PSI - Issue 23

Aneta Nowak-Michta et al. / Procedia Structural Integrity 23 (2019) 77–82 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Intruduction

The compressive strength (f c ) directly depends on the microstructure of the hardened cement paste. It is assumed in engineering practice that the strength of properly cured concrete is a consequence of the w/c ratio and the degree of compaction. Whereas the degree of compaction is the result of workability. The presence of pores in concrete significantly reduces compressive strength (Neville 2012). Pores in concrete: come from air entraining, are blisters of trapped air, are spaces remaining after removal of excess water or are a side effect of superplasticizers (SP) (Łukowski 2016, Neville 2012). Air entraining (AE) is a function of the density coefficient and voids from air entraining influence the structure the same as the remaining voids (pores accidentally caught, gel pores or capillary pores). The average drop in compressive strength is 5.5% per 1% of the air present in the mix (ACI 201.2R-01 2001, ACI Education Bulletin E4-03 2003, Du and Folliard 2005, Fagerlund 1997, Łukowski 2016, Neville 2012, Pigeon et al. 1996 , Ramachandra n 2001). AE causes a reduction in the number of capillary pores, which in effect makes a lower drop in strength in air entrainned concretes (AEC) with a low strength (Łukowski 2016, Neville 2012). AE acts in the mixture on the principle of the effect of the bearing balls, thus increasing its workability. The introduction of 5% of air, depending on the properties of the mixture, increases the density index by about 0.03 to 0.07, and the slump by 15-50mm (Neville 2012). SP liquefies the AEC mix, causes it to compact, and its negative effect is AE. SP reduces the effectiveness of the air entraining admixture (AEA) and consequently changes the porosity structure (Łukowski 2016, Neville 2012, Nowak-Michta 2015) responsible for shaping the compressive strength of concrete (Neville 2012).

Nomenclature A

total content of air in hardened concretes (%)

A 300 AE AEA AEC

micro air content (%)

air entraining

air entraining admixture air entrained concretes

D α L f c

density (kg/m 3 )

specific surface (mm -1 ) spacing factor (mm) compressive strength (MPa)

I D density indicator (%) NAEC non air entrained concretes SP superplasticizer Vp

total air content in concrete mix (%)

 f c

strength change (%)

2. Experimental section

2.1. Purpose and scope of research

The aim of the research programme was adopted to determine the effect of SP and AEA on the compressive strength in air entrained concretes (AEC) and non air entrained concretes (NAEC). The basic compositions of concrete mixes were constant (Table 1), the content of paste in all concretes was 30.71%. The water-cement ratio was also constant and its value has been determined experimentally and it was 0.46. The pre assumed highest value of the w/c ratio will be in the range of 0.45-0.55, for which, with the quantity of cement not exceeding 400kg/m 3 , consistency class S1 will be obtained. The main distinguishing feature of the composition of the concrete mixes were the contents of SP and AEA, which are given in the Table 2.