Documents

ARTICULO

Description
Description:
Categories
Published
of 7
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
   Procedia Engineering 195 ( 2017 ) 205 – 211  Available online at www.sciencedirect.com 1877-7058 © 2017 The Authors. Published  by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ ). Peer-review under responsibility of the organizing committee of the 18th International Conference on Rehabilitation and Reconstruction of Buildings 2016 doi: 10.1016/j.proeng.2017.04.545 ScienceDirect  18th International Conference on Rehabilitation and Reconstruction of Buildings 2016, CRRB 2016 Long-term compressive strength of mortars modified with hardening accelerating admixtures Jan Pizoń a * a Silesian University of Technology, Akademicka 5 str. 44-100 Gliwice, Poland Abstract  Nowadays producers of fresh concrete and reinforced concrete precast elements wants to shorten time of curing. This is  beneficial for the sake of organization of building and manufacturing process. Constructional elements gain strength faster. They can be demoulded, transported and loaded earlier. Economic benefits are gained because formworks may be rent for shorter  periods and moulds for precast elements may be filled up with next element earlier so the production is more efficient. The shortening of curing time may be achieved by usage of hardening and set accelerating admixtures. Other benefit is possibility of concreting during winter time. Besides obvious advantages showed before there are drawbacks of such admixtures. One of the most dangerous is lowered final compressive strength in comparison with non-modified concrete. Long-term (28, 90, 180 and 360 days) compressive strength of cement mortars were tested. Samples were made with usage of Portland cement and cement with 35 % ground granulated blast furnace slag addition. In case of CEM I 52.5R decline of long-term compressive strength caused by hardening accelerating admixtures is always visible but its scale is not very large and depends on type of admixture. In case of cement with slag addition the decrease of compressive strength does not occur after modification by accelerator based on CSH crystal seeds in both dosages (50 % and 100 % of maximal allowed by producer) and for calcium nitrate in half of maximal dosage. According to statements above it is not allowed to say that every hardening accelerating admixture cause decline of compressive strength in long period of time in case of every kind of cement. Performed examinations are part of PhD thesis entitled ‘The possi  bility of early compressive strength enhancing of cements with ground granulated blast- furnace slag addition’.   ©  2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of the 18th International Conference on Rehabilitation and Reconstruction of Buildings 2016. Keywords:  Hardening accelerating admixtures; ground granulated blast-furnace slag; compressive strength * Corresponding author. Tel.: +48663767227  E-mail address:  jan.pizon@polsl.pl   © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ ).Peer-review under responsibility of the organizing committee of the 18th International Conference on Rehabilitation and Reconstruction of Buildings 2016  206  Jan Pizo ń  / Procedia Engineering 195 ( 2017 ) 205 – 211 1.   Introduction  Nowadays producers of fresh concrete and reinforced concrete precast elements wants to shorten time of curing. This is beneficial for the sake of organization of building and manufacturing process. Constructional elements gain strength faster. They can be demoulded, transported and loaded earlier. Economic benefits are gained because formworks may be rent for shorter periods and moulds for precast elements may be filled up with next element earlier so the production is more efficient. There are many methods to shorten time of curing. First one is usage of Portland cement with hydration heat, speed of set and hardening process and gaining strength and early strength is highest among all kinds of commonly used cement [1, 2]. Unfortunately Portland cement does not satisfy sustainability conditions and manufacturers more often use cements with mineral additives. Usage of blended cements is better for environment safety but it does not lower the cement manufacturing process [3]. The shortening of curing time may be achieved by usage of hardening and set accelerating admixtures. Other  benefits are possibility of concreting during winter time. Some of such admixtures lower freezing point of water in cement paste pores and enlarge temperature of fresh concrete by enhanced hydration heat exhale rate during the most crucial phase of hardening, when the flexural strength is low and freezing water may destroy structure of concrete from inside [4, 5, 6]. Besides obvious advantages showed before there are drawbacks of such admixtures. One of the most dangerous is lowered final compressive strength in comparison with non-modified concrete [7]. European standard EN-934-2 [8] for set accelerating admixtures requires compressive strength after 28 days to be at least 80 % of compressive strength of reference concrete and at least 28 th  day compressive strength after 90 days. Compressive strength of concrete modified with hardening accelerating admixtures is demanded to be not less than 120 % of compressive strength of reference concrete after 24 hours in 20 °C and at least 90  % of one after 28 days. Some of those admixtures may cause enhanced shrinkage or increased corrosion risk of reinforcing steel (especially chloride compounds) or concrete (in terms of alkali-silica reaction) [3, 4, 9, 10]. Those dangers may be lowered by usage of  blended cements containing GGBFS. Mortars with such cement have lower size of air voids (the air content is almost the same) in comparison to Portland cement ones. Cement matrix is tighter and the velocity of chloride ions migration is lower. Third possibility is lowering of water content. Compressive strength is in inverse proportion to water quantity [1] Slag addition also improves workability, and connected with lowered water-cement ratio, may lead to additional compressive strength improvement [1,11]. Although plasticizing agents may be required and initial setting time may  be delayed by secondary ingredients but summary hydration heat in first 41 hours is enlarged by usage of such admixtures [6, 12, 13]. Performed examinations are part of PhD thesis entitled ‘The possibility of ea rly compressive strength enhancing of cements with ground granulated blast- furnace slag addition’.   2.   Experimental 2.1.    Range and target The scope of performed experiments was to define the scale of the problem of lowered long-term compressive strength by accelerating admixtures. Mortars were made of Portland cement and cement with GGBFS addition. Tests were conducted in temperature of and 20°C. Mortars were examined after 28, 90, 180 and 360 days of curing. Research consisted of compressive strength test of hardened mortars. 2.2.    Materials Table 1. Chemical composition and specific surface of CEM I and GGBFS.  207  Jan Pizo ń  / Procedia Engineering 195 ( 2017 ) 205 – 211 Constituent CEM I 52.5R GGBFS Ignition losses 1.95 - Unsolved parts 0.42 - SiO 2  Al 2 O 3  Fe 2 O 3  CaO MgO SO 3   Na 2 O K  2 O Cl Blaine surface [cm 2 /g] 20.54 5.14 2.63 64.12 1.36 2.69 0.17 0.81 0.06 4230 37.35 7.30 1.22 43.90 5.73 0.62 - - 0.03 3870 Portland cement CEM I 52,5R obtained from one of the Polish cement plants and cement with regulated GGBFS content (S) were used. Amount of GGBFS in mortars were 6 %, 20 % and 35 %. This is the normative range of CEM II/A-S and CEM II/B- S. Chemical composition and Blaine’s specific surface of constituents are given in table 1. Sand for mortars preparation was standard graded, accordingly to EN 196-1. Table 2. Admixture characteristics Symbol Characteristics Dosage range [% c.m.] Dry mass [%] CF Calcium formate based hardening accelerating admixture 0.2  –   5.0 50 CSH CSH crystal seeds based hardening accelerating admixture 2.0  –   4.0 20 CN TEA Calcium nitrate based hardening accelerating admixture Trietanolamine based hardening accelerating admixture 1.0  –   3.0 1.0  –   2.0 < 5 < 5 Table 3. Composition of mortars Cement GGBFS content Admixture type Adm. quantity [% c.m.] w/c ratio CEM I 52,5R - - - 0.5 6, 20, 35% - - - ACC1  –   ACC5 50% - ACC1  –   ACC4 100% 35% ACC1  –   ACC4 50% 35% ACC1  –   ACC4 100% Water-cement ratio was 0.5 for mortars without admixtures. Amount of water was reduced due to water content in admixtures in mortars modified with admixtures. 4 types of non-chloride accelerating admixtures were used. They were added in quantity of 50 % and 100 % of manufacturer recommended dosage. Portland cement mortar was modified with 2.5 % (by cement mass) of calcium chloride  –   the most efficient accelerator for concrete  –   for comparison of results with other admixtures. Calcium chloride usage is limited due to reinforcing steel corrosion risk. Characteristics of accelerators are given in table 2. Exact composition of mortars is given in table 3.  208  Jan Pizo ń  / Procedia Engineering 195 ( 2017 ) 205 – 211 2.3.    Methods Examinations were provided in accordance to standard EN 196-1:2006 Methods of testing cement, determination of strength. Components and surrounding temperature during samples forming was stable and equal to 20±1°C. Specimens were cured in stable condition s of climatic chamber. Air temperature was equal to 20±1°C and relative humidity was equal to 60%. 3.   Results and discussions Results for non-modified mortars are predictable. Compressive strength after 28 days is higher for lower ground granulated blast furnace slag content. In later terms compressive strength of the specimen with 6% of GGBFS is the highest. Compressive strength of mortar with 20% of slag is close to Portland cement one. Mortar with 35% of slag obtains almost 28th day compressive strength of Portland cement one after 90 days (fig. 1). It can be said that mortar with low addition of ground granulated blast furnace slag may successfully replace Portland cement mortars. Fig. 1. Compressive strength of cement mortars without and with addition of 6, 20 and 35% of ground granulated blast furnace slag Compressive strength of hardening accelerating admixtures modified Portland cement mortars after 28 days is higher or similar to non-modified ones. This concern maximal dosage and half of it of admixtures based on calcium formate, calcium nitrate, calcium chloride and CSH crystal seeds (ACC 1  –   3 and ACC 5). In case of TEA based on (ACC 4) compressive strength is lowered (fig. 2 and 3). In longer terms increments of compressive strength is lower in every case and strength itself is always lower than of non-modified mortar. It is noticeable that mortars modified with admixtures ACC 1 and ACC 2 in half of maximal dosage reach similar compressive strength as calcium chloride (ACC 5) modified mortars. CaCl is considered to be the most effective accelerating admixture.
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks
SAVE OUR EARTH

We need your sign to support Project to invent "SMART AND CONTROLLABLE REFLECTIVE BALLOONS" to cover the Sun and Save Our Earth.

More details...

Sign Now!

We are very appreciated for your Prompt Action!

x