Research on Fracture Toughness of C 120 Ultra-High-Performance Concrete in Kingkey Financial Center Project

2.1. Raw Materials. Following raw materials are used in this research: (1) cement (Nanjing Xiaoyetian PII52.5), (2) microballoon (Made in Kunming), (3) ganister sand (from Zhunyi, Guizhou), (4) Fine aggregate (sea sand, desalted, FM2.6∼2.8), (5) coarse aggregate (macadam, 5– 10, 10–20 mm in diameter), (6) Water reducer (BASF poly carboxylic acid, solid content: 40%, made in Guangdong), (7) water (Shenzhen tap water), and (8) polypropylene fiber (Grace 19 mm long fiber and short fiber provided by Shenzhen Lijian Concrete Company).


Mix Proportions.
Please refer to Figure 1.This test aims to see what the difference is in concrete with long fibers and short fibers, so the compositions of every group of test pieces are the same, except fiber types and quantities.

Mixing Process.
Get certain amount of raw materials ready.At first, put gelatinizer and sand into a mixer and run for 1 minute; then add water and additive, mix for 2 minutes; lastly, put in coarse aggregate, mix for 1 minute.For materials that are too sticky, mix 1 more minute after adding coarse aggregate.Find Mix Proportions in Table 1.

2.5.
Curing.Upon being demoulded, specimens were transferred to the caring unit and soaked into calcarea hydrica solution (lime water) for 28 days.Finished specimens were immediately sent to the Building Material Lab of the Civil Water Conservancy Institute of Tsinghua University.

Main Mechanical Property Tests
Results of cube compression test, prismoid compression test, and static modulus of elasticity test have been listed in Figures 1 and 2. Equipment used in the tests included YE-200A hydraulic pressure test machine, YJR-5 static digital    strain gauging unit, and YEW-200A dropping electrode compression-testing machine (Tables 2 and 3).

P-CMOD and σ-w Curve Charts for 4 Types of Concrete.
Through tests, we got the P-CMOD (loading-crack opening displacement) curve charts for basic concrete, concrete with 1 kg long fiber, concrete with 2 kg long fiber, and concrete with 2 kg short fiber, on which we came up with the O-W (softening relation) curve charts (Table 4).

Analysis
(1) From Figures 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15, we could tell that P-CMOD and O-W curves were similar for the same type of concrete, which meant test piece's dispersion was not large.(2) 4 types of concrete were almost the same, except fiber content, which only presented difference after cracking.Therefore, the cracking load for all 4 types of concrete was similar.
(3) As fiber content increased, concrete's tensile strength almost remained the same.
(4) As fiber content increased, concrete's cracking energy mounted too.When the fiber content remained the same, fiber type played a role in cracking difference.
Test results suggested that cracking energy rose in order of basic concrete < concrete with 1 kg long fiber < concrete with 2 kg long fiber < concrete with 2 kg short fiber.
(5) Characteristic length of brittleness "l ch " notates the brittleness of concrete.The smaller the number is, the more brittle the concrete is.From the test results, we learnt that as fiber content increased, concrete's toughness enhanced too.Characteristic length of brittleness varies in order: basic concrete < concrete with 1 kg long fiber < concrete with 2 kg short fiber < concrete with 2 kg long fiber, and toughness varies in order:basic concrete < concrete with 1 kg long fiber

Type
No. E (GPa)  < concrete with 2 kg short fiber < concrete with 2 kg long fiber.

Conclusions
(1) C120 ultra-high-performance concrete's fracture toughness can be enhanced by adding fiber, and as fiber content increases, cracking energy mounts too.When fiber content stays the same, fiber type makes difference too.
(2) Characteristic length of brittleness notates the brittleness of concrete.The smaller the number is, the more brittle the concrete is.As fiber content increases, concrete's toughness enhances too.

Table 2 :
Compressive strength test results of cube specimens.

Table 3 :
Results of compressive strength and static modulus of elasticity of prism specimens.

Table 4 :
Concrete fracture parameter test results.