanced rheological properties of SCC result in easily flowable concrete (as shown in Fig. 3.1), which can provide many benefits. For instance, SCC can quickly fill the gaps between congested reinforcements, and it substantially reduces the tedious surface levelling requirement of poured concrete. These unique properties are governed by an advanced mixture design and accurate quality control during production and placement. Therefore, the number of workers at the construction site is reduced. SCC is less forgiving in terms of the variability of its properties and proportion of constituents compared to traditional concrete. Very accurate raw material testing and batching are required for SCC mixtures during production at concrete plants, and any changes and adjustment at worksites should be limited. The effect of mixture design on fresh and hardened properties of self-compacting concrete was explained in Chapter 1. This section briefly explains methods of adding chemical admixtures and controlling the moisture content of aggregates, as well as mixing requirements and other quality control measures during the production of SCC.
3.1.1 Chemical admixtures Chemical admixtures such as high-range water reducers or superplasticizers, viscosity modifying agents, shrinkage reducing and air-entraining admixtures, accelerators, retardants and mineral admixtures have been used in SCC (Şahmaran et al., 2006; Self-Compacting Concrete: Materials, Properties, and Applications. https://doi.org/10.1016/B978-0-12-817369-5.00003-9 Copyright © 2020 Elsevier Inc. All rights reserved.
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Fig. 3.1 Smooth flow and self-levelling of SCC during casting. Courtesy of Ioannis Sfikas of Mott MacDonald.
Huang et al., 2018; Yu et al., 2014). Among the chemical admixtures, superplasticizers are essential ingredients for controlling the flowability or rheology of SCC. Polycarboxylate-based (PC) superplasticizers are the most common water reducing agents in SCC. Many different commercial grades of PC with different functionalities and polymer structures have been tailor-designed for SCC. The chemical structure of PC influences the workability retention and rheological performance of SCC (Felekog˘lu and Sarikahya, 2008). Superplasticizers can also change the rheology of SCC at varying temperatures (Schmidt et al., 2014). The quantity of chemical admixtures is often very low (<1 wt% of the total mass of concrete), but the increased performance at such a low concentration is phenomenal. Chemical admixtures are the most expensive ingredients for self-compacting concrete. Therefore, finding the minimum required chemical admixture for achieving specific properties is a common practice for concrete suppliers to produce cost-effective SCC. Small quantities of admixtures are often added to the concrete mixer manually whereas other ingredients such as cement and aggregates are normally transferred to the mixer via a conveyer belt. Chemical admixtures can also be dissolved in water and then pumped to the concrete mixer which can result in a more homogenous distribution of admixtures in concrete and a better performance. However, it is recommended that some admixtures are
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not to be added simultaneously. For instance, it is beneficial to add an accelerator before the addition of a superplasticizer to enhance strength development. Also, some admixtures are not effective when they are used together. For instance, using a calcium chloride-based accelerator in a concrete mixture, which has a sulphonate-based plasticizer and an air-entraining admixture, can increase autogenous shrinkage (Shanahan et al., 2016). It must be noted that the concrete mixture becomes saturated with admixtures at a certain ratio, meaning that adding more amounts of admixtures will not further enhance the properties of concrete. For instance, the adsorption isotherm of most superplasticizers on cement particles will be equilibrated at a certain concentration. Hence, no further adsorption (and plasticizing effect) will occur by increasing the concentration of superplasticizers (Yoshioka et al., 2002). The plasticising effect of water-reducing admixtures can be affected by the presence of supplementary cementitious materials (SCM) such as slag, resulting in a lower adsorption rate (Alonso et al., 2013). Also, the maximum benefits from the chemical admixtures for SCC are achieved by well-diluting and well-distributing them within the mixture. Sufficient mixing time and the dilution of admixtures within the water before mixing with concrete concrete mixture can help to achieve better performance at lower concentrations, thereby reducing costs associated with the use of expensive admixtures. Moreover, highly agitated mixing after adding the chemical admixture can result in excessive air entrainment, which can adversely affect the surface finish of SCC by the creation of bugholes during placement.
3.1.2 Moisture content Self-compacting concrete is highly susceptible to a small fluctuation of water content in the mixture design than traditional concrete. The changes in the moisture content of aggregates, especially in fine aggregates, can considerably affect the rheology, strength and durability of SCC (Shen et al., 2015; Poon et al., 2004). Excess water in an SCC mixture can cause bleeding and segregation. On the other hand, very dry aggregates can absorb water from the mixture and adversely affect the flowability of concrete. Therefore, controlling the moisture content of aggregates and an adequate method for the correction of water content are very important in SCC production. A study by Ortiz et al. (2009) showed the effects of the storage conditions for aggregates, i.e. the effects of the environmental temperature and the moisture content of aggregates on the workability of concrete. The moisture content of aggregates (especially if stored outside) must be monitored and controlled to produce highperformance SCC. In a hot and dry condition, entrapped water in aggregates evaporates quickly, thereby increasing the water absorption of aggregates. After the addition of aggregates to a mixer, a portion of water from the mixture will be absorbed by the aggregates. This loss of water, although it can be minimal, reduces the flowability of concrete (Poon et al., 2004). On the other hand, under high humidity and rainy conditions, aggregates are saturated with water so any excess water in the mixture can cause segregation and bleeding, as well as reductions in strength and durability. Segregation and reduction in the robustness of SCC are caused by a small increase in water content that comes from aggregates with saturated moisture content (Shen et al., 2015).
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Therefore, it is a routine quality control practice (especially for SCC) to measure the moisture content of aggregates daily or before production. Automatic moisture content measurements by sensors have also been used in some concrete batching plants. However, these sensors must be checked for accuracy and should be calibrated regularly. Ding and An (2017) developed a real-time method for estimating the aggregate moisture content of SCC mixtures, which can guide batch weight adjustment. In this method, the estimated moisture content, as determined by slump flow, shows the excess or lack of moisture in comparison with the moisture content in the reference mixture design. It must be noted that excess water can result from washing batching equipment or the truck-mixer drum, which can affect the properties of SCC. Therefore, it is recommended to include any water either from the aggregate moisture content, washing and so on in the final mixture design. The tolerance for changes in water content for SCC is considerably lower than that of normal concrete. Therefore, these quality control measures are essential to produce high-quality and durable SCC. Controlling the moisture content and water absorption of aggregates in the case of using recycled aggregates is more critical as these aggregates tend to absorb more water. Hence, the required flowability, strength and cracking-resistibility can be compromised (Poon et al., 2004; Ji et al., 2013; de Oliveira and Vazquez, 1996).
3.1.3 Concrete mixers The types of mixers, the sequence of adding mixture components and mixing time can affect the homogeneity and uniformity of SCC (Hemalatha et al., 2015). Selfcompacting concrete has lower yield stress and higher plastic viscosity compared to traditional concrete (Petit et al., 2007). Therefore, a very efficient mixer with an adequate mixing time is required to achieve high-quality SCC that can satisfy any standard requirements. It must be noted that concrete mixtures for SCC production are usually comprised of a higher amount of cement and fines which can adversely affect mixing quality compared to the mixtures with a higher portion of aggregates. One common problem stems from adhering and hardening a stiff paste of cement and fines to the drum or blade of a mixer, which can reduce the quality of SCC and affect the preparation of subsequent mixtures in the same mixer. Therefore, it is recommended that larger aggregates with a portion of water and superplasticizer are added to the drum before the addition of other mixture constituents to prevent the paste from sticking to the drum or blades. The mixing time is also critical and can increase the performance of SCC, which is attributed to a higher amount of fine materials in SCC that require more time and energy to be dispersed homogeneously in the mixture. A study by Ngo et al. (2016) showed that monitoring the mixing power evolution can be used as a tool to determine the amount of water and chemical admixtures needed for controlling the rheology, flowability and self-compactness of concrete in ready-mixed concrete plants. The particle size distribution and type of mixture components (e.g. the addition of supplementary cementitious materials) can also affect the rheology (yield stress and viscosity) of concrete (Kashani et al., 2014, 2019). Mixtures with a higher yield stress and viscosity (less flowable) require higher energy for mixing.
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Free-fall mixers, although typical for traditional concrete, may not provide enough energy and agitation for effectively mixing of SCC. As a result, some specifications do not allow the use of free-fall mixers to produce SCC. Mixers that can provide a higher shear rate to concrete by well-arranged blades and higher power can produce concrete with a very low yield stress, which is more suitable to achieve self-compacting properties (Hemalatha et al., 2015). Forced action mixers which blades and paddles that rotate for mixing are more effective in the production of a uniform SCC mixture over a shorter period (de Schutter et al., 2008). Truck-mixers, although useful for mixing, are not the most efficient method for preparing SCC. The added time of mixing at the back of the truck after production in concrete plants needs to be considered to optimize the total mixing time before placement. It must be noted that the flow properties of SCC can be adversely affected if there is a large gap between the production and placement of concrete.
3.1.4 Quality control at concrete batching plant A consistent and guaranteed availability of aggregates and other concrete constituents must be taken into consideration to produce SCC because a series of trials for mixture development, testing and optimization must take place to achieve the self-compacting properties. The introduction of a new concrete constituent with different properties often means that the mixture design procedure should be repeated. Therefore, it is vitally important to secure a consistent supply chain of materials for SCC production. The rheology and flow properties of concrete must be checked to avoid any loss of mechanical properties and durability and to prevent an improper surface finish. For instance, low viscosity and a high amount of water can cause bleeding, and subsequently cause segregation and surface imperfections such as scaling (de Schutter et al., 2008). An adequate flow rate is necessary for the filling capability of SCC. However, a very high flow rate induces additional pressure on formwork that may cause movement and displacement during the discharge of SCC. Both fresh and hardened properties of concrete must be checked and compared against specific standards of SCC based on the region and type of application. It must be noted that concrete properties achieved in smaller batches in the laboratory can be different from concrete batches prepared in larger mixers. The main reason is the difference in quality and time of mixing, as well as the type of mixer. Therefore, the amount of water and admixtures need to be revisited in upscaling the mixture design from the laboratory scale to a production plant. Barluenga et al. (2017) developed and tested a series of methods that permit real-time measurement during pumping namely, early age and hardened stages of SCC for quality control purposes. They measured the real-time pumping pressure and discharge rate and conducted non-destructive testing (NDT) on freshly casted concrete, such as ultrasonic pulse velocity (UPV), to assess the properties of SCC, which were compared with the results obtained from the laboratory. They found that pumpability can be affected by the discharge rate rather than the slump flow. Pumping resulted in up to 16% reduction in the compressive strength of SCC due to some level of air entrainment. Prolonged mixing during production can cause excessive air entrainment (especially for the mixtures with high amounts
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of admixtures), which subsequently results in bugholes on the surface of concrete that endanger the aesthetics and durability of concrete. The standard tests for SCC were discussed in Chapter 1. The required values in terms of flowability and mechanical properties can be different among many regional standards, guidelines and specifications. As discussed above, the moisture content of aggregates and the inclusion of any additional source of water (e.g. from washing) needs to be controlled, and the total water added to the mixture needs to be adjusted accordingly. The amount of bleeding water can be used as an index for quantifying the segregation resistance of SCC during quality control. Visual Stability Index (VSI) (AASHTO-T-351, 2014) is commonly used for the visual evaluation of segregation in SCC, and also in traditional concrete. This test determines an index for the visual distribution of coarse aggregates after spreading the concrete. Generally, after conducting slump flow test, a VSI can be evaluated and assigned to concrete. The VSI values are defined as follows and shown in Fig. 3.2: 0. Highly Stable: no evidence of segregation or bleeding. 1. Stable: no evidence of segregation and slight bleeding observed as a sheen on the concrete surface. 2. Unstable: a slight mortar halo (10 mm) or aggregate pile in the centre of the concrete spread. 3. Highly Unstable: segregated by an evident large mortar halo (10 mm) or a large aggregate pile in the centre of the concrete spread.
VSI 0
VSI 1
VSI 2
VSI 3
Fig. 3.2 Visual Stability Index (VSI) values of 0–3 for controlling the segregation of SCC (AASHTO-T-351, 2014).
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VSI values of 0 or 1 indicate an acceptable SCC mixture design without any segregation. A VSI value of 2 is an indication of unstable concrete that is not recommended for SCC applications. A VSI of 3 indicates highly unstable concrete with heavy segregation that must not be used as an SCC. The required power for mixing SCC (higher power can indicate a stiffer mixture that may not be self-compacting) can also be used for predicting the rheological properties and quality control of concrete during production. Testing frequencies are determined based on the specific guidelines for SCC production, which may be for every load, at the beginning of the day or every other load.
3.2
Transport, pumping, placing and finishing
3.2.1 Transport Self-compacting concrete has higher fluidity compared to traditional concrete. Hence, there is a higher chance of spillage during transport. Additional caution is advised by reducing the batch size on the back of a truck, as well as ensuring the water-tightness of the drum. Extreme weather conditions (very high or low temperature) can affect the self-compacting properties of concrete. Under such conditions, the transport duration must be minimized by choosing non-peak hours in congested areas and also by choosing the closest concrete production plant to the placement site. Overall, the average time that an SCC mixture can spend on the back of a truck before placement needs to be considered and the mixture design needs to be optimized accordingly. Otherwise, flowability properties might not be achieved. For example, a small percentage of retardants may be used to slow down the cement hydration and setting time for long distances of transport under hot conditions. Usually, fresh SCC can maintain its self-compacting behaviour for up to one hour and then start to show more semi-solid behaviour compared to the high fluidity required for SCC (de Schutter et al., 2008). However, the time-span that SCC can maintain self-compacting properties after mixing can be very different based on the mixture design, type and amount of admixture, temperature, humidity, the intensity of mixing and the amount of SCM versus Portland cement.
3.2.2 Pumping, placing and formwork An advantage of SCC is its excellent flow properties, which results in easier pumping and placing of SCC compared to traditional concrete. After discharging, selfcompacting concrete can flow up to 10 m in horizontal directions (de Schutter et al., 2008). The excellent flowability of SCC also results in a much higher filling capability compared to traditional concrete, i.e. it can quickly fill inaccessible voids between reinforcements and formwork. The placement rate of SCC can occur over a short amount of time. Therefore, it is essential that all formwork, linings, reinforcing steel and any other embedded objects are secured and tightened before placement. SCC can be placed with chutes, buckets and pumps. Pumping is the most common method of SCC placement because of excellent flowability without segregation
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Fig. 3.3 Pumping of SCC from a truck using a crane pump at a building site in Melbourne, Australia.
(Roussel et al., 2007). Fig. 3.3 shows the pumping of SCC from a truck using a crane pump at a building site in Melbourne, Australia. The diameter and total length of the pipelines, as well as the number of concrete pumps, need to be determined based on the rheological properties of SCC and job requirements on-site, i.e. the maximum permissible drop height, flow distance and structural requirements. The placement rate of SCC needs to be optimized based on its thixotropic behaviour, i.e. changes in rheological behaviour as time passes, which can be predicted and measured (Petit et al., 2007). The concrete setting time and reinforcement conditions are other factors for determining the placement rate. Increasing the pumping rate of SCC can result in a higher loss of pressure compared to traditional concrete. During the rapid pumping and placement of SCC, a fair amount of air entrainment into concrete occurs. Specifically, a long-distance free-fall can produce a large number of air bubbles during the placement of SCC. The escape of these bubbles to the surface is sometimes challenging. Therefore, such a practice must be eliminated if possible. In order to limit air entrainment and segregation, the pump-head should be placed under the concrete surface inside the formwork. For placement of very large vertical objects, the concrete flow should happen slowly to provide sufficient time and a short distance to the open surface for releasing the air bubbles. A good SCC mixture should push air out by its weight without any vibration. However, the level of air
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content in fresh SCC must be checked to ensure that the durability of concrete is not compromised. On the other hand, concrete can be pumped from the bottom of the formwork in order to reduce the chance of air entrainment compared to the free-fall method. The location of the injection point should be around the centre point of the bottom of the formwork in order to have an equalized horizontal flow of concrete. However, the application of this method can cause higher pressure at the injection point, i.e. the bottom of the formwork. Higher fluidity of SCC can also allow concrete to escape where the joints are not tight enough, thereby resulting in a considerable loss of materials and waste production, as well as imperfections on the surface after removing of the formwork. SCC requires a lower pumping pressure. Hence, concrete can be easily pumped over longer distances and heights compared to traditional concrete (de Schutter et al., 2008). A preferred placement process is a continuous one in which multiple concrete trucks feed into the hopper of a pump. This method maintains uniform SCC flow and limits surface marks and colour variations. Most concrete mixture designs for high-rise buildings are self-compacting or behave similarly to SCC. The highperformance concrete for tall buildings often consists of fibres and lower water content. Mixture designs of high-performance concrete to make it self-compacting allow much better pumpability in addition to easier compaction, placement and a better surface finish. The excellent flow properties and pumpability of SCC allow for bifurcated (branched) pipelines, which results in higher concrete output and faster placement of concrete, especially for high-rise structures. SCC can also be placed with a skip and crane similar to traditional concrete for low-volume applications. It must be noted that fresh SCC can quickly run down a slope. Therefore, the slope needs to be close to zero for foundation applications. Special grades of SCC can be designed to resist flow on a slope of <4%, but the filling capability will be compromised to some extent (de Schutter et al., 2008). In some SCC applications where steep or curved surfaces are required, formwork can be designed to be placed on top of the concrete while it is fresh (right after pouring) in order to manoeuvrer fresh concrete and achieve the final geometries required. Before and during the placement of SCC, the following quality control procedures need to be conducted on-site: (1) Evaluation of the requirements for placing SCC either via a chute, a bucket or a concrete pump. (2) Formwork design considerations to withstand vertical and lateral pressures. (3) Risk management strategies associated with the interrupted placement of SCC. (4) The maximum rate of concrete placement to avoid air entrainment and to limit the pressure on the formwork. (5) Visual checks on potential segregation inside the formwork. (6) Surface finishing and management of surface defects. (7) Curing requirement based on environmental conditions, including temperature and humidity.
Another advantage of SCC is reduced noise during placement as no vibration is required. Due to the elimination of formwork vibration for SCC compared to traditional concrete (vibration of SCC can cause segregation and bleeding), lighter
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formwork can be used provided that it can withstand the pressure of concrete. It must be noted that the higher flow rate of SCC compared to traditional concrete can cause a dynamic pressure, in addition to the hydrostatic pressure of placed concrete, and this must be taken into consideration for formwork design. Also, the longer setting time of SCC compared to traditional concrete (Bouzouba^a and Lachemi, 2001) causes higher lateral pressures over a more extended period.
3.2.3 Surface finish of SCC SCC is generally used for architectural concrete because the surface finish of SCC is of high quality, often more appealing with sharp edges compared to traditional concrete. The improved surface finish is attributed to the self-levelling and filling capabilities of SCC, which allows concrete to flow smoothly, and thereby fill holes. The surface finish of traditional concrete often has discolouration because of hydration by-products and segregation. Other imperfections such as sand textured areas, honeycombing (aggregate bridging), and some problems caused by mortar loss can also occur (de Schutter et al., 2008). Using SCC can increase the chance of eliminating these surface imperfections. However, a well-balanced concrete mixture with optimized rheological properties is required to achieve a high-quality surface finish for SCC, i.e. aesthetic appeal for exposed architectural use. Mixtures with a lower viscosity, i.e. higher slump flow allow for entrained air to escape more efficiently and thereby provide a better surface finish. The quality of formwork surfaces, type and amount of releasing agent, as well as production and placement methods also affect the surface finish of SCC. Three main types of surface imperfections of SCC are bugholes, honeycombing and surface cracking. These issues are explained below and some solutions to rectify these problems are provided. Bugholes are small cavities which result from air bubble entrapment between the concrete and formwork, or the trace of bubbles escaping from the free surface of concrete during the hardening stage, whereby the self-levelling property is no longer available. It was discussed previously that the method of placement, formwork and mixture design of concrete is essential to reduce air entrainment. There is a higher chance of air escape while the concrete is still plastic, which can recover its surface. The interrupted delivery of concrete, very long or very short flow length, very high viscosity, the rough internal surface of formwork, inappropriate application or choice of releasing agent are the main causes of bugholes. Image analysis has been developed for the quantitative evaluation of bugholes on the surface of concrete (Liu and Yang, 2017). Different grades of bugholes on the concrete surface, based on the Concrete Industry Board of American Concrete Institute, are shown in Fig. 3.4. Imperfections in the internal surface of the formwork can cause severe flaws in the surface finish of SCC compared to traditional concrete whereby these imperfections are less noticeable. In order to achieve a better surface finish, a permeable lining inside the formwork is typically used for escaping air bubbles and for limiting bugholes in the surface finish (Kothandaraman et al., 2016). However, the excellent flow properties of SCC can produce a good surface finish even in the case of using steel formwork
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Fig. 3.4 Different grades of bugholes on the concrete surface based on the Concrete Industry Board of American Concrete Institute. Courtesy of Liu, B., Yang, T., 2017. Image analysis for detection of bugholes on concrete surface. Constr. Build. Mater. 137, 432–440.
with an impermeable surface. Dry uncoated wooden formwork can excessively absorb water, which may result in staining discolouration or retarding of the surface of the concrete. The surface cleanliness of the formwork and use of an appropriate type and amount of releasing agent are also crucial in order to achieve a better surface finish. Dirty formwork can increase the chance of producing a rough surface finish. Also, using wax- or oil-based releasing agents, as well as the application of a thick layer of a releasing agent, increase the formation and retainment of air bubbles on the surface of concrete, which creates the bugholes. A thin layer of a water-based releasing agent, which is applied evenly on the surface of formwork, is highly recommended to achieve the highest quality surface finish. It must be noted that releasing agents should fully adhere to the internal surface of formwork. Detachment of the releasing agent and then blending it with SCC can cause severe reductions in mechanical properties and durability, as well as issues with the surface finish of concrete. The incompatibility of hydrophobic releasing agents (oil-based) with water can cause stabilization of air bubbles in concrete after casting, which can potentially increase the air content and porosity of the concrete. The amount of releasing agent should be adequate in order to easily remove the formwork and to create a perfect surface finish but should be controlled to prevent other issues. The excess amount of releasing agent can also run to the bottom of formwork due to gravity (especially for a sprayed releasing agent) and then blend with the concrete mixture. In this case, air-entrainment will be considerably higher because of the high pressure of SCC placement and bubble stabilization of releasing agents. Honeycombing happens in the concrete surface when the mortar between aggregates is missing, and voids between the aggregate are apparent. It is caused by leakage of the mortar from the formwork, especially at the joints. It can also result from aggregate bridging, which impedes the flow of mortar between aggregates. Insufficient mortar, using large aggregates and higher viscosity of concrete can cause honeycombing. A better gradation of aggregates and mixture design with lower viscosity and tight formwork joints can help to avoid honeycombing. An example of honeycombing on the surface of the concrete is shown in Fig. 3.5.
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Fig. 3.5 An example of honeycombing on the surface of concrete. Courtesy of V€olker, C., Shokouhi, P., 2015. Clustering based multi sensor data fusion for honeycomb detection in concrete. J. Nondestruct. Eval. 34, 32.
Drying and plastic shrinkage can cause surface cracking of SCC. Rapid surface drying and the lack of appropriate moist curing conditions, placement at higher temperatures which causes fast evaporation of water, and deep formwork with reinforcements close to the surface can also cause surface cracking. The mixture design of SCC is based on a higher volume of paste and lower water to cement ratio, which results in less water bleeding at the surface of SCC. Therefore, SCC is highly susceptible to moisture loss and surface cracking due to the fast drying of the surface. The addition of short fibres is a practical solution to control the surface cracking of concrete because of drying shrinkage (Barluenga, 2010). An example of surface cracking of concrete due to drying shrinkage is shown in Fig. 3.6.
Fig. 3.6 An example of surface cracking of concrete due to drying shrinkage. Courtesy of Barluenga, G., 2010. Fiber–matrix interaction at early ages of concrete with short fibers. Cem. Concr. Res. 40, 802–809.
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The effect of temperature must also be considered to achieve the highest quality surface finish of SCC and to avoid surface cracking. A higher temperature causes faster evaporation of water which results in the loss of adequate flow and filling capability of concrete. Therefore, the travel time of concrete on the back of a truck on a hot day must be taken into account in the production and mixture design of SCC. In most cases, a small increase of the water to solids ratio is recommended to compensate for water loss due to evaporation and in order to avoid the loss of flowability of SCC. Higher temperatures and lack of humidity can also cause rapid evaporation of water from the surface of concrete after placement. As a result, the concrete surface may crack, or bugholes may be created due to surface boiling (extremely hot temperature). In order to address these problems, the surface of concrete must be kept wet by regular water jetting or membranes that can hinder water evaporation from the surface of the concrete. Fog misting and evaporation retardants are other useful treatments to prevent surface cracking issues. On the other side, very low and freezing temperatures result in delayed hydration and setting of concrete. The delayed setting may cause segregation due to gravity forces or the freezing of water, which are required for the hydration of concrete. Therefore, the use of accelerators or warming up the formwork (if possible) may help to avoid problems caused by the placement of SCC at very low temperatures. The excellent flowability of SCC is attributed to a lubricant layer made of cement, fine aggregates, cementitious materials and water, which enclose the mass of concrete (Choi et al., 2013). During placement, this lubricant layer is moved to the top surface, or it is placed adjacent to the formwork. After removing the formwork, the hydrated lubricant layer is the surface finish of the concrete, which gives it an appealing and smooth appearance. However, there is a chance that this lubricant layer on the surface of concrete can contain a high amount of water, especially for mixtures with less superplasticizers. The higher water content in the top surface of concrete can cause problems such as drying shrinkage cracking (Barluenga et al., 2015). After the evaporation of water, the top layer of SCC with a high amount of water may create a porous and permeable surface which can cause higher chloride penetration, carbonation and efflorescence. It must be noted that surface defects not only affect appearance but can be signs of longer-term durability problems. The surface of concrete acts like a barrier against the intrusion of chemicals, carbon dioxide and salt-contaminated water (Kothandaraman et al., 2016). The penetration of harmful substances that adversely affect its durability properties is mainly controlled by the surface permeation of concrete (Basheer et al., 2001). For example, bugholes in the surface can be good entry points for the abovementioned chemicals to enter the structure of concrete and cause degradation and cracking of concrete after a few years. On the other hand, extensive bugholes on the surface of casted SCC can delay the production schedule as additional surface treatment is often required to cover these imperfections. Fig. 3.7 shows samples of SCC surface finish with imperfections caused by bugholes. A few techniques to classify the surface finish of SCC for quality control based on image analysis have been introduced (da Silva and Sˇtemberk, 2013; Liu and Yang, 2017). These methods are based on quantitative measurements of the size and numbers of bugholes per area of concrete by transforming natural images into a binary digital form (black and white), which can be suitable for quantitative computer processing.
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Fig. 3.7 The surface finish of SCC: (A) perfect surface finish (B) imperfect surface finish with many bugholes of various sizes. Courtesy of da Silva, W.R.L., Sˇtemberk, P., 2013. Expert system applied for classifying selfcompacting concrete surface finish. Adv. Eng. Softw. 64, 47–61.
3.3
Conclusions
Controlling the production, placement and finishing of self-compacting concrete are more complicated than traditional concrete. Many constructability and appearance issues, as well as inferior mechanical and durability properties, are attributed to
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neglecting standards and guidelines designed explicitly for SCC. The following factors are essential to achieve high-quality production and placement of SCC: l
l
l
l
l
l
l
l
a well-studied concrete mixture design; controlling moisture aggregates and the fluctuation of water content in the mixture; well-diluted and distributed chemical admixtures; effective mixing time with enough agitation; limiting water evaporation during delivery; choosing the right method of placement to limit air entrainment; well-designed formwork to resist the hydrostatic pressure in SCC with a proper lining and releasing agent to provide a perfect surface finish without bugholes; and proper curing to avoid surface cracking.
Therefore, robust quality control measures must be followed. The excellent flowability of SCC compared to traditional concrete makes pumping the best method of placement. However, there is a high chance of air entrainment due to a higher flow rate, which can result in surface defects and segregation. Normally SCC has fewer surface imperfections compared to traditional concrete, but it would be susceptible to bugholes, honeycombing and cracking.
References AASHTO-T-351, 2014. Standard Method of Test for Visual Stability Index (VSI) of SelfConsolidating Concrete (SCC). American Association of State Highway and Transportation Officials. Alonso, M.M., Palacios, M., Puertas, F., 2013. Compatibility between polycarboxylate-based admixtures and blended-cement pastes. Cem. Concr. Compos. 35, 151–162. Barluenga, G., 2010. Fiber–matrix interaction at early ages of concrete with short fibers. Cem. Concr. Res. 40, 802–809. Barluenga, G., Puentes, J., Palomar, I., 2015. Early age monitoring of self-compacting concrete with mineral additions. Constr. Build. Mater. 77, 66–73. Barluenga, G., Gimenez, M., Rodrı´guez, A., Rio, O., 2017. Quality control parameters for on-site evaluation of pumped self-compacting concrete. Constr. Build. Mater. 154, 1112–1120. Basheer, L., Kropp, J., Cleland, D.J., 2001. Assessment of the durability of concrete from its permeation properties: a review. Constr. Build. Mater. 15, 93–103. Bouzouba^a, N., Lachemi, M., 2001. Self-compacting concrete incorporating high volumes of class F fly ash: preliminary results. Cem. Concr. Res. 31, 413–420. Choi, M., Roussel, N., Kim, Y., Kim, J., 2013. Lubrication layer properties during concrete pumping. Cem. Concr. Res. 45, 69–78. da Silva, W.R.L., Sˇtemberk, P., 2013. Expert system applied for classifying self-compacting concrete surface finish. Adv. Eng. Softw. 64, 47–61. de Oliveira, M.B., Vazquez, E., 1996. The influence of retained moisture in aggregates from recycling on the properties of new hardened concrete. Waste Manag. 16, 113–117. de Schutter, G., Bartos, P.J.M., Domone, P., Gibbs, J., Gibbs, R., 2008. Self-Compacting Concrete. Whittles Publishing, Dunbeath. Ding, Z., An, X., 2017. A method for real-time moisture estimation based on self-compacting concrete workability detected during the mixing process. Constr. Build. Mater. 139, 123–131.
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Felekog˘lu, B., Sarikahya, H., 2008. Effect of chemical structure of polycarboxylate-based superplasticizers on workability retention of self-compacting concrete. Constr. Build. Mater. 22, 1972–1980. Hemalatha, T., Ram Sundar, K.R., Murthy, A.R., Iyer, N.R., 2015. Influence of mixing protocol on fresh and hardened properties of self-compacting concrete. Constr. Build. Mater. 98, 119–127. Huang, F., Li, H., Yi, Z., Wang, Z., Xie, Y., 2018. The rheological properties of self-compacting concrete containing superplasticizer and air-entraining agent. Constr. Build. Mater. 166, 833–838. Ji, T., Chen, C.-Y., Chen, Y.-Y., Zhuang, Y.-Z., Chen, J.-F., Lin, X.-J., 2013. Effect of moisture state of recycled fine aggregate on the cracking resistibility of concrete. Constr. Build. Mater. 44, 726–733. Kashani, A., San Nicolas, R., Qiao, G., Van Deventer, J.S.J., Provis, J., 2014. Modelling the yield stress of ternary cement–slag–fly ash pastes based on particle size distribution. Powder Technol. 266, 203–209. Kashani, A., Ngo, T.D., Mendis, P., 2019. The effects of precursors on rheology and selfcompactness of geopolymer concrete. Mag. Concr. Res. 71 (11), 557–566. Kothandaraman, S., Kandasamy, S., Sivaraman, K., 2016. The effect of controlled permeable formwork liner on the mechanical and durability properties of self compacting concrete. Constr. Build. Mater. 118, 319–326. Liu, B., Yang, T., 2017. Image analysis for detection of bugholes on concrete surface. Constr. Build. Mater. 137, 432–440. Ngo, H.-T., Kadri, E.-H., Kaci, A., Ngo, T.-T., Trudel, A., Lecrux, S., 2016. Advanced online water content measurement for self-compacting concrete production in ready-mixed concrete plants. Constr. Build. Mater. 112, 570–580. Ortiz, J., Aguado, A., Agullo´, L., Garcı´a, T., Zermen˜o, M., 2009. Influence of environmental temperature and moisture content of aggregates on the workability of cement mortar. Constr. Build. Mater. 23, 1808–1814. Petit, J.-Y., Wirquin, E., Vanhove, Y., Khayat, K., 2007. Yield stress and viscosity equations for mortars and self-consolidating concrete. Cem. Concr. Res. 37, 655–670. Poon, C.S., Shui, Z.H., Lam, L., Fok, H., Kou, S.C., 2004. Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete. Cem. Concr. Res. 34, 31–36. Roussel, N., Geiker, M.R., Dufour, F., Thrane, L.N., Szabo, P., 2007. Computational modeling of concrete flow: general overview. Cem. Concr. Res. 37, 1298–1307. _ O., € 2006. The effect of chemical admixtures and Şahmaran, M., Christianto, H.A., Yaman, l. mineral additives on the properties of self-compacting mortars. Cem. Concr. Compos. 28, 432–440. Schmidt, W., Brouwers, H.J.H., K€uhne, H.-C., Meng, B., 2014. Influences of superplasticizer modification and mixture composition on the performance of self-compacting concrete at varied ambient temperatures. Cem. Concr. Compos. 49, 111–126. Shanahan, N., Buidens, D., Riding, K., Zayed, A., 2016. Effect of chloride-based accelerator in the presence of water-reducing and retarding admixture on autogenous shrinkage. J. Am. Ceram. Soc. 99, 2147–2158. Shen, L., Jovein, H.B., Shen, S., Li, M., 2015. Effects of aggregate properties and concrete rheology on stability robustness of self-consolidating concrete. J. Mater. Civ. Eng. 27 04014159.
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Shi, C., Wu, Z., Lv, K., Wu, L., 2015. A review on mixture design methods for self-compacting concrete. Constr. Build. Mater. 84, 387–398. Yoshioka, K., Tazawa, E.-I., Kawai, K., Enohata, T., 2002. Adsorption characteristics of superplasticizers on cement component minerals. Cem. Concr. Res. 32, 1507–1513. Yu, J.J., Yang, W.K., Zou, J.M., Qiu, R.W., 2014. Apparent quality control for the selfcompacting concrete. Appl. Mech. Mater. 584–586, 2137–2141.
Further reading V€ olker, C., Shokouhi, P., 2015. Clustering based multi sensor data fusion for honeycomb detection in concrete. J. Nondestruct. Eval. 34, 32.