CN103760036B - A kind of method of testing of steel fiber reinforced concrete destructive test initial cracking load - Google Patents

Abstract

The present invention relates to the method for testing of a kind of steel fiber reinforced concrete destructive test initial cracking load, belong to fracture parameters of concrete technical field of measurement and test.During steel fiber reinforced concrete coped beam test specimen of the present invention cracking, the stress intensity factor of front edge area, prefabricated crack reaches Fracture Toughness of Concrete, and steel fibre action of crack arrest does not plays, the stress intensity factor of external load generation and Fracture Toughness of ConcreteK _ICEqual, in the stress intensity factor that external applied load produces with external loadK ‑POn curve relation figure, the stress intensity factor numerical value that external load producesK _ICCorresponding external load value i.e. can determine that initial cracking load.Method of testing equipment needed thereby of the present invention, analysis and processing method are simple, and expense is low, and precision is high.

Description

A kind of method of testing of steel fiber reinforced concrete destructive test initial cracking load

Technical field

The invention belongs to fracture parameters of concrete technical field of measurement and test, be specifically related to a kind of employing coped beam three point bending test and survey Determine the method for testing of steel fiber reinforced concrete initial cracking load.

Background technology

According to double-K fracture criterion, in Fracture Process of Concrete, fracture propagation can be divided into the crack initiation in 3 stages, i.e. crack, split The stable extension of seam and the unstable propagation in crack, the critical stress intensity factors of this unstable propagation is fracture toughness K_IC, Fracture toughness reflects concrete material resisting crack extending capability, is by engineering structure Crack Control and judges that its crack is expanded The Important Faults parameter of exhibition stability.

The determination method of concrete initial fracture toughness, is typically all and first determines that initial cracking load calculates public affairs further according to initial fracture toughness Formula is obtained, and therefore, initial cracking load determines that the determiner of initial fracture toughness, and what initial cracking load determined whether accurate directly determines The accuracy quality of the fracture toughness that surely ftractures.

Fracture mechanics test is used to determine that concrete initial cracking load is frequently with following several method:

(1)P-δ_mCurve method

Concrete for hydraulic structure destructive test code (DL/T5332-2005) proposes according to P-δ_m(load and crack mouth open position to curve Move curve) ascent stage in be changed into the turning point of curved section as initial cracking load from straightway, but at P-δ_mOn curve accurately Determine that turning point is the most difficult, be only applicable to large and medium water power station engineering normal concrete and grinding coagulation soil cracking load really Fixed.

(2) photoelastic coating method

The method is that Photoelastic coating higher for optical sensitivity is pasted onto test specimen surface, and uses specific light path incident illumination Elastic patch, after test specimen stress deformation, uses photo-elastic instrument directly to survey crack under the loads at different levels reading to show on each test specimen photoelastic Coatings Regional area isochromatic fringe order, and with camera shooting record isochrome provision pattern, judge finally according to color change on paster Test specimen cracking determines cracking load.

(3) laser speckle photography method

The method uses laser instrument to launch laser, forms specific light path by special optical device and irradiates surface of test piece, and uses camera Recording light is shown up.Use camera double-exposure continuously step by step, i.e. egative film before and after loading is exposed face once respectively, it is thus achieved that under loads at different levels Laser specklegram.Then each double-exposure laser specklegram use Point-by-point analysis observe the change of its Young fringe, accordingly Judge crack occurrence and cracking load.

(4) supercritical ultrasonics technology

The method uses nonmetal ultrasonic device to launch ultrasound wave, is interacted with test specimen by ultrasound wave, analyzes reflection, transmission Wave property with scattering, it is considered to ultrasonic energy attenuation change, sets up ultrasound wave wave amplitude with load(ing) point change in displacement graph of a relation, according to This judges crack occurrence and cracking load.

(5) electric resistance strain film method

The method uses at test specimen crack tip front edge bonding foil gauge, after concrete cracking, and answering of seam end both sides concrete Change can be released, and this principle stagnant occurs back in strain value, pastes highly sensitive little gauge length foil gauge in pre-formed joint end symmetry, Initial cracking load is determined by its strain hysteresis characteristic.

Use these methods to determine initial cracking load, experimental technique and testing equipment required higher, the component less for strain and Structure, insufficient sensitivity, process of the test is complex, and process and analysis to result of the test also require that higher, conventional test bar It is difficult under part determine accurate cracking load.

Steel fiber reinforced concrete is mix in normal concrete that the short steel fibre of random distribution formed a kind of novel heterogeneous compound Material, the steel fibre of these random distributions can hinder the extension of inside concrete microcrack and the formation of macrocrack effectively, Significantly improve the tension of concrete, bending resistance, shock resistance and anti-fatigue performance, there is preferable ductility；Steel fiber reinforced concrete Fracture propagation is by steel fibre retardation, different from concrete, and it is complicated that the stress-strain field of crack tip changes relatively concrete, During fracture initiation, steel fibre meeting bridging cracking face, cause closure of crack surface, particularly when straining less, use existing technology Means are difficult to the most fixed, and process is complex.

Summary of the invention

It is an object of the invention to provide a kind of action of crack arrest, easy and simple to handle and can accurately determine steel fibre considering steel fibre The method of testing of concrete by three-point bending destructive test initial cracking load.

For achieving the above object, the present invention adopts the following technical scheme that

The principle of the present invention: according to Theory of Fracture Mechanics, the stress intensity factor of front edge area, coped beam test specimen incipient crack increases To a certain marginal value Fracture Toughness of Concrete, incipient crack extends, i.e. test specimen cracking, and the action of crack arrest of concrete cohesive strength is transferred Being undertaken by steel fiber reinforced concrete closing force, fracture open displacement increases, fracture propagation.During this, front edge area, crack stress Intensity factor K_I, external load produce stress intensity factorIt is reverse that crack process zone steel fiber reinforced concrete closing force is formed Stress intensity factorThere is a following relation:

$K_I=K_I^p-K_I^f$ ①

In formula: K_IFront edge area, crack stress intensity factor, Mpa m^1/2；

The stress intensity factor that external load produces, Mpa m^1/2；

The reversal of stress intensity factor that crack process zone steel fiber reinforced concrete closing force is formed；

Before test specimen ftractures, steel fibre does not play action of crack arrest, the reversal of stress intensity that steel fiber reinforced concrete closing force is formed because of Son is zero, i.e.When steel fiber reinforced concrete coped beam test specimen ftractures, the stress intensity factor of front edge area, prefabricated crack Reaching Fracture Toughness of Concrete, steel fibre action of crack arrest does not plays, and stress intensity factor and concrete fracture that external load produces are tough Degree K_ICEqual, that outer load produces with external load stress intensity factorOn curve relation figure, what external load produced should Force intensity factor value K_ICCorresponding external load value i.e. can determine that initial cracking load, and it comprises the following steps:

(1) one group of edge of a knife being arranged symmetrically in concrete coped beam undercut both sides, it is pre-that another group edge of a knife is arranged symmetrically in beam The both sides that incipient crack processed is most advanced and sophisticated, the edge of a knife that clip tens(i)ometer grooving is clamped on coped beam；

(2) on coped beam, loading load continuously, realizing automatic data collection by computer data acquisition system, until being added to Can stop after peak load loading, external load and crack tip opening displacement relation curve P-δ can be obtained_tWith external load with split Sealing opening displacement relation curve P-δ_m；

(3) pouring steel fiber reinforced concrete coped beam test specimen simultaneously, pouring according to corresponding match ratio and be not spiked with steel fibre coagulation Soil test specimen as a comparison, according to Fracture Toughness of Concrete K in concrete for hydraulic structure destructive test code DL/T5332-2005_ICMeter Calculate formula, determine Fracture Toughness of Concrete K_IC；

(4) being processed by data can be by the P-δ of Real-time Collection_tWith P-δ_mCurve through P-e transformation isCurve,With K on graph of a relation_ICValue is done parallel with axis of abscissas straight line, its withCurve intersection, crosses its intersection point o₁Do horizontal stroke The vertical line of coordinate axes, itself and the intersection point o of axis of abscissas₂Corresponding load is the gravity sum of initial cracking load and test specimen；Its In, described P-e relation is the relation of external load and fracture propagation length.

K in described concrete for hydraulic structure destructive test code DL/T5332-2005_ICComputing formula be: K_ICComputing formula For:

$K_{IC}=\frac{1.5(F_Q+\frac{mg}{2}\×{10}^{-2})\×{10}^{-3}\×s\×\sqrt{a_0}}{{\mathrm{th}}^2}f\left({\mathrm{\α}}_1\right)$ ②

Wherein, $f\left({\mathrm{\α}}_1\right)=\frac{1.99-{\mathrm{\α}}_1(1-{\mathrm{\α}}_1)(2.15-3.93{\mathrm{\α}}_1+2.7{{\mathrm{\α}}_1}^2)}{(1+2{\mathrm{\α}}_1){(1-{\mathrm{\α}}_1)}^{3/2}}$ ③

${\mathrm{\α}}_1=\frac{a_0}{e}$ ④

In formula, F_QInitial cracking load, kN, i.e. test specimen P-δ_mThe ascent stage of curve is changed into corresponding to the turning point of curved section Load；

Quality between m test specimen bearing, kg；

G acceleration of gravity, 9.8m/s²；

S span, m；

a₀Incipient crack length, m；

H height of specimen, m；

T specimen thickness, m；

E fracture propagation length, m.

DescribedCurve specifically determines that step is:

A. fracture propagation length e is determined: fracture open displacement refers between fracture faces measured under an objective referential Distance, it can specifically refer in particular to crack tip opening displacement δ_t, crack mouth opening displacement δ_m, under the conditions of three point bending test, Whole loading procedure, δ_tAnd δ_mAnd geometric similarity relation perseverance is set up between fracture propagation length e, the δ that can record according to test_t And δ_mFracture propagation length e is calculated according to geometric similarity relation:

$e=\sqrt{{\left(\frac{a_0}{{\mathrm{\δ}}_m/{\mathrm{\δ}}_t-1}\right)}^2-{\left(\frac{{\mathrm{\δ}}_t}{2}\right)}^2}$ ⑤

In formula: a₀For incipient crack length, mm；

B. the stress intensity factor that three point bending specimen external load producesAccording to Theory of Fracture Mechanics, under three-point-bending conditions The stress intensity factor that external load produces can use following formula to determine:

$K_I^p=F\·\frac{3sP}{2{\mathrm{bh}}^2}\sqrt{\mathrm{\π}(a_0+e)}$ ⑥

Wherein, $F=\frac{1.99-{\mathrm{\α}}_2(1-{\mathrm{\α}}_2)(2.15-3.93{\mathrm{\α}}_2+2.7{{\mathrm{\α}}_2}^2)}{(1+2{\mathrm{\α}}_2){(1-{\mathrm{\α}}_2)}^{3/2}}$ ⑦

${\mathrm{\α}}_2=\frac{a_0+e}{h}$ ⑧

In formula: P external load, including test specimen self gravitation effect, kN；

B test specimen cross-sectional width, mm；

H test specimen depth of section, mm；

S span, mm；

C. determineCurve: by P-δ_t、P-δ_mCurve obtains further through the conversion of P-e relationCurve calculates public affairs Formula.

The positive beneficial effect of the present invention:

Testing equipment the most of the present invention is simple, it is not necessary to use optics, acoustic equipment can complete test, without pasting strain Sheet, Photoelastic coating, required testing expenses are few.Load transducer can select suitable off-the-shelf hardware, clip according to test requirements document Tens(i)ometer may be used without finished product or oneself makes, the test that whole experimental test process is the highest to the requirement of testing equipment, common Under the conditions of just can complete test job.

2. the present invention is according to acquisitionCurve, i.e. can determine that cracking load, analyzes and processing method is simple, and precision is high, The cracking load maximum that method of testing of the present invention determines with Theory of Fracture Mechanics method differs all less than 2.0%.

Accompanying drawing explanation

Fig. 1 is that schematic diagram arranged by three point bending test clip tens(i)ometer；

Fig. 2 is fracture propagation length schematic diagram；

Fig. 3 is external load-fracture propagation length P-e curve relation figure；

Fig. 4 is stress intensity factor-load that external load producesCurve relation figure.

Detailed description of the invention

Below in conjunction with some specific embodiments, the present invention is further described.

Meaning representated by Fig. 1 label: the 1-edge of a knife, 2-clip tens(i)ometer.

Embodiment 1

See Fig. 1-4, use the test fracture open displacement of coped beam three point bending test, coped beam a size of l × b × h=515 Mm × 100mm × 100mm, prefabricated incipient crack length a₀=40mm, steel fiber reinforced concrete is MF05, i.e. steel fibre Volume fraction is 0.5%, and contrast specimen MF05-0 is not spiked with steel fibre, and Fracture Toughness of Concrete result of the test is shown in Table 1.

Concretely comprise the following steps:

(1) one group of edge of a knife being arranged symmetrically in concrete coped beam undercut both sides, it is pre-that another group edge of a knife is arranged symmetrically in beam The both sides that incipient crack processed is most advanced and sophisticated, the edge of a knife that clip tens(i)ometer grooving is clamped on coped beam；

(2) on coped beam, loading load continuously, realizing automatic data collection by computer data acquisition system, until being added to Can stop after peak load loading, external load and crack tip opening displacement relation curve P-δ can be obtained_tWith external load with split Sealing opening displacement relation curve P-δ_m；

(3) pouring steel fiber reinforced concrete coped beam test specimen simultaneously, pouring according to corresponding match ratio and be not spiked with steel fibre coagulation Soil MF05-0 test specimen as a comparison, according to Fracture Toughness of Concrete in concrete for hydraulic structure destructive test code DL/T5332-2005 K_ICComputing formula, determine Fracture Toughness of Concrete K_IC；

(4) being processed by data can be by the P-δ of Real-time Collection_tWith P-δ_mCurve through P-e transformation isCurve,With K on graph of a relation_ICValue is done parallel with axis of abscissas straight line, its withCurve intersection, crosses its intersection point o₁Do horizontal stroke The vertical line of coordinate axes, itself and the intersection point o of axis of abscissas₂Corresponding load is initial cracking load P_cWith the gravity sum of test specimen, rise Split load P_cThe results are shown in Table 1.

Embodiment 2

The present embodiment method of testing is same as in Example 1, somewhat different: steel fiber reinforced concrete is MF15, i.e. steel fibre Volume fraction is 1.5%, and contrast specimen MF15-0 is not spiked with steel fibre, fracture test of concrete initial cracking load P_cThe results are shown in Table 1。

Method of testing of the present invention is verified

According to Theory of Fracture Mechanics to test result analysis, steel fiber reinforced concrete closing force σ and crack tip opening displacement δ_t Meet formula 9. relation:

σ=a₁δ_t ²+a₂δ_t+a₃ ⑨

In formula: a₁、a₂、a₃For constant.

When σ is zero, i.e. there is not action of crack arrest in steel fibre, the corresponding test specimen cracking moment, front edge area, crack stress intensity because of Sub-theoretical value reaches Fracture Toughness of Concrete, and therefore, can use the relational expression that 9. formula determines, inverse σ is δ x when zero, then root The P-δ recorded according to test_tCurve, by δ_tDetermine the initial cracking load P of correspondence₀, it is shown in Table 1.

Table 1 initial cracking load test result

Test specimen is numbered	Steel fiber volume ratio	a1	a2	a3	P0/kN	Pc/kN	Test specimen is numbered	Steel fiber volume ratio	KIC/MPa·m1/2
										MF05	0.5%	-52.499	69.592	-11.481	1.948	1.916	MF05-0	0.0	1.233
MF15	1.5%	-75.364	84.404	-17.248	2.430	2.386	MF15-0	0.0	1.491

As known from Table 1, the initial cracking load maximum that method of testing of the present invention determines with Theory of Fracture Mechanics method differs and is all less than 2.0%, therefore use the method for patent of the present invention to determine that initial cracking load has enough accuracy.

Claims (3)

1. the method for testing of a steel fiber reinforced concrete destructive test initial cracking load, it is characterised in that steel fibre coagulation During soil coped beam test specimen cracking, the stress intensity factor of front edge area, prefabricated crack reaches Fracture Toughness of Concrete, Steel fibre action of crack arrest does not plays, the stress intensity factor of external load generation and Fracture Toughness of Concrete K_ICIt is equal, The stress intensity factor of outer load generation and external loadOn curve relation figure, the stress that external load produces Intensity factor numerical value K_ICCorresponding external load value i.e. can determine that initial cracking load, concretely comprises the following steps:

(1) one group of edge of a knife being arranged symmetrically in concrete coped beam undercut both sides, another group edge of a knife is arranged symmetrically with On the edge of a knife that the two side clamp type tens(i)ometer groovings most advanced and sophisticated in beam prefabricated incipient crack are clamped on coped beam；

(2) on coped beam, load load continuously, realize automatic data collection by computer data acquisition system, directly Can stop loading after being added to peak load, external load and crack tip opening displacement relation curve can be obtained P-δ_tWith external load and crack mouth opening displacement relation curve P-δ_m；

(3) pouring steel fiber reinforced concrete coped beam test specimen simultaneously, pouring according to corresponding match ratio and be not spiked with steel fibre The concrete test specimen as a comparison of dimension, according to coagulation in concrete for hydraulic structure destructive test code DL/T5332-2005 Soil fracture toughness K_ICComputing formula, determine Fracture Toughness of Concrete K_IC；

(4) being processed by data can be by the P-δ of Real-time Collection_tWith P-δ_mCurve through P-e transformation isBent Line,With K on graph of a relation_ICValue is done parallel with axis of abscissas straight line, its withCurve intersection, Cross its intersection point o₁Do the vertical line of axis of abscissas, itself and the intersection point o of axis of abscissas₂Corresponding load is crack initiation lotus Carry the gravity sum with test specimen；Wherein, described P-e relation is the relation of external load and fracture propagation length.

The most according to claim 1, the method for testing of steel fiber reinforced concrete destructive test initial cracking load, its feature exists In, K in step (3) concrete for hydraulic structure destructive test code DL/T5332-2005_ICComputing formula is:

$K_{IC}=\frac{1.5(F_Q+\frac{mg}{2}\×{10}^{-2})\×{10}^{-3}\×s\×\sqrt{a_0}}{{\mathrm{th}}^2}f\left({\mathrm{\α}}_1\right)$

Wherein, $f\left({\mathrm{\α}}_1\right)=\frac{1.99-{\mathrm{\α}}_1(1-{\mathrm{\α}}_1)(2.15-3.93{\mathrm{\α}}_1+2.7{{\mathrm{\α}}_1}^2)}{(1+2{\mathrm{\α}}_1){(1-{\mathrm{\α}}_1)}^{3/2}},$

${\mathrm{\α}}_1=\frac{a_0}{e}$

In formula, F_QInitial cracking load, kN；

Quality between m test specimen bearing, kg；

G acceleration of gravity, 9.8m/s²；

S span, m；

a₀Incipient crack length, m；

H height of specimen, m；

T specimen thickness, m；

E fracture propagation length, m.

The most according to claim 1, the method for testing of steel fiber reinforced concrete destructive test initial cracking load, its feature exists In, step (4) determinesThe step of curve is:

A. fracture propagation length e is determined: according to testing the δ recorded_tAnd δ_mCrack is calculated according to geometric similarity relation Extension length e:

$e=\sqrt{{\left(\frac{a_0}{{\mathrm{\δ}}_m/{\mathrm{\δ}}_t-1}\right)}^2-{\left(\frac{{\mathrm{\δ}}_t}{2}\right)}^2}$

In formula: a₀For incipient crack length, mm；

B. the stress intensity factor that three point bending specimen external load producesAccording to Theory of Fracture Mechanics, three-point bending Under the conditions of external load produce stress intensity factor following formula can be used to determine:

$K_I^p=F\·\frac{3sP}{2{\mathrm{bh}}^2}\sqrt{\mathrm{\π}(a_0+e)}$

Wherein, $F=\frac{1.99-{\mathrm{\α}}_2(1-{\mathrm{\α}}_2)(2.15-3.93{\mathrm{\α}}_2+2.7{{\mathrm{\α}}_2}^2)}{(1+2{\mathrm{\α}}_2){(1-{\mathrm{\α}}_2)}^{3/2}}$

${\mathrm{\α}}_2=\frac{a_0+e}{h}$

In formula: P external load, including test specimen self gravitation effect, kN；

B test specimen cross-sectional width, mm；

H test specimen depth of section, mm；

S span, mm；

C. determineCurve: P-δ_t、P-δ_mCurve obtains further through the conversion of P-e relationCurve.